texts: In a chemical reaction, is energy always either lost or gained by the reactants? As much as I am concerned, changes in energy can occur during the absorption of heat or the emission of light or heat (these are the most often occurring). Is every chemical reaction accompanied by these conditions? Aren't there any chemical reactions without these conditions? Usually when we observe common chemical reactions (sugar that is melting in the water), we don't see any emissions or absorptions. Are these changes so trifling that we can't feel them (while they actually exist), or there are reactions with no emissions or absorptions?


texts:
I was thinking about how Daniell cell works and got into a serious doubt.

Let's consider a half cell of Zn. In standard conditions, it'll have an electrode potential of -0.76V with respect to SHE. If I now increase the zinc ion concentration, I know from Nernst equation that it's electrode potential (still with respect to SHE) should become less negative. Intuitively also, it's tendency to get reduced will be more due to Le Chatelier's principle, so more reduction potential as predicted by Nernst equation.
So far so good.

But, if I see it like this, then the results seem to get flawed:
When I increase zinc ion concentration, in the new equilibrium state that will be attained, the absolute potential of the electrolyte will be greater (due to more positive (zinc) ions), and a little bit (safe to neglect it's consequences) of negative charge on Zn electrode will be decreased, with a little deposition of Zn.
Zn2+ + 2e-  <--->  Zn
So the net effect would be of electrolyte solution becoming more positive. Now, the definition of  "reduction" potential is the (absolute) potential difference between the reduced and the oxidised species (along with some additive constant that makes SHE's electrode potential 0V, but it's of no concern here as we're seeing where the electrode potential (of Zn) increases or decreases, NOT its absolute value); hence, E = V(Zn electrode) - V(Zn2+ in electrolyte). So this should get more negative as the potential of electrolyte increases. (Potential of the electrode also becomes more negative, but its effect is marginal.)

So what is the flaw in this perspective?


texts: Could the density of an aqueous solution be less than 1g/ml ? For an example, can 0.98g/ml be taken as a density for an aqueous sulphuric acid solution ?


texts: Conformational analysis determining forces

texts:
So the determining forces in the conformational analysis (what will be the more stable state and what will be the more excited state) according to my textbook are:

Steric interaction
Electron repulsion
Interaction of bonding and anti bonding molecular orbitals (for example $\sigma$ and $\sigma^{*}$ in ethane)

Electron repulsion is quite clear. Filled (molecular) orbitals repulse each other. I have no clue however what is meant with steric interaction, my textbook makes a small remark about it saying it is a repulsive force when atoms come to close to each other. It notes that in the conformation of ethane this hardly plays a role because the hydrogen atoms are to small to interact. But isn't this the same as electron repulsion? What else makes (neutral) atoms want to avoid each other?

Then the last part of bonding and anti bonding orbital interaction (overlap) stabilizing the staggered conformation of ethane is also not so clear to me. How does partially putting electrons in a higher energy molecular orbital (anti bonding) make a molecule's conformation lower in energy?


texts:
Can you please explain to me why are A and E not relevant resonance structures of the molecule F?
And is B aromatic?


texts: How can ionic substances have an infinitely-repeating lattice structure?

texts: The title's pretty much it. At school, we're taught that ionic substances are composed of an infinitely-repeating lattice, with atoms at fixed angles from other atoms. How is this possible when I can get an individual grain of salt? Clearly if there's an individual grain, it can't be infinitely repeating.


texts: Dynamic equilibrium in unsaturated solutions

texts:
One standard definition of equilibrium in beginner chemistry is that for a reversible reaction, (dynamic) equilibrium has been achieved when the rate of the forward and backward reactions are equal. We also define the equilibrium constant for a given reaction in terms of a ratio involving the concentrations of products and reactants at equilibrium.

Consider adding a small amount of solid $\mathrm{AgCl}$ to a sample of pure water, where the amount added is small enough that the solution does not become saturated. The associated reaction is

$$\mathrm{AgCl}(\text{s}) \rightleftharpoons \mathrm{Ag^{+}}(\text{aq}) + \mathrm{Cl^{-}}(\text{aq}), $$

with $K_{\mathrm{sp}}\approx 1.77\times 10^{-10}$. After the added amount of salt has dissolved, the concentrations of all species are constant since all of the $\mathrm{AgCl}$ has dissolved and if any solid $\mathrm{AgCl}$ is produced by the reverse reaction, it will be shortlived and will dissociate back into ions almost instantly.

Is it not therefore true that the forward and backward reaction rates are equal at this point? By the simple definition of equilibrium given above, we would then conclude that the system has reached equilibrium. On the other hand, the reaction quotient $Q$ is less than the equilibrium constant $K_{\mathrm{sp}}$ since the saturation point has not yet been reached, and therefore the system has not reached equilibrium.

I am seeking an explanation of why this apparent contradiction arises – I expect it may be due to the basic definition of chemical equilibrium simply being inadequate, and that for equilibrium to be 'properly' achieved (in the sense of the equilibrium constant), the forward and backward reaction rates must approach each other smoothly rather than abruptly as they do in this example due to the $\mathrm{AgCl}$ running out before saturation is achieved. Or perhaps at equilibrium, the reaction rates are necessarily equal, but the converse is not true.  You can assume that I'm familiar with thermodynamics and statistical mechanics.

Edit: The equilibrium constant here is calculated subject to the constraint that both a solid and an aqueous phase must coexist as pointed out in Charlie Crown's answer. This is also alluded to in Buck Thorn's answer where they note that there is no equilibrium with the solid below the solubility limit, and in a comment by Karl below their answer where they state that there is no dynamic equilibrium when the concentration of the solid is zero. Hence the apparent contradiction arises here because the static equilibrium achieved before the solubility limit has been reached is not the same as the dynamic equilibrium which $K_{\mathrm{sp}}$ refers to.


texts:
I'm doing exercises on hybridisation, and I was given this molecule:

I'm wondering about this (electron deficient) oxygen atom. My intuition says it should be ${sp^2}$ like the answer says, but honestly I only see that it's $sp$ hybridised. Is it because you ask what it would look like if it was not electron deficient?


texts: Modeling of photocatalytic and photoelectric devices on a macroscopic device level

texts:
As a computational materials chemist studying photocatalysis, how close to realistic systems can your modeling get?

I have a sort of high-level question about the theoretical modeling of photocatalysis. Most of the computational work on photocatalysis involves DFT calculations looking at band gaps, free energy profiles, and reaction pathways. I have also seen a handful of papers that use reactive MD to probe how the surrounding water molecules get affected by adsorbed species.

As a computational chemist or materials engineer, and you want to take your work closer to a real system, what is the logical next step? Would it be device modeling, where you take inputs from DFT (such as overpotentials), and then try to calculate the I-V curves, for instance? Are there any other multi-scale modeling techniques or coarse-grain methods that can be used?

I guess this question can be extended more generally to different classes of applications, such as $\ce{CO2}$ sensors, other electrochemical reactions, and so on. I'm not really sure what the next step would be to go beyond DFT in order to model more macroscopic length and time scales.


texts: Molar conductivity of transition metal complexes solutions

texts:
Which of the following has the highest molar conductivity?
(A) Diaminedichloroplatinum(I)
(B) Tetraamminedichlorocobalt(III) chloride
(C) Potassium hexacyanoferrate(II)
(D) Hexaaquochromium(II) bromide

Since mobility depends on the size, charge and the number of particles, how can we determine the extent of molar conductivity from the given data?
The solution provided to me considered only the number of ions that each gives. But is this justified? Because they all have different sizes, charges and thus different mobility.


texts: Is it possible for graphite which has shed off metal rods during water electrolysis to bond with the hydrogen being produced to form hydrocarbons?

texts:
I recently conducted a simple experiment for school involving the electrolysis of water.
I changed the concentrations of an ionic compound to determine its effect on the rate at which the products of the lysis of water are formed. I used graphite rods for the electrodes. These shed quite heavily.
I noted this down in my qualitative data, but I was wondering whether this excess graphite may have bonded with some of the hydrogen produced at the cathode to form hydrocarbons?


texts: What are good examples of questions that make college students think about chemistry concepts?

texts:
I am teaching the second semester of general chemistry at the college level. The course tends to get bogged down in calculations, and sometimes we forget about the big picture and what is going on at the atomic level. What are some good questions that would make students think deeply about the following topics:

Intermolecular forces
Kinetics of chemical reactions
Equilibrium and direction of reactions
Acid/base reactions
Redox reactions (electrochemistry)


texts: Confusion between bases (not alkali) and other -OH containing compounds

texts: Since some bases are not alkaline, we can't check whether they give $\ce{OH-}$ ions or not. Ethanol also has a $\ce{-OH}$ group. Why isn't it a base?


texts: What is the relationship between emission and fluorescence?

texts:
A molecule exposed to a photon with some energy is put into an excited state, and emits a photon of some energy when it returns to ground state.
The photon that provided the energy for excitation corresponds to a particular wavelength, at which we see a peak in an absorption spectrum.
The photon that was released when the molecule returned to its ground state also corresponds to a particular wavelength, at which we see a peak in an emission spectrum.
Fluorescence is when a molecule emits a photon of lower energy when it returns to ground state than what it absorbed initially to be put in an excited state. That photon corresponds to a particular wavelength, at which we see a peak in the fluorescence spectrum.
But isn’t the emission of a lower energy photon the case for all molecules, since all molecules lose some energy in form of vibration and rotation before they reach ground state? How is fluorescence distinguished from emission?


texts: How to select a schematic of the coordinate vs energy representing a rearrangement reaction?

texts:
Here's the question

For a reaction to take place the net energy should go down, and thus option (d) can be discarded.
We are now left with options (a), (b), and (c).
A $3^{\circ}$ carbocation is more stable than a $2^{\circ}$ carbocation, therefore the 3rd dip in the graph should be lower than the 2nd dip as more stable means lower energy.
Thus now we are left with options (b) and (c).
How do you decide between (b) and (c)?


texts: In Finkelstein reaction, how does precipitate formation favor forward reaction?

texts:
In my textbook it's stated that:

  $\ce{NaCl}$ and $\ce{NaBr}$ thus formed is precipitated in dry acetone. It facilitates the forward reaction according to Le Chatelier's principle.

Is this related to common ion effect in any way? I'm not familiar with Le Chatlier's principle. Please explain this in simple words as I'm in high school.

How does formation of $\ce{NaCl}$ and $\ce{NaBr}$ (both insoluble) prevent backward reaction from taking place? Does it mean $\ce{NaI}$ will be produced again?


texts: What do the stereochemical labels D and − mean in tartaric acid?

texts:
Why is the isomer of tartaric acid above labelled as ᴅ and (−)?

it rotates plane-polarized light clockwise
it rotates plane-polarized light anticlockwise
it is an equimolar mixture of the enantiomorphs
its configuration is related to that of ᴅ-glyceraldehyde

If I am not wrong, the ᴅ in ᴅ-glyceraldehyde refers to (+)-glyceraldehyde, and likewise ʟ-glyceraldehyde is (−)-glyceraldehyde. The (+) and (−) labels indicate that these isomers are dextrorotatory and levorotatory, respectively.
So in ᴅ-(−)-tartaric acid, what does the ᴅ mean, if not dextro, and what does the (−) mean, if not levo (the minus sign indicating anticlockwise rotation of plane-polarized light)? How can one molecule be both?


texts: Equilibrium reaction where the number of moles of reactants and products are the same

texts:
Le Chatelier's principle says that for an equilibrium reaction where all reactants and products are gases a change in pressure will change the number of moles of the product such to counteract the change in pressure.
My textbook also states that this only affects reactions in which the number of reactants and products are different.
I'm trying to think of a gaseous equilibrium reaction where the number of moles of reactants and products are the same.
All suggestions welcome.


texts: Pressurized Line and Rigid Tank Question

texts:
Using a pressurized line, a fluid is charged in a rigid tank with volume
$2m^3$. Derive the expression and calculate:

The internal energy $u_t$ of the fluid in that tank supposing the initial mass is
, and specific enthalpy $h_i=1670kJ/kg

Assuming a fluid is an ideal gas, derive and calculate the tank temperature using the specific heat ratio of k = 1.67 and the initial temperature of $T_i=400 ^{\circ}C$

Assuming a fluid obeys ideal gas law and a flow is choked derive and calculate the tank pressure as a function of time using $\dot{m}=0.5kg/s,t=2s,k=1.67, T_i=400^{\circ}C$.

