texts: Does the amine used to generate the enolate affect the selectivity of an Evans aldol reaction?

texts:
In an Evans aldol reaction, the enolate is typically generated by addition of a boron Lewis acid (e.g. $\ce{(n-Bu)2BOTf}$) and an amine, which acts as a weak base. I'd like to know whether there's any difference between

Triisobutylamine, $\ce{(i-Bu)3N}$
N,N-Diisopropylethylamine (Hünig's base), $\ce{(i-Pr)2NEt}$
Triethylamine, $\ce{Et3N}$

in the selectivity of the enolate formation and subsequent aldol reaction. I'm thinking about the different $\mathrm pK_\mathrm a$ values (1>2>3) and the different steric demand (1>2>3). Probably the base with the highest $\mathrm pK_\mathrm a$ will be kinetically fastest. Also, the cis-enolate should be favoured as it avoids a destabilising syn-pentane interaction between the terminal methyl group and the auxiliary. However, do all of the bases lead to the same ratio of cis/trans-enolate formation (and hence diastereoselectivity in the subsequent aldol reaction)?


texts:
I'm working on preparing SBF in my school's lab however a couple of components are missing which are $\ce{NaHCO3}$ (0.350 g) and $\ce{K2HPO4}$ (0.228 g) .
Is it possible to replace these components with $\ce{NaCO3}$  and $\ce{KH2PO4}$ ?

The following link has the ions concentration of SBF:

https://www.irsm.cas.cz/materialy/cs_content/2002/Helebrant_CS_2002_0000.pdf

I'm not a chemist and I would really appreciate your view on this.


texts:
I have a bottle of salt water I collected on a local beach. I want to see what is in it, apart from water and salt. If I boil the whole bottle and wait for it to evaporate, mostly salt will remain. I want to see everything except salt that is still in the pot after I boil the water. Is this possible with common household items and commonly available (and safe) chemicals?

Disclaimer: I know nothing about chemistry.


texts: Geminal coupling constant of formaldehyde protons in NMR

texts: I read in a book that the protons in formaldehyde have the largest known positive geminal coupling constant (41 Hz). I thought only couplings between non-equivalent protons can be seen in the spectrum. Since formaldehyde has C2v symmetry and (if I remember correctly) coupling constants cannot be calculated from theory accurately, how come this coupling is apparent in the spectrum?


texts: What happens to the "leftover" Ions in the Mass Analyzer

texts:
I am very intrigued by what happens to the ions which do not make it to the detector in mass spectrometers, do they just pile up? I have never seen anyone clean our quadrupole, is this done? or do they just stay there indefinitely? I am talking of a triple quadrupole detector in a GC instrument, which deals with very small quantities, but after thousands of injections there must be a considerable mass of analyte on the analyzer. Even more so considering that most ions which go through the quadrupole do not make it to the detector.
 Thanks.


texts:
I prepared some PDMS mixing a Sylgard 186 (elastomer) and a curing agent with 10:1 ratio. I sucked all the bubbles and I sandwiched the PDMS between two glass slides to get flat surfaces. Finally I heated up at 60C for 6 hours. Now I would like to get my PDMS by removing one of the two glass slides, but it is practically impossible...
Does anyone have some suggestions to overcome this issue?


texts: Question about reading NIST datasheets & Gibbs energy of reaction

texts:
I'm peeping the NIST thermo tables for $\text{H}$ and $\text{H}_2$, and I'm perplexed.
Seems to me that using
$$G=[\Delta H^\circ_f+H-H^\circ(T_r)]-T\cdot S^\circ$$
(well, actually correcting the entropy for partial pressures in a mixture of ideal gases) obtains some bewildering values for simple reactions.
Consider $\text{H}_2 \rightarrow 2\text{H}$ at standard pressure and standard temperature. If I'm calculating it correctly (which I am sure I am not), using the values at $298.15^\circ K$ in columns 2 ($S^\circ$), 4 ($H-H^\circ(T_r)$), and 5 ($\Delta H^\circ_f$) from the links above, I find that
$$G_{\text{H}_2}=0\frac{\text{kJ}}{\text{mol}}−298.15^\circ \text{K}\cdot130.680\frac{\text{kJ}}{^\circ \text{K}\;\text{mol}}\approx-38962\frac{\text{kJ}}{\text{mol}}$$
And
$$G_\text{H}=217.999\frac{\text{kJ}}{\text{mol}}−298.15^\circ \text{K}\cdot114.716\frac{\text{kJ}}{^\circ \text{K}\;\text{mol}}\approx-33984\frac{\text{kJ}}{\text{mol}}$$
Which is great, the reaction isn't spontaneous. Except... if I recall correctly,
$$G_\text{mixture} = \sum^\text{species}_j \mu_j n_j$$
Where $n_j$ is the number of mols of that species and for ideal gases at constant pressure & temperature,
$$\mu_j = H_j-TS_j = G_j$$
That would imply the mixture of one mol of $\text{H}_2$ has a total gibbs of the aformentioned $-38962\frac{\text{kJ}}{\text{mol}}$, but the mixture of two mols of $\text{H}$ (pressure neglected) would be $2\times(-33984\frac{\text{kJ}}{\text{mol}})=-67969\frac{\text{kJ}}{\text{mol}}$, and now supposedly I've found that hydrogen gas spontaneously decomposes at standard temperature. That's clearly wrong.
(I've even done the calculations for a reaction considering pressure, and the doubling of mols of mixture post-reaction isn't enough to drive it into non-spontaneity except at absurdly high pressures)
So my question is: do I misunderstand how the Gibbs works for mixture of gases, how to read values off of the NIST themo tables, or both?
Thank you!


texts: Why do silver nitrate and sodium hydroxide react to produce silver(I) oxide?

texts:
I did an experiment in class combining the aqueous solutions of $\ce{AgNO3}$ and $\ce{NaOH}$. I predicted that $\ce{AgOH}$ would form but it didn't, instead a brown precipitate. I found out it was silver oxide. But why does this happen?

I researched and found out that silver is a noble metal and is thus resistant to oxidation, so why is it oxidised in this case?

Also why is $\ce{AgOH}$ not formed? I looked at the reactivity series and apparently silver can't displace $\ce{H+}$ ions out of solution, is that why?


texts:
I'm currently learning about substitution and elimination reactions and to do so I need to learn about bases and nucleophiles.

Apparently a good base is an unstable species.

What does stability even mean in chemistry?

I just need to conceptually understand and wrap my head around this concept of a compound or atom or species being 'stable' or 'unstable'. Please don't explain it in complicated terms.


texts:
I understand the diazomethane resonance forms that have two double bonds between $\ce{C=N}$ and $\ce{N=N}$ in the structure and also the form that has one single between $\ce{C-N}$ and triple between the two nitrogens ($\ce{N#N}$) in the structure.

Why is not possible for diazomethane to have a double bond between $\ce{C}$ and $\ce{N}$ as well as a triple bond between $\ce{N}$ and $\ce{N}$ as a resonance form?


texts: Expression comparing the Solubility of Ionic compounds

texts:
In chemical bonding, I am taught an expression that compares the Solvation Energy and the Lattice Energy of an ionic compound which is being dissolved in a polar solvent (could be water, could be something else).
The expression is:
$$\left\lvert r_+ -r_-\right\rvert$$
where $r_+$ denotes radius of cation and $r_-$ denotes radius of anion making up the compound. When this difference is large, solvation energy dominates and when this difference is small, lattice energy dominates. But I don't the reason behind these deductions. Can someone explain?


texts: How does Le Chatelier's principle apply to a fuel tank full of hydrogen?

texts:
A nano-tube packed fuel tank is full of Hydrogen, and there is a steady pressure of Hydrogen in the tank. While hydrogen gas is being removed from the fuel tank to power the car, the pressure in the fuel tank drops very little for some time. Explain this observation using Le Chatelier's principle.

My attempt at a solution: We look at the equilibrium $\ce{H2 (\text{absorbed by car}) <=> H2 (\text{in tank})}$. Initially, this equilibrium lies to the right, so it moves to the left. But this just means that the hydrogen in the fuel tank is being used up, and so the pressure should drop.


texts: At what concentration would copper sulphate crystallise at 25 degrees Celsius?

texts: I am doing an experiment on how increasing concentration of copper sulphate affects the voltage in a galvanic cell. The voltage increased as temperature did until 2M where it started to decrease. I know the reason it does that but I can't find any website saying for 25 degrees Celsius it would start decreasing. I hope that makes sense and thanks for ur help!


texts:
While doing computational chemistry for both organic and inorganic compounds, an energy optimization of that compound is done before starting a study.

