texts: Why must a voltaic cell take place in a conductive solution?

texts: In order for a voltaic cell to function, its electrodes must be placed in a conductive electrolyte solution, most often solutions consisting of their metal ions. While I understand the use of the metal ions (allows for the plating to occur on the cathode), why must the electrolyte be conductive? Thanks!


texts: Why do methylamine and ethylamine have nearly the same basicity constant?

texts:
It is known that ethyl has a stronger inductive effect than methyl.
Its positive inductive effect is greater than methyl.

Then why does ethylamine has a $\mathrm{p}K_\mathrm{b}$ of $3.35$ and methylamine has the $\mathrm{p}K_\mathrm{b}$ of $3.34$
($\mathrm{p}K_\mathrm{b}$ values in water)
Shouldn't the $\mathrm{p}K_\mathrm{b}$ of ethylamine be lower than methylamine?


texts: Why can't we use orbital correlation diagrams for sigmatropic rearrangements?

texts:
In Ian Fleming's Molecular Orbitals and Organic Chemical Reactions book, he describe the use of correlation diagram as a method for predicting whether pericyclic reactions will be allowed/forbidden on symmetry argument (he calls this a basis for the Woodward–Hoffmann rules).

  Correlation diagrams have given us a convincing sense of where the barriers come from for those reactions that we have been calling forbidden.

Whilst there are given examples of cycloadditions and electrocyclic reactions there are no examples of sigmatropic rearrangements, and on the internet a few sources say the diagram is not appropriate for these sigmatropic rearrangements.

I am curious why a correlation diagram does not work for sigmatropic rearrangements, since the the Woodward–Hoffmann rules are able predict which are allowed and forbidden.


texts: What is the mechanism that dihydrolipoic acid undergoes to be oxidized by FAD?

texts:
The mechanism for dihydrolipoic acid oxidation in the pyruvate hydrogenase complex is not clearly demonstrated in any Lehninger textbook, other than the fact that FAD becomes FADH2. I am wondering what intermediates the FAD/FADH2 undergoes, my textbook just does skips over the details.

In the diagram below, the mechanisms are mostly all clear, except for the arrow I've highlighted with orange.


texts: Role of Alcohols(EtOH,MeOH) in nitro reduction used for Metal(Sn,Fe)

texts:
I'm trying to reduce an aryl nitro group.
I understand that these reactions are radical reactions by the metal, but I don't understand
why alcohols are used as the solvent. Are they used as a source of protons, or just because they are good solvents?


texts: The effect of a voltmeter on the electron flow of a voltaic cell

texts: Due to a voltmeter’s strong resistance nature, how could electrons flow from anode to cathod? It is more reasonable to assume that a voltmeter would stop the redox reaction because the electrons could never reach the positive electrode. When I measure the potential energy between two electrodes with a voltmeter, is the reaction happening? If the reaction is not taking place, how come the potential difference exists?


texts: When our hard water is boiled, it leaves a blue-green residue in our kettle.

texts: The inside of our kettle is lined in a blue-green deposit. Our water is very hard, but what minerals could cause the coloration?


texts: vapor pressure of one part of binary compound

texts: Let's say we have a binary compound AB in an enclosed container, and let's say there's a tendency to form B-vacancies so that B atoms/molecules escape and form a vapor. Let's assume there's enough of AB that the pressure saturates before AB$_{1-x}$ decomposes too much. Would the vapor pressure of B be the same as for the pure element B?


texts: What is the proper nomenclature for CH₂O₂?

texts:
Can you tell me what's the nomenclature of $\ce{C H_2 O_2}$?

What is its IUPAC name?


texts: Why do we need to add FeBr3 for bromination of anisole, but not for bromination of phenol?

texts:
I understand that we need to add $\ce{FeBr3}$ while brominating benzene in order to polarize the bromine molecule. However, we do not need to add such a Lewis Acid in the case of phenol, because it is activated by the $\ce{-OH}$ group, and so polarizes the bromine molecule on its own. Anisole is also highly activated, however we still add $\ce{FeBr3}$ in order to brominate it. Why?
(An answer which compares bromination in phenol and anisole will be highly appreciated.)


texts: What is the balanced equation for the reaction of potassium permanganate, iron sulfate, and sulfuric acid?

texts:
I'm trying to balance the following redox equation. I think this is happening in acidic solution, the textbook doesn't specify anything more. Can you please help me understand if I got the half reactions correct?
$$\ce{KMnO4 + FeSO4 + H2SO4 -> K2SO4 + MnSO4 + Fe2(SO4)3 + H2O}$$

I found out that manganese is reduced as the oxidation number goes from $+7$ to $+2$ and iron gets oxidized as the oxidation number goes from $+2$ to $+3$.
I wrote the two half reactions, \eqref{Q:red} for the reduction and \eqref{Q:ox} for the oxidation. Did I get them right?

\begin{align}
\tag{A}\label{Q:red}\ce{KMnO4 + H2SO4 &-> K2SO4 + MnSO4}\\
\tag{B}\label{Q:ox}\ce{FeSO4 &-> Fe2(SO4)3}\\
\end{align}

I started working on the reduction half-equation and this is what I came up with after balancing atoms and charge:

$$\ce{10 e- + 10 H+ + 2 KMnO4 + 3 H2SO4 -> K2SO4 + 2 MnSO4 + 8 H2O}$$

Now comes the trouble. How do I balance iron and sulfur in the oxidation half-reaction \eqref{Q:ox}?


texts: Diffusion and potential gradients for two similar atoms

texts:
Studying biochemistry I noticed that equilibrium is reached for K and Na separately (not taking into account electric potential), as would be the case I assume for any molecule or atom. This is of course entropy.

Eg: My understanding is, you have two types of atoms, and a membrane separating them, with different concentrations on each side. Each atom will reach equilibrium with itself across the membrane, as if the other type of atom doesn't exist (barring the influence of charge).

Why is it that, for instance, a K atom is so keen to reach equilibrium with another K atom in an area of lower concentration, and yet seemingly ignores the Na, even if the Na is in high concentration where the K is low? Both Na and K still have the same charge, and would also collide with each other I imagine? Why do identical atoms behave like this but non-identical ones with very similar properties don't?

Update #2:

My understanding was incorrect regarding K and Na having separate forces. The two atoms seem to be treated the same, except for selective permeability.

I'm still not understanding why the two atoms have separate gradient potentials, if they can be treated as the same. I made the following images to illustrate my point a little clearer.

There is a compartment with a left and right side, separated by a wall containing two selectively permeable channels that may open or close. Only Na can cross the green channel when it is open, only K can cross the orange channel when it is open.

First, the assumption: The main assumption behind all this is that, in answering this question, we can treat Na and K as being the same. They are effectively two marbles with identical physical and chemical properties, just with different colours. This is what I have come to understand based upon the answer by orthocrestol.

The cell on the left has a -70mv potential energy compared to the right compartment, caused by invisible negative ions which will always remain on the left side.

All that has been summarized by the discussed assumption is seen in figure 1.

Now in figure 2, we open the selective channel for Na. Since Na has a gradient potential, and there is also a potential difference, the positive Na flows into the left hand compartment.

In figure 3, an equilibrium is reached at an arbitrary value of -20mv. On average, no more Na will move to the left or the right, it's at equilibrium.

Figure 4, channels closed. Now this is where my understanding ends.

Apparently now, from what I gather, in figure 5 if we open the selective channel for orange (K+), since K+ has a gradient potential, it moves to the right. In the process, it takes its positive charge with it and restores more or less the original potential difference of -70mV on the left hand side.

But, if this is what happens, this must surely invalidate the marble analogy of the two atoms being the same. Because if they're the same, then whether we open the Na channel (which is already at equilibrium) or the K channel should make no difference, because once gain, they're the same atom with a different colour (except for being separately permeable via their selective channels).

The only difference that should be apparent is that if we leave both channels open, then the colours will eventually reach an equilibrium on both sides. But if an equilibrium is already reached for the charge (which it has in figures 3 and 4), I'm failing to grasp how opening then closing a different channel suddenly causes that equilibrium to change (as in figure 5 and 6). The explanation is the K potential gradient, but this is still the same marble as Na is it not? How come the potential difference changes?

Please direct me where my thinking has gone wrong, or if this is what even happens. And if it is, how is this possible? Many thanks.


texts: Can you approximate the change in entropy of a reaction?

texts:
I found a problem in which I am given a hypothetical reaction in which from 2 liquids (reactants) I get 4 moles of gas and 1 mole of a solid substance (products):

$$\ce{A (l) + B (l)  -> 4C (g)  + D (s)}$$

I am asked about the change in entropy (an approximation of it), so I am curious if there is some kind of approximation for the change in entropy of a reaction based on the change in the amount of substance of gaseous compounds (since I know that the highest influence on entropy is that of gases).


texts: Why does sodium polyacrylate solidify, on addition of water?

texts:
Sodium polyacrylate is a white powder with a characteristic odor. But when we add water to this powder, instead of forming an aqueous solution, it instantaneously  solidifies and forms more amount of powder like substance, also called "hot ice".
Why and how this phenomenon occur?

YouTube video link


texts: Acidity and alkalinity versus inorganic carbon concentration

texts:
Alkalinity represents the amoount of carbonates in water. When I compare concentration of inorganic carbon measured by TOC-analyzer, with concentration of inorganic carbon calculated from alkalinity,they are in pretty good accordance with each other. But, when there is also acidity present, then the better correlation is achieved with adding the amount of acidity to alkalinity value before multiplying with 12.01. pH is around neutral (6.8)

What is the basis of this? Why this adding gives better estimation of inorganic carbon concentration?


texts: Acid-Base Conjugate Pairs with multiple Products

texts:
Identify the acid-base conjugate pairs in the following reactions:
  $$\ce{H_2SO_4 + HNO_3 <=> HSO_4^- + NO_2^+ + H_2O}$$

The obvious acid base conjugate pair is $\ce{H_2SO4}$ and $\ce{HSO_4^-}$.

If $\ce{NO_2^+}$ and $\ce{H_2O}$ were together, the second acid-base pair would be quite obvious. But the two aren't. What would be the conjugate-acid pair of $\ce{HNO_3}$ in this case?


texts: Is it easier to mix a solution that is in the liquid or in the vapor phase?

texts:
In the article on dilution, Wikipedia has the following statement:

The same direct relationship applies to gases and vapors diluted in air for example. Although, thorough mixing of gases and vapors may not be as easily accomplished.

I was wondering if there is any supporting evidence for the second statement in the quote (or for the opposite statement). I would think that gases mix quickly, given how quickly gas expands into a vacuum (I am thinking of the classic picture of half a container filled with gas, and a divider that is removed). On the other hand, I don't know how effective mechanical mixers are for the gas phase. For the liquid phase, it is common to mix large volumes with a magnetic stir bar whose volume is comparatively small.
Is it easier to get a homogeneous mixture for a liquid phase than a gas phase? I am excluding immiscible liquids from the question because they will never become a homogeneous solution. To focus the question a bit, I am interested in the rate of mixing; to make it comparable, the mole ratios should be equal in the comparison of the liquid phase case and the gas phase case. Mixing could either be by diffusion, or sped up by a mixer appropriate for the system.


texts: Which is correct 5-methylbenzyl-3-carbaldehyde-1-oic acid or 3-formyl-5-methylbenzoic acid, MarvinSketch recognizes them both as correct?


texts: Has it been observed that the dihydrogen molecule dissociates into ions?

texts:
As we know from the improved methods valence bond theory (VB), as well as molecular orbital theory (MO-LCAO), the wave function of the $\ce{H2}$ molecule consist of two terms, which belong to covalent and ion structure, respectively:
$$\psi =\psi_\mathrm{cov} + \lambda\psi_\mathrm{ion}.$$

According to quantum mechanics, there is a nonzero probability $\propto\lambda^2$ for the ionic state wave-function term. Such reaction should lead to the $\ce{H2}$ molecule dissociation products $\ce{H+}$ (proton) and anion $\ce{H-}$:
$$\ce{H2 -> H+ + H-}.$$

Have such processes been observed in nature?


texts:
Hello here is a question that is a bit confusing I hope you can help me solve it and understand it:

  Determine the hydronium and hydroxide ion concentrations in a solution
  that is $10^{-4} M \ce{Ca(OH)2} \text{ AND } 10^{-4} M \ce{HCl}$

Note: the "and" is actually capitalised in my book..