Assume that charging occurs over a short period of time.
Starting with the control volume energy rate balance,
$$\frac{dE_{cv}}{dt}=\dot{Q}_{cv}-\dot{W}_{cv}+\sum_{i}\dot{m}_i(h_i+\frac{V_i^2}{2}+gz_i)-\sum_{e}\dot{m}_e(h_e+\frac{V_e^2}{2}+gz_e)$$
we have, after ignoring kinetic energy and potential energy effects, taking into account that no mass is exiting the tank, and also assuming the tank is rigid and insulated, we have
$$\frac{dU_{cv}}{dt}=\dot{Q}_{cv}-\dot{W}_{cv}+\sum_{i}\dot{m}_ih_i-\sum_{e}\dot{m}_eh_e$$
Upon integration and solving numerically, we have
$$\int_{0}^{t}\frac{dU_{cv}}{dt}dt=h_i\int_{0}^{t}\frac{dm}{dt}dt\Rightarrow U_{cv}=mu=mh_i\Rightarrow h_i=u=1670\frac{kJ}{kg}$$
For part 2, using the ideal gas model we have
$$u(T)-u(T_{i})=c_v(T-T_i)=RT_i$$
$$h(T)-h(T_i)=c_p(T-T_i)=RT_i+R(T-T_i)$$
I'm not sure where to go from here.


texts: What is the theoretical justification for state averaging in SA-CASSCF?

texts:
According to the molpro manual, section 7.3:

  In order to compute excited states it is usually best to optimize the
  energy average for all states under consideration. This avoids
  root-flipping problems during the optimization process and yields a
  single set of compromise orbitals for all states.

Is see that SA-CASSCF is quite standard in the literature, even in cases when root-flipping wouldn't necessarily cause an issue, and I don't understand why making such compromise to obtain a single set of orbitals makes theoretical sense. Isn't it better to find the set of orbitals and the CI vector that minimizes the energy for each of the roots of interest?


texts: What is the major product in the reaction of 2,3-dichlorobutane with sodium amide in liquid ammonia?

texts:
I am not able to decide which among 1,3-butadiene and 2-butyne is the major product of this reaction. Will there be even a major product out of the two or will there be a mixture?

According to me, the product formed from the 1st mechanism should be the major product as it is more stable (two double bonds in a conjugate alkene are more stable than a triple bond). But I also think that product formed from mechanism II can be formed faster as the hydrogen being picked up in the first step of mechanism II is more acidic (due to the electron withdrawing effect of electronegative chlorine atom) as compared to the hydrogen being picked up in mechanism I.

Another factor, which seems to favor the formation of product by mechanism I, is the picking of hydrogen atom in the 2nd step which is easier from an sp3 carbon than from an sp2 carbon (in mechanism II).


texts:
We were asked the reaction between 1 mole acetone and 1 mole $\ce{Br2}$ in basic medium. Since only a limited amount of bromine was given I got the final answer as 1-bromopropan-2-one (mono bromo acetone). However, in the given solution the entire haloform reaction was shown with $\ce{CHBr3}$ and acetate ion as the final product.

I am not sure how a single mole of $\ce{Br2}$ is sufficient for this(at least 3 moles are needed, I believe).


texts: Will Raman photon counts/Spectral resolution increase if laser beam becomes parallel?

texts:
Most Raman spectrometer has the laser converging into focus with the sample at center like this.

Usually how many millions of H2O atoms will be covered by one spot size?
What will happen if the laser beam would be made parallel instead of converging. Will it increase the counts of the spectrum? Will it improve the spectral resolution? Or will both become worse?
Water Raman:


texts: Why aren't soldering tips coated with platinum?

texts:
The tips in a soldering iron tend to oxidize quite quickly (therefore becoming unuseable) if left heated and unused for more than a few seconds.

Why aren't they coated with platinum or some other material that sticks to solder and doesn't dissolve in it? The cost of the coating should pay for itself fairly quickly.


texts: How to prepare 4-aminophenyl zinc iodide?

texts:
I'm trying to follow a published synthetic procedure from Direct preparation of new organozinc reagents, aminophenylzinc iodides, and their applications:[1]

$$\ce{H2N-Ph-I + \underset{(Rieke)}{Zn} -> H2N-Ph-ZnI}$$

However, I can not manage to reproduce the results.
The Rieke zinc was prepared by lithium naphthalide and $\ce{ZnCl2}$.

[1] Jung, H.-S.; Kim, S.-H.; Tetrahedron Lett. 2015, 56 (8), 1004–1006 (DOI: 10.1016/j.tetlet.2015.01.053).


texts:
What's the strongest known organic acid? Is it maybe trifluoromethanesulfonic acid or tautomer of pentacyanocyclopentadiene?

EDIT: Since the question was reactivated I thought about formalising it. I can think of a few categories of compounds: neutral (actually created or only studied in silico) and cationic (also two options). Second Dave's answer and ANM's one do the trick for two groups, but two other options remain (still prefer answers without boro/silico/etc. stuff).


texts: What is the hybridization state of E (= Sn, C) in ECl₂?

texts:
What is the hybridization state of tin in tin(II) chloride, $\ce{SnCl2}$, and is the explanation similar for dichlorocarbene, $\ce{CCl2}$?

For $\ce{SnCl2}$, I read in my book that it is sp² but I am not able to explain it. I am not sure about the carbene.

I might have a misunderstanding about hybridisation, I think that the energy required for hybridization has to be compensated by the energy released in forming bond between hybridized orbitals.


texts: How important is it that geometry be optimized at a high level of theory?

texts:
I've been doing research with a computational chemist for a little while now and in one of our projects we are dealing with a rather large system and because I go to a relatively small university, our resources are limited. Thus, when we began to ramp up the calculations in the project, we opted not to re-optimize our structure at CCSD(T)/def2-ATZVPP despite the fact we are doing the rest of our calculations at that level of theory with that basis set. The reason we chose not to optimize is we have a limited amount of time/resources and we estimated the optimization would take around 45-50 days to complete which would basically hog a computer for that long and stall the other projects we are working on.

I'm wondering how significant of an error, if much at all, that will introduce?

To give a few more details, we are studying vibrational properties of the system using a local mode approach. The structure we have is optimized at MP2/def2-ATZVPP.

Regardless, I'm just looking for a general understanding of how important the optimized structure might be. It seems like it would be less important, but could be quite dependent on what one is studying, than other things based on the fact that no system is truly at that optimized structure, but is only in that geometry on average. Is that correct at all?


texts: Is there a relationship between an electron's energy and its distance from the nucleus?

texts:
I have read that in Bohr's model of the atom electrons in the same atomic orbital have the same energy, and that this energy (as a result of the electron existing in a fixed atomic orbital) is related to the distance of the electron from the nucleus. However, using the quantum mechanical model of the atom, in which an orbital is the area of space around an atom where the probability of interacting with an electron is greatest, it is implied that there is no relationship between an electron's energy and its distance from the nucleus. Is there no relationship, or have I misunderstood something else?

Also, it is often mentioned that an atom is 99.99% empty space etc., but since the probability of interacting with an electron is described by its wave function (which suggests it is possible, though unlikely, to interact with an electron very close to the nucleus), this suggests to me that this isn't necessarily true. What am I misunderstanding?


texts:
By oxidation of 9 g of glucose it is released 150 kJ of heat. Calculate the standard enthalpy of derivation of glucose (in kJ/mol), if the given enthalpies of the reactants are ΔfH (H2O(l)) = -285,8 kJ/mol; ΔfH (CO2(g)) = -393,5 kJ/mol. Mr (glucose) = 180 (The answer is -1075.8)

$$\ce{6 CO2 + 6 H2O -> C6H12O6 + 6O2}$$

$$n(\ce{C6H12O6})=\frac{\pu{9g}}{\pu{180 g mol^-1}}=\pu{0.05 mol}~\ce{C6H12O6}$$

Now, $$\frac{\pu{0.05 mol}}{\pu{150 kJ}}=\frac{\pu{1 mol}}{x}\implies x=\pu{-3000 kJ/mol}$$

$$\Delta_\mathrm rH=\Delta_\mathrm fH(\ce{H2O(l)})+\Delta_\mathrm fH(\ce{CO2(g)})-\Delta_\mathrm fH(\ce{C6H12O6(s)})$$
$$= 6 \times (\pu{-285.8 kJ/mol}) + 6 \times  (\pu{-393.5 kJ/mol}) - (\pu{-3000 kJ/mol}) = \pu{-1075.8 kJ/mol}$$

Did I take the right steps?


texts: What will form in the hydrolysis of urea in urine?

texts: I just wanna know what is the end product of hydrolysis of urea in urine. I know that the hydrolysis of urea will form ammonia and a subsequent rise in pH but will the ammonia react with the water from urine to form ammonium and hydroxide?


texts: Reference for electronegativities of different metal oxidation states

texts:
A long time ago I was researching the effect of the self regulatory response in Fe and Co. I found that my results made sense based on the idea of the electronegativity of the ions considered. I found a  webpage that listed the relationship between the different electronegativities for different oxidation states of Fe and Co.

My problem is I didn't save the webpage and I don't know where I found it. I am not a chemist so I don't know in which kind of books I could find a list of values of electronegativities for different oxidized ions.

It basically said that the relationship between the electronegativities was something like

$$\ce{Co^2+} < \ce{Fe^2+} < \ce{Fe^3+} < \ce{Co^3+}$$

Note: I don't remember it well, so what I wrote could be lies. The important thing was that there was a flip of order in electronegativities for Fe and Co when changing oxidation state.

Does anyone know where I could find that kind of information? I tried googling, but I am not finding the webpage and don't remember the terms I used to find it in the first place.

I never understood the why of the order either, and I have seen chemist webpages that tell you which ion is more electronegative by just looking at it. If someone could explain that too I would appreciate it.

Edit:

I forgot to add that it was in octahedral complexes. I was able to find this pdf online (page 32) but it doesn't state a reference.


texts:
Why is AlCl3 or FeCl3 might be preferred for chlorination reactions and AlBr3 or FeBr3 for bromination reactions? Why not use AlCl3 for bromination reactions?

Source: https://www.masterorganicchemistry.com/2011/07/22/reagent-friday-aluminum-chloride-alcl3/


texts: Result of glucose with acetic anhydride and pyridine?

texts: What is the result when glucose is mixed with excess acetic anhydride and excess pyridine?


texts:
I'm looking to find whether water or methanol is more polar, and I'm getting conflicting answers.