For example, if we do drug docking studies using aspirin, first we would make aspirin's chemical structure, and then we would optimize it. So what does "energy optimization" mean in the context of organic and inorganic compounds.

Is energy optimization different for organic and inorganic materials (e.g.,aspirin or gold nanoparticles, respectively)?


texts:
I am having some difficulty in writing the correct term symbols for diatomic molecules in their ground states.

For instance, the $\ce{S2}$ molecule has a MO configuration in its valence shell $\ce{3s\sigma^2_u 3s\sigma^2\mbox{*}_g 3p_z\pi^2_u 3p_y\pi^2_u 3p_x\pi^2_u 3p_y\pi^1\mbox{*}_g 3p_x\pi^1\mbox{*}_g}$

Now, for its term symbol I would say (erroneously) that I have two unpaired pi electrons, hence $\ce{\Lambda}=2$, so I would obtain a $\ce{\Delta}$ symbol.

As for the spin multiplicity, it would be 3. And the spin-orbit coupling term would be therefore S+2=3. Concerning simmetry, I would say that it's g, since the last occupied shell is g (still, I don't understand why Ar2 should be g if the last shell is u. Maybe because having wto electrons in a u shell I should consider u*u=g? If that is the case, it's correct to say that every molecule with an even number of electrons in the last energy level, and hence every homonuclear diatomic, is g?)

Actually, it looks like the correct term symbol for the $\ce{S2}$ molecule is $\ce{^3\Sigma _g}$, but I don't understand why. Can you help me please?


texts: How can the number of moles in the product side be more than reactant?

texts:
A rigid stainless steel chamber contains $\pu{0.32 bar}$ of methane, $\ce{CH4}$, and excess oxygen $\ce{O2}$, at $\pu{200.0 °C}$. A spark is ignited inside the chamber, completely combusting the methane. What is the change in total pressure within the chamber following the reaction? Assume a constant temperature throughout the process.

I wrote out the following chemical equation:

$$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$

I know I have $\pu{32 kPa}$ of methane, so using up all the moles of methane would mean that pressure decreases all the way to $0$.

The solution writes the following: it multiplies the 1 mol of $\ce{CH4}$ by $1$ to get the number of moles of $\ce{CO2}$ and by $2$ to get the moles of $\ce{H2O}$ produced.

Why can we do that? I would assume that by multiplying in such a way, you're implying that all of the $\ce{CH4}$ is being used up to create $1$ mol of $\ce{CO2}$. This means that you have no more moles to spare for creating $\ce{H2O}$.

I know my conceptual understanding is wrong, but this is what I thought while doing the question and the solution doesn't intuitively make sense to me.

I was wondering if someone could clarify my misunderstanding for me.


texts:
To let you know where I'm coming from: I haven't studied chemistry since high school. I am now studying something else, but I have a dataset coming from your field and I would like some help in understanding just the very very basics of what it is.

What I have is $\ce{NOx}$, nitric oxide and nitrogen dioxide in an engine exhaust and the equivalence ratio. It's from the dataset ethanol in the package SemiPar in R, and the original reference is Brinkman (1981). You'll find a figure at the bottom.

What I want: I want to be able to, extremely briefly, explain what this is describing.

My understanding of the equivalence ratio is that if it is below 1, then there is too little air (compared to the stoichometric mix). If it is above 1, that means that there is too much fuel, and not all of it can be used leading to what is called incomplete combustion.

Is this correct? Would you be able to provide a better explanation? Imagine you're trying to explain it to a five-year-old, that's about how much of this I know!

If you've read this far, please also help me with appropriate tags for the question. I'm not sure what to choose.

Reference:

Brinkman, N.D. (1981). Ethanol fuel – a single-cylinder engine study of efficiency and exhaust emissions. SAE transactions Vol. 90, No 810345, 1410–1424.


texts: About how long do polymer chains have to be for their physical properties to resemble the polymer more than the monomer? (Or are at least comparable)

texts:
For example: According to wikipedia, Styrene is a liquid at room temperature, whereas isotactic Polystyrene does not melt until the 200s celcius. This is a simple property but quite a large change. I am curious how long polymer chains have to be (how many monomer units are present) before you can just consider them to be functionally identical to the kind of Polystyrene generally used in practical situations?  I know that there are different forms of polystyrene such as puffed/extruded, films etc, this is just an example.

The purpose behind this question is that I am considering doing some molecular dynamics/computational work on polymers and the longer the chains are, the more computer time is needed. So I'm wondering how large the molecules should be to reflect "real behavior".

Note: I'm sure it's a bit of a spectrum. You can probably make a smooth function between chain length and melting point, but I doubt it's a linear function... there's probably a point where it basically stops changing.

Edit: While I cited melting point I am actually more interested in electronic/orbital properties in case that helps.


texts: How are the boiling points of Tungsten and other metals determined and proved?

texts:
The boiling point of Tungsten is 10,030 degrees Fahrenheit. How was this determined and proved? And more generally how are the boiling points of metals determined and proved?

Is it really so simple as "heat it up as high as you can and see if (and at what temperature) it evaporates"?

I feel like there is a more sophisticated process, but I can't really guess at what that might be, especially if the boiling point was determined years and years (100+?) ago.

The Internet is really good at telling me the boiling point of a lot of (or all, I haven't exhaustively checked) elements and alloys, unfortunately it seems it's not very good at telling me how these seemingly arbitrary temperatures were determined.


texts: Does "covalent" AgCl (aq) exist and if so, how to quantify its concentration?

texts:
I am reading Ionic Equilibrium by James N. Butler.  Using silver(I) chloride in excess aqueous chloride as an example it speaks of several forms:  $\ce{AgCl (s)}$ i.e. the ionic crystalline salt, $\ce{Ag+(aq)}$ and $\ce{Cl^{-}(aq)}$ the aqueous ions from dissociation, and finally $\ce{AgCl(aq)}$, which Butler purports to be "dissolved covalent silver chloride molecules as distinct from solid salt or dissolved ions."

Does this "molecule" of $\ce{AgCl}$ really exist in solution?  If so, is there a way to quantify the concentrations of this covalent species?  Equilibrium solubility product constants seem to give the concentration of the aqueous ions only.


texts:
I have been reading a journal article on ab-initio pseudo-potentials, and I need some help understanding it. The article is C. F. Melius and W. A. Goddard, III. Phys. Rev. A 1974, 10, 1528.

A summary of the article for those that do not have access follows. The authors present a way to construct a pseudo-potential. Basically, their argument goes like this (according to my understanding):

  Suppose that we are using the Hartree Fock method. We can represent
  the ground-state wavefunction of some atom as a function of the valence
  orbitals, core orbitals, and spin.

  $$\Psi= f (\phi_{core}, \varphi_{valence},X)$$

  Note: in the article they use a weird letter instead of $f$.
  After we obtain the ground-state occupied orbitals, we solve for the valence orbitals.
  We obtain the valence orbital by solving the variational equations
  $$H^{HF}\varphi_{i}=\epsilon_i\varphi_{i}$$ Where $H^{HF}$ is the Hartree Fock Hamiltonian and is given by
  $$H^{HF}=-\frac{\nabla^2}{2}-\frac{Z}{r}+2J_{\alpha}-K_{\alpha}$$
  For the valence orbital, $J_{\alpha}$ is equal to the classical Coloumb potential due to a charge density corresponding to the $\phi_{core}$. The exchange operator $K_{alpha}$ is an integral operator resulting from the antisymmetric form of the wavefunction.

  The authors then state that the valence orbital of the ground-state wavefunction is NOT determined uniquely by the variational principle. This is because the solutions of the variational equations may not yield a wavefunction that is purely a valence orbital. In other words, the mixing of core orbitals with a valence orbitals does not change its energy. Hence the solution $\varphi_i$ may be of the form $$\varphi_i=\varphi_{valence}+\sum_i c_i\phi_{core}$$
  The authors then impose the constraint that $\varphi_{i}$ be orthogonal to other occupied orbitals.