So what I understood from this question is that they are both in one solution. Now I thought of two ways to solve it:
A) the concentrations are equal and they are both strong which means they we're neutralise each other completely and end up being a neutral solution and both $\ce{H3O+}$ and $\ce{OH-}$ will be $10^{-7}$.

B) the calcium base has 2 $\ce{OH-}$ while HCl has 1 H+ so the base concentration is double that of the acid which means that the solution is going to be basic but I don't know how can I solve this to find the actual concentrations.

Which way is right and when does an acid and base really react to become neutral?


texts: Why photons with longer wave length demonstrate more active scatter and transmit that shorter length?  This troubles me to understand why NIR and Infra-Red goes through diffused media when compared to other colors. My thought is lean to think that be the other way around. For example, X-ray are short waves but goes straight though a biological tissue.  Please, comment


texts: Series of dilution without a constant dilution factor?

texts:
I have to make a series of dilutions though the dilution factor is not constant.

I'm trying to understand how to carry out this dilution in the most efficient way as not to waste much solution.

Series of dilution

Initial dilution: 1/250

1/250 -> 1/500 -> 1/1000 -> 1/2000 -> 1/3000 -> 1/4000 -> 1/5000 -> 1/6000

I need to end up with 25µl in each tube. I've attached my solution in an image below though i'm not sure i'm doing this correctly as i would have to discard so solution.

My solution as depicted in the image:


texts: Do individual crystals resulting from evaporating a mixture of two ionic compounds contain both cations/anions within the crystal lattice?

texts: I have wondered this for awhile. Let's say I dissolve, without loss of generality, potassium chloride and sodium chloride in water, then evaporate the water. Would any of the resulting crystals consist of randomly dispersed potassium and sodium cations within the lattice or would each resulting crystal be purely potassium chloride/sodium chloride?


texts: Predicting the stability of coordination compounds using Crystal field theory

texts:
The question is this:

On the basis of CFT, predict with justification which of the following two complex ions would be more stable [Mn(CN)6]4- or [Fe(CN)6]4-.

Attempt at solution:

Both the complexes are low spin. Filling in the d electrons we get 5 electrons in t2g for Mn2+ and 6 electrons in t2g for Fe2+. There are no electrons in eg for both.

Mn2+ has one unpaired electron while Fe2+ has completely filled t2g level. From here I was in a dilemma because Fe has fully occupied t2g level as well as a higher charge density because the effective ionic size of Fe2+ in low spin is 61 pm while Mn2+ in low spin has size 67 pm. However, due to Jan Teller effect the Mn compound will gain more stability due to the unsymmetrical filling of the t2g level. I know that Jan Teller distortion for t2g is less pronounced, so I was wondering if it was enough to make the complex more stable than Fe2+ complex?


texts: Is mixing ammonia and bleach spray in the same parcel of air considerably dangerous to health?

texts:
I've read a few articles advising not to mix bleach and ammonia in the same room because it makes chloramine gas. But my boyfriend and I are having an argument - he thinks that since those articles don't specifically say that mixing them in the same room (in our case, our small, poorly-ventilated bathroom) is bad, the only situation in which we shouldn't do this is spraying them directly on the same surface.

I don't see any reason why the chemical reactions producing chloramine gas wouldn't occur if they're mixing in the same room, from our bleach cleaner and glass cleaner spray bottles, though they would be less concentrated. However, he would only trust an answer from a chemistry expert. Please advise!


texts: What drives the 'internal return' in SNi?

texts:
I just learnt that alcohols can undergo $\mathrm{S_N}i$ with $\ce{SOCl2}$ (without pyridine) to achieve retention:

However, what I really do not understand is the 'internal return' step (forming 'intimate ion pair'). What drives the departure of oxygen + attack from chloride? Electronegativity of oxygen is more negative than chlorine, and bond strength of $\ce{C-O}$ is $\pu{358kJ/mol}$, $\ce{C-Cl}$ is $\pu{339kJ/mol}$. It seems to me that it is more stable to stay at $\ce{C-O}$ bond then forming $\ce{C-Cl}$ bond.
Moreover, what causes $\ce{Cl}$ to attack the carbocation on the same face? Like $\mathrm{S_N1}$, it can attack from the front and back (both sides of empty $\mathrm{p}$-orbital), what cause this to differ from $\mathrm{S_N1}$? Also, is primary/methyl alcohol unable to undergo $\mathrm{S_N}i$ due to carbocation formation?


texts:
How does borax  react with NaOH?
My textbook says that sodium peroxoborate is formed. I couldn't find the reaction anywhere else.
Can anyone verify as well as explain it?


texts: Consider a neutral atom that has $n$ protons and $n$ electrons. Is it possible to remove a proton from the atom via some technique such as bombardment with another particle or applying an appropriate amount of energy to the system?


texts: Why is the tropylium anion non-aromatic?

texts:
Recently I came across a question asking which compounds are anti-aromatic. This was one of the options:

I’m confused as this is not an answer. I think the lone pair electrons on the carbanion participate in resonance and so there are 8π electrons which, follows the 4nπ electrons rule for anti aromaticity.
So why isn't it anti-aromatic?


texts: Can freezing temperatures cause chalk to fragment?

texts:
I am a physics student, with little experience in chemistry. I am researching for a project which involves freezing and its capabilities on humans and different minerals like chalk.
I have been researching chalk for the last couple of months as part of my project. Can the cracks generated cause the mineral to break into fragments under freezing conditions?
It would be appreciated if anybody can explain this to me or even send/link me some papers that I can read further about this effect.


texts: What are some compounds that release carbon dioxide when heated, hydrated, or reacted with sodium, aluminum, silicon, or iron?


texts: Does "higher symmetry" mean any symmetry group of higher order?

texts:
What is the precise definition of a higher symmetry in group theory? I see this term scattered around, such as in Cotton's Chemical Applications of Group Theory:

  ...In the cyclic groups each of the operations, Cn, Cn2, Cn3, ..., , Cnn-1, constitutes a class by itself and we continue to use this notations. However, in all other groups of higher symmetry, the number of classes spanned by these operations will be reduced in the following way...

Does higher symmetry strictly mean a supergroup? Or is it a quantitative statement about something like the number of symmetry elements or number of irreducible representations (which are not always ordered the same way, as in the case of C3v and S4).

I am failing to find intuition because, if it does mean strictly supergroups, we can't compare the "amount" of symmetry in C2 compared to C3 (or Td compared to Ci even!).


texts: What is the hybridisation of titanium in the hexaaqua- and hexaamminetitanium(III) complex?

texts:
I want to check if their hybridizations are as follows:

$\mathrm{sp^3d^2}$ in  $\ce{[Ti(NH3)6]^3+}$ and $\mathrm{d^2sp^3}$ in  $\ce{[Ti(H2O)6]^3+}$.


texts: pH calculation in case of amphiprotic ion

texts:
$100\ \pu{ml}$ of $0.1\ \pu{M}\ \ce{H3PO4}$ is titrated with $0.5\ \pu{M}\ \ce{NaOH}$ solution till 2nd equivalence point. Then resultant solution was mixed with $10\ \pu{ml}$ of $0.5\ \pu{M}\ \ce{HCl}$ solution. The pH at 2nd equivalence point will be?
  Given: $K_1 = 10^{–3}\ \pu{M}, K_2 = 10^{–8}\ \pu{M},K_3 = 10^{–13}\ \pu{M}$

My attempt:

I found the volume of $\ce{NaOH}$ (equivalents of $\ce{H3PO4} = $ equivalents of $\ce{NaOH}$).

$$2\times 100\times 0.1 = 0.5 \times V\\
V=40\ \pu{ml}$$
Since the solution is diluted now, the concentration doesn't remain the same. So $[\ce{Na2HPO4}]=0.071\ \pu{M}$.

Now $\ce{Na2HPO4 -> 2Na+ + HPO4^2-}$

For this I tried to find the hydrolysis constant. I am getting it to be

$$K_\mathrm h=\frac{K_\mathrm{w}^2}{K_1 K_2}$$

The formula given to evaluate the pH after the 2nd equivalence point is $$\frac{\mathrm{p}K_2+\mathrm{p}K_3}{2}$$ How to derive this?


texts:
Tritium, symbol $\ce{T}$, is hydrogen with a mass of $3\ \mathrm{amu}$. It is radioactive and undergoes $\beta$ decay. Which of the following could be the only products after a quantity of $\ce{HTO}$ undergoes decay?

$\ce{HeOH}$
$\ce{H2O}$, $\ce{O2}$ and $\ce{He}$
$\ce{H2O}$, $\ce{H2}$ and $\ce{He}$

The answer is 2 only, however I am finding it difficult to understand how I could have worked this out. Tritium undergoes beta decay, so: $$\ce{^3_1H+ -> ^3_2He^{2+} + e- + \bar {\nu}_e}$$
I had assumed it wouldn't be possible for the electron emitted in beta decay to become an orbital electron. I have seen the following question: What does HTO decay into?, but it doesn't answer my specific query. I could state that the helium ions produced gain 2 electrons from the oxygen ions to become helium, with these oxygen atoms then reacting to form $\ce{O2}$ and some of the hydrogen combining with oxygen to form water, leading to answer 2, but this feels somewhat arbitrary. What approach could be taken to solve this problem?


texts:
I think I have trouble interpreting reaction enthalpy in these kind of situations:

I am given that $\Delta _rH^0 = 25,9$kJ/mole in the reaction:
$$\ce{1/2 H2 + 1/2 I2 \rightarrow HI}$$
which I interpret as: if this reaction takes place where 1 mole HI forms, the the produced heat is equal to 25,9kJ.

Now suppose we double the amounts, so that we have the reaction
$$\ce{H2 + I2 \rightarrow 2HI}$$
then clearly the difference in enthalpy should be doubled and be equal to 51,8kJ. Then why do chemists write that  $\Delta _rH^0 = 51,8$kJ/mole and not $\Delta _rH^0 = 25,9$kJ/mole? Because I would think that we can calculate the enthalpy change like this: $2mole\times25,9$kJ/mole.

There is cleary some fundemantel thing I'm missing here or misinterpreting, so I would like some help.


texts: Does calcium nitrate decahydrate dehydrate or decompose upon heating to 100 degrees Celsius?

texts: Consider a sample of $\ce{Ca(NO3)2.10H2O}$. To remove water, I need to heat it to $\pu{100 ^\circ C},$ but will this high temperature decompose it or will I get the anhydrous salt?


texts: An unknown glassware piece from our cupboard

texts:
We have multiple of these. It is a small glass funnel with splits into 3 spouts from it. Now, we are thinking it might be similar to a cow receiver, however, I have never seen one this small or delicate. It's 11cm in length.

We have put 3 test tubes underneath and ran water through and it does appear to distribute it relatively evenly.