My textbook says "the polarity of a bond is quantified by the size of its dipole moment." According to this table, at 20 °C, the dipole moments of methanol and water are 2.87 D and 1.87 D. This implies to me that methanol is more polar.

But there seems to be another measure called "polarity index," which I can't seem to find a definition for online. This table says the polarity indices of methanol and water are 10.2 and 5.1, which implies to me that actually water is more polar.

So what's the difference between the dipole moment and the polarity index? How is the latter defined? And is methanol or water more polar?


texts: In most of silver electroplating (or electrorefining) methods, it is mentioned that silver anode is oxidized in aqueous solution of silver nitrate, and that silver is deposited at the cathode (regardless of cathode material, let's assume it's also silver for the sake of simplicity). What I don't get is why would silver anode be oxidized before $\ce{OH-}$? The reduction potentials are ~0.8 V for silver and ~0.4 V for $\ce{OH-}$, at standard conditions.


texts:
From my AP Chem Prep Book:

What should you do if you spill sulfuric acid on the countertop?
A) Neutralize the acid with vinegar.
B) Sprinkle solid NaOH on the spill.
C) Neutralize the acid with NaHCO3.
D) Neutralize the acid with an Epsom salt $(\ce{MgSO4})$ solution.

The answer given was C — stating that it should be $\ce{NaHCO3}$ because it is a weak base solution.
However, I do not understand why the strength of the base is of relevance (given the situation) and how to determine the strength of the base — at least relative to other bases.


texts: Representing woody biomass concentration for kinetic reactions

texts:
Woody biomass is composed of cellulose, hemicellulose, and lignin components. The following diagram provides representative formulas for each component:

Biomass can be represented by it's density, ultimate analysis, and composition. For example, beech wood can be represented by its density, $\rho = 700~\mathrm{kg/m^3}$, its ultimate analysis by weight percent such as $\ce{C}=49.05\%$, $\ce{H}=5.83\%$, $\ce{O}=45\%$, $\ce{N}=0.12\%$, and its composition by weight percent such as $\text{cellulose}=48\%$, $\text{hemicellulose}=28\%$, and $\text{lignin}=24\%$.

The following diagram represents a kinetic scheme for cellulose, CELL, which can be represented by the formula $\ce{C6H10O5}$:

The reaction rates for each pathway are first-order Arrhenius equations where $A$ is the pre-factor in $\mathrm{s^{-1}}$, $E$ is the activation energy in $\mathrm{kcal/kmol}$, $T$ is the temperature, and $\mathcal{R}$ is the universal gas constant:

$$
K = A\cdot \exp{\left(\frac{-E}{\mathcal{R}T}\right)}
$$

To apply the cellulose kinetic reaction to beech wood, I used the density of the wood, $700~\mathrm{kg/m^3}$ to calculate the initial concentration of cellulose as 48% of the wood density, $336~\mathrm{kg/m^3}$.  Using the initial mass concentration of cellulose, the reaction rates at a certain temperature can be applied to that concentration. This approach will give the mass concentration for each group (G1, G4, LVG). Notice that each group represents various chemical species on a mole basis.

Since the individual chemical species are represented on a mole basis, it seems like there are two approaches to solve this problem:

Mass Basis Approach

start with an initial mass concentration of the biomass
divide that mass conc. into cellulose, hemicellulose, and lignin components based on the weight % composition
apply the reaction rates on a mass basis for each component to determine the mass concentration of each group
finally use the mass concentration of the group to calculate the individual chemical species (as mass or moles?) in that group

Mole Basis Approach

represent the biomass as moles of C, H, O, N using the ultimate analysis
divide the biomass into cellulose, hemicellulose, and lignin components
apply the reaction rates on a mole basis to determine the chemical species
atomic balances for C, H, O, N should be checked at each step

How do I attempt to solve this problem on a mole basis?
Or is it best to treat it initially on a mass basis then convert to moles to solve for the individual chemical species?


texts: Is the largest number of molecules in 36 g of water or 54 g of dinitrogen pentaoxide?

texts:
Recently my teacher asked us which of the following has the largest number of molecules? He gave us two options:

$\pu{36 g}$ of $\ce{H2O}$
$\pu{54 g}$ of $\ce{N2O5}$

I'm stuck, because both of them are having two moles of each of the respective molecules. And technically, the number of molecules equals the amount of substance multiplied by Avogadro's number.

So shouldn't $\pu{2 mol} \times N_\mathrm{A} = N(\text{molecules})$ be the same number of molecules for both of them?

But yet, the teacher said that water is the correct answer.

And I'm still not able to figure out why?


texts: If I understand everything correctly, carboxylic acids behave like acids because oxygen is much more electronegative than hydrogen, so it takes his electron and releases H+. The problem is that alcohols have an OH group too, so why don't they behave like acids?


texts:
I saw

on the Internet and I'm curious as to what compound is represented by it.


texts: How to model iso-surface of molecular orbitals of singlet and triplet excited states

texts: I've modelled the iso-surface of the highest occupied molecular orbital of a molecule with DFT in Gaussian.If I were to model the iso-surface of the molecular orbital of the singlet and triplet excited states, how would I do that?Using tdDFT, I've already calculated the excitation energy of both the triplet and singlet manifold.


texts: Are there molecules that take the shape of every platonic solid?

texts:
Methane is the simplest molecule I know of that takes the shape of a platonic solid — in this case, the tetrahedron. Wikipedia has a page on platonic hydrocarbons that includes molecules with tetrahedron, cube, and dodecahedron shapes. There are lots of molecules that have octahedral geometry (e.g. sulfur hexafluoride).

Are there any molecules that form an icosahedron? The closest I've found is $\ce{C_60}$, buckminsterfullerene, which takes the form of an Archimedean solid, the truncated icosahedron.

To be explicit: I am interested in molecules with atoms positioned at the vertices of platonic solids; whether or not there are direct bonds between those atoms is not relevant.


texts: Which is the correct name: 2,3-dimethyl cyclopentene or 1,5-dimethyl cyclopentene?

texts:
According to the lowest locant rule, 2,3-dimethyl cyclopentene should be the correct IUPAC name as the sum of the locants of the substituents(2+3=5) of 2,3-dimethyl cyclopentene is lower than the sum of the locants of the substituents(1+5=6) of 1,5-dimethyl cyclopentene.

But my book says that the correct name of the compound is 1,5-dimethyl cyclopentene. I guess this is due to the fact that methyl gets the 1st locant position. But I am doubtful about this.

What is the correct name?


texts: Why is Cr(2+) a stronger reducing agent than Fe(2+) in water?

texts: According to me, $\ce{Fe^{2+}}$ should be a better reducing agent because $\ce{Fe^2+}$ - after being oxidized - will attain a stable $\ce{d^5}$ configuration, whereas $\ce{Cr^2+}$ will attain a $\ce{d^3}$ configuration. I think the half filled $\ce{d^5}$ configuration is more stable than the $\ce{d^3}$ configuration. Why is this not so?


texts:
This recent question about the gauche effect led me to read this rather older answer. The problem is that I notice I am still confused.

To take the case of 1,2 difluoroethane, I drew these orbitals:

Figure1: orbitals of $\ce{C2H4F2}$ in the gauche (1,2) and anti (3,4) confromations

Now I understand that 1 (gauche) leads to more stabilization than 3 (anti). But 2 (gauche) leads to less stabilization than 4 (anti). I don't know which effect is strongest: the destabilization in 2 or the stabilisation in 1. So I don't know why the gauche conformation (1,2) leads to more stabilization than the anti (3,4).

Did I draw something wong? Did I make a mistake in reasoning? Did I miss something else? Is this one too many error-acknowledgement questions?


texts: Why do compounds like SF6 and SF4 exist but SH6 and SH4 don't?

texts: Both $\ce{SF6}$ and $\ce{SH6}$ and $\ce{SF4}$ and $\ce{SH4}$ have the same central atom and the same hybridization, but my teacher specifically mentioned that $\ce{SH6}$ and $\ce{SH4}$ don't exist. I've looked everywhere but I can't figure out why? I'd appreciate some insight into the problem.


texts: What is the difference between "molecular mass", "average atomic mass" and "molar mass"?

texts:
I don't understand the difference between "molecular mass" and "average atomic mass". They seem like the same thing to me. Is it that average atomic mass is just the weighted average of the "weights"/masses of the isotopes whereas the molecular mass is the average of "weights"/masses of the average atomic masses of each element in the molecule.

Also, what's the difference between molar mass and molecular mass? Is it just that molar mass is expressed in Daltons and molecular mass is expressed in g/mol?

The hardest part about chemistry is keeping track of which people are using which terms, and which terms are outdated. Is this one of these "oh, we use this term now," type of thing?


texts: What does the number mean in Sylgard 170, Sylgard 184, etc?

texts:
My lab uses a lot of PDMS, a silicon-based organic polymer, to make microfluidic devices. PDMS is available in many different formulations, and we usually use Dow Corning's Sylgard 184. Recently we got a batch of Sylgard 170 (which is supposed to cure to a solid much faster than 184), and apparently there's a whole bunch of different Sylgard types with numbers ranging from 160 to 577.

My question is, what does that number in the name of each different Sylgard type mean?

I can't seem to find a clear answer online. I know it's not directly related to curing speed, since 182 cures slower than 184. My guess is that it refers to the number of monomer residues in the polymer. Does anyone actually know?


texts: Why a low conductance ion such as lithium is used in batteries?

texts: Lithium ions have a relatively low conductance (Wikipedia) so why are they used in batteries, and not sodium, potassium, etc.?


texts: What are the optimal conditions for the complete acid hydrolysis (using HCl) of potato starch?

texts:
I'm hoping someone can provide the specifics on the optimal conditions required for the complete acid hydrolysis (using $\ce{HCl}$) of potato starch?

I will be using an autoclave (~$\pu{15 psi}$ @ $\pu{120 degrees Celsius}$) to facilitate hydrolysation.

Specifically I'm after for the following information:

Concentration of dry starch required for the starch slurry.
Concentration of $\ce{HCl}$ required to adjust $\pu{pH}$ to required level.
What is the required $\pu{pH}$ for hydrolysation?
Time required to complete hydrolysation under above autoclave conditions.

Plus anything else that I may have missed....

Thanks!


texts: What are the similarities and differences of adiabatic and diabatic states?

texts:
The idea of an adiabatic potential energy surface or adiabatic state is closely related to the Born-Oppenheimer approximation. This has been discussed on this site before in some detail. People frequently talk about diabatic states as well. It seems based on the reading I have done that a diabatic state is not really counter to an adiabatic state as one might expect from the names, but is sort of a different representation when looking at processes involving multiple potential energy surfaces.

So, my question is, how are adiabatic and diabatic states/surfaces/processes (what word should I be using here??) similar and how are they different? Can both pictures be used to describe the same processes all of the time or only some of the time (or perhaps I'm mistaken and they are never the same)?

Also, I think this is just terminology, but where does a nonadiabatic process fit into all this? I assume this just means there is a crossing of surfaces and the Born-Oppenheimer approximation breaks down, but that isn't quite the same as what I'm asking above I think.


texts: Ring expansion in the reaction of (hydroxymethyl)cyclobutane and HBr

texts:
I saw a question on how to predict the product of a reaction between (hydroxymethyl)cyclobutane and HBr. The correct answer was given as 1-bromocyclopentane, however, I do not understand why a ring expansion would occur in the reaction.