What I do not understand is this. How does orthogonality allow for a unique solution? Also later on, the authors state "Although this orthogonality restriction has the desired consequence of leading to a prescription for a unique valence orbital, there is no reason to consider $\varphi_i$ to have a special significance over any other combination of $\varphi_i$ with the various core orbitals." What do they mean by this statement?


texts: What is the difference in (alc) and (aq) states in reaction mechanism?

texts:
In these reactions between alkyl halide and KOH in different states the product is different only because of the change in state of KOH from aqueous and alcohol .

Can someone explain the mechanism which causes this drastic change?


texts: What is the difference between normal phase and reverse phase high-performance liquid chromatography?


texts: What exactly is hydrolysis reaction in organic chemistry?What mechanism does it follow?

texts:
According to my study material:

  Hydrolysis is a special type of nucleophilic substitution ($\mathrm{S_N1}$) where water acts as both nucleophile and a solvent molecule.

Is this definition correct? If so how to solve the following problem (picked up from my textbook exercise which has no solution)?

  When alkyl bromides (listed here) were subjected to hydrolysis in a
  mixture of ethanol and water ($80\ \%\ \ce{C2H5OH} / 20~\%\ \ce{H2O}$)
  at $55~\mathrm{^\circ C}$ the rates of reaction showed the following order:
  $\ce{(CH3)3CBr} > \ce{CH3Br} > \ce{CH3CH2Br} > \ce{(CH3)2CHBr}$.

  Provide an explaination for this order of reactivity.

But my doubt is how is this order $\ce{(CH3)3CBr} > \ce{CH3Br} > \ce{CH3CH2Br} > \ce{(CH3)2CHBr}$ possible?

I mean I know the first compound will form a tertiary carbocation. That is okay for $\mathrm{S_N1}$ but how will the second compound undergo $\mathrm{S_N1}$ (because methyl carbocation is very unstable)? Moreover if really $\mathrm{S_N1}$ is taking place the fourth compound should form a more stable carbocation compared to the third compound. These things are confusing me. Is hydrolysis really $\mathrm{S_N1}$ or something else?

According to me the order should have been $(1)>(4)>(3)>(2)$.


texts: Why does sweetening milk need less sugar as compared to tea?

texts:
If I have a cup of milk and a cup of tea; to get them to a sweetness I like, I usually add less sugar to the milk as compared to the tea.

Why is it that milk needs less sugar to be just as sweet as tea?


texts: Changing Zinc acetate dihydrate to Zinc acetate

texts:
For some Chemistry work in the laboratory, I needed Zinc acetate but laboratory only has Zinc acetate dihydrate. Is there any way to convert hydrate form to anhydrous form? Maybe by heating or any other method?
If by heating, how much time and temperature I should give?


texts: Fast and complete ways to remove chloride from solution (sub ppb levels)

texts:
Main question: We are trying to reduce the chloride concentration in a solution to sub-ppb levels. I have to dissect a fruit fly in this solution so I am interested in removing any chloride that is released during that process. Is there a method that will reduce chloride below 1ppb in a timescale of minutes? The best I have found is the addition of silver nitrate, which will form silver chloride precipitate, but I am not sure how fast or how thorough this process is.

Background: I have been performing an experiment where I inject current into a cell of a fruit fly for a research project. For various reasons, we believe the cell membrane in one fruit fly line is more permeable to chloride than another fruit fly line and are trying to confirm this. We inject with 20nA of current for about 2 minutes and can't change these parameters much. We hope to completely replace the chloride anion in our bath with another anion, to see if this changes the readings we get from our injection. For that small of a current, I believe I need less than 1ppb chloride concentration for it not to participate much in carrying charge. It is very difficult to find a replacement salt that doesn't have a minor chloride impurity, so we are hoping to find ways to remove the chloride.


texts:
Most solubility references (e.g. this one) say that there are "few" insoluble salts of alkali metals.

Are there any exceptions?


texts:
Why does the temperature of a tyre rise when it is pumped up rapidly?

The way my textbook explains this is that $V\propto T$, so, an increase in volume causes an increase in temperature.

However, I think that this justification is flawed, because air is a real gas instead, and follows (approximately) the van der Waals' equation of state: $\left(P+\frac{a}{V^2}\right)(V-b)=RT$. I'm not not sure how I can neglect the $a/V^2$ term in it, considering the volume in a tyre is not that high.

Moreover, I actually had thought the reverse statement to be true. Pumping air "rapidly" implies the conditions are adiabatic i.e. heat exchange is negligible. So, $TV^{\gamma-1}=\text{constant}$ holds, implying an inverse relation between volume and temperature.

So, why actually does pumping a tyre increase its temperature? And what is the fault in my reasoning above?

Source: KS Verma; Physical Chemistry for JEE (Advanced): Part 1; Exercise 5.1 Question 6


texts: What is the difference between K and Kp in the equilibrium equation?

texts: In this equilibrium equation, $$K = K_p\cdot (RT)^{-\Delta n}$$ what does $K$ represent in comparison to $K_p$? It seems to me that they both are equilibrium constants, so how are they different?


texts: How do scientists check which kind of unit cell is present in a crystal?

texts:
There are many types of unit cells like:

Tetragonal        Monoclinic      Orthorhombic        Rhombohedral
Hexagonal         Triclinic       Cubic

But how do scientists identify the type when a sample of the compound is given to them?


texts: Why is the value of "a" is positive instead of negative in Van der Waals equation?

texts:
Higher value of a means higher atttraction force between molecules. Attraction force causes the pressure to be less than expected. Now the actual pressure becomes $P + \frac{n^2a}{V^2}$ .
Which means if a increases the neat pressure increases too. But isn't it the other way around?
(I assume here P is the pressure exerted on the wall of the gas container considering there is no intermolecular force)


texts:
It was written in my textbook that the spatial arrangement of hydroxy groups was determined by Fischer by studying certain properties. But I wanted to know to how the relative configurations of hydroxy groups were determined.
Glucose is dextrorotatary but that could be possible with many arrangements.


texts: Molecular (infrared) vibrations: General relationship between molecular mechanics force constants and harmonic frequencies (in internal coordinates)?

texts:
In molecular mechanics method simple molecules, say symmetrical tri-atomics (H$_2$O), have potential energy surface governed by distances $r_1,r_2$ and angle $a$ like

$V (r_1,r_2,a_{121}) = K (r_1 - r_0)^2 + K (r_2 - r_0)^2 + K_a (a - a_0)^2$

The non-linear tri-atomic will have 3 harmonic frequencies $f_1,f_2,f_3$.

I understand that Hessian matrix seems to be important. The Hessian elements is $\frac{\partial^2 V}{\partial r_1^2}=2K$ and $\frac{\partial^2 V}{\partial r_2^2}=2K$ and $\frac{\partial^2 V}{\partial a_{121}^2}=2K_a$.

Is there a general relationship how harmonic frequencies are related to force constants? Does this relationship involve further approximations (besides the simple molecular mechanics PES)? The variables are internal coordinates and not the normal coordinates, right?

The water case I have just taken as illustrative example...

Thank you in advancement!
Miranda Christina


texts: How many protons does phenolphthalein lose in basic solutions?

texts: So, I've looked it up and Wikipedia claims that two protons are released by Phenolphthalein when pH is over 8.2, but I've seen other sources that claim only one proton is lost. So, is it really just one or two? Which form is responsible for  its colour? (I would expect a fully deprotonated PhPh in more basic solutions, so I guess the PhPh that released two protons is the one responsible for the solution's colour)


texts: Question on assumption in Michaelis-Menten kinetics

texts:
In Michaelis-Menten kinetics, it is assumed that the substrate concentration is much higher than the enzyme concentration. In which step of the derivation of the Michaelis-Menten rate law is this assumption used?

From the explanations given in different sources, I think this assumption is used in expressing the total enzyme concentration as the sum of free enzyme and enzyme-substrate complex. Is my understanding correct? What happens when the enzyme concentration is in the same range as that of substrate concentration?
Why is it important to make this assumption?