What could it be?


texts: Melting and boiling points of transition elements

texts:
The melting and boiling points of transition elements increases from scandium ($1530~\mathrm{^\circ C}$) to vanadium ($1917~\mathrm{^\circ C}$). They increase because as we go across the group, we have more unpaired (free) electrons.

But at chromium ($1890~\mathrm{^\circ C}$) however, the melting point decreases even though it has more unpaired electrons than the previous atoms. Why does this happen?


texts: Algorithm used by PubChem to determine the 2D layout assignment for Compound entries?

texts:
The PubChem database provides 2D structures for entries in its Compounds database:
ftp://ftp.ncbi.nlm.nih.gov/pubchem/Compound/ (ftp link to download examples)

On the page linked below, under the "NCBI-generated records" heading, it's mentioned that the data processing of records for the Compound database "includes 2D coordinate layout assignment":
https://pubchem.ncbi.nlm.nih.gov/docs/subcmpd_summary_page_help.html#DataProcessing

What algorithm does PubChem use to generate these 2D structures?

I'm familiar with similar algorithms used in RDKit (e.g. compute2dCoords defined in rdkit/Code/GraphMol/Depictor/RDDepictor.cpp), but I'm specifically interested in a resource that describes the algorithm that PubChem uses.

Thanks very much!


texts: Is polylactic acid (PLA) soluble in ethylacetate?

texts:
I'm searching for polylactic acid (that used in 3D printing) solvent, that is less toxic. It's used to postprocessing to polish object.

Dichloroethane is toxic, acetone and dichloromethane less, but enough. Tetrahydrofurane and chloroform usage is limited in Russia.

As I know there is D- and L- polylactic acid, I don't know what type is it, but can test using acetone.

Can I use ethylacetate for this purpose? Or there is another solvents?

Sorry about my english

Useful links:
https://www.researchgate.net/post/What_is_the_best_solvent_for_polylactic_acid2/amp
https://www.researchgate.net/post/Solubility_of_PLA/amp
https://groups.google.com/forum/m/#!topic/tvreprapug/1wTmQZJMc9g


texts: What proportion of a combustion engine's output energy comes from the air?

texts:
When trying to compare the efficiency and range of electric engines vs gasoline engines, it seemed at first that gasoline had a higher energy density than batteries. But then I realiced that a significant proportion of the energy of combustion must come from the oxygen in the air, whereas in an electric engine all the energy comes from the batteries, so it's not a fair comparison.

So, assuming a typical air-fuel ratio of 14:1, what proportion of the energy produced by combustion comes from the air and what proportion from the fuel itself?

PS: Please assume usual values of any other variables if you need to.


texts: Electrolysis of water with aluminum at the negative pole

texts:
I am doing electrolysis of water to collect the hydrogen and I'm running into a problem: on the positive side I'm using graphite from a pencil (I cannot use any other material because it corrodes too fast) and on the negative side I was using copper. Copper was working really fine, the water was clear and no problem. But I don't have copper with a big surface area so I decided using aluminum (from coca cans) and my solution started to get dirty because of the aluminum and the aluminum is slowly disappearing.

Why? Any other eletrode that I used for the negative pole was not corroded (only on the positive pole everything corrodes). Why is aluminum making my water dirty? Do you think if I use iron this problem will not happen?


texts: Can potassium carbonate be used to calibrate hygrometers?

texts:
Context:
In the process of trying to calibrate a number of hygrometers, with Sensirion MEMS electronic sensors, I came across the method of using wet salt: an airtight container will stabilize at a predictable relative humidity over time, given a particular salt and temperature.
For a list of salts and their RH values at particular temperatures see this page on the website of Vernier, a maker of STEM education equipment. The various salts suggested for this method repeatably produce a range of low to high relative humidities, also allowing for two-point calibration.
I require my hygrometers be calibrated close to 40% RH. With a predictable 43% RH that makes potassium carbonate the ideal candidate. However, when I wettened potassium carbonate with water it underwent an moderate exothermic reaction. Googling what was going on, I read "potassium carbonate dissociates completely in water into potassium (K+) and carbonate ions (CO32-)."
Which makes me wonder:
Can potassium carbonate be used at all for the wet salt calibration method, or does its swift dissociation render it useless for this application?


texts: How to determine the composition for a sucrose/ zinc nitrate solution from its freezing point?

texts: A $4~\mathrm{g}$ mixture of sucrose ($\ce{C12H22O11}$) and zinc nitrate ($\ce{Zn(NO3)2}$) is dissolved in $150~\mathrm{g}$ of water. If the resulting solution freezes at $-0.768~^\circ\mathrm{C}$, what is the mass percent of sucrose in the mixture? Assume that zinc nitrate dissociates completely in water, $K_\mathrm f = 1.86$. (Answer: $2.8\%$)


texts: Naming ester as a substituent to carboxylic acid

texts: Can carboxylic acid functional group -COOH and ester's functional group R-COO-R' be in one structure? If yes, carboxylic acid will be the parent chain and ester will be the substituent as c.a. is higher than ester in the priority table. Then, how do we name the structure where ester becomes a substituent? Thank you.


texts: Where does pepsin cleave a human type 1 collagen molecule? Does it occur at multiple locations or only one?


texts: How can I define a torsion force field for $n$ points in space?

texts:
I would like to define a model for torsion force field between $n$ point in space.

I thought to generalize the OPLS model:

$$
\sum_\mathrm{dihedrals} \left( \frac {V_1} {2} \left [ 1 + \cos (\phi-\phi_1) \right ]
                + \frac {V_2} {2} \left [ 1 - \cos (2\phi-\phi_2) \right ]
                + \frac {V_3} {2} \left [ 1 + \cos (3\phi-\phi_3) \right ]
                + \frac {V_4} {2} \left [ 1 - \cos (4\phi-\phi_4) \right ] \right)
$$

How can I make them to lie in the same plane allowing fluctuations of $\pm10^\mathbf{o}$?


texts: Effect of dilution on the degree of dissociation of an electrolyte

texts:
My book says that on dilution, the degree of dissociation of an electrolyte increases. Consider the reaction:

$$\ce{A(aq) <=> C(aq) + D(aq)}$$

Let $\ce{A}$ be the electrolyte that splits into its ions $\ce{C}$ and $\ce{D}$.
According to my book, on dilution, the concentration of $\ce{C}$ and $\ce{D}$ will decrease. Hence according to the Le Chatelier's principle, the reaction must proceed in forward direction.

Won't the concentration of $\ce{A}$ also decrease? In that sense, shouldn't the reaction proceed both in the forward and backward direction?

Please do let me know where am I going wrong.


texts:
As everyone knows a diamond won't last forever. It will eventually turn into a graphite over a long period of time, as the free energy of graphite is lower than that of diamond, hence making the reaction energetically favorable.

However, is it possible to do the reverse, by using graphite and the applying energy onto graphite so as to synthesize diamond. If so, how is it done and can it be done in a lab by a chemist? Does that mean that people who managed to do it can become rich? As a diamond costs so much more than a graphite. Shouldn't we synthesize diamonds in this way? Can I become  rich by synthesizing diamonds and selling them?


texts:
Can acid mine drainage form acid rain?

Since normal acid rain is formed via combustion of fossil fuels, the sulphur dioxide and nitrogen dioxide are all in the gas phase, whereas in acid mines the sulphuric acid is in solution.

Here's the reaction for acid rock drainage if it helps:

$$\ce{FeS2 + 3.75 O2 + 0.5 H2O → Fe^3+ + 2 SO4^2- + H+}$$


texts: I am highly allergic to propylene glycol and polyethylene glycol.  I am finding many hidden sources of both and MANY synonyms for PG and PEG.  Is there a practical way to tell how closely related another chemical might be to my allergens using such resources as the U.S. Household Products data base?  For example:  glycerin's alternate name is propandiol.  Propandiol is also an alternate name for propylene glycol.  I'm very confused about sodium laureth sulfate in particular.  It has PEG listed as an alternate name, but is not an alcohol like the glycols.  Thank you for any clarification you can give me!


texts: Why do we have the concentrations powered to the coefficients in the equilibrium expression?

texts:
$$\ce{A -> 2B} \qquad K_\mathrm{c} = \frac{[\ce{B}]^2}{[\ce{A}]}$$

Why do we have a power that is the same as the coefficient?
Assuming $K_\mathrm{c} = 1$, shouldn't the quantities of the mixture be $2B = A$ (in other words, the amount of $\ce{A}$ in the mixture equals two times the amount of $\ce{B}$ in the mixture)!? So why do we have $[\ce{A}] = [\ce{B}]^2$ rather than $[\ce{A}]=2[\ce{B}]$?


texts: Number of hyperconjugative structures in aromatic systems

texts:
I know that to find the number of hyperconjugative structures in aliphatic systems we use the formula $a + 1$ (where $a$ is the number of α-hydrogens in that compound)

However, my teacher answered to the above question as:

  No, this formula is not valid for aromatic systems.

Why is it so? What is the exact formula? Is it that we don't count its original structure as a hyperconjugative structure?


texts: Esterification and dehydration synthesis

texts:
I know there are many more terms out there like polymerization, condensation, etc. But the important thing is whether they can be differentiated or not.

In Esterification, which I just learned, needs to have a dehydrating agent when involving bad leaving group, such as $\ce{-OH}$.They are typically Brønsted-Lowry acids. Correct me if I'm wrong.

How does heterolysis work in the case of $\ce{CH3COOC2H5}$. [the reverse of the esterification of ethanoic acid and ethanol]


texts: Why does delocalization of π electrons bring stability?

texts:
My book mentions that

  Delocalization of $\pi$ electrons makes a compound stable as the process of delocalization involves loss in internal energy. This is the synopsis of resonance.

But how does shifting of $\pi$ electrons let the body lose internal energy? What is the cause?

While studying resonance, I observed that one structure possesses charge while in other no charge. But in the final structure, there is, according to the book, partial charge present.

Eg.
In phenol, three resonating structures contain '+' charge on $\ce{O}$ . Thus, in the final structure, there is a slightly positive charge on oxygen and also, small negative charge on the main body.

Now my question is, why there is partial negative charge on the body? And why does oxygen carry partial positive charge? What is the cause?


texts: Acid-base chemistry - determining stability

texts:
I am learning about acid-base reactions and how to identify whether an acid is strong or not by looking at the conjugate base. The more stable a conjugate base the stronger the acid.

However, two of the factors that determine conjugate base stability seems contradictory to me.

I understand the bigger the atom, the more stable the base so $\ce{HI}$ is more stable than $\ce{HF}$.
The smaller the orbital the more stable the base. So $\ce{sp}$ is more stable than $\ce{sp^3}$.

Now why is a bigger atom more stable than a smaller one, but a smaller orbital  more stable than a bigger orbital?


texts: How does current start to flow when galvanic cell is first connected?

texts:
Using the Daniell Cell as an example, where in the steady state we expect zinc to be oxidised and copper to be reduced spontaneously due to the overall reduction in Gibbs free energy, how does the system "know" to start moving electrons from the anode to the cathode? What is the underlying physical mechanism?
When the two half cells are first connected via the external circuit, it seems that they would not yet "know" anything about each other; each half cell would not already "know" it is anode or cathode (e.g. zinc could be the cathode in a different galvanic cell) and so how would the system immediately start driving the electrons in the right direction?
In other answers on this site (e.g. https://chemistry.stackexchange.com/a/28638/98580) I've seen statements like "the metal atoms of one half-cell are able to induce reduction of the metal cations of the other half-cell; conversely stated, the metal cations of one half-cell are able to oxidize the metal atoms of the other half-cell".
However, as a physicist this is somewhat unsatisfying, as the phrase "induce reduction" seems to bundle up a whole lot of physical processes in the half cells and the external circuit into a very high level concept. Fundamentally, it must come down to electrical forces and thermodynamics to explain the overall behaviour of the system.
My understanding is that prior to connecting the external circuit, each electrode is in equilibrium with its electrolyte, undergoing both oxidation and reduction at the same time; so each electrode could both push and pull electrons to/from a connected wire. The moment the external circuit is connected, do electrons start flowing from the zinc electrode to the copper electrode because zinc has a greater "tendency" (than copper) to push electrons onto a connected wire, and copper has a greater "tendency" (than zinc) to pull electrons from a connected wire? That would then sound like the galvanic cell gets started via a statistical process and is sustained by electrical charging of the electrodes thereafter.


texts:
Is 2,3-dimethylprop-1-ene a possible organic structure?