My attempt: I would think that the final product would by (bromomethyl)cyclobutane. This is due to the fact that HBr would first protonate the alcohol, causing it to become a good leaving group. Afterwards, an $\ce{S_N2}$ reaction between this protonated alcohol and $\ce{Br-}$ would take place, as the substrate is a primary alcohol, where no ring expansion/carbocation formation could occur as the steps are concerted, resulting in (bromomethyl)cyclobutane.

Why is this mechanism wrong, and why does a ring expansion occur instead?


texts: Should I plate cast iron with copper first before nickel plating?

texts:
I have some antique handplane lever caps I would like to retore back to their former glory. They are made of nickel plated cast iron. The plating  pin holed and flaked in areas, creating rust spots.

I am planning on stripping the nickel using hydrochloric acid and electrolysis. I've heard that nickel has a much better affinity for copper than iron. Would it be a good idea to first plate the lever caps with copper and then plate them with nickel for a more durable finish?  Any other suggestions for improving the quality if the finish?


texts:
Why are diatomic oxygen molecules STILL reactive especially with metallic elements like sodium and copper even at room temperature?

You would think that since the two oxygen atoms already have the much needed 8 valence electrons when they bonded with each other that they wouldn't need to react with anything else but that doesn't seem to be the case.


texts: Why are multiple-quantum coherences (coherences with changes in magnetic quantum numbers, $|\Delta m| \neq 1$) unobservable in regular NMR experiments?


texts: What is the difference between dynamic and static electronic correlation

texts: I am confused. In the static correlation, we use combination of Slater determinants to account for electronic correlation (known as CI or configuration interaction) and the wave function is represented as a sum of various SD where the HF SD is the base function and higher excited SD contributes some parts. Similarly, in dynamic electronic correlation, we use basis set expansion to account for higher excitation orbitals. What is the fundamental difference between these two correlations? Both are essentially expanding the electronic space to allow more relaxation. What am I missing here? Thanks


texts:
Methane is burned with air in a continuous steady-state combustion reactor to yield a mixture of carbon monoxide, carbon dioxide and water.
The feed to the reactor contains: $\pu{7.8 mole \% }\ce{CH4}$, $\pu{19.4 mole \% } \ce{O2}$, $\pu{72.8 mole \% } \ce{N2}$. The conversion of methane is $90\%$ and the gas leaving the reactor contains a ratio of $\pu{8 mol}\,\ce{CO2}/\pu{1 mol}\,\ce{CO}$.

My attempt:

$\ce{2 CH4 + 3 O2 -> 2 CO + 4 H2O}$

$\ce{CH4 + 2 O2 —> CO2 + 2 H2O}$

Both of these reactions have $90\%$ conversion.
From the ratio, can I say that approximately $88.88\%$ of the methane in the feed produces $\ce{CO2}$ while $11.11\%$ produces $\ce{CO}$?


texts: Can the empirical formula be defined for berthollides? Is it equal to the molecular formula, or it can't be defined?


texts: Why do we use J(J+1) instead of J² in the rigid body rotor energy equation?

texts:
I looked for the reason, but it seems to be taken for granted that it must be $J(J+1)$ instead of $J$.

I'm making use of the analogy of the particle in a box energy where

$$E=\frac{n^2h^2}{8mL^2}$$


texts: Why polymers have a range over which they melt instead of having a specific melting temperature?

texts: Does it have something to do with the fact that all chains are not of the same length?


texts:
How would we assign denticity to bridging ligands?
For eg, consider $\ce{NH^-_2}$, it has 2 lone pairs and can hence act as a bridging ligand, making 2 metal-ligand bonds. So I thought it should be bidentate. However, my professor says it is monodentate, why so?


texts: Which group is more suitable for metal nanoparticles functionalization?

texts:
I have synthesized bimetallic nanoparticles (gold core with a silver shell). I have two choices for functionalizing them, through thiol group or amine group.
Which of them is more attracted to this kind of nanoparticles?


texts: Why does the solubility of some salts decrease with temperature?

texts: It is well known to us that the Solubility of solute in a solution increases with the increase in the temperature because, when the temperature increases the molecules of the solvent gain more kinetic energy. Thus the molecules move randomly and having greater distance from each other which is responsible for the large voids between them and it gives more space for the solute molecules between them to come in. However, there a few salts  like cerium sulphate , lithium carbonate sodium carbonate monohydrate, etc. whose solubility decreases with the increase in temperature. How it is so ?


texts:
Is there a way to turn azeotropic hydrochloric acid into a gel?

I am trying to remove barnacles from the bottom of a boat and normal hydrochloric acid works, but it is tedious because in liquid form it just runs off, so I have to keep applying it over and over, kind of wearing the barnacle away.

If I could turn the HCl into a gel then it could just be applied for a few minutes and remove the whole barnacle.


texts: Meanings of isopropyl, n-butyl, isobutyl, s-butyl and t-butyl and differences between them

texts:
I am having a hard time learning organic chemistry because I cannot understand the meanings of isopropyl, n-butyl, isobutyl, s-butyl and t-butyl. There is no proper explanation given in my book about them. I have also read pages on the internet but I ended up confusing myself more.

My questions:

What are they? Are they alkyl groups (substituents) like methyl and ethyl or just a portion of the molecule (not a substituent)?
How was their name obtained? (I believe it was obtained on the basis of their structure and i would like to know on what basis)?
How are the names for such type of groups for higher alkanes obtained?
Are these names a part of IUPAC nomenclature?


texts:
Given the possibility that metallic hydrogen may have recently been produced, and the massive media bloviation that followed, I am curious about some of the "properties" that I heard from multiple media outlets.

Does metallic hydrogen decompose to mono-atomic hydrogen, then recombine to diatomic hydrogen?
How much energy is produced by the decomposition?
How certain are we that it will be stable(ish) at STP?


texts: Calculating singlet-triplet gap and open-shell singlet character

texts:
I am trying to calculate the singlet-triplet energy gap ($S_0-T_1$) of a conjugated system (18 atoms). For the singlet $S_0$ state, assumed to be closed shell, the Gaussian options were

opt=tight b3lyp/6-311g(d,p) integral(ultrafinegrid)
...
0 1
...

and for the energy of the triplet $T_1$ state

opt=tight rob3lyp/6-311g(d,p) integral(ultrafinegrid)
...
0 3
...

Does the energy difference of those two calculations have any physical significance? (i.e. should have I used  ...td-ub3lyp...0 3 ... instead?)
I believe a system I am currently working on may have some open-shell character for the singlet ground state so I performed the following with the optimized geometry I got from a TD calculation of a triplet state:

#p ub3lyp/6-311+g(d,p) scf=(tight) guess=(mix,always) gfinput gfprint iop(6/7=3)
...
0 1
...

For the stability check, do I have to use the guess=(mix,always) on they keyword route as well?


texts: Half cells: difference between E(cell) = E(cathode}-E(anode) & E(cell) = E(oxidation) + E(reduction)

texts:
When do I use each of the above? If I use one or the other they give me difference results.

For example, I'm trying to put the Volts into an equation to find standard free energy change.

$$\Delta  G^\circ= -nFE$$

If I use the cathode minus anode I get a different answer to the oxide + reduct.


texts: Why is a comma used to separate hydrogen gas and platinum when using cell notation?

texts:
I have come across many online articles and books where I see the following:

  Gaseous electrode

  Gases such as $\ce{H2}$ and $\ce{Cl2}$ can act as anode and as cathode by loosing electrons ans by accepting electrons, respectively. For example,

  $\ce{Pt,H2 | H+}$ (acts as oxidation potential)
  or
  \begin{align}
\ce{0.5 H2 (g)(\pu{1 atm}) &-> H+ (aq) + e-} \\
E_\mathrm{cell} &= E_\mathrm{cell}^\circ -\frac{0.059}{1}\log{[\ce{H+}]} \\
 &= 0 + 0.059\mathrm{pH}
\end{align}

Aren't the two species in different phases? If so, aren't they supposed to be separated by a '|' and not by a comma?


texts: Are orbitals always filled in from closest to nucleus to farthest away?

texts:
In which orbital, 4f or 6s, would an electron have a greater likelihood of being near the nucleus.

I figured that the 6s would be closest, since it is filled in first when using the Aufbau principle.
However, does that necessarily imply that it is closer to the nucleus? Or would 4f be closer, since its principal quantum number is smaller?


texts: Why is the boiling point of para hydroxybenzoic acid lower than that of ortho hydroxybenzoic acid?

texts: As per the literature at 760mm of mercury, p-hydroxy benzoic acid has lower boiling point ($\pu{336^\circ C}$ - ChemSpider) than o-hydroxy benzoic acid ($\pu{372^\circ C}$ - ChemSpider). What is the reason behind this although for the former we can see intermolecular H-bonding?


texts: Does Zinc become black & dull after reacting with dil. HCl?

texts: In the reaction where $\ce{Zn}$ reacts with $\ce{HCl}$ to give off Hydrogen gas & Zinc Chloride solution, does the zinc turn a dull black color after a while? If so, why?


texts: For a compound to be aromatic, each atom in the ring must be a part of conjugation. Boron here has no lone pair, so how can 1H-borepine be aromatic as I was told by someone?


texts: How to add hydrogens to a crystal structure?

texts: PDB protein crystal structures most of the time lack hydrogen atoms. Usually I use PyMOL (h_add) but it does not always seem to be giving the correct result. What can I use for accurate hydrogen addition?


texts:
Disclaimer: Sorry if this is a noob question, I am a high school chem student.

So I was reading my textbook in preparation for the midterm, and it talked about exceptions to the octet rule.

I understand that they exist, and what they are, but how are the chemicals structurally stable?

I also know that they are reactive, but that still doesn't help.

How do they exist?

I was thinking something like only under high pressure/heat, but I can't find anything to back that up.  Amazingly, google hasn't help.

Can somebody please explain?


texts: Is so called "Polymerized LiNb (Lithium Niobocene)" a possible compound?

texts:
The so called "Polymerized LiNb (Lithium Niobocene)" is a fictional compound.

I heard about this substance being used for an interesting purpose. It reacts to mechanical stress or pressure by creating an electric charge (piezoelectricity) and, conversely, deforms, as if external pressure was applied, when voltage is applied. Apparently it was far more "piezoelectric" than any other similar compounds, i.e. it deformed far more drastically than other piezoelectric materials.

Does such a compound, or similar compounds, exist?


texts: Second Ionisation Energy in d Block elements

texts:
The second ionisation energy is expected to be highest for element
  with atomic number:

  22
  24
  25
  30

I had expected the answer to be option 2 because the second electron has to be removed from the stable half filled $\ce{3d}$ configuration. But the answer is option 4.


texts: Why is it referred to as "carbonation" and we drink "carbonated" beverages when carbonate is $\ce{CO3}$ while $\ce{CO2}$ (carbonite?) is present in carbonation?


texts: Synthesis of ethyl methyl ketone from 2-(4-hydroxyphenyl)ethyl methyl ketone

texts: Is it possible to reduce raspberry ketone (4-(4-Hydroxyphenyl)butan-2-one) to ethyl methyl ketone? How would I do this in practice?


texts: Electrophilic substitution on disubstituted benzene rings

texts:
If I have a strong deactivating group (like nitro) and a weak activating group (like methyl) attached to the benzene ring, which group's preference will direct the next substitution on the benzene ring? For example, if a chloro group is attached to meta-nitrotoluene, where will it substitute?