Could someone explain?


texts: Is it possible to calculate the 'overall' redox potential of a system?

texts: For a solution where many redox reactions occur (e.g. cytosol, soil solution), can we consider anything as 'overall' redox potential of a system? Is there a formula to quantify it in relation to the amount of each of the constituents?


texts: Effect of Electronegativity on Bond Angles in NH3 and NF3

texts:
I am reading a book: Concise Inorganic Chemistry by J.D.Lee (Fifth Edition)
In the chapter-The Covalent Bond, the author says:

  $\ce{NF3}$ and $\ce{NH3}$ both have structures based on a tetrahedron with one corner occupied by a lone pair. The high electronegativity of $\ce{F}$ pulls the bonding electrons further away from $\ce{N}$ than in $\ce{NH3}.$ Thus repulsion between bond pairs is less in $\ce{NF3}$ than in $\ce{NH3}$. Hence the lone pair in $\ce{NF3}$ causes a greater distortion from tetrahedral and gives an $\ce{F-N-F}$ Bond angle of 102°30', compared with 107°48' in $\ce{NH3}$. The same effect is found in $\ce{H2O}$ (bond angle 104°27') and $\ce{F2O}$ (bond angle 102°)

I am not able to understand why the pulling of the bonding electrons (in $\ce{NF3}$) by F results in reduced repulsion between the bond pairs and how does this ultimately lead to a smaller bond angle ($\ce{F-N-F}$) in $\ce{NF3}$?


texts: Why do lone pairs of electrons have a higher repulsive force?

texts:
I was just wondering why lone pairs of electrons are more repulsive than bonded pairs of electrons.

I have read that lone pairs have orbitals which are shorter and more round, compared to bonded pairs. I assume, therefore, that this difference in repulsive force could be due to the increased surface area that these orbitals provide, which would cause more repulsive force to be directed towards electrons.

Of course this is just a presumption that I made, which is probably incorrect.


texts: Where does hydrogen evolution occur when aluminium amalgam is placed in water - aluminium or mercury?

texts:
Let's say a drop mercury is placed on clean aluminium sheet; mercury amalgam is formed. The solubility of aluminium in mercury and vise versa at room temperature is fairly low. If the entire sheet is now placed under water, hydrogen evolution is observed. Where is this hydrogen evolution from - the aluminium rich surface on the edges or the mercury rich surface on the drop?

(For reference, please look up videos of aluminium amalgam online, say on YouTube.)


texts: How to explain that an increase in temperature favours an endothermic reaction?

texts:
I'm trying to explain how a change in temperature favours an endothermic/exothermic reaction from a collision theory point of view, but I'm not really familiar with the concept just yet. For an increase in temperature, can I say that:

  An increase in temperature leads to a greater frequency of collisions between reactants and products, leading to an increased rate of reaction. Since the rate of the endothermic reaction increases more than the exothermic reaction, the equilibrium shifts towards the reactants/products.

If this is right, I'm struggling to see why the endothermic reaction rate will increase more with this logic. The exothermic reaction has a lower activation energy, so shouldn't its rate of reaction increase more than endothermic since it has a lower energy requirement to complete the reaction? Unless the increase in particles with enough energy for the exothermic reaction is comparatively lower to the increase in particles with enough energy for the endothermic reaction (aka proportion)?


texts:
My question is based on the Preparatory Problem Set for the 47th IChO Problem 26 - Holy War against Four Horsemen of the Apocalypse (it is on page 36). It is written that cooling of Y combustion products from 150 to 0 degrees Celsius does not result in any condensed phase, and addition of silver chloride to the supernatant of this combustion product provide some precipitate, so I concluded that this organic compound have either (or both) chloride and bromide, since iodine is too large for this substance. After calculating some possible answers, I've concluded that compound Y is $\ce{C3H2BrClO2}$.

My question is, if I combust compound Y ($\ce{C3H2BrClO2}$), will I get some products that aren't condensed when I cool these combustion products?


texts: The main difference between using AlCl3 and FeCl3 in Friedel-Crafts alkylation

texts:
What is the main difference between using $\ce{AlCl3}$ and $\ce{FeCl3}$ in Friedel-Crafts alkylation?

Is that $\ce{FeCl3}$ disfavors carbocation rearrangement in Friedel-Crafts alkylation?


texts: Complete Combustion of Inorganic, Hydrogen Containing Covalent Compounds in the Atmosphere

texts:
From what I've seen in alkanes, their complete combustion creates carbon dioxide, which has carbon in its highest possible oxidation state.  I've been wondering what complete combustion would look like in similar compounds, such as ammonia, phosphine, hydrogen sulfide, and silane.

After looking at through several search results of The combustion of <compound> and <compound>'s reaction with oxygen, I was surprised to only see that the search results for silane and phosphine generally showed that the combustion products attained their maximum oxidation state.

For example, the general result for ammonia seems to be nitrogen monoxide or nitrogen dioxide, which make sense as products for incomplete combustion, but I would think that a complete combustion would produce dinitrogen pentoxide.  Hydrogen sulfide's combustion also seems to form sulfur dioxide or disulfur dioxide rather than sulfur trioxide.

This really confused me, since I was expecting the main products of all these reactions to be fully oxidized nonmetallic oxides.  My question is, how can one predict what the fully combusted products of an inorganic, hydrogen containing compound look like?

Links for my ammonia query:

http://www.questionsolutions.com/ammonia-gas-reacts-molecular-oxygen-gas/
https://answers.yahoo.com/question/index?qid=20150615223643AArPdtu
https://answers.yahoo.com/question/index?qid=20080504104957AAgQrL3

Links for my hydrogen sulfide query:

https://chemiday.com/en/reaction/3-1-0-151

https://www.sciencedirect.com/science/article/pii/S0082078473800463


texts: Corey -House synthesis vs Wurtz reaction

texts: In Corey House synthesis the first step is formation of Alkyl Lithium. why can't we react alkyl lithium with alkyl halide to produce alkanes. Rather we choose to prepare Gilman's reagent? Does it have something to do with the reactivity of alkyl lithium?


texts: How long burned tobacco/cigarette smoke remain toxic?

texts:
It's hard to evaluate Environmental Tobacco Smoke lifecycle, it can certainly react, be absorbed by organisms, or be diluted in rain, and be emitted again from the ground.
But in the end, I'm interested to understand (as a non expert) if the toxicity (cytotoxicity, genetoxicity) of the compounds decrease over time (especially how are they eliminated) in a global outdoor environment where we suppose emissions would stop

Several resources list the components Chemistry and Toxicology of Cigarette Smoke, Comparison between Marijuana and Cigarette Smoke, CAEPA (page6):

Gas and particle phase contain roughly the same constituents as explained in the first paper, some compounds are 1,3-butadiene, acrolein, ammonia, aromatic amines, benzene, CO, isoprene, nicotine, nitrosamines, PAHs (polycyclic aromatic hydrocarbons), pyridine, toluene, hydrogen cyanide...

What could be their half-life in outdoor environment?


texts: Effect of halogen atom on Markovnikov's rule

texts: I know that the Markovnikov's rule is because of the stability of carbocations. If a halogen atom (electron-withdrawing group) is directly attached to the carbon atom carrying a positive charge, would it destabilize the carbocation (for example dichloroethene reacts with HBr)and 'break' the Markovnikov's rule?


texts: What does negative pressure mean in the context of this manometer?

texts:
I have a basic question on a closed manometer:

I see that in this case the pressure of gas is lower than that of the liquid, which is why the liquid is higher on the left side. Now, what I don't understand is why do we say that the gas is at a negative pressure?  Wouldn't that mean there's no gas at all? Also, how do we know that it's negative? Usually, I only take the difference between the highest point and the lowest.


texts: How to achieve clean dishes and glasses?

texts:
I heard that there is a standard about cleaned glass, namely

  Chemically clean glass supports a uniform film of water, with no hanging droplets visible.

I tried to clean my dishes at home using detergent. After several efforts, the ceramic-made dish looks like a uniform film of water just after finishing the cleaning. If I hold the dish for a while, the water starts to concentrate at regions, forming a droplet.

In addition, in this video
http://www.youtube.com/watch?v=m7liHlmv5Kc
at 2 min, there is water droplet in the flask.

My questions are:

(i) Does the clean standard have any time range? Does it apply to arbitary time scale?

(ii) If the answer to question (i) is yes, how should I do to make the dishes so clean that there is absolutely no water concentrating at certain regions? I do not have a dishwasher.

P.S. Video demonstration is highly appreciated.


texts:
I have bought a bottle of denatured alcohol. On the web I've found these solutions typically contain the following:

$85\%$ ethanol
$10\%$ water
$3\%$ methanol
$1\%$ aceton
$1\%$ pyridine

I would like to get rid of the acetone. How can I remove it? (and how do I know if it is in my solution?)


texts: Does chlorine trifluoride react with candlewax?

texts:
I've run across references to chlorine trifluoride quite a lot. It pops up fairly regularly on the Worldbuilding SE, and whatever idea people have with it is usually shot down very quickly - For obvious reasons.