I am not sure how to approach this, what would the correct name of this molecule be?


texts:
I've read that when a certain amount of UV light is shone on a metal surface, electrons are ejected (the photoelectric effect).

Are these electrons from the metal atoms themselves? and in case they are - what happens to the metal atoms seen as they have lost an electron? (Are they chemically different?)


texts: I understand that deflagration is slower and smoother than a detonation, but can't a deflagration be used to initiate detonations of other explosive agents?


texts: Axial vs equatorial attack of Grignard reagent on dioxan-5-one and dithian-5-one

texts: How to account for the diastereoselectivety (axial vs equatorial attack) of these reactions?


texts:
The question is the electron configuration of $\ce{Fe2+}$.
Why does it lose the electron of $4s^2$ first instead of losing $3d^2$?
I consider the pattern of the losing electrons in the $p$ orbital. The electron in $p_y$ first but not the $p_z$ in the $Mn$.


texts: Do thermodynamically favourable chemical reactions always proceed?

texts:
I wanted to know some common parameters which decide the feasibility of any chemical reaction.
And can we make a non favourable reaction favourable.


texts: The magic of ink tubes - Hydrophilic polymers?

texts:
First Chemistry SE post. I'm trying to make a capillary ink tube similar to the ones they used to have in chart recorders (here's an example of a capillary tube patent).

That is, an ink reservoir and a writing nib of some sort (probably felt tipped) connected with a long thin tube. What is the nature of that tube?

Physioscribe Chart Recorder - We found one at a local tech salvage place, but minus the critical ink tube portion

I tried some plastic tube experiments to see if the tube would draw water-based ink up out of a reservoir, but it did not. Afterward, I research the material, and found the tube was made of teflon. Teflon is hydrophobic and so apparently discouraged capillary action.

So here are a series of questions:

Is there a thin tube material that will draw water-based ink up out of a reservoir?
What would this thin tube be made of? Some hydrophilic polymer? Something such as polyvinyl alcohol, poly vinyl pyrrolidone, or cellulose?
Are there common suppliers that make tube of this material? And where?
Finally, is there something I am totally missing that would work better?

Thanks for the help.


texts: What is the mass of 150 ml of 40% sucrose, and what is the mass of 150 ml of 20% sucrose? I don't know exactly how to figure it out without using a device to tell me the mass. All I know is the mass of 10 ml of 40% sucrose, which is about 15.7 grams.


texts: Why is trans-1,2-dimethylcyclohexane an enantiomer?

texts: Can't the two be superimposed over each other after ring inversion.


texts:
Where's the lone pair on the cyanide anion in this MO diagram?

In the traditional Lewis picture there are two lone pairs on the cyanide ion. In the MO diagram though there appear to be no non-bonding orbitals that are populated ...

Also how come there appear to be two sigma bonding orbitals populated when in $\ce{NC^-}$ there is only one sigma bond?


texts: Hydrogenation of pyridine compared to other heteroaromatic rings

texts: Why does pyridine require the lowest temperature for hydrogenation, even though it has it has the highest resonance energy amongst pyridine, pyrrole and furan? Is it just because of the increase in the absolute pressure or does the HCl also effect the reaction somehow?


texts: Restrictions on transitions in Spectroscopy

texts:
Just for the sake of an example, let's assume a transition from one energy state to a higher energy state in atomic absorption spectrum

If the energy difference between the ground state and first excited state is E and that between ground state and second excited state is 3 E (just for say...). Now if an incident photon has an energy of 2 E so;

(Assuming the electron is initially in ground state)

Will the electron show transition to the first excited state and emit a photon of energy E

OR

Will the electron not show any transition and not accept the photon at all, since the energy of the photon does not exactly match with the energy difference between any two specific states?

The example of atomic absorption spectrum is taken up just for the sake of explaining the question, the question is rather general and pertains to any kind of spectroscopy experiment in which their occurs transition from one state to another by absorption of EM waves.


texts: PTFE-to-PTFE or PTFE-to-Copper pressure seal

texts:
I am currently working on an engineering project with a friend. Without giving too much away, I have two parallel strips of Zirconium-Copper alloy sandwiched between an upper and lower piece of smooth PTFE. Between those two strips of metal runs a channel, and I wish to run liquid nitrogen (or another cryogenic coolant) in there.

My question is will a pressure seal alone (light pressure being applied from top and bottom uniformly) be enough to keep the liquid nitrogen in the channel? Will PTFE seal when pressed against itself, or when pressed against a metal?

To be clear, this is oil-free PTFE without any filler.

Some data on the materials:

PTFE data sheet

Zirconium Copper Alloy

An image of the setup. PTFE is in white, copper in orange... This is just a crude representation.


texts: What does unactivated, activated and deactivated aryl chloride means in cross-coupling reactions?

texts:
When I read this literature https://pubs.acs.org/doi/pdf/10.1021/jo990408i, it used unactivated aryl chloride for Buchwald-Hartwig amination.

What does unactivated, activated and deactivated aryl chloride means in cross-coupling reaction?


texts: Sodium benzoate in soft drinks reacting to produce benzene?

texts:
Regarding chemical synergies, I read: if someone ingests various products containing sodium benzoate (for example soft drinks that exist) AND if one has had some quantity of vitamin C there might be 'reactions' that produce benzene, a carcinogen.

Some people have told me you'd have to drink a lot of pop for this to happen (assuming you have had some vitamin C during the day), but how much pop is a lot?

Given one has had a glass of orange juice say, how much pop would one have to drink before a benzene molecule is formed?


texts: Is hydrogen bond the attraction between a hydrogen atom bonded to a highly electronegative atom (N, O, F) and a lone pair? If this is true, in aqueous solutions, are there always hydrogen bonds between the solute molecules and water molecules as long as the solute molecules have lone pairs of electrons? So, even oxygen gas forms hydrogen bonds when dissolved in water because the oxygen has lone pairs?


texts: Why is magnesium sulfate acidic? Why are my calculations showing that it is basic?

texts:
Let's say I was dissolving $\pu{1 M}~\ce{MgSO4}$ into water and I wanted to find its $\mathrm{pH}$. I would go right to the reaction of the dissociation of $\ce{Mg(OH)2}$:
$$\ce{Mg(OH)2 <=> Mg^2+ + 2OH-}$$
The $K_\mathrm{sp}$ expression for this reaction is:
$$K_\mathrm{sp}=[\ce{Mg^2+}][\ce{OH-}]^2$$
According to wikipedia (see sources at bottom), the $K_\mathrm{sp}$ for $\ce{Mg(OH)2}$ is $\pu{5.61*10^{-12}}$
$$\pu{5.61*10^{-12}}=[\ce{Mg^2+}][\ce{OH-}]^2$$
My molarity of $\ce{MgSO4}$ will be the same as that of $\ce{Mg^2+}$, which is $\pu{1 M}$. Substitute that in.
$$\pu{5.61*10^{-12}}=[\ce{OH-}]^2$$
And then solving for $[\ce{OH-}]$ we get:
$$[\ce{OH-}]=\pu{2.37*10^{-6} M}$$
Therefore:
$$\mathrm{pOH}=5.63$$
and
$$\mathrm{pH}=8.37$$
So my final solution of $\ce{MgSO4}$ will be basic. However, on stack exchange it says that the $\mathrm{pH}$ of $\ce{MgSO4}$ is between 5.5 and 6.5. The post does not explain why, but that the general consensus is that that is the range of the $\mathrm{pH}$.
What did I do wrong?
Sources:
https://en.wikipedia.org/wiki/Magnesium_hydroxide
Is magnesium sulfate basic, neutral or acidic?


texts: How to calculate Revelle factor using only pH, DIC and TAlk for seawater?

texts:
I have the temperature, salinity, $\mathrm{pH},$ dissolved inorganic carbon (DIC) and total alkalinity (TAlk) values for an estuarine water sample. I want to calculate the Revelle factor.

The Revelle factor can be calculated from the partial pressure of $\ce{CO2}$ and DIC. But I don't have the value for partial pressure of $\ce{CO2}.$ I want to know whether it is possible to calculate Revelle factor using $\mathrm{pH}$, DIC and Talk values. And if it is possible then I want to know how to do it.


texts:
According to List of oxidation states of the elements, silicon has a possible oxidation state of $-4$.

Now, I've been looking everywhere for a compound that contains Si(-IV), but I cannot find any definitive references. Perhaps one of the synthetic carbonyls of silicon has this OS? I've seen references to $\ce{[Mo(CO)_4]^{-4}}$ and $\ce{[W(CO)_4]^{-4}}$ in my research, but silicon never shows up.

I am also uncertain as to how these carbonyls come together, and whether or not they actually complex in the same way for metalloids as for the transition metals.


texts: Rotational Constant for CO2 from an IR Vibrational-Rotational spectra

texts:
I have been trying to calculate the rotational constants ($B$) for $\ce{CO}$ and $\ce{CO2}$ from IR Vibrational-Rotational spectra.

I know that for $\ce{CO}$ the peak spacing is approximately $2B$ (ignoring effects due to centrifugal distortion) however for $\ce{CO2}$ it seems the peak spacing represents $4B$ instead when I compare my results to literature?

Does anybody have any explanation as to why this is, or could provide reading on this subject to explain why the peak spacing for $\ce{CO2}$ is $4B$?


texts: What does addition of iodine and base do this case?

texts: I figured that the first reaction just adds a bromine to the meta position but I don't know what step ii) does. The only thing I can think of is the base gets rid of the alpha hydrogen next to the carbonyl. Then perhaps the negative carbon now attacks iodine? But that seems a bit off to me.


texts:
I was reading an article about a chemical reaction, and I came across the phrase:

  The oxygen atom at this point has three bonds and has a net positive charge

How can this happen? Oxygen has 2 missing electrons in the valence shell. Therefore it can only form 2 bonds at the most, if both are sigma bonds.

Does it mean the 3rd bond is not covalent? Can it happen with a hydrogen or an ionic bond?


texts: What is the definition of current efficiency?

texts:
What is the exact definition of current efficiency of an electrolytic cell? I've looked online and there is no suitable answer to it.

I thought current efficiency would be the amount of electrons that actually participate in a desired reaction. For example, if we are electrolyzing a an aqueous solution of $\ce{FeCl3}$ and we have a $\pu{10A}$ current running for, say, $\pu{100 s}$, at $60\%$ current efficiency, two possible products are $\ce{H2}$ gas and reduced $\ce{Fe}$ metal. Current efficiency of $60\%$  means that half of the electrons that pass through this circuit would reduce the $\ce{Fe}$ instead of reducing $\ce{H2}$.