If I have both strong activating and also deactivating groups in the benzene ring, which position will be preferred for the next electrophilic substitution? For example, if I want to attach a chloro group to the meta-nitroaniline, where will it substitute?


texts: Comparing the rates of hydrolysis of different haloalkanes

texts:
The title may not completely explain my exact doubt so I'll explain it here:
My teacher gave me the following problem during a lecture about SN1 reactions and I had to determine the order of the rate of reaction ($k$):

As in the rate determining step of an SN1 reaction, the halogen (leaving group) would leave to form the carbocation. I simply looked at which $\ce{C-X}$ bond would easily break and concluded that:
$k_1 > k_2 > k_3$ where $k_i$ is the rate of the given reactions respectively.
I had another approach to this by looking at acid-conjugate base relations.
As $\ce{HI}$ is a strong acid, it's conjugate base, $\ce{I-}$ would be a weak base hence making it a good leaving group (Weak bases are generally good leaving groups).
Which was correct as my teacher further explained. But he mentioned some more points, which I think, weren't suitable to be talked about but it still confused me.

He mentioned the solvation of the respective halide ions ($\ce{X-}$) in a polar protic solvent (here, $\ce{H2O}$):
$$\ce{Cl-} > \ce{Br-} > \ce{I-}$$
as the iodide ion is larger in size, the water molecules don't surround and 'trap' it as much as they surround the chloride or bromide ion.
But here's where things got a bit confusing. Technically, if the chloride ion is solvated the most, it is trapped by the solvent and isn't a naked nucleophile anymore while the iodide is. So Iodide would have more tendency to attack the substrate back.
Looking at it this way, the chloride ion was seeming to be a better leaving group which it obviously isn't supposed to be.
So changing the way of looking at this is changing the conclusions as well. The former approach leads to the correct answer, which is:
$$k_1 > k_2 > k_3$$
Yet, the latter seems to indicate:
$$k_3 > k_2 > k_1$$

What's going on here? I would really appreciate if someone could clear out this confusion.


texts:
What is the criteria to identify the order of heat of combustion among different organic molecules?
(like the main points to be noted while selecting the order)

The answer being Z>Y>X.
The method tried is that for the bonds to break easily , if enough of electrons are there it would break easily so order being X>Z>Y,
Please tell where I am wrong.


texts: Would Zirconium (IV) Silicate be stable in a hot oxygen atmosphere?

texts: As per the title, really. I potentially need to protect a bit of pipework from hot oxygen (800-1000 K), and I've seen this refractory paint for sale. It says it's good up to 1750 C, but hot oxygen is another kettle of fish. I'd have thought since it's formed from oxides at high temperature and it doesn't lose any oxygen in the process that it'd be fine? Just thought I'd check with the experts here before burning a hole in my kit!


texts: Proportionality between number of absorbed photons and optical density

texts:
I am to show that the number of absorbed photons only is proportional to the optical density at low optical densities.

I do not know how to do this, but this reminds me of the linear range of absorbance vs concentration; at high concentrations, the relationships is no longer linear. I also feel that the "number of absorbed photons" is another way of saying "absorbance", and that "optical density" is another way of saying "concentration times optical path length", but I am not sure. I cannot find references that clarifies this for me, so I would appreciate some guidance.


texts:
My chemistry  book says that heat capacity varies with temperature. There is no further discussion.
Does the heat capacity of a given amount of substance really vary with temperature? If yes, why is it so?


texts:
So I was practicing for exams, and this is the specific question.

I get that structure 2 is possible through polymerization and 3 doesn't fit to be the polymer of cyclohexene. I'm kinda consfused about 1 tho, I see that the double bonds have broken and joined with another cyclohexene, but I'm not sure about how that works. Like, is it similar to the addition of electrophiles..if so why isn't this seen with non-cyclic alkenes?


texts:
So we say that 1 Amu (u) = 1/12 of the mass of a Carbon 12 atom.

Im not sure about this, but I believe that first we measured  the mass of 1 carbon 12 atom (This includes the mass defects that occur during the formation of nucleus) and then defined that 1 atomic mass unit has a value of 1/12 of the carbon 12 atom mass... Therefore one Carbon 12 atom has exactly 12 atomic mass units.

Until here everything is fine.

Now my question is... To define all the other elements isotopes atomic mass did we first measured each isotope atom mass and then said " If 1/12 of the Carbon 12 atom is equal to X grams and this X grams are equal to 1 atomic unit the then my isotope atom which has Y grams must have Y atomic mass units " did we work it out this way or did we use calculation to define other isotopes mass based on carbon 12 atomic mass ?

Basically what I want to know is, do we need to have the value of an atoms mass in grams or kilograms before we know how many Amu´s it has, or can we calculate it ?


texts: What exactly causes equilibrium to shift when pressure is decreased?

texts:
Take the reaction:

$$\ce{N2(g) + 3 H2(g) <=> 2 NH3(g)}$$

If pressure is decreased, equilibrium shifts to the left because it has more moles. I understand the basics of why this is, i.e. due to Le Chatelier's principle, but is there an actual explanation as to why the system does this (e.g. using collision theory)? What's actually happening to cause the shift?

My chemistry teacher says she doesn't know of an explanation.


texts: What is the oxidation number of the oxygen in BaO2

texts:
According to the book for a binary compound, first we assign the element with greater electronegativity its oxidation number (oxygen always -2 except in peroxides). So, we have
-2 x2 =-2. So, each oxygen would have an oxidation number of -2. The book says the answer is -1.

The only thing I can think of, is since barium is an alkaline metal, it has an oxidation number of +2, so each oxygen would be -1. But, it contradicts itself because in the rules. It says for a binary compound, "the element with greater electronegativity is assigned a negative oxidation number equal to its charge in simple ionic compounds of the element."


texts: Why is the chemical formula for vinegar CH3COOH what it is?

texts:
I am quite new to chemistry, but I would like to think that I have mastered the basics. This would include naming and writing out chemical formulas. However, once I had seen the formula for vinegar ($\ce{CH3COOH}$), I was bewildered.

It would have made sense to me that the formula would be more, say, "compacted" into just $\ce{C2H4O2}$. What is the reasoning behind this odd way of writing such formulas and when should I use it (it seems like this could be a huge gap in my knowledge)


texts: How to justify this contradiction in acid-base equilibrium of methanol and water?

texts:
Let's talk about the following compounds - water and methanol.

We know that methanol is slightly more acidic than water, because water's ability to donate a proton as an acid is reduced due to extensive hydrogen bonding.
The $\mathrm{p}K_\mathrm{a}$ values are approximately $15.5$ and $15.7$, for methanol and water respectively.
Now consider their conjugate bases, i.e. methoxide ion and hydroxide ion. It is very clear that methoxide is a stronger base compared to hydroxide, due to the +I (inductive) effect of the methyl group (electron releasing).

How is this possible? Isn't it contradictory?
It is a well known fact that a strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.

This is a consequence of  $\mathrm{p}K_\mathrm{a}$ + $\mathrm{p}K_\mathrm{b}$ = $\mathrm{p}K_\mathrm{w}$, where Kw is the ionic product of water, a constant at a given temperature.


texts: Why is the relative atomic mass of carbon not exactly 12?

texts:
Relative atomic masses of atoms of all chemical elements are numbers without units, being the value of proportion compared to  $\frac{1}{12}^\text{th}$ the mass of the carbon atom.

But the relative atomic mass of carbon is never 12! Instead, it is 12.01 or, more accurately,  12.011.

Why is this so?


texts: Why is Electrochemistry often the method of choice for determining Ksp values?

texts: I don't understand the relationship they have with one another. Please, someone explain.


texts: Determine standard enthalpy of formation of salt given other reactions

texts:
I'm a little confused by this question, perhaps my understanding of enthalpy isn't as good as it should be.

  For the compound $\ce{KNO3}$, which of the following reactions is used to determine the standard enthalpy of formation, $\Delta H^\circ_\mathrm f$?

  $$\ce{K(s) + N (g) + O3(g) -> KNO3 (s)\\
2K + N2 + 2O3 -> 2KNO3\\
2K + N2 +3O2          -> 2KNO3\\
K  + 1/2 N2 + 3/2 O2  -> KNO3}$$

I know that the $\Delta H^\circ_\mathrm f(\ce{KNO3})= \pu{−494.6 kJ/mol}$ from reading a table, I'm just confused about how do I get from that number to determine the formation of a compound.


texts: Why does chloromethane have a larger dipole moment than chloroform?

texts:
Why does $\ce{CH3Cl}$, methyl chloride, have a larger dipole moment than $\ce{CHCl3}$, chloroform?

Let us consider $\ce{C-Cl}$ bond moment to be $x$ and that of $\ce{C-H}$ bond to be $y$.

In $\ce{CCl4}$, the dipole moment of any three $\ce{C-Cl}$ atoms is balanced by the dipole moment of the fourth $\ce{C-Cl}$ bond dipole moment. This means the resultant of dipole moment of three $\ce{C-Cl}$ bonds in tetrahedral structure is equal to $x$, which is equal and opposite to the dipole moment $x$ of the fourth $\ce{C-Cl}$ bond.

Now, when we replace one of the chlorines with a hydrogen, the net dipole moment equals $x+y$, (as they are in the same directions). Similarly if we carry out the same procedure for $\ce{CH3Cl}$ it can be proved that its dipole moment is also $x+y$.

Where have I gone wrong?


texts: Why are the melting and boiling points of the group 1 metals lower than the group 2 metals in the same period?

texts: I thought it was because the group 1 metals are smaller than the group 2 metals but the answer sheet says it's because there are more valence electrons and a stronger positive charge in group 2 metals. But why does it matter if there's more valence electrons and a stronger positive charge?


texts:
I just learned about rate and order... I just want to confirm if these three ideas are correct:

If the first elementary step is the slow step, will the stoichiometric coefficients of the reactants in the rate determining step (RDS) be EQUAL to the order the concentration of the reactants in the rate equation is raised to?
e.g.
$$\begin{align}
\ce{2A + B &-> C} &(\text{slow})\\
\ce{C + B &-> D} &(\text{fast})
\end{align}$$

Will the order with respect to $[\ce{A}]$ be 2 and $[\ce{B}]$ be 1 if this proposed mechanism is correct?

If the balanced equation is the ONLY elementary step in the reaction mechanism, the stoichiometric coefficients of the reactants will be the same as the order the concentration of the reactants are raised in the rate equation
If the second elementary step is the slow step, will the stoichiometric coefficients of the reactants in the rate determining step (RDS) be different to the order the concentration of the reactants in the rate equation is raised to?


texts:
I'm an electrical engineer trying to understand a failure in a plastic part that I'm working with.

I am using a plastic called polysulfone- and the data sheet has a lot of words ending in "amine" that are listed as not-being compatible with said plastic.

I want to bond this plastic with an epoxy, but the very-vague datasheet states that the hardener contains 80% "Modified Cycloalipathic Amines".

With this admittedly limited information, should I look for a different adhesive?