One of the things that comes up in almost any web search about chlorine trifluoride is that it doesn't react with "candle wax" pretty much at all. (Example here)

Is this true? If it is, why? What is so special about candle wax that makes it immune to this substance that reacts extremely well with almost anything on, in, or around Earth?

(Worth noting: I'm not a chemist, so a basic overview would be appreciated in addition to any extensive scientific answer. Said extensive scientific answer is also appreciated because I'm sure there's others that would like the answer to this as well.)


texts: Alkene reaction with acid via SN1 reaction

texts:
I want to ask a question about alkenes and their reactivity with acids.

From high school studies, I recall that if you had an acid $\ce{H3O+}$ i.e. $H^+$ that it would react with an alkene as shown below:

However, when I was given a lecture today on Allylic alcohols in a $\ce{S_{N}1}$ reaction, the acid does not react with the alkene at all:

As you may understand, attempting to google or find an equivalent mechanism to explain this observation didn't yield a successful result.

The two mechanisms in my opinion appear to contradict each other, but I couldn't work out how the second mechanism for the Allylic Alcohol functioned whereby the alkene did not react with the acid. My assumption was that it was easier to protonate the alcohol group than react with the acid (with maybe a reasoning along the lines of pKa) but I couldn't find anything sufficient to justify my idea.

Why does the acid not react with the alkene group in the allylic alcohol shown above?


texts:
My brother and I were talking about how he needed to take his beers out of the freezer because they would burst open. This made me think of something no professor ever addressed... Obviously work is occurring on the beer can, but oxymoronically it's due to the removal of heat energy. The secret I imagine is in the phase change itself, which likely takes its own energy (which would normally be given off as heat) and uses it to expand (do work).

My questions extend on this idea...

What if we replace the beer can with a 5 inch thick steel capsule. Does the capsule prevent the water from freezing because there isn't enough room to expand?
What about if you take even more energy away? Would absolute zero finally break the capsule?
Can a strong enough container simply prevent water from phase changing to a solid altogether, regardless of temperature?
Normally increasing the pressure of a substance will cause it to freeze (in chemistry class my teacher made water boil at room temperature with a vacuum pump removing pressure). With water though, it expands while freezing... So paradoxically if you have water in a strong capsule and keep increasing the pressure (instead of temperature), will the water eventually freeze or simply remain very cold liquid?

Thanks!


texts: Strange batch of potassium nitrate with lower than expected melting point

texts:
I'm part of a rocket building team and I'm working with propellants. We use saltpeter as the oxidizer, bought as fertilizer and purified by recrystallization. Recently we bought it from another source, that identified the chemical as potassium nitrate, as usual, but when we heated it to make the propellant it simply melted at a temperature of $231~^\circ\rm{C}$, $100$ degrees lower than expected.

It works as an oxidizer, as making the propellant with this chemical gives good results, so it must be something similar to $\ce{KNO_3}$. Maybe there is something working as flux? It tastes like saltpeter (I eat a lot of materials we have in the workshop) and has that characteristic high temperature-dependent solubility.

Do you have any idea of what could it be, and what can I do to find out? Have in mind that our workshop doesn't have much in terms of lab equipment, only thermometers, various things to heat stuff and some solvents for a variety of applications. Also, note that I'm a telecommunications engineering student, so I don't have too much of deep knowledge of chemistry.


texts: Measure ethanol volume fraction in a beverage

texts:
Given a sample of an alcoholic beverage (be it beer, wine, etc..), what are possible methods to find out the ABV (alcohol by volume) of it with moderate accuracy?

As fas as I know, some methods exist already but are not really suitable in my condition:

Distillation. Impractical because of the apparatus needed
Boiling temperature based. Not accurate if exact condition of sugar is not known
Estimation by measure of specific gravity before and after fermentation. I've found it to be inaccurate with the data I have access to

I was thinking about a process similar to the tritation method used to find water hardness, but I'm not sure what kind of reactions would be involved for this kind of test, nor the nature of the titrant that would be required.


texts: Can MCM-41 (mesoporous materials) be used for ion exchange?

texts:
Wikipedia, and this journal article talk of a microporous and mesoporous material MCM-41. Although MCM-41 has dynamic sized pores which can be changed, but it is hydrothermally unstable.

Can this microporous material be used for ion-exchange similar to zeolites? What is the property that typically allows a substance to act as an ion-exchange resin?


texts: Why is energy released during the process of adsorption? How is this energy change observed and how is it calculated?


texts:
Today a friend showed me something we could not explain:

She had a pan which she used for years to heat water. On the bottom it had a thick layer of calcium carbonate. She forgot the pan on the hot plate. So all the water evaporated.

Then the calcium carbonate layer at the bottom got really black. Is this possible if it was pure $\ce{CaCO3}$ or must there have been other organic compounds to explain the black colour?

May I have an explanation what happened chemically as well?


texts: Why does benzoyl peroxide homolytically cleave to give phenyl radical

texts:
Benzoyl Peroxide cleavese rapidly to give out $\ce{CO2}$ and phenyl radical.

What I could not understand was why phenyl radical is formed because it is not at all resonance stabilized and nor any electronic effects are acting on it that could help in its stabilization then why is it being formed ?? Wouldn't it be too unstable ??

I mean after first cleavage, (Before $\ce{CO2}$ leaves) it should be more stable due to resonance (and that too equivalent resonating structures)


texts:
In order for a compound to be analysed in mass spectrometry it must first be ionised, forming an adduct ion. In many papers (e.g 1 [PDF], 2) these adduct ions are described in the form:

[M+H]+
[M+Na]+
[M-2H]2-

Where M is the analyte, the other content of the square brackets is the ion, and outside the brackets is the resulting charge. The IUPAC Gold Book also mentions this format for protonation without, as far as I can tell, describing it further.

Is there some reference actually describing this convention - ideally encompassing more complex scenarios such as dimerisation or neutral losses as well - around?


texts:
I am currently studying to complete the Chemistry CLEP exam, I ran into this question and was thoroughly confused by the phrase

  12.04x10^23 atoms of H_2

According to the question, this corresponds to 12.04X10^23 molecules (2 moles) of H_2, or 24.08x10^23 atoms (4 moles) of Hydrogen. The use of the word "atom" over "molecule" seems useless, confusing, and possibly wrong.

I'm not sure if this question is written poorly or if I have made a mistake in my grammar either way, any help is appreciated. Thank you!


texts:
This is the procedure I carried out in class:

Reduction of nitrobenzene by using tin and $\ce{HCl}$ acid
After the completion of the reaction, $\ce{NaOH}$ added to neutralize excess acid and dissolve any $\ce{Sn(OH)2}$
Steam distillation is carried out to obtain suspension of phenylamine
Salting carried out where $\ce{NaCl}$ salt is added. Aqueous layer discarded
Solvent extraction: dry ether added and mixture shaken vigorously. Aqueous layer discarded
Distillation under reduced pressure carried out and solid phenylamine left in flask

My questions:

Instead of carrying out distillation under reduced pressure at the end of the procedure to remove the ether, could I have just carried out simple distillation?
Why is steam distillation suitable in part 3? Could I have used vacuum distillation here?


texts: Is selenocysteine C3H6NO2Se or C3H7NO2Se?

texts:
I'm a beginner in chemistry, and recently I am interested in selenocysteine.

While most websites (Wikipedia, ChemSpider) say it is $\ce{C3H7NO2Se}$, PubChem says it is $\ce{C3H6NO2Se}$.

Which one is correct? Or are both reasonable?


texts:
Suppose I have a white paper originally. Then it is placed in a trunk, where soda spills occasionally and gasoline gas/gasoline also spills due to no gas tank cover. Over time, this paper becomes discoloured.

How do I make it white again? What equipment do I use?


texts: Aqueous copper and how it reacts with NH3

texts:
I have learnt that you can test whether an unknown metal is copper by doing a simple test tube reaction. The reaction involves adding aqueous $\ce{NH3}$ to the test tube which contains the copper.