However, there is another way to think about it, in that it means 60% of the theoretical mass yield of $\ce{Fe}$ comes out of the reaction instead of my original definition, which is different because the reduction of iron (III) requires 3 electrons rather than only 2 electrons for the reduction of $\ce{H2O}$/$\ce{H+}$ ions. So what is the actual definition?


texts:
How is the specific heat of water so high?
I was reading the hyperphysics article on it but could not understand it properly.


texts:
My professor has an anti-fetish for carbocations. In any mechanism that we draw with a carbocation, the maximum earnable credit is automatically halved. In any case, he draws all mechanisms involving carbocations as concerted reactions; he never wants us to draw an carbocation by itself. If a carbocation could be drawn (as in the case of the formation of an acetal; specifically the step in which water leaves) then we must have electrons flowing from somewhere else to give the carbon an octet.

Now, is this valid? Are all mechanisms involving carbocations concerted? Excluding the obvious case of SN1; I'm pretty sure he'd accept that carbocation (and he hasn't touched on SN1 ever).


texts:
I am looking in to analytical techniques for a future project. I may have a mixture of metals, i.e. an alloy, which I am thinking XRF spectroscopy would be the best quantitative technique. I could even purchase a handheld XRF analyser.

However, I believe that the XRF technology does not detect light elements such as oxygen. Therefore, if I have a mixture of metal oxides in a sample; what is the best quantitative analytical technique for me to identify each oxide?

I have had a look at XPS but I am confused about how I can work out which oxide I have in my sample. For example, if my sample contains Lead oxide and Copper oxide. How do I know whether it is Cu(II) oxide or Cu(I) oxide?


texts: UV/Vis spectroscopy transition reference

texts:
There are mainly fundamental transitions happening during excitation and UV/Vis spectroscopy (as n- > σ* ,π->π* ,n- > π ,..).  I am looking for a good reference that shows these transitions for molecules (for example shows n- > σ* for acetone , π- > π* for benzene,... ).

Actually I am talking about tables. I can do guesswork for most of the molecules based on their structures but definitely I need references for publication.

Update: Mostly interested in these molecules: Acetone, Acetaldehyde , Benzene , Naphthalene, Anthracene , Methylene Blue, Phenanthrene , 1-Butanol


texts: Thermodynamics behind organic chemical reactions?

texts: I took organic chemistry a few years ago at my university, and although I went through the motions to pass the exams, I didn't understand the thermodynamic reasoning behind why the reactions actually occurred. For instance, I memorized that Claisen Condensation reactions occur between two esters in the presence of a strong base, but I don't fully grasp WHY the reaction happens in the first place.  Why are the products more stable than the reactants in cases like these? Does this reaction always occur whenever these components are in a mixture, or is there a sort of catalyst to trigger it? I feel similarly confused for many other organic reactions (e.g., Aldol condensations, Fischer esterification, Sn2, etc). I suppose I'm wondering if there is a general thermodynamic reasoning I'm missing in organic chemistry.


texts: How does cullet reduce the production temperature in glass production?

texts:
In my book, I read that broken glass pieces are reused in the production of glass, known to be cullet. This on one hand saves the production cost and also reduces the production temperature.

My question is: how does adding cullet reduce the production temperature? I see no reason why the materials would react any differently with or without cullet.


texts: I have a basketball support pole that is 5" in diameter welded from iron pipes that supports the hoop. There is a vertical component welded to a horizontal one that supports the backboard and hoop. The pole developed two rust holes approximately 1 1/2" in diameter in the horizontal section. Based on responses I have seen, rust cannot form without the presence of moisture and oxygen. Therefore, I sprayed an expansive foam to occupy the air voids within the pipe (only in the horizontal portion where the rust holes are). I sprayed a permatex rust neutralizer on the exterior location of the two holes. I plan on closing the holes with J B Weld (a very strong 2 part resin). Lastly, I will paint the poles with Rustoleum protective enamel paint. The thought is to seal the pipe from air and slow or discontinue the rusting process. I even considered spraying the pipe before painting with the rubberized Flex Seal to completely encapsulate the pipe. This may not be necessary but your opinion would be helpful. Please comment on the statements I have made in terms of accuracy and procedure. Thank you for your thoughts in advance.


texts: What gas would be suited best to form a density gradient in a long tube?

texts:
I want to set up an aero-optics experiment using a sealed ~5 m length of pipe with a gas inside. The gas must form a density gradient, and must be transparent enough to view a target at the other end of the pipe.

Barometers at the top and bottom of the pipe will measure density.

The purpose of the experiment is to replicate the density structure of earth's atmosphere to establish what affect density gradients alone have on light passing through.

Question: What gas would be best to use for this experiment?

Would a moderate amount of boiling water in the bottom of the pipe do the job? Are there better gasses that could be used? My thinking is once the gas is added and the pipe is shaken up to distribute the gas around, then as it settles back down it would form some kind of gradient for a short while.


texts: Why is there no exothermic crystallization peak visible during cooling of PLA and no endothermic melting peak visible during a second heating of PLA?

texts: I am doing lab research on DSC measurement (Heat-cool-heat) for understanding melting characteristics of polymers (PLA, PA6 and PE). During cooling i saw exothermic peak and during second heating i saw endothermic peak on PE and PA6. I don't understand why there is no crystallization peak during cooling and no melting peak during second heating on PLA. I have posted DSC measurement graph of PLA done by DSC Q-series. The figure is shown in exothermic up formation and crystallisation is exothermic process and melting is endothermic process. Any guidance will be really helpful.


texts: Benzaldehyde and Cyanide - if you can smell one can you smell the other?

texts:
Cyanide and benzaldehyde both have an almond-like smell.

Some people cannot smell cyanide (specifically, HCN). Can they also not smell benzaldehyde?

In other words, do they both trigger the same receptors?


texts:
why is white phosphorous given to have ZERO standard heat of formation??

while it is well known fact that white phosphorous is highly reactive and is obviously less stable than red and black phosphorous

also here=Which allotrope of phosphorus is kinetically the most stable?

it says that black phosphorous is thermodynamically more stable

also what is the relative stability of the allotropes [black red white purple]


texts:
From my experience so far with chemistry, it seems like every reaction that happens is to attain stability. So shouldn't a lower entropy be good? Since less disorder means more stability.
But by Gibbs Helmholtz equation, a negative entropy change value favors a positive $\Delta G$.... But shouldn't a reaction which leads to stability be spontaneous?


texts: What is a suitable acid to remove limewash from masonry brick

texts:
Diluted Hydrochloric acid is often touted as the only thing to remove limewash from masonry but the process is dangerous and can adversely affect the bricks, color changes can readily occur.

Is there a viable alternative? I noticed that a kettle descaler product used formic acid and wondered if that could be used instead.


texts: Order for acidic strength of hydrides across a period

texts:
I have learnt that in a group , say group $15$, the acidic strength increases on going down the group as

$NH_3 < PH_3<AsH_3<SbH_3<BiH_3$

This is due to decrease in the thermal stability on going down the group. But we can also reason it as follows : on going down the group , the electronegativity of the central atom decreases , and the bond between the central atom and hydrogen becomes less polar , so the bond strength also decreases hence it becomes easier to lose a hydrogen atom thus increasing the acidity.

Now if we compare the hydrides of a period, that is

$PH_3 , HCl , H_2S$ , according to the above logic , the order should’ve been $PH_3>H_2S>HCl$ because the electronegativity increases across a period. However , the correct order seems to be the reverse. Could you please explain why ? Is there a flaw in my reasoning?


texts:
I am working on Li-ion batteries, and I need to use γ-$\ce{MnO2}$ (electrolytic manganese dioxide). I have googled and read quite a few papers, but I am not very sure of those methods. I'd love to know if you are aware of some! The ones which I encountered, again and again, was the one involving precipitation of $\ce{MnO2}$. But is it possible to precipitate the γ form? Also, I think the commercially available $\ce{MnO2}$ is the α form. Is there any way to convert it to γ form? I am particularly looking for a way to synthesize  $\ce{MnO2}$ using electrolysis.

I know that I can also break a few batteries to get my $\ce{MnO2}$, but it would be heavily contaminated by carbon. Commercially available  $\ce{MnO2}$ is very expensive.


texts: How does a simple alkaline battery work?

texts: The basic + - electrolyte set up. Modern batteries and the like. How exactly do they work?


texts: What is the correct electron dot notation of carbon?

texts:
My book says that the dot notation for Lewis structure of carbon has 2 possible ways:

two dots on the right, one on the top, one on the left
one dot on all four sides

Why are both of these correct? Isn't only the first one correct by Hund's rule? By the way, we are now learning Lewis structures and the book randomly used the 2nd dot notation of carbon.


texts: Why does de Broglie wavelength lambda = c / frequency?

texts:
For a chemistry problem, I want the velocity of the de Broglie wave to agree with one of these values:

A) $c^2/v$
B) $h\nu/mc$
C) $mc^2/h$
D) $v\lambda$

We know that
$$\lambda = \frac{h} {mv}$$
But I am confused by the next step in my book:
$$\lambda = \frac {c} {\nu}$$
After which:
\begin{align}
\frac{h}{mv} &= \frac{c}{\nu} \\
\implies v &= \frac{h\nu}{mc}
\end{align}
Why is the de Broglie wavelength $\lambda = c/\nu$? In every context I've seen it used, $c$ is the speed of light. But I don't think the velocity of the matter wave could equal the speed of light, because the matter wave is only as fast as the particle – which, having mass, cannot travel at the speed of light.
What am I doing wrong here? Does $c$ moonlight in quantum chemistry as the velocity of the matter wave, separate from its connotations in physics?


texts:
I am trying to make a copper sulfate electroplating solution. I came across a document (Schlesinger and Paunovic 2011) which mentions the amount of copper sulfate that should be added to water to create such a solution. In Table 2.1 on the bottom of page 35, it has two columns: one for conventional solutions and one for high throw solutions.

I read another paper (Nikolova and Watkowski) which talks about high throw copper electroplating process. All I could glean from the paper was that high throw solutions enable higher aspect ratios for copper PCBs.

Please reply if anyone knows what high throw solutions are.

References:

Nikolova, Maria, and Jim Watkowski. “INNOVATIVE HIGH THROW COPPER ELECTROPLATING PROCESS FOR METALLIZATION OF PCB.”

Schlesinger, M., and M. Paunovic. 2011. Modern Electroplating. The ECS Series of Texts and Monographs. Wiley. http://books.google.com/books?id=j3OSKTCuO00C.


texts: what wavelength should I use to measure the transmittance of iodine colouring?

texts: I am doing an experiment on how pH buffers may affect amylase's break down of starch into glucose in a starch solution. I am using iodine reagent to identify how much starch is left after amylase is added. I am wondering, to which wavelength should I set the spectrophotometer to in order to measure the absorbance and percent transmittance values of the solution after iodine solution is added to measure the amount of starch?


texts: Verification of variants of the Ebullioscopic formula

texts:
Wikipedia cites the following formula for the ebullioscopic constant $K_{\mathrm{b}}$ regarding the elevation in boiling point of a volatile solvent upon addition of a non volatile solute (https://en.wikipedia.org/wiki/Ebullioscopic_constant):

(All symbols carry their usual meaning, unless specified otherwise)

  $$K_{\mathrm{b}} = \frac{RT^2_{\mathrm{b}}}{\Delta H_{\mathrm{vap}}}$$

But my textbook cites another formula :

  $$K_{\mathrm{b}} = \frac{RT^2_{\mathrm{b}}}{L_{\mathrm{v}}}$$

Where $L_{\mathrm{v}}$ is the latent heat of vaporization. Since there exists the equation $$\Delta H_{\mathrm{vap}} = {mL_{\mathrm{v}}}$$

my textbook further modified its formula to:

  $$K_{\mathrm{b}} = \frac{RT^2_{\mathrm{b}}m}{1000\Delta H_{\mathrm{vap}}}$$

Where $m$ is the mass of solvent in grams.