The process temp is $5$ to $85~^\circ\mathrm{F}$ ($-15$ to $29~^\circ\mathrm{C}$).


texts: Why basicity of oxides increases down a group?

texts:
Take group $13$ oxides as an example. The elements in this group are $\ce{B,Al,Ga,In,Tl}$.
Boron oxide $\ce{B2O3}$ is acidic and $\ce{Al2O3}$ and $\ce{GaO3}$ are amphoteric while $\ce{In2O3}$ and $\ce{Tl2O3}$ are basic in nature.
What is the reason for this?


texts: Would adding phosgene to ortho-xylene lead to the formation of 2-indanone?


texts: Potassium Atom can act as reducing agent, yet ion cannot?

texts:
I am currently looking through a revision sheet, however it has caught me on a question 'Explain why a potassium atom can act as a reducing agent yet a potassium ion cannot'.

I have done brief searches, however none have given the answer I'm looking for. So please, why is this?

Also, if the explanation doesn't already include it; is this only the case with Potassium, and how is this to be determined? (How is it known whether it is referring to a cation or anion?)

Thanks,


texts: Why isn't a polar protic solvent used in the Finkelstein reaction?

texts:
Usually, in a nucleophilic substitution reaction, a stronger nucleophile replaces a weaker nucleophile.

In any polar protic solvent, the order of nucleophilicity is $\ce{F-}<\ce{Cl-}<\ce{Br-}<\ce{I-}$ (due to stronger solvation of the smaller anions).

While in any polar aprotic solvent, the order of nucleophilicity is $\ce{F-}>\ce{Cl-}>\ce{Br-}>\ce{I-}$.

Why does the Finkelstein reaction use acetone in spite of the fact that it is a polar aprotic solvent? Wouldn't using a polar protic solvent be better since it would make $\ce{I-}$ the strongest nucleophile of the group.


texts: Ionic radius of Al[3+] compared to K[+], Ca[2+], S[2-], and Cl[-]

texts:
I came across this question:

Which of the following is bigger?
$$\ce{K+, Ca^2+, Al^3+, S^2-, Cl-}$$

Now, I know that the radius decreases as the period progresses to the right and increases as we go down on the P.T. So, I get to this:
$$\ce{S^2- > Cl- > K+ > Ca^2+}$$
Also, I know that $\ce{Al^3+ < Al^0}$ and that $\ce{Al^0 > S^0}$. But I can't figure out how to connect that to the inequality above.


texts: Why is a phosphorus atom with three different substituents chiral, but nitrogen isn't? Nitrogen inverts fast, while phosphorus obviously much slower. How is that explained?


texts:
What's the degree of freedom of a critical point of a single component system?

In the critical point the difference of density between the liquid phase and the vapour phase is so small the are indistinguishable, hence considered as single phase. So according the gibbs phase rule (. F=C - P +2) the degree of freedom should be 2.

But critical point is a point. And the degree of freedom of a point in the phase diagram should be zero( for example, the triple point).

So what's the correct degree of freedom   of the critical point?


texts: Topic in Salt hydrolysis that I couldn't understand

texts:
Salts can be made up of all combination S of ( SA ,WA) and (SB , WB)
I understand how to deal with salts of strong acids with both strong and weak bases and also of strong bases with weak and strong acids but I want to understand how can we find the pH for the salts of weak acids and bases ? Also If you could explain the relationship of hydrolysis constant and the dissociation constants it would be appreciated.
Thank you in advance


texts:
I tried to google this but 'element' as a programming term complicated the search.

I'm looking for a site or article that covers how one would find each element in nature, in its most abundant form. For example, what's the best natural source/form of Sulfur? Beryllium? Tugsten? etc.

Any help is appreciated.


texts: How to remove fluorine from 2-[2-(3-bromophenyl)ethenyl]-5-fluoro-1,3-benzoxazole?

texts:
I tried to develop a synthesis strategy for 2-[(E)-2-(3-bromophenyl)ethenyl]-1,3-benzoxazole below. I was able to find the synthesis plan shown on the picture below, however my target molecule is one without a fluorine on the benzoxazole. Hence, my questions is: would this reaction go the same way if I were to react the benzoxazole without the halogen (F)? If not, is there a way to remove it from the target molecule?
Here's a link to the synthesis plan I currently have:

And here's my target molecule:

edit:
Here's the reagents and the procedure as stated on reaxys:

With  N-benzyl-N,N,N-triethylammonium chloride; sodium hydroxide in  >dichloromethane; water
Inert atmosphere;
2.1. Synthesis
General procedure: The SBOs were prepared by the base-catalysed >condensation of the appropriate 5-halogeno-2-methylbenzoxazole with the >requisite aromatic aldehyde under phase transfer conditions. In a typical >experiment, equimolar quantities (5 mmol) of the starting materials were >dissolved in dichloromethane (20–50 ml) in the presence of >benzyltriethylammonium chloride (3 mmol) and stirred magnetically under a >nitrogen atmosphere as an aqueous solution of sodium hydroxide (50percent, >w/v, 5 ml) was added dropwise over a period of 10 min. After being stirred >for 2–36 h until analytical thin layer chromatography indicated that the >reaction was complete, the mixture was diluted with water (50 ml) and the >SBO was extracted with dichloromethane (3×20 ml), dried (MgSO4), filtered, >evaporated under reduced pressure and recrystallized from aqueous methanol >or ethanol.


texts:
Chemical elements are said to be immutable at normal energies, but not at the energies that are orders of magnitude higher.

Given that nucleus at normal energies gives us the Periodic Table and at higher energies becomes unstable, can we say that (with fusion) we will be able to transform an arbitrary chemical element into some other chemical element (if we have enough energy)?


texts:
In the process for electrolysis for zinc, three things must be done:

Leaching the ZnO
Purifying the ZnSO4 by means of cementation
Performing the electrolysis

However, step 2, as it is presented by Wikipedia, is quite vague. In particular, it says:

  The purification process utilizes the cementation process to further purify the zinc. It uses zinc dust and steam to remove copper, cadmium, cobalt, and nickel, which would interfere with the electrolysis process.

However, when looking it the definition of cementation, a crude summary of what happens is that one metal gets oxidized in order to reduce the other ion. I am assuming that this means that the zinc sulfate is getting reduced. However, what is reducing it? What equations are involved? I apologize if this question does not make sense, as chemistry is not particularly my strong suit, but I believe that this has gone unanswered online so far.


texts: Why is bromine produced at the anode when aqueous sodium bromide undergoes electrolysis?

texts:
The electrode potential for Bromine is $\pu{+1.07V}$ whereas for Oxygen to be produced it is $\pu{+0.40V}$.

$$\ce{O2 +2H2O + 4e- -> 4OH-}$$

As the electrode potential for bromine is higher, it means bromine should be reduced rather than $\ce{OH-}$ ions.

I'm aware that the concentration affects the electrode potentials, but the question simply states 'an aqueous solution of NaBr' with no reference to concentration whatsoever, so I can only assume standard solutions.

Also, why do we not look at the electrode potentials of other possible reactions at the anode, like;

$$\ce{O2 + 4H+ + 4e– -> 2H2O}~~~~~~~(E=\pu{+1.23V})$$ or

$$\ce{HO2- + H2O + 2e– ->3OH–}~~~~~~(E=+\pu{0.88V})$$ or

$$\ce{H2O2 + 2H+ + 2e– -> 2H2O}~~~~~~~ (E=\pu{+1.77V})$$.

Why are these not viable?


texts: If alloys are homogeneous mixtures, why can't we separate their components?

texts: An alloy is a material composed of two or more metals or a metal and a nonmetal. And, they are usually formed by heating the elements to their melting points, and then cooling them, so that the components mix. Now, why doesn't this works backwards i.e. if we heat the alloy again to melting point of their constituents, and they should separate?


texts: Can two electron occupy the same spatial spot in a statistical way?

texts:
My friend claimed that if we have a two-electron atom in the ground state, and somehow we get the "real" electronic wavefunction $\psi(\mathbf{x_1},\mathbf{x_2})$ of the system, the statistical probability of finding two electrons (spin-up and spin-down) at the same spot $\mathbf{x_0}$ is nonzero, and the repulsion potential between them looks like a cusp condition when an electron is near the nucleus.

I think if two electrons occupy the same place, then the repulsion must be infinitely large, which is not possible. So I believe the probability should vanish here. However, he argues that in the dynamic case, two electrons can never be in the same place, but statistically, the probability obtained by squaring the wavefunction is not zero.

I'm pretty confused now. Who is correct?

Update:
The link here seems to contradict the answers below. I do not quite understand the statement "while at large distances between the electrons the conditional probability is greater than half of the corresponding unconditional one". Can someone give a hint?

Update 2:
Some helpful references mentioned by DavePhD:

Fitzpatrick's Quantum Chemistry "On the other hand, in the spin-singlet state, there is an enhanced probability of finding the two electrons at the same point in space (because of the final term in the previous expression). In other words, the two electrons are attracted to one"

Advances in Quantum Chemistry volume 1, page 121: for the beryllium triplet P state two opposite spin electrons being at the same point is the most probable configuration

A statement in Atkins Physical Chemistry "The other combination [opposite spins] does not vanish when the two electrons are at the same point in space".


texts: Electrolysis used to cause a Chemical Reaction?

texts:
Electrolysis is used to separate compounds like Water into Hydrogen and Oxygen.

Can it be used to induce a synthesis reaction rather than splitting reactants apart? For example, could you use electrolysis to recombine the Hydrogen and Oxygen?


texts:
Calculate the solubility of $\ce{Ag3AsO4}$ in $\pu{0.02M}~\ce{K3AsO4}$ neglecting the activity coefficients. Find the relative error. $K_\mathrm{sp}(\ce{Ag3AsO4}) = \pu{6.0e-23}$

I know how to calculate the relative error but I get a very complicated equation finding the concentration solubility product constant ($K'_\mathrm{sp}$). There should be a quicker way to solve this since it is a midterm question.
I tried this:
$$\ce{Ag3AsO4 -> 3Ag+ + AsO4^3-}$$
$$K_\mathrm{sp} = 27x^4$$
$$x = \pu{2.78e-6}$$
$$[\ce{Ag+}] = 3x = \pu{8.34e-6M}$$
$$[\ce{AsO4^3-}] = x = \pu{2.78e-6M}$$
$0.02~\mathrm{M}\ \ce{AsO4^3-}$ comes from $\ce{K2AsO4}$. So there should be an equation like:
$$K_\mathrm{sp} = (\pu{8.34e-6} - 3x)^3 \times (\pu{2.78e-6} + 0.02 - x) = \pu{6.0e-6}$$
And things get complicated. After finding $x$, I will also have found the final concentrations of silver and $\ce{AsO4^3-}$ ions. Then I will read the activity coefficients of them from the appendix table.
I have two questions:

Is my method true or false?

What is an easier way of solving this problem?


texts:
Benzoic acid is a weak acid, so it does not completely dissociate in water. I know that the dissociation equation of benzoic acid is:

$$\ce{C6H5CO2H <=> H+ + C6H5CO2-}$$

The question I'm having trouble with is:

  Would the dissociated form be more or less likely to dissolve in water?

Is this basically asking if the ions on the right are more likely to dissolve when compared to $\ce{C6H5CO2H}$?