The $\ce{NH3}$ as it is dissolved in water will form $\ce{NH4+}$ ions and $\ce{OH-}$ ions. The $\ce{OH-}$ ions react with the copper forming a precipitate $\ce{([Cu(H2O)4(OH)2])}$ but many other metals form a precipitate with $\ce{NH3}$ as well such as $\ce{Fe^2+}$, $\ce{Fe^3+}$ etc. So to make sure that it is copper present in the test tube you add an excess of $\ce{NH3}$ as this causes a ligand exchange reaction to occur producing $\ce{[Cu(H2O)2(NH3)4]2+}$.

Why doesn't this occur with the other metals? Why doesn't this occur for say $\ce{Fe^2+}$ to form $\ce{[Fe(H2O)2(NH3)4]2+}$ which would also be soluble?


texts: why type of adsorption takes place during catalysis? It can't be physisorption because it involves formation of chemical bond. It shouldn't be chemisorption too because it is characterised by desorption, where as desorption takes place in chemisorption. So basically what kind of adsorption is involved in catalysis?


texts:
Graham’s law of diffusion states the rate of diffusion between two gases is inversely proportional to their molar masses. Now, this is helpful when we are focused on two gases. But that is rarely the case in a beaker or even the atmosphere.
So how do you measure the rates of diffusion of many gases moving at once? Do you measure each rate of effusion (inversely proportional to molar mass) and just order them, then use stoichiometry to determine which one will be dispersed quickest based on the amounts present?


texts:
How are nodes in a molecular orbital different different from those of atomic orbitals?

eg. For a Px-Px Pi bond, we would say that the nodal plane is the Y-Z plane (correct me). How about for a Pi* (antibonding) molecular orbital?

I think it should be similar to an atomic orbital, but please confirm.


texts: Atomization enthalpies of transition elements

texts:
So, my book says that transition elements have higher enthalpies of atomization than other elements (say s- or p- block) because of stronger metallic bonding, primarily due to large number of unpaired electrons in the d-orbitals.

Now, if that is the case, then the atomization energy for $\ce{Ni}$ should be less than for $\ce{Fe}$ and $\ce{Co}$, as $\ce{Ni}$ has only 2 unpaired electrons compared to $\ce{Fe}$ and $\ce{Co}$ (4 and 3 respectively).

Actually, it is just the opposite.

$$\begin{array}{cc}
\text{Metal} & \Delta_\text{atom}H(\ce{M})\text{ / }\mathrm{kJ~mol^{-1}} \\ \hline
\ce{Ni} & 430 \\
\ce{Co} & 425 \\
\ce{Fe} & 416
\end{array}$$

Why could this be?


texts: Why is a reaction more favorable if one of the products is not soluble/sparingly soluble?

texts: I have heard people go so far as to say that the "driving force" of a reaction is the formation of the precipitate. Why is this so? Whilst the process is almost certainly exothermic (I think - please clarify) the entropic effects are surely unfavorable.


texts: Which reactant to use to calculate yield?

texts:
A chemist mixes 24.0 g $\ce{H2}$ with 8.0 g $\ce{N2}$. Assuming the reaction goes to completion, which reactant should she use to calculate the yield?

$$\ce{3H2 + N2 -> 2NH3}$$
I believe it should be nitrogen. With 24 g of $\ce{H2}$, we have 11.9 mol of $\ce{H2}$. With 8 g of $\ce{N2}$, we have 0.3 mol of $\ce{N2}$. So, when I express the moles of $\ce{N2}$ in terms of $\ce{H2}$, I see that I only need 0.9 moles of $\ce{H2}$ – which means that $\ce{N2}$ is the limiting reacting and then used to calculate the theoretical yield, right?


texts:
I'm not sure if this is a silly question, but I was sitting here with a cup full of cheezey poof balls thinking, "My goodness, it's like an amazing cheesey delicious liquid - huge water molecules!"

Of course my next thought was, "Wait a minute - water has two hydrogen atoms bonded to an oxygen, so that's not quite right. They wouldn't be round like this."

Then I started thinking about the diagrams we see in chemistry textbooks, etc., and how the atoms are always pictured as round balls. How do we know this is accurate? Is it possible for the atoms to be configured in more complicated shapes (e.g. not solid, crystalline, some type of lattice)?


texts: Understanding basicity of heterocyclic compounds

texts:
I want to know the reason why,

Pyrrolidine

is more basic than,

Piperidine

Mainly I want  to know what affect  does  the ring structure have on basicity  and how does 5 membered rings and 6 membered  rings containing a electron rich atom,  affect the basicity?


texts: Entropy of free expansion and isothermal process

texts:
Let me tell what I understand of the concepts on which the question is based first.
Heat is a flow of energy that takes place due to a temperature gradient.
When doing reversible isothermal expansion of an ideal gas, even though there is no net change in the system's temperature before and after the process, each time we take a differential pressure off the system, there is a differential work done by the gas which creates a differential temperature difference in the system and surrounding and so the differential heat flow, which is the added to get the total heat exchanged in the process.
Then there is the case of free expansion. In this case, even though the gas expands, as it is done against $0$ pressure there is no work done in any step, so there is no, not even differential, flow of heat in any of the steps involved in the process. Also, as the final and initial temperatures are the same in this process, we can safely say that there is a possible way for reaching the final state
achieved in free expansion, from the same initial states, isothermally.
Question: Let's take two process into account for going from state $(P_1, V_1, T)$ to state $(P_2,V_2,T)$. One via an isotherm and other via free expansion. Entropy for the first process (isothermal) is going to be $\Delta S=\int \frac{dq}{T}$ . As T is constant, the integral becomes $\Delta S=\frac{Q}{T}$. As for the second process (Free Expansion), $dQ$ is $0$ for every step and $T$ is constant for every step. this yields $\Delta S=0$ . But entropy is a state function, so the value for both processes $1$ and $2$ should be the same. So what is going on here? Is there something wrong with my understanding of free expansion?


texts:
What does it mean to add "1 equivalent" of something? I looked it up on google and couldn't really find a good explanation for it.

Thanks!


texts: Can the 1st generation Grubbs Catalyst be used for ring closing metathesis on non-terminal alkenes?

texts: I'm wondering if the 1st generation Grubbs catalyst can be used to do ring closing metathesis on a non-terminal alkene.


texts: Is the relative natural abundance of isotopes of an element the same everywhere?

texts: Wherever you find potassium, the isotopes are present in a set percentage that exists the same everywhere in nature, but why is that? Does it have something to do with how the element is produced?


texts:
Several books that I have read say that in $\ce{H2O2}$ the degree or extent of H-bonding is 6 as each H atom would form 1 and O atom would form 2 H-bonds respectively. However if we look at HF the no.of H-bonds formed by it is only 2 (because in overall mixture of molecules the extra lone pairs present in F would out no. the no. of available H atoms with partial positive charge)

My question is that going with the same logic even $\ce{H2O2}$ should form only 4 hydrogen bonds as the no. of lone pairs in O would outnumber the H atoms available then why is the degree of H bonding in $\ce{H2O2}$ 6 but not 4?


texts: Does selenium sulfate corrode or otherwise react with metals?

texts:
I was just watching a cartoon called Motorcity, and they featured a real compound called selenium sulfate.

While I'm sure that the compound blowing up a detached steel [car part] on contact is just cartoon wackiness, I'm curious whether it actually does anything bad to any metals, especially since a close compound, selenium sulfide, is commonly used in shampoos.


texts: I have a steam distillation setup with the flask for boiling the açaí powder with distilled water. Are you able to extract the essential oil from the boiling material?


texts: Molecular orbitals of heteronuclear diatomic molecules

texts:
For drawing MO diagram for heteronuclear diatomic molecule, how do I know which is lower in energy than the other?

For example, given a heteronuclear diatomic molecule $\ce{NO}$, which one is lower in energy?


texts: Number of stereoisomers of pentane‐2,3,4‐triol

texts:
What will be the number of stereoisomers of $\ce{CH3CH(OH)CH(OH)CH(OH)CH3}?$

By formula $2^n - 1 + 2^n + 1/2$ for $n$ being the odd number of chiral centres, it's $6;$ four optically active and two optically inactive.
Optically active  when RRS, SSR, RR, SS, but I am not getting how we get two optically inactive.


texts: What is the difference between a glassy, amorphous and a vitreous phase?

texts: I have been reading about some inorganic materials lately, and these three terms confuse me. If they all mean the same thing, would still one of them have preference over others?


texts:
I'm studying molecular motors for a project of mine and I'm trying to understand what a Fjord region is.

It's sort of labeled in the image below but I'm not entirely sure which part.