Now my question is, which one's correct? The only difference between the equation on Wikipedia and the one in my textbook is the mass term $m$. I can't seem to find any reliable references on this :/. So a good reference would be appreciated.


texts:
Why doesn’t aluminum have negative oxidation states? In almost all compounds of aluminum  which I know of has positive  oxidation state.

It has valence EC [Ne] 3s2 3p1 , my teacher told it usually forms +1 and +3 oxidation state by completely sharing p orbital electron or both s and p orbital completely(and +2 o.s. is always average of aluminum)

So why can’t aluminum or any 13th group element for that matter accept electron and make its p orbital half filled i.e form compounds with negative oxidation state?


texts: Composition of arcanite (potassium sulfate) agricultural grade. Potassium content of random drawn sample?

texts:
I bought potassium sulfate from this source:
Potassium sulfate granulate
I'm using it as a potassium additive in hydroponics. The composition is listed as K2O = 50%, S = 18%. This equals about 41.5% K (50* (mass of K / mass of K2O) Calculated from mass percentages (mass of K2 / K2SO4) potassium sulfate should contain about 44.9% K and and 18% S. So based on this calculations I concluded that the potassium content is within the margin of error tolerable for fertilizer application. (About 3.4% for potassium)
However after further reading I came across an article which suggested that raw materials should be as homogeneous as possible because otherwise a sample drawn randomly could have huge variations in the content of the chemicals involved. How high could the deviation be with respect to potassium?f I think the source material is arcanite
EDIT: The security data sheet states that the purity of the granulate is guaranteed to be between 90-99%


texts:
Now I (and most of us here, I guess) have grown up around "Mercury's toxic! Avoid handling it! Do NOT screw around with this thing, if you value your life" and other equally heartening pieces of advice regarding mercury. Apparently mercury poisoning makes you, really lose it.

Now a while back I chanced upon a video on Youtube (I checked today, and now there're multiple videos on the same thing) where someone was messing around with elemental mercury … with his/her (I don't remember) bare hands.

Naturally I was taken aback. A quick check provided sufficient evidence to ascertain that he/she hadn't died as a result.

Now I posed this to a teacher of mine, and after some deliberation, he concluded that it isn't the mercury metal that's toxic. He told me that it's the various compounds of mercury that are formed by elemental mercury once it gets in your system (i.e- once you drink it) which are responsible for the toxicity. So apparently touching metallic mercury with your bare hands isn't an issue, since the metal isn't subjected to all those metabolic reactions that go on inside the body; heck you can even gargle it and live (provided you spit it out afterwards) since mercury's fairly inert.

So what I'd like to know is

Is it true that metallic mercury by itself, isn't toxic?

*Spin-off questions:

Is there any substance to my teacher's claim? How come those asses that performed this mundane task survived and now live (seemingly) normal lives without going insane? Were they already insane prior to touching the mercury? Should I just go and touch some mercury, partly to discover if it is indeed possible to touch metal mercury safely (and partly just to let out all the stress that built up while typing out this question)?


texts: Where's my experimental error coming from?

texts:
I’m teaching chemistry to a home-ed 15 year old.  Occasionally we do some practical work, since I believe very strongly that physical science learning should have some real lab work to bring it alive, and also to teach things like observational skills and attention to detail.

My background is a PhD in organic chemistry, so I’m familiar with hazard assessment, and all the lab work I do with my student is something I’ve done myself already.  Any time she touches the apparatus it’s under close supervision by myself.

So the practical involved taking hydrated copper sulphate, heating it to drive off the water, weighing before and after, and thus calculating the number of water molecules of crystallisation, based on the respective molecular weights of the anhydrous salt and water.

I have a porcelain evaporating dish to contain the hydrate during heating, electronic scales that will weigh to 300 g in increments of 10 mg, and commercially bought copper sulphate that is allegedly 99.5 % pure.

I started with the hydrate, heated it in the evap dish over a flame (using a gas cooker ring) until it had all turned greyish white, then allowed it to cool and weighed again.

The results were:

Starting wt hydrate: 13.03 g

Wt after heating: 8.22 g

Hence water lost: 4.81 g

So that’s 4.81/18 = 0.267 mol water, and
8.22/(63.5 + 32 + 64) = 0.0515 mol anhydrous copper sulphate

Hence 0.267/0.0515 = 5.18 mol water per mol copper sulphate.

So my question: why not 5.0?

The electronic scales (quite cheap) were an obvious first candidate for a source of error.  I’ve compared its readings at weights of about 2, about 20, and about 50 g, with another similar model, and they agree to within 10 mg (one digit in the final decimal place) at all weights.

I could perhaps have not heated the hydrate enough, and not driven off all the water – but then I’d have less than 5, not more, waters per mol.

Or the hydrate, as supplied, could be a little wet.  I haven’t investigated that yet, but I plan to weigh a sample before and after leaving in a warm airing cupboard overnight.

If you’ve read this far, I’m already grateful, but I’d be even more so, if anyone could suggest where the difference between 5 and 5.18 is coming from.

I did it a second time, and got 5.16.

The difference between 5.00 and 5.16 is a weight error of about 200 mg, and I was really careful, so this seems unlikely.

The weight after cooling of the evap dish is constant.

Any ideas?

Edit (2):

There was no sign of $\ce{CuO}$ after the heating.  No trace of black, and indeed we dissolved the anhydrous salt in water (for growing some seed crystals later), and there was no insoluble residue.
I’ve now heated the dish over a hot blue natural gas flame for ten minutes.  Its weight before cooling was 0.02 g (on my scales) less than its empty weight cold.

Edit (3)

I weighed out some of the hydrate into another dry dish, flattened it out as much as possible within the confines of the dish to expose as much surface area as possible, and left it in the airing cupboard for 24 h at 26 degrees Celsius.  Then I re-weighed.

Pre drying wt: 42.68 g

Post drying wt: 41.98 g

Assumed water content: 42.68 − 41.98 = 0.7 g (!!!)

So assuming that 26 degrees is not warm enough to cause partial loss of water of crystallisation, that suggests that at best my hydrate is only 98.36 % pure, not 99.5 as it says on the bottle (typo corrected above).

So repeating my calculation above:

Wt before: 13.03 × 0.9836 = 12.82 g hydrate
Wt after: 8.22 g
Water lost: 4.60 g

Moles water: 4.60/18 = 0.256

Moles $\ce{CuSO4}$: 8.22/159/5 = 0.052

Ratio: 4.96

I’m much happier with that!

Many thanks.


texts: Why is it difficult to detect dopamine using $\ce{^1H}$ Magnetic Resonance Spectroscopy (MRS) in the brain (in vivo)?


texts: How can eosin be acidic and negative at the same time?

texts:
The broadest acid-base definition is done by Lewis and he states that a lewis acid is:
"a compound or ionic species which can accept an electron pair from a donor compound."

And all the sources say that eosin is an acid and a negatively charged molecule.

I think eosin cannot accept a pair of electrons if it's already negatively charged, thus, it should be considered as a base.

A clarification on this subject would be perfect.


texts: Work done in Irreversible Isothermal Expansion

texts:
This is the graph for the Isothermal Expansion of an Ideal Gas. Sorry for the extremely basic doubt, but I just could not understand how the expansion was Isothermal if the graph was a straight line. The pressure is changed once suddenly and then remains constant.
Why don't we consider this as an Isobaric expansion instead of Isothermal if the graph is not even an isotherm? If a gas is expanding but the pressure has to be constant then why doesn't the temperature increase?


texts: why is the pressure exerted by ideal gas on the walls of the container more than that of real gas

texts:
I have learnt that ideal gases have no intermolecular forces. So even at high pressure the volume gets completely compressed.
But in case of real gases,  we have both attractive and repulsive forces. At far distance,  attractive forces dominate.
But at high pressure , when the particles are closer to each other,  the repulsive forces dominate over the attractive forces.
So my question is,  if the repulsive forces dominate over the attractive forces at close range,  then real gas should exert more pressure on the walls of the container.  With this one more doubt arises,  how do gases liquify if repulsive forces are dominant?


texts: Why is Aniline a poor substrate for Nitration reaction?

texts:
I have been asked to find out which one of the following is a poor substrate for Nitration reaction:

Upon encountering this question, my first reaction was the option c, as I know that the nitration mechanism involves the substitution of a positively charged Nitronium ion ($\ce{NO2+}$) and hence a $\ce{-OCOCH_3}$ group that shows $\mathrm{-M}$ effect will not be helpful for that process as it reduces the electron density in the benzene ring.
But the solution states that the answer is aniline (given in option a). I do not understand how is that possible. Even in the case when we use mixed acid as the reagent, it will form para- and meta-substituted nitro compounds. But, I do not know why and how does it behave as a 'poor substrate' in any case of nitration.
Please explain.


texts: Effect of temperature on benzenediazonium during diazotization

texts: What will benezendiazonium ion become if it is not maintained at a temperature below $5\ °\mathrm{C}$ during diazotization?


texts: Does trans-decalin really have a plane of symmetry?

texts: My teacher told me that trans-decalin (see below) is achiral due to the presence of both, a centre of symmetry and a plane of symmetry. But I could not spot the plane of symmetry untill now. Can someone point out the plane of symmetry in a diagram?


texts: A problem regarding the first law of thermodynamics

texts:
2 moles of an ideal gas are in a closed system with a temperature of $25\ \mathrm{^\circ C}$.
The gas is cooled under constant pressure and afterwards heated gas under constant volume.
During the cooling process the gas has lost $1000\ \mathrm J$ of heat to the environment.
It is known that the molar heat capacity of the gas is $C=25\ \mathrm{J/(^\circ C\ mol)}$
What is the work done by the system during the cooling and heating combined?

Official answer: $333\ \mathrm J$.
The problem: I understand how to get to this solution, however I don't understand why an alternative method yields an incorrect answer. Here's what I tried:
First, during the heating process there is no work done because the volume is constant.
$Q=-1000\ \mathrm J$
We know that:
$\Delta E=\frac32\cdot n\cdot R\cdot\Delta T$
Now, $Q=n\cdot C\cdot \Delta T$, therefore:
$-1000=25\times2\times\Delta T$ and therefore $\Delta T=-20$
So $\Delta E=\frac32\cdot n\cdot R\cdot\Delta T=\frac32\times2\times8.314\times(-20)=-498.84\ \mathrm J$
By the first law of thermodynamics: $\Delta E=Q+W$, therefore $-498.84=-1000+W$ and $W=501.16\ \mathrm J$
Could anyone please tell me where is my mistake?


texts:
This might sound funny but my friend asked me why most - if not every - faeces are of a brown or brownish color?
Why not grey or slightly bluish or green for example?
Is there any simple scientific explanation for this?

Thanks


texts: Why bath sonicators don't come with adjustable power setting?

texts:
All the bath sonicators I saw online don't have a power level setting.
(Probe sonicators, on the other hand, do have a power setting.)
Why is that?


texts:
I've been learning how the Jahn Teller Distortion effects the orbitals in Metal complexes and how the splitting of eg and t2g orbitals happens.
But the book mentions that the effect is strong for electron configurations where the eg orbital has 1 or 3 electrons. I can't figure out what that's the case. Why isn't the effect strong with 2 electrons in the eg orbital as well?


texts:
I have a question regarding a reversible elementary reaction:

If we have an equilibrium where $$\ce{A<=>[k_1][k_{-1}]B}$$ , then $$-d[A]/dt= k_1[A]-k_{-1}[B]$$.