If this is what the question is asking, I think yes, the dissociated form will be more likely to dissolve as they are very polar when compared to $\ce{C6H5CO2H}$ and they can be surrounded by polar water molecules. But I'm not sure.


texts: Available datasets to study using Hückel theory

texts:
I'm trying to do some tests using graph Laplacians based on Hückel theory. This, however, is said to be only applicable for conjugated hydrocarbons. My questions are:

I have been working with QM9 dataset and I was wondering if QM9 passes the filter (being consist of conjugated hydrocarbons only). How about ZINC database?
Are there any datasets available that can be analyzed using Hückel theory (Preferably with their SMILES representations included, and some property value reported, for testing a prediction algorithm)?


texts: Mechanism of an epoxide ring opening with trimethylaluminium

texts: Am I right that there is a rearrangement reaction here? After coordinating the lewis acid to the oxygen, how do I come up with the correct rearrangement step?


texts: Nucleophilic substitution in polar protic/aprotic solvents

texts:
I am confused as to why nucleophilic substitution reactions occur. Does a stronger nucleophile substitute a weaker nucleophile in nucleophilic substitution reaction?
For example, I reasoned that reaction
$$\ce{R-Br + KF ->[DMF] R-F}$$
is correct because $\ce{F-}$ is a stronger nucleophile than $\ce{Br-}$ in polar aprotic solvent.
Therefore,
$$\ce{R-Br + KF ->[Polar protic solvent]}$$
should give no reaction because $\ce{F-}$ is a weaker nucleophile than $\ce{Br-}$ in a polar protic solvent. Is my reasoning correct?


texts: Why do people often capitalize element names?

texts:
According to IUPAC rules, names of chemical elements should not be capitalized. See Wikipedia’s take on the issue:

  According to IUPAC, chemical elements are not proper nouns in English; consequently, the full name of an element is not routinely capitalized in English

However, I commonly see people (even in research articles) capitalize the element names, and I wonder why. Is there a historical trend of doing so? Did the rule change at some point? Are element names capitalized in other languages?


texts: Choosing electrodes for corrosive environments

texts:
Background
I'm building a watering robot for my household plants. For the purpose of measuring the moisture content of the plant I intend to use a resisitive approach where I pass an electric current through two electrodes in the soil.
I initially used a cheap off-the-shelf probe with a metal I don't know what it is but it corroded very quickly in the soil. I have learned since then that soil is quite corrosive to metals. I'm aware of a capacitive method of measurement that doesn't require the metal to be in contact with the soil but I haven't found a lot of information on how that works and I've already built most of the project around the resistive approach.
I'm not good with chemistry, and I'm trying to figure out what material I should use for these electrodes. As the plants I'm growing are eventually going to end up for personal consumption I am concerned with toxicity and if these materials will react with the soil and produce some thing that can be absorbed by the plant's leafs and fruit.
Research Done
I read that Graphite, Palladium, Platinum and Gold are the three best materials from a corrosion resistance point of view with Silver and Titanium coming in close. [1]
My understanding is that Platinum and Gold are noble metals and they are inert and are normally found in their pure form in the earth so they would not react with the soil at all under normal circumstances. So these metals would be my first choice if availability and cost was not a concern.
I managed to find some information on titanium since we use it in medical implants, it's not directly toxic invivo. Although I did find this site claiming you can have titanium poisoning (also claiming platinum poisoning). But to be honest I'm not sure if I consider that page a credible source, it seems to be fear mongering designed to sell 'home tests'. I found some research papers detailing that lab rats had adverse effects on lung development when exposed to TiO2 nano particles, but no studies to show any actual adverse effects on humans, some recommended caution however.
As for Graphite I found that it really depends on the alloy and basically I felt like it would be too hard for me to make an informed decision so I discounted that, also I'm kind of afraid that it might be too brittle for the application. But this may also be lack of knowledge on my part.
I know you can get platinum plated titanium mesh for electrolysis applications (but I only found alibaba online and that seemed sketchy) but they are too big and unsuitable for my application. And if I could find platinum coated titanium rods, I guess that would be optimal but I have had zero lucky finding such things (2-4mm thick, 7cm long or there about)
Questions
So I turn to you guys: Gold and Platinum are apparently the best choices. But would titanium be good enough? Is it safe? Should I still consider Graphite? Can the soil react with Titanium to produce TiO2? Do you know of a place online where I can purchase these things in Europe?


texts: Determining the redox half-reactions of single displacement reactions

texts:
For the following double displacement half reaction, the half reactions can be determined easily using the oxidation numbers.
$$\ce{MnO4- {(aq)} + IO3- {(aq)} -> MnO2 {(s)} + IO4- {(aq)}}$$
We know this is the reduction half reaction because the oxidation number of $\ce{Mn}$ changes from +7 to +2:

$$\ce{MnO4- {(aq)} -> MnO2 {(s)}}$$
and we know this is the oxidation half reaction because the oxidation number of $\ce{I}$ changes from +5 to +7:
$$\ce{IO3- {(aq)} -> IO4- {(aq)}}$$

However, for the following reactions, this strategy cannot be used. So how do we determine the half reaction equations for the following reactions:

  $\ce{Mn(OH)2 {(s)} + H2O2 {(aq)} ->T[Base] Mn(OH)3 {(s)}}$
  $\phantom{iiiiiiiiiiiiiiiii}\ce{MnO4^2- {(aq)} ->[Acid] MnO2 {(s)} + MnO4- {(aq)}}$
  $\phantom{iiiiiiiii}\ce{IO3- {(aq)} + I- {(aq)} ->T[Acid] I3 {(aq)}}$
  $\phantom{iiiiiiiiii}\ce{P {(s)} + PO3^3- {(aq)} ->T[Base] HPO3^2- {(aq)}}$


texts:
$\ce{VO2}$ is a weird metal which becomes a conductor at temperatures higher than $\pu{67 ^\circ C}$, but below $\pu{67 ^\circ C}$, it is an insulator!

How is this possible anyway? This is against solid state theory and how are we supposed to create band gaps with it?


texts: When using the Arrhenius Equation, beyond what range would the values of k, A , and Ea be extreme

texts:
In the Arrhenius Equation
$$k = A \mathrm e^\frac{-E_\mathrm a}{RT}$$
with:

$k$ defined as the rate constant
$A$ as the pre- exponential factor
$E_\mathrm a$ as the activation energy (in$\ \mathrm{\frac{kJ}{mol}}$)
$R$ as the universal gas constant (in$\ \mathrm{\frac{J}{mol\ K}}$)
$T$ as temperature (in $\mathrm K$)

beyond what range would the values of $k$, $A$, and $E_\mathrm a$ seem unreasonable for any given order or type of reaction?  Specifically is there a simple way to tell if a reaction is as fast as an explosion or as slow as paint drying for verifying if kinetics calculations seem reasonable.


texts: What units are used in Gaussian 16 for dipole derivatives output?

texts:
I ran a frequency analysis employing Gaussian 16 on MP2/6-31+G(d,p) level of theory with the keyword iop(7/33=1) in order to have access to dipole derivatives with respect to the individual normal mode coordinates. An example line from the output reads
Dipole derivative wrt mode XX:  5.93205D-01 -1.47564D+00  1.93547D-02

Does anybody know in which units the dipole derivatives are actually written and can, ideally, point me at a corresponding documentation?
Up to know I read it should be something like $\sqrt{\pu{km}/\pu{mol}}$, but I have no clear evidence that this is the case. I would expect something involving Debye and atomic mass unit, like $\pu{D}/\sqrt{\pu{u}}\,a_0$.


texts: Is there a name for the opposite reaction to the dissolution?

texts: When atmospheric $\ce{CO2}$ reacts with water to form $\ce{H2CO3}$, this is called dissolution, isn't it? What term would you use for the opposite reaction when it occurs at atmospheric pressure (e.g. due to temperature change)? For a solid it would be precipitation, but here it is for a gas.


texts: Is there a known value for the molar extinction (absorption) coefficient for amylose and amylopectin iodine complexes?

texts:
I am doing a colorimetry experiment with starch, and have a theoretical model based on Beer's law. To solve it, I need a known value for the molar extinction coefficients of amylose and amylopectin iodine complexes. Are these known values, and where can I find them?
While experimentally finding them would not be too hard, I cannot obtain pure amylose and amylopectin, so I cannot do this myself. I have done some research, but could not find many values(lack of research skills and access to scientific databases).
Any direction to papers or sources would be greatly appreciated.
Alternatively, a reliable and commonly available replacement for a starch standard would also solve my problem, if such a thing exists - this would make things much easier for me. This is a senior school experiment, so a compromise between accuracy and availability is expected.


texts: Ca and Mg hydrated radius in clay flocculation

texts:
Ca has an atomic radius of $197\mathrm{~pm}$ and a Pauling electronegativity of $1.00$.

Mg has an atomic radius of $197\mathrm{~pm}$ and a Pauling electronegativity of $1.31$.

Since Mg is smaller and has a higher electronegativity, one would think Mg would have a smaller hydrated radius than Ca, but it doesn't.  Would someone explain to me why this isn't so?  I have read an explanation pertaining to charge density, but I'm not understanding why Ca would have a larger charge density if it is bigger with less electronegativity.

The goal of understanding this is to understand why the addition of Ca to soil will make it drain better and be looser, less sticky, etc.  And conversely, why highly respected agricultural consultants say soils high in Mg (low in Ca) are "sticky".  Any help in understanding this would be well appreciated.


texts:
Are all higher trans-cycloalkenes chiral as trans-cyclooctene is?

This question supplements my previous question: Why is trans-cyclooctene chiral? See also the follow-on question: Do more double bonds cause a bigger number of stereoisomers in cycloalkenes?


texts: Why does iodine have different colors in different media?

texts: It is violet in trichloromethane, but brown in water. why? In the Shriver Atkins book, they tried to explain this using Molecular Orbital Theory which was not clear to me


texts: Why is the binding angle of HOF lesser than that of H2O, despite the lone pairs and size of the flourine?

texts: Why is the binding angle of HOF(101°) lesser than that of H2O?(104.5°) Since fluorine has unpaired electrons wouldn't it repel hydrogen more strongly than the corresponding hydrogen of H2O? Also, the size of fluorine is bigger than hydrogen, which also should have made the repulsion greater.


texts: Acid-catalyzed hydrolysis of diphenyl malonate

texts:
What are the complete, acid-catalyzed hydrolysis products (with application of heat) of diphenyl malonate?

According to me, they are phenol, carbon dioxide, and acetic acid.

The aqueous acid and heat conditions indicate to me that we are returning from the carboxylic acid derivative (ester) to the carboxylic acid. The mechanism involves protonation of the carbonyl oxygen and nucleophilic attack by water. This ejects alcohol (well, in this case, phenol).

The heat induces the malonic acid intermediate to decarboxylate and this results in the production of carbon dioxide and acetic acid.

Is my analysis correct?


texts: Which d orbitals are involved in "d3s hybridisation"?

texts: I was reading about hybridisation and came across a point saying that $\mathrm{d^3s}$ hybridisation involves only $\mathrm d_{xy}$, $\mathrm d_{yz}$, and $\mathrm d_{zx}$ orbitals and not $\mathrm d_{x^2-y^2}$ nor $\mathrm d_{z^2}$. Why is that so?


texts:
Why do some covalent compounds form lattice while others do not? I'm not only talking about amorphous solids but covalent compounds that form molecular lattice in general. For Example: Compounds like SiO2, SiC and AlN form covalent lattices while some covalent compounds like methane and SiF4 form molecular lattices. The question is - Why do only some covalent compounds do this and also, is there a way to identify which compound forms covalent lattice and which forms a molecular lattice? If so, then how?