I would like a suggestion of a book or article that may help me understand what it is?


texts: Is the Co-ordination number of Cobalt always 6 in every co-ordination compound?

texts:
For example in the following  compounds

CoCl3.6H2O
CoCl3.6NH3
CoCl3.5NH3
CoCl3.4NH3

in all of these complexes, we took the
a)oxidation state of Cobalt as +3 ( since there are 3 Cl- ions and NH3 and H2O are neutral ligands )
b) Co-ordination number of Cobalt  as 6
and the answer to writing the compounds was:
1.[Co(H2O)6]Cl3 or [Co(H2O)     5Cl]Cl2.H2O

[Co(NH3)6]Cl3

[Co(NH3)5Cl]Cl2

[Co(NH3)4Cl2]Cl

My question is this: Why do we take the co-ordination number of Cobalt always six?
Can't Co take other co-ordination numbers like 4? or 5?
So for the following compound
CoCl3.4NH3
Can't we write it as [Co(NH3)4]Cl3 instead of
[Co(NH3)4Cl2]Cl
Also how to predetermine the C.N of the central metal atom just by looking at anions and ligands? (cases when no coordination sphere is marked like in my above examples)


texts: Why formulate drugs as HCl salts when HCl is in stomach acid?

texts: I know there are issues of formulation and industrial processing that make it advantageous to produce many amine containing drugs as their salts rather than as freebases. And if giving the drug intravenously, the increased water solubility is clearly an advantage. But if giving orally, is there not sufficient HCl in the stomach to have this occur in vivo? Is it just that some of these drugs are too non polar to even go into solution in the stomach acid? Or do we just need to avoid neutralizing too much of the stomach acid with a drug not intended for that purpose?


texts: More ions form pairs when concentration of acid increases, does that decrease the speed of reaction?

texts: Electrical conductivity decreases as an acid becomes more concentrated because more ions form pairs. However, does that affect the speed of reaction of the acid as there is a smaller concentration of Hydrogen ions?


texts: Understanding sodium hydroxide drain cleaner's exothermic dissolution of aluminum; does the aluminum need to be oxidized before it dissolves?

texts:
I'm not a chemist but I remember that the standard HF/nitric silicon etch used in micro-fabrication supposedly worked in two steps; the HF would dissolve the thin SiO2 oxide layer on the silicon surface, exposing silicon atoms, and the nitric acid would then re-oxidize the bare silicon, allowing the HF to then dissolve it.
Or at least that's how it was explained to me.
Despite being challenged here, Wikipedia explains that the aluminum bits in sodium hydroxide-based drain cleaner are included to produce heat via exothermic liberation of hydrogen gas so that the heat can loosen some of the clogging materials. The article provides the following two steps.

Breakdown of Aluminum Oxide: $\ce{ \ \ \ Al2O3 + 2NaOH + 3H2O → 2Na [Al(OH)4]}$

Oxidation of Aluminum metal: $\ce{ \ \ \ 2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2}$

Is what is happening similar to my understanding of the HF/nitric etch of silicon; it is the oxidized aluminum that is removed by the solution, and only because it happens to also oxidize the freshly exposed aluminum metal that the oxide is removed?

Snapshot of some drain cleaner with aluminum bits


texts: Chemicals that form coloured complexes with Pb2+

texts:
I’m currently conducting a photo-spectrometric analysis of lead concentration in water and was looking into using Eriochrome Black t as an indicator for Pb2+ (due to the complex forming a reddish colour). Because Eriochrome is relatively expensive, would there be any alternate chemicals that can complex lead in solution of the form Pb(OH)NO3, such that colour is present. This is because the source of lead is from a dissociation reaction of lead (II) nitrate when it is dissolved in water. The resulting lead is complexed.

All the best!


texts: Comparing axial and equatorial bond lengths in square pyramidal structures

texts:
I found this image on wikipedia:

It got me wondering why the axial bond angles were shorter than the equatorial ones. I found this extract elsewhere on the internet -
Extract: (ww.nrcresearchpress.com)

For $AX_{5}E$ molecules which have a square pyramidal shape the extra repulsion exerted by the lone pair would be expected to cause the neighboring four bonds, i.e., those at the base of the square
pyramid to be slightly longer than the fifth bond to the apex. Experimental evidence on
molecules of this type is scanty (5). The four square-planar bonds are longer than the
single bond to the apex in $SbCl_{5}^{2-}$ but they appear to be shorter in $SbF_{5}^{2-}$.

The extract is from a research paper from 1961. It states that the rule isn't followed in $SbF_{5}^{2-}$, and that there's "scanty" evidence anyway.
Now, given that the paper is pretty old, I was wondering if there has been any new evidence collected on this subject. I essentially would like to understand why the bond length of BrF5 follows this order, and why it isn't followed by $SbF_{5}^{2-}$. I'd appreciate some references as well for evidence and perhaps other researched examples.
Note: This isn't a duplicate of this, since that question doesn't address the deviation in $SbF_{5}^{2-}$.


texts: Why is it only electrons that take part in chemical reactions?

texts: I learnt that only electrons take part in chemical reactions and the protons do nothing (neutrons are electrically neutral, so they do nothing). Why is this so if the protons are positively charged and without them, electrons in the other atom won’t be attracted (and since electrons are like charges I do not think they would interact with each other)?


texts: What's with the weird definition for the Calorie?

texts:
I came across this definition for the calorie, which I found... strange:

"The (15°) calorie is the heat required to raise the temperature of one gram of water from 14.5°C to 15.5°C."

Subsequent searches on-line revealed that this was (one of) the standard definitions for the Calorie.

Now two issues came to my mind after I read that definition:

1) Why is the term 'heat' and not 'energy' used?

From what I recall from my Thermodynamics classes at school last year, heat is energy in transit. I'm under the impression 'heat' is more of a 'surface phenomenon' if you will; there's 'heat' at the junction between two surfaces, but no matter how much you 'heat' up a body it can never contain heat (strictly speaking).

2) What's so special about the 14.5°C and the 15.5°C range?

Seriously, why that particular range? Why not 0°C to 1°C, or even 1°C to 2°C? Heck even 14°C to 15°C would go down (mildly) well with me, but 14 .5 °C to 15 .5 °C? What's with the inordinate fondness for the 0.5°C? A 1°C difference is a 1°C difference in temperature, so why specify any range at all? Why not define the calorie as 'The energy required to raise the temperature of one gram of water by 1°C' ?

I didn't have this question before because I, for all practical purposes, only used to see the calorie as being roughly equal to 4.19 Joules.

Would anyone happen to have the answer to those two queries?

EDIT-

I've already seen this particular question, and I believe it is necessary to state that this does NOT answer my question. My question is distinct from it in the following respects:

1) I'm asking for a clarification regarding the use of the term 'heat' over energy.

2) I want to know why a generalization of the definition, replacing the arbitrary temperature range with a '1° difference'  wouldn't suffice.

3) I want to know why the range is demarcated by two fractions (14.5° and 15.5°) instead of integers.


texts: Relative stability of carboxylic acid derivatives

texts:
This picture depicts the relative stability associated with resonance structure.

My questions are:

1.1. Teacher says anhydride is less stable because of two reasons above.
but it seems there's no reason the resonance of anhydride would be less likely compared to acetate.
1.2. and about reason #2, the actual structure is the hybrid of these resonances so it's hard to agree with this reason.

Why won't Cl give its e- pair to adjacent carbon for resonance structure.

Someone says it's about electronegativity but it contradicts with resonance associated with oxygen. And someone says it's about the high energy orbital of Cl (n=3) but I can imagine a acetyl fluoride and this logic doesn't solve this problem anymore.


texts: Does Ammonium Acetate, a salt of a weak base and weak acid, undergo both hydration and hydrolysis in aqeous solution? What is meant by the statement that " Some hydration reactions are hydrolysis"


texts: Mismatch of plane of symmetry vs Gold Book's definition for a pseudochiral carbon

texts:
Here is pentane-2,3,4-triol:

At first glance, without deciding the configurations at each chiral carbon, we can clearly see that this molecule has a plane of symmetry, perpendicular to the plane of paper and passing through the $\ce{-OH}$ and $\ce{-H}$ on the 3rd carbon atom.