If we take $$[B]=[B]_0 + ([A]_0 - [A])$$, the equation above becomes $$d[A]/dt= -k_1[A]+k_{-1}([B]_0+([A]_0 -[A])$$

So far, I've taken the integrating factor to be $$u=\mathrm{e}^{(k_1+k_{-1})t}$$

and emerged with the following equation:

$$[A]=\frac{k_{-1}}{k_1+k_{-1}}([B]_0+[A]_0)(1-\mathrm{e}^{-t(k_1+k_{-1})}) + C \mathrm{e}^{-t(k_1+k_{-1})}$$

I have tried to put $$[A](t=0) = [A]_0$$ and the solution appears to be $$C=[A]_0$$

Are there further approximations required?

My lecturer has produced the final equation as

$$[A]=\frac{k_{-1}+k_1\mathrm{e}^{-t(k_1+k_{-1})}}{k_1+k_{-1}}([B]_0+[A]_0)$$

When we take $$t->\infty$$ our equations match, however.


texts:
I am struggling with naming this structure either (E)-2-bromo-1,4-difluoro-3-iodobut-2-ene or (E)-3-bromo-1,4-difluoro-2-iodobut-2-ene.

I am aiming at the question whether there is also a prioritization for IUPAC nomenclature given by the order of atomic numbers as we have it for the CIP nomenclature. Or do we just have a prioritization given by the alphabetic order in the case of halides?


texts:
Sorry if this seems like a silly question but it has me confused a lot. So I have just started taking Chemical Reaction Engineering, and was studying about continuous-stirred tank reactor (CSTR).
So in my textbook it's stated that "It is normally operated at steady state and is assumed
to be perfectly mixed; consequently, there is no time dependence or position
dependence of the temperature, concentration, or reaction rate inside the
CSTR. That is, every variable is the same at every point inside the reactor.
Because the temperature and concentration are identical everywhere within the
reaction vessel, they are the same at the exit point as they are elsewhere in the
tank. Thus, the temperature and concentration in the exit stream are modeled
as being the same as those inside the reactor"
This has me confused a lot, if concentration isn't changing with time, then does the chemical reaction has to occur instantaneously?
So the reactant is instantaneously and uniformly mixed throughout the reactor upon entry, but what about it's conversion into a product?. Because if that's taking time then the statement "concentration isn't changing with time" doesn't make any sense to me.
Can someone please clarify?


texts: X-ray bombardment of CaCl2 and its resultant chemical changes

texts:
If one were to take $\pu{1 mol}$ of pure liquid Calcium Chloride (at $\pu{600 ^\circ C}$) and bombard it with $\pu{7.65 \times 10^{14} Ci}$ ($\pu{2.83 \times 10^25 particles}$) of x-rays ($\pu{75 keV}$), how the calcium chloride become chemically altered in anyway? Would the radiation cause the calcium and/or chloride ions to further ionize into a form that would not bond with the other ions?

I ask this because my purpose requires that the liquid $\ce{CaCl2}$ (not aqueous) would be able to keep the calcium and chlorine from precipitating out of the liquid.


texts: Calclulation of MP2 first order wave function

texts:
My question is, how can I calculate the MP2 first order wavefunction, and thus construct the MP2 density matrix? Now here is what I understand so far:

The density matrix is given as:

$$P_{ij}=\sum_{a}C_{ia}(C_{ja})^*$$

Here $C$ is the wave function coefficient matrix. To get the density matrix for the MP2 first order wavefunction, their coefficients need to be found.

The first-order wave function is given as [1]:

$$\left|\left.\varPsi_{i}^{(1)}\right> \right.=\sum_{n}c_{n}^{(1)}\left|\left.\varPsi_{n}^{(0)}\right> \right.$$

Now this gives:

$$\left< \left.\varPsi_{n}^{(0)}\right|\varPsi_{i}^{(1)}\right> =c_{n}^{(1)}$$

Which by some more steps, that can be seen in Szabo and Ostlund leads to:

$$c_{n}^{(1)}=\left< \left.\varPsi_{n}^{(0)}\right|\varPsi_{i}^{(1)}\right> =\frac{\left< \varPsi_{n}^{(0)}\left|\mathscr{V}\right|\varPsi_{i}^{(0)}\right> }{E_{i}^{(0)}-E_{n}^{(0)}}$$

In Szabo and Ostlund the energy is derived to have the expression:

$$E_{i}^{(2)}=\sum_{n}'\frac{\left< \varPsi_{i}^{(0)}\left|\mathscr{V}\right|\varPsi_{n}^{(0)}\right> \left< \varPsi_{n}^{(0)}\left|\mathscr{V}\right|\varPsi_{i}^{(0)}\right> }{E_{i}^{(0)}-E_{n}^{(0)}}$$

Though it can also be written as:

$$E_{i}^{(2)}=\sum_{n}'c_{n}^{(1)}\left< \varPsi_{i}^{(0)}\left|\mathscr{V}\right|\varPsi_{n}^{(0)}\right>$$

Now forwarding to the energy expression for the MP2 energy:

$$E_{0}^{(2)}=\sum_{i<j,a<b}\frac{\left< \varPsi_{0}^{(0)}\left|\mathscr{V}\right|\varPsi_{ij}^{ab}\right>\left< \varPsi_{ij}^{ab}\left|\mathscr{V}\right|\varPsi_{0}^{0}\right>}{E_{0}^{(0)}-E_{ij}^{ab}}=\sum_{i<j,a<b}\frac{\left< ij\left|\right|ab\right>\left< ab\left|\right|ij\right>}{\varepsilon_{i}+\varepsilon_{j}-\varepsilon_{a}-\varepsilon_{b}}$$

Now I would like to think that I could identify the wave function coefficients by comparing the two above equations to get:

$$c_{ijab}^{(1)}=\frac{\left< ab\left|\right|ij\right>}{\varepsilon_{i}+\varepsilon_{j}-\varepsilon_{a}-\varepsilon_{b}}$$

I would have expected that I would have found a vector of $c$ such that I would be able to construct the coefficent matrix as:

$$C^{MP2}=c\cdot c^T \Rightarrow P^{MP2}_{ij}=\sum_{a}C^{MP2}_{ia}(C^{MP2}_{ja})$$

But instead I got a four index coefficent out. Where did my line of thinking go wrong? And how can I proceed to construct the first order wave function, and thus get the density matrix?

References to old papers where this have been done explicit is also appreciated, because I have not been able to find any thing but recent papers about MP2 and density matrix fitting, which is not what I am looking for.

[1] Szabo, A.; Ostlund, N. S. Modern Quantum Chemistry: Introduction to Advanced Electronic Structure Theory; Dover: Mineola, NY, 1989.


texts: Calculating the molecular mass of an element using relative molecular mass in several compounds

texts:
The relative molecular masses of A, B, and C were found to be $101.30$, $87.30$ and $144.90$ respectively, compared to an assigned value of exactly $2.00$ for $\ce{H2}$. Subsequently, these substances were subjected to elemental analysis and were found to contain $37.2\%$, $64.8\%$ and $78.0\%$ of $\ce{X}$ respectively by mass.
Find the molecular mass of element  X, found in A, B & C.

I found the relative molecular mass of X in each of the three compounds:
In A, $$101.3 \times 0.372 = 37.68$$
In B,
$$87.3 \times 0.648 = 56.57$$
In C,
$$144.9 \times 0.78 = 113.022$$
I calculated the HCF of these three values. However my answer doesn't match with the actual answer. What am I doing wrong here?


texts: I have read that teflon coatings undergo decomposition at temperatures above 300 degree Celsius. What do they decompose to? And what other conditions apply for the decomposition? Is this decomposition harmful for humans as cooking goes beyond that temperature?


texts: Can we predict Dextrorotatory and levorotatory character of an organic molecule?

texts:
Can we predict in which direction a given molecule will bend plane polarized light?

If I have a molecule with R form in R-S nomenclature does it mean that the molecule will be dextrorotatory or is it to be found by experiments like finding out rate of a reaction by experiments?


texts: Lets say there is $\ce{CH3-OH + NaH}$. The reaction leads to $\ce{CH3-O- Na+ + H2}$. However $\ce{H-}$ from the metal hydroxide is also a strong nucleophile, so why can't the reaction lead to $\ce{CH4 + OH-}$? I realize that $\ce{OH-}$ is a bad leaving group, but then again, an alkoxide isn't a good leaving group either. An alkoxide's $\mathrm{p}K_\mathrm{a}$ is even higher than $\ce{OH-}$, so its a worse leaving group.


texts: α-Halo ketone and strong base: Favorskii vs SN2

texts:
Let us consider an α-halo ketone, with at least one α-H on the α carbon not containing the halogen. If I add a strong base, what is likely to happen: $\mathrm{S_N2}$ or Favorskii rearrangement?

I have learnt that α-halo carbonyls undergo excellent $\mathrm{S_N2}$ due to the lowered energy of its LUMO. And the Favorskii is a characteristic of the α-halo carbonyls. How to decide between the two?


texts: Why is the intensity of the IR spectra of IBr higher than that of ICl?

texts:
Why is the intensity of the IR spectra of $\ce{IBr}$ higher than that of $\ce{ICl}$?
I know that the IR spectra intensities are proportional to the derivative of the dipole moment with internuclear distance. However, how would one be able to deduce (without looking at the IR spectra) that the IR spectra for $\ce{IBr}$ is higher than $\ce{ICl}$ in the gas phase?
I would have thought it would be the other way around since $\ce{Cl}$ has the higher electronegativity.


texts:
I got in Paula Bruice under the section Mechanism for Mercuric ion catalysed Hydration of Alkyne

Terminal alkynes are less reactive than internal alkynes towards addition of water. Hydration will take place if Hg²+ is added to acidic mixture.

Why are Terminal Alkynes less reactive than internal alkynes?


texts: A problem in solubility of salts with multistep hydrolysis of an ion possible

texts:
Calculate the solubility of $\ce{MnS}$ in pure water, $K_\mathrm{sp}(\ce{MnS}) = \pu{2.5e-10}$. Assume hydrolysis of $\ce{S^2-}$ ion. $K_1$ and $K_2$ for $\ce{H2S}$ are $10^{-7}$ and $10^{-14}$, respectively.

Let unknown amount of $\ce{MnS}$ be dissolved in $\pu{1 L}$ of water, so $\ce{Mn^2+} = s~\mathrm{moles}$ and $\ce{S^2-} = s~\mathrm{moles}$ where $s$ is the solubility of $\ce{MnS}$ in $\pu{mol L-1}$.

Now let $\ce{S^2-}$ participate in hydrolysis by dissociating its $x$ moles and combining with $\ce{H2O}$ to form $x$ moles of $\ce{HS-}$ and $x$ moles of $\ce{OH-}$, so final moles of $\ce{S^2-}$ is $(s-x)$.

I considered only $K_\mathrm{h1}$ as it is $10^7$ times larger than $K_\mathrm{h2}$, $K_\mathrm{h1}$ is $K_\mathrm{w}/K_\mathrm{a2}$ which is equal to $1$. So I got $s^2 = (s-x)$ and we know that $K_\mathrm{sp} = s(s-x)$. On substituting I got the following equation:

$$K_\mathrm{sp} = s^2 - s^{1/2} \cdot K_\mathrm{sp}^{1/2}$$

I am stuck here. I know that $K_\mathrm{sp} = \pu{2.5e-10}$, so I tried solving for $s$, but could not find the correct answer.

Can you please point out my conceptual mistake?