NOTE: I've looked this up over Concise Inorganic Chemistry by JD Lee and also on the internet but have found nothing.


texts:
So here's my problem:

What is the osmotic pressure, noted in mm of $\ce{H2O}$, of a solution with 125 micrograms of vitamin B12 ($\ce{C63H88CoN14O14P}$) in 2.50 mL of $\ce{H2O}$ at 25 degrees Celsius.

The "hint" that comes with the problem says: Weak osmotic pressures are often measured and given in mm of $\ce{H2O}$. Use here the density of water (1,00 g/mL) and of $\ce{Hg}$ (13,6 g/mL). In this case, the constant $R$ has a value of $\pu{62,36 mm Hg * L / mol * K}$.

Here:

$$
2.50 \,\text{microgram} = 0.000125 \,\text{grams}
$$

$$
pi = cRT
$$

$$
\frac{(\pu{0.000125 g * 1 mol / 1355.3652 g})}{(\pu{2.50 mL / 1000})} * \pu{62.36 mm \ce{Hg} * L / mol * K * 298.15 K} = \pu{0.685890194 mm \ce{Hg}}
$$

So, I blocked here and I saw that the solution was:

$$
\pu{0.685890194 mm \ce{Hg} * 13.6 g/ml / 1.00 g/ml = 9.32 mm \ce{H2O}}
$$

So, here's two questions:

Why did I have to use R = 62.36 and not the usual R = 8.31?
Also, what's happening in the conversion exactly? How was I supposed to figure out what they did to convert from mm $\ce{Hg}$ to mm $\ce{H2O}$?


texts: Where does magnetic inequivalence come from?

texts:
As the question says, I'm having trouble understanding where does magnetic inequivalence in $\ce{^1H}$ NMR come from. All the sources I've found say that two protons are magnetically equivalent if all $J$ values to all other spins in the molecule are identical. So for example, in 1,2-dibromobenzene, the $\ce{H^3}$ and $\ce{H^6}$ are not magnetically equivalent because $J(\ce{H^3, H^4}) \neq J(\ce{H^6, H^4})$ and $J(\ce{H^3, H^5}) \neq J(\ce{H^5, H^6})$. I understand how to apply the rule, but I don't understand why does this affect the spectra. For example, assuming a not unusual $J_\mathrm{ortho} = \pu{7 Hz}$ and $J_\mathrm{meta} = \pu{2 Hz}$, then:
$J(\ce{H^3, H^4}) = \pu{7 Hz} = J(\ce{H^5, H^6})$, and
$J(\ce{H^3, H^4}) = \pu{2 Hz} = J(\ce{H^5, H^6})$. So they are both split the same way, but in reality they also split each other because of their magnetical inequivalence. This is what I find hard to grasp: if they are being in overall affected the same way, by the same type of atoms, in the same positions, how do they "know" that they are, in fact, different.

I guess related to this would be, why do all protons in ethene give the same signal? If they are also related differently between each other? ($\ce{H^1}$ and $\ce{H^{$1'$}}$ have different coupling constants to each $\ce{H^2}$ and $\ce{H^{$2'$}}$, cis and trans, respectively).

PS. Maybe related to this, although I can open a new question if it's not, of course, is why are diastereotopic protons different in NMR – i.e., how does the molecule know that "left" and "right" are different in, for example, (2R)-2-bromobutane, where the only difference between the protons in the $\ce{CH2}$ centers is that they "feel" the substituents to their left or right (looking at all its possible staggered Newman projections). Also, correct me if I'm wrong, these are not even chemically equivalent. Why would left and right make it so different (yet not so for enantiotopic protons)? I realize this might be a completely different questions, but something tells me they might be related?


texts: How does total depth affect the colloid gradient?

texts:
When a colloid reaches diffusion–sedimentation equilibrium, it creates a gradient where the solute concentrates toward the surface or the bottom of the vessel based on its density relative to the solvent even though it never completely separates.
This got me wondering, if you were to compare two vessels of different heights containing the same colloid solution, would the taller vessel have the same gradient slope, resulting in a greater difference between the concentration at top and bottom of the vessel, or would the slope become more shallow resulting in the same difference in concentrations, or would it be something in between?


texts: Does oxygen catalyze the decomposition of sucrose?

texts:
When a bottle of sports drink that contained sucrose is opened, is it expected that the sucrose contained therein will split into its constituents (fructose and glucose) more rapidly than it will via acid hydrolysis?

I understand that in acid hydrolysis there is nucleophilic substitution (provided the acid is protic). Does a similar phenomenon occur when the contents of the bottle are exposed to oxygen?

I am trying to interpret some unexpected results I obtained from an experiment, and I assume what I have detailed here is what happened. However, I cannot find any relevant information to confirm or deny my hypothesis.

RESULTS:

The experiment showed 0.72 g of glucose was present per 100 mL of solution. The bottle label states that it is 0.5 g per 100 mL.


texts: Is there a light cured resin (UV or another) that I could make in my house with easy to find chemicals?

texts:
Is there a light cured resin that I could make in my house with easy to find chemicals? It does not matter if the finished product have practical or commercial applications as long that is not too toxic.

I want to use it for SLA 3D printing of prototypes.


texts:
I melted about 2 kg of gallium and put it in a plastic container, in order to make crystals. After letting them grow and extracting them, I let the gallium freeze at the room temperature, which is below the melting point.

However, for some reason there is a small quantity of gallium (10-20 g) that is not freezing after days. I extracted the liquid then froze it in a refrigerator. But if I put it outside again, it melts, so this is not supercooled gallium, a well documented phenomenon.

It behaves like normal gallium with slightly lower melting point, something like a Galinstan eutectic. However this is 99.99% pure gallium (that was fully solid) and I am pretty sure that I never contaminated it...

Any idea what is happening?


texts: Global reactivity parameters from open shell DFT calculations using Koopman's theorem

texts:
Koopmans' theorem is a useful approach to calculate the global reactivity parameters from the HOMO-LUMO energies. My question is, does it apply to the open-shell systems where we get two sets of singly occupied (alpha and beta) HOMO-LUMO energies?

I am working on rare earth systems. Rare earth (RE) elements generally show +3 oxidation states which result in open shell configurations for $\ce{Ce^3+}$, $\ce{Eu^3+}$, $\ce{Gd^3+}$ etc. I am performing unrestricted open shell DFT calculations for these systems, as restricted open shell calculations for RE systems require larger computational time and cost. In unrestricted open shell calculations, we get two sets of alpha and beta HOMO and LUMO. My question is, can I apply Koopmans' theorem here to calculate the global reactivity parameters?

Could anyone help with some literature on the reactivity parameters for open shell systems? Thank you!


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$\rm 20\,cm^3$ of ethyne, $\ce{C2H2}$, is reacted with $\rm 500 \,cm^3$ of oxygen.
  The equation for the reaction is:

  $$\ce{2C2H2(g) + 5O2(g) → 4CO2(g) + 2H2O(l)}$$

  What is the total volume of gas remaining at the end of the reaction?

  (all volumes are measured at room temperature and pressure)

My effort: I think the answer should be $510\,\mathrm{cm^3}$ because the volume of the leftover oxygen is $450\,\mathrm{cm^3}$ and the volume of the products is $60\,\mathrm{cm^3}$ but the answer is $490\,\mathrm{cm^3}$. Can someone please explain why?


texts: Delocalization of π-electrons in nitrate ion

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Benzene and nitrate ion are given in my textbook as examples for the delocalization of π-electrons. Benzene, due to symmetry of its resonating structures, is simple enough. We assume that σ-electrons are localized and π-electrons are delocalized in the ring.
Each carbon atom promotes one electron from its $\mathrm{s}$ orbital to the empty $\mathrm{2p}$ orbital. It hybridizes two $\mathrm{p}$ orbitals with the $\mathrm{s}$ to form three $\mathrm{sp^2}$ orbitals which it then uses to form three σ-bonds, two with other carbons and one with a hydrogen.
The remaining unhybridized $\mathrm{p}$ orbital then sticks out above and below the plane of the ring. The electrons from all the six unhybridized $\mathrm{p}$ orbitals of the six carbons are then delocalized above and below the plane of the ring.
However, when I try to apply a similar reasoning to the nitrate anion, problems arise. The resonance Lewis structures of nitrate ion are:

Now, assuming again that only the π-electrons are delocalized, we would expect that only two electrons are delocalized (since there is only one double bond). But my textbook claims that each atom is $\mathrm{sp^2}$-hybridized, and as in benzene, one unhybridized $\mathrm{p}$ orbital per atom sticks out above and below the plane of the molecule.
Assuming this to be true, I presumed that two electrons would be localized and tried to arrange the remaining 22 electrons into a Lewis structure with $\mathrm{sp^2}$ hybridized atoms. I couldn't find any viable structures with only three lone pairs around each atom.
What is the exact mechanism for delocalization of electrons in nitrate? Is there a general scheme to this mechanism that applies to all such similar molecules such as $\ce{CO3^2-}?$


texts: I understand that k alone is the equilibrium constant and know how to find this number or when given it I can take the log of this number, but what is the purpose of this number? What does the log of k tell us in relation to the reaction? Why do we find log k?


texts: Hydrolysis of Aspartame with a warmed alkali

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Aspartame has this structure below

Shouldn't the peptide group be oxidised along with the ester group?

Then why is the answer to the question below B?

EDIT : I'm sorry I misunderstood a friend's marking, the answer is A. I deeply apologise for my stupid mistake.

Thank you very much!


texts: Why is lowering of vapour pressure (Po - Ps) temperature dependent? And why is relative lowering of vapour pressure (1 - Ps/Po) independant of temperature?


texts: Why are heterogeneous catalysts calcined?

texts: Most of the literature I read says to calcine heterogeneous catalysts when preparing them. When synthesizing heterogeneous catalysts, what is the reason for calcining them?


texts: Fundamental physical forces behind osmosis

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I wonder which fundamental forces act behind osmosis process? Sum of these forces oppose gravity force, like in this figure:

So I wonder what are these forces?


texts: Why does increasing electronegativity difference result in decreasing density?

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I've been collecting data and there's a very strong inverse relationship between electronegativity difference and density ($\pu{R^2}$ value of roughly 0.95 from my data). Is there any chemistry that could explain these results?

This doesn't make much sense to me because I would have thought increasing electronegativity difference would result in more polar bonds and thus more polar molecules, which would result in stronger inter-molecular forces and thus smaller volume (and hence more mass being packed into a smaller volume).


texts: What is a “spectator mode” in this analysis of molecular vibration modes

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I'm reading a paper in which I found this line:

  The other modes have relatively small contributions for the displacements of the heavy atoms and can be considered as spectator modes for the internal conversion

I cannot find a definition or explanation of term “spectator mode”. What is it exactly?


texts: Regioselectivity for electrophilic aromatic substitution with multiple strongly electron withdrawing groups

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At which position will electrophilic substitution on p-cyanonitrobenzene occur?

As both are -M groups, there are two possible sites of reaction, meta to each of the groups.
The answer is meta to $\ce{CN-}$. Why is this so? According to me it should be meta to $\ce{NO2}$, because $\ce{NO2}$ is the stronger -M group?


texts: Nodes of orientation of electron orbitals

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As you can see the orientation of 2 p orbital there is a nodal plane in between the electron density pictures.... So doesn't the electron move from left to right side in2 p X orbital.
I think that electrons moved around the nucleus in circular paths