Sorry, if this question is of homework type, I really tried solving this before posting.


texts:
In this German Wikipedia article about optical activity, I found the beautiful picture of starch granules:

Unfortunately, I have no idea what I am looking at. What are these crosses and why are they blue (i.e., same as the background color)?


texts:
I've heard that in some alien planet there might be life based on Silicon chemistry, but have we ever discovered molecules that are similiar to carbon organic compounds? Something like chains or rings...

P.S. I'm not asking if silicon-based life is possible, i'm just asking if there actually are silicon organic compounds(in the sense that they are similiar to their carbon counterparts)


texts: Writing equilibrium expressions with aqueous species

texts:
If we have the reaction: $$\ce{A +BC_{(aq)} <=> D}$$ where $\ce{A}$ and $\ce{D}$ are generic compounds (neither is pure solid nor pure liquid) and $\ce{BC}$ is an ionic compound dissociated in water (consisting of $\ce{B+}$ and $\ce{C-}$ ions), would the $K_{\mathrm{c}}$ expression be written as

$$K_{\mathrm{c}}=\frac{[\ce{D}]}{[\ce{A}][\ce{BC}]}$$

or

$$K_{\mathrm{c}}=\frac{[\ce{D}]}{[\ce{A}][\ce{B+}][\ce{C-}]}?$$


texts: Brownian motion of colloids at high density

texts: Colloidal particles in dilute suspensions tend to perform a universal type of Brownian motion at finite temperature, according to the Stokes Einstein relation $D\approx\frac{k_BT}{6\pi \eta r}$ in the case of spherical colloids at low density. When the concentration is high enough and the colloids start to aggregate, however, the diffusion constant of colloids within the aggregate will change. This is true not only because of confinement of colloids by their neighbors, but also because of the fact that proximity of colloids to a stationary aggregate should actually change the diffusion constant from its value when colloids are isolated. This can be seen from the fact that the liquid phase is essentially incompressible, and also that the effects of viscosity would be amplified. My questions are, is this effect well understood, and if so, what would be some illuminating resources to learn about it in more detail? Full answers would also be much appreciated.


texts: What is the reaction between nitric acid and fluorine?

texts: Nitric acid, $\ce{HNO3}$, is a strong oxidising agent as well as fluorine, $\ce{F2}$. What will be the reaction when pure $\ce{F2}$ gas is bubbled through 100% pure $\ce{HNO3}$. While searching, many products are suggested including $\ce{HF}$, $\ce{O2}$, $\ce{H2O}$, $\ce{NO}$, $\ce{NO2}$, $\ce{NF3}$, $\ce{NO2FO}$, etc.. What is correct?


texts: What is the term for passing an eluate back through an adsorption column?

texts:
In my experiment I am passing a solution through an adsorption column, collecting the eluate, then passing the eluate back through the same column.

Is there a concise term which describes this process?


texts: Estimating maximum size of magnetite particles in this Nile Red ferrofluid video?

texts:
In the Nile Red video Making ferrofluid from scratch a suspension of small magnetite ($\ce{Fe^{2+}Fe_2^{3+}O_4^{2−}}$) particles is produced from a combination of ferrous chloride and ferric chloride.
The nanoparticles are then given a coating of oleic acid ($\ce{CH3(CH2)7CH=CH(CH2)7COOH}$) to provide steric stabilization to prevent aggregation, then dissolved in kerosene.
After about 23:20:

…the magnetite isn’t truly dissolving, it’s going into suspension. This is because the nanoparticles aren’t soluble in the solvent, but they’re small enough that they can still easily get dispersed.
The oleic acid coating that helps with this, by preventing two particles from coming too close together, and by also allowing them to interact with the non-polar solvent. The result of this is a stable suspension known as a colloid, where it’s almost like the magnetite being dissolved.

As an exercise I'd like to estimate the maximum size of the oleic acid-coated magnetite particles that can be stable in this solution. Some considerations are:

While the magnetite particles themselves have a high density (over 5 g/cm3), the oleic acid molecules might be arranged radially making the volume of the complete particle much larger than the magnetite core.

While large magnetite particles would sink quickly to the bottom due to gravity, the thermal agitation (Brownian motion) at room temperature is sufficient to keep these in colloidal suspension if they are small enough. This suggest that there might be some ratio of $k_B T$ with some expression of gravitational potential energy.

Question: How would I go about estimating the maximum size of the magnetite nanoparticles that could remain in stable suspension in this case?


texts: Why is Promethium the only radioactive Lanthanide?

texts:
Why is it that only Promethium (Pm) is the only radioactive lanthanide?

I was trying to figure out what percentage of all radioactive elements in the Periodic Table the f-block accounts for, when it struck me that Promethium was the only radioactive lanthanide shown. I found this a bit odd, since I expected to see a couple of other radioactive elements there as well, especially since Promethium is (somewhat) near the middle of the series.


texts: Why can't carboxylic acid + NaH + tert-butyl bromide react to create an ester?

texts:
So, the picture I showed above asks for which reagents can form the ester product. I was wondering why D can't form it? I was thinking that first, the $\ce{NaH}$ would react with the carboxylic acid to form the basic form of the carboxylic acid (where the $\ce{-OH}$ becomes $\ce{-O^-}$). Then, the $\ce{-O^-}$ of the carboxylic acid would attack the tert-butyl bromide in a $\mathrm{S_N1}$ reaction where the bromide would leave first. Why is this not possible?

Also, does F not work because Fischer esterification does not occur in basic/neutral conditions?


texts: Explaining color differences using bond conjugation

texts:
I'd like to ask about how the color differences between the dyes methyl violet and malachite green can be explained. Looking at the conjugated bonds of each, methyl violet seems to be more conjugated than malachite green with two $\ce{NMe_2}$ auxochromes, which causes a bathochromic shift, suggesting that methyl violet's absorption peak to have a longer wavelength compared to malachite green's absorption peak.

However, after having collected data using a spectrophotometer, methyl violet appears to absorb at a shorter wavelength ($\pu{593 nm}$) compared to malachite green ($\pu{623 nm}$), with both dyes solvated in $50~\%$ ethanol.

How can this color difference be explained?

Methyl violet:

Malachite green:


texts: How to determine at temperature and pressure to keep Cl2 a gas?

texts:
I have chlorine tanks and they will be used industrially. When I use them, they will be exposed to all sorts of varying temperatures. I was wondering if there was any way that when I use them, I could figure out at what pressure I could release the $\ce{Cl2}$, depending on the temperature, so that it remains a gas?

I already know it only takes $\pu{6.8 atm}$ at $\pu{70^\circ C}$ to liquefy $\ce{Cl2}$ to a volume of $\pu{0.322812 L}$, and its boiling point is $\pu{-33.97 ^\circ C}$ at $\pu{1 atm}$.


texts: A boy in Memphis built 1 year ago a nuclear fusion reactor at his home . How did he achieve so high temperatures in order for the fusion to take place?The fusion that took place was 2 atoms of hydrogen-2 were fused into helium.What I know is that hydrogen-2 can only be made by fusion of 2 hydrogen-1 atoms and then the unstable radioactive diproton which is produced is converted into hydrogen-2 and 1 positron.Where did he find so much energy? And if we pressure a gas very hard,can  we fuse 2 hydrogen atoms into 1 diproton?


texts: How to determine the temperature dependence of equilibrium constants?

texts:
Given $K_c=2.8\times10^2$ at $727~^\circ\mathrm{C}$ for
  $$\ce{ 2SO2 + O2 <=> 2SO3},$$
  calculate $K_c$ and $K_p$ when 8 grams of $\ce{SO2}$ result in 0.32 grams
  of $\ce{O2}$ forming at equilibrium (temperature is not $727~^\circ\mathrm{C}$).

Now at first I thought this was straightforward :

determine $K_c$ at this temperature from the equilibrium conditions
apply Van 't Hoff equation to determine the temperature
calculate $K_p$ from $K_c$ and the temperature

However right after that part of the question the next part asks

  Is the new temperature higher or lower than $727~^\circ\mathrm{C}$?

This implies that it is possible to calculate $K_p$ without calculating the temperature? With the new $K$ values then provide a qualitative argument for the temperature increasing or decreasing without having to calculate it?


texts: Mechanism of hydrogen evolution on metal-oxide electrodes

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I am looking into the mechanism of different electrocatalytic reaction in acidic media on metal-oxide electrodes.

Reduction of on such electrodes generally happens with the help of electrons from the conducting band of the electrode, which is mainly d bands of the metal. That made me wonder about the mechanism of hydrogen formation: I assumed hydrogen is formed from protons chemisorbed on the surface. However, protons are likely to attach to surface oxygen ions, not metal ions, therefore it seems difficult to receive electrons from the conducting band.

Different of water-related electrocatalysis seems very extensively studied, including many atomistic simulations, however, I haven't found much on hydrogen evolution on oxide electrodes. Is there an established mechanism for hydrogen formation?


texts: What is the exact meaning of GAMESS (US) SCF density convergence criteria?

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In Gaussian the SCF convergence criteria is set to $10^{-N}$ by

SCF=(Conver=N)

Note that this criteria applies to the root mean square change in density matrix between two SCF cycles, while the maximum change convergence criteria is set to $10^{-N+2}$. For instance, for the default case of $N=8$ the output says

 Requested convergence on RMS density matrix=1.00D-08 within 128 cycles.
 Requested convergence on MAX density matrix=1.00D-06.
 Requested convergence on             energy=1.00D-06.

Now in GAMESS (US) the SCF convergence criteria is set to $10^{-N}$ by

$SCF CONV=1.0d-N $END

However, neither manual, nor the output do not explicitly specify does this criteria refers to the root mean square change in density matrix or to the maximum change in it? Judging from the default value (1.0d-05), it is the last one, the maximum change in density matrix, otherwise it would be too small for a reliable convergence. Thus, SCF=(Conver=N) in Gaussian corresponds to $SCF CONV=1.0d-(N+2) $END in GAMESS (US). But I'm not sure, so can anyone confirm that?

P.S. I tried to look at the source code, but it is an old FORTRAN mostly undocumented spaghetti mess with good-old six-lettered totally undescriptive variable names. :D


texts:
I know that anions such as $\ce{CH3COO-}, \ce{OH-}, \ce{CN-}$ are basic in nature, since they are conjugate bases of weak acids. Similarly, cations such as $\ce{NH4+}, \ce{H3O+}$ are acidic in nature , since they conjugate acids of weak bases.

However, we also have another case whereby, anions such as $\ce{HSO3-}, \ce{HS-}$ are amphoteric in nature. This is due to the fact that their conjugate acids are polyprotic acids. Thus, one can have the two reactions:

$\ce{HS- + H2O <=> S^2- + H3O+}$ and $\ce{HS- + H2O <=> H2S + OH-}$

Since, for this case, we got amphoteric anions from polyprotic acids, is there a possibility that we can do the same for polyprotic bases so as to get amphoteric cations? i.e. Can we have a conjugate acid of a polyprotic base, which can be regarded as an amphoteric cation? I haven't seen any such examples of amphoteric cations yet, hence I am asking this question.


texts: What should be the nomenclature of this organic compound?

texts:
I am confused with my teacher's explanation for the nomenclature of this organic compound.

He said that we should start numbering from the sulphonic acid at the bottom of the structure. But one of the students asked if we should start numbering from the other sulphonic acid and with this method, the double bond will be on number $1$ (as shown in the image below). But my teacher didn't give a reason for that.

So isn't the student saying correct ?


texts: Does aluminum oxide react with rubidium?

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I have rubidium vapor inside of the vacuum chamber. Inside the vacuum chamber, there are two flat stainless steel coated with aluminum oxide at temperature ~100 °C.

Since rubidium is active, I am worrying that it will interact with the aluminum oxide $\ce{Al2O3}$ coating. Any ideas?