However, if we then sit down to compute the R/S configuration at carbons 2 and 4, we find them to be R and S respectively. Thus, going by the definition of a pseudo-chiral carbon, given in Gold Book as:

  a tetrahedrally coordinated carbon atom bonded to four different entities, two and only two of which have the same constitution but opposite chirality sense.

we would infer that the 3rd carbon is indeed pseudo-chiral. This is confirmed by ChemSketch which generates the name of the above compound as "(2R,3s,4S)-pentane-2,3,4-triol".

Thus, we have observed that using the plane of symmetry to check chirality, we end up declaring the 3rd carbon to be achiral, rendering the molecule optically inactive. However, if we use the Gold Book's definition, we end up declaring the 3rd carbon to be pseudo-chiral, rendering the molecule optically active.

Another fact confirming that the molecule is optically active is that there exist two isomers, of two opposite configurations of the central carbon atom -  (2R,3s,4S)-pentane-2,3,4-triol and (2R,3r,4S)-pentane-2,3,4-triol. Here are their structures:

I have looked at several questions on pseudochirality here, yet I have been unable to resolve this conundrum. I believe I am using a wrong fact or a misguided inference, but whatever is it, I am unable to grasp it, and thus need help resolving the conundrum.


texts:
My interpretation of this image is that $\Delta G^\circ$ is equal to the difference between $G^\circ$ of the products and $G^\circ$ of the reactants.

But, I've also read $\Delta G^\circ$ as being given for calculating the difference in free energy between a point when $Q=1$ and the equilibrium position. That is, $\Delta G^\circ=-RT\ln(K)$.

Which one of these is the accurate interpretation?


texts:
How to form Schiff's Base at both the ends of the thiosemicarbazide?
$\ce{NH2}$ of hydrazine part reacts easily to form Schiff's base i.e. imine. But $\ce{NH2}$ attached to $\ce{C=S}$ does not. How to do that?
One approach is by using NCS but that forms imine at $\ce{NH2}$ attached to $\ce{C=S}$ only and not at the other end.


texts:
In the orange box, I think that the blue-carbon should be a carbocation instead. So, option A should be correct.
But, the answer is B. Why is it so?


texts: When finding the liquid density which gives the most accurate results, a 25 ml Pycnometer or a graduated 10 ml cylinder?

texts:
Its 10 ml graduated cylinder with 25 ml of pycnometer, and we are measuring the density of the liquid(pure liquid) in each of the materials and we are suppose to compare the densities found , in terms of accuracy

Excuse me guys am doing a a research project on density, these are the two methods i used to find the accuracy, been searching but i have no progress. Feel free to give suggestions.


texts: How can magnesium hydroxide be a strong base and be sparingly soluble at the same time?

texts:
A strong base means it completely dissociates forming $ OH^- $ ions when added to aqueous solution.
Sparingly soluble means it dissolves very little when added to aqueous solution.
Yet magnesium hydroxide is said to be both. These two properties seem to contradict each other. How can this be?
A very similar question was asked here, the most relevant answer states that

Some strong bases like calcium hydroxide aren't very soluble in water.
That doesn't matter - what does dissolve is still 100% ionised into
calcium ions and hydroxide ions. Calcium hydroxide still counts as a
strong base because of that 100% ionisation.

But I don't understand the difference - how can something dissolve and not be ionised? (wouldn't that imply a covalent structure like glucose?)


texts:
I've tried this:

But its hows that the valence of Fe in normal state is 4, when in the periodic table it isn't. Why does this happen? Can you show me the answer with configuration formula in the states with energy?


texts: What are the bubbles and crystals found in sodium acetate hot packs after use and storage in the solid form?

texts:
When sodium acetate trihydrate hot packs are regenerated soon after activation, they regenerate to a clear liquid upon boiling (typically with an air bubble that was not present in the new pack)

On the other hand, when activated hot packs are stored in the crystallized form for a while, regeneration by boiling in water yields a mixture of clear liquid and very thin clear glossy plate-like crystals almost like mica sheets, plus large air (gas) pockets.

I suspect that the bubbles arise due to diffusion of room air through the flexible plastic envelope of the pad. When a new pad is activated and sodium acetate trihydrate crystals form, small gas spaces are left between the crystals which are necessarily filled only with water or acetic acid vapors which have low vapor pressure at room temperature. The low gas pressure in between the crystals (I estimate < 3 kPa) would create a large driving force for gas diffusion through the polymer envelope.

The crystals could be sodium acetate anhydrous formed because of water vapor loss by diffusion through the polymer envelope, but I'm not as sure on this. Perhaps they are sodium hydroxide formed because of acetic acid loss by diffusion? The liquid remaining around the crystals can still be activated to crystallize like a new hot pack, so the crystals are definitely not the trihydrate.

I'm looking for a reality check on these speculations, ideas on how to find out what the crystals are (ideally without needing sophisticated lab equipment), and advice for restoring hot packs containing these flat crystals to the initial clear liquid state. Photos below. Many thanks!


texts: Safety in handling of concentrated ammonium fluoride

texts:
I recently saw a protocol for extracting organic materials that are bound in the silica matrix formed by diatoms.  The extraction protocol used 10 molar ammonium fluoride ($\ce{NH4F}$).

I briefly considered using the protocol and then endeavoring to remove the ammonium fluoride (and water) by lyophilization or with a rotary evaporator.

However, I am concerned about the safety of doing this.  My first worry is that any glass equipment or vessels in the lyophilizer or rotary evaporator systems would be damaged by the fluoride.  My second worry is that even if the equipment used did not contain any glass, dangerous solutions of hydrogen fluoride (i.e. hydrofluoric acid) could be formed in the cold trap of the equipment.

My first question is, am I right that it is a bad idea to evaporate ammonium fluoride in glass equipment?

My second (but related) question is, even if glass is not used, what are the safety risks of evaporating ammonium fluoride under vacuum?  Imagining that a glass-free apparatus could be found, is there a way to do the evaporation safely?


texts: What is Markovnikov's rule? How can we use it to predict the products of additions to alkenes? And is it always obeyed, or are there exceptions to this rule?


texts:
I was having trouble drawing the Lewis structure for $\ce{NCO^-}$. I found this image online:

But why is structure number one preferred over structure number two? Why does carbon prefer to have a triple bond with nitrogen, rather than have a double bond with nitrogen and a double bond with oxygen?


texts: Two common fluids that don't mix inside a labyrinth

texts:
I'm not sure in which SE site put this question.
Imagine a typical child toy, where a bunch of spheres move by gravity through a labyrinth.
Now imagine a sealed small labyrinth with two fluids. I'd like to reproduce the concept of one fluid moving through another forming small bubbles, but I don't know which common fluids might be useful. Theose two fluids should not mix, the viscosity must be low (so the bubbles can move easily), both fluids must have different colors, they must have different density, the bubbles should not stick to the plastic of the labyrith, it should work at room temperature, and -if possible- I'd like to see many small bubbles instead of a single big bubble.
Which two common fluids might be useful? I tried air inside water (few bubbles and movement too fast), I also tested water inside kitchen oil, but it moves too slow and the colors aren't quite different.


texts: Different products using different indicators

texts:
A question states that 0.1 M solution of compound$$\ce{Na2CO3.NaHCO3.2H2O}$$ is titrated against 0.05M HCL, and that a different amount of HCL is used according to the indicators(phenolphthalein or methyl orange) used.

How does the indicator used affect the titration?


texts: Which of the following statements is true about the trend down group 7?

texts:
Which of the following statements is true about the trend down group 7 (halogens) with increase in atomic number?

  The reactivity increases.
  The atoms get smaller
  The boiling points decrease
  The melting points increase
  The reactivity decreases

For me D and E must be right but only one can be right.


texts: Why is HCl a stronger acid than Acetic Acid?

texts:
Drawing the conjugate bases,  Oxygen is more electronegative than Chlorine so negative charge on Oxygen is more stable than Chlorine,  Thus My argument that Acetic Acid is more acidic.

Also the O¯ ion is in resonance with another Oxygen so that should also add to the acidic nature.

Where am I going wrong?


texts: Is water the only substance that expands in both gaseous and solid states?

texts:
I have been told that water is the only substance that expands in both solid and gaseous states!

Is this true? If not, what other substances can do this?


texts: Is this molecule possible? And if so, what is it called? (Cyclic groups)

texts:
I saw a little chemistry pun with a couple of hydrocarbons included, and it had me thinking if they were actually possible  and if so, what would the name given to it be.

The hydrocarbon I am curious about:

For those wondering what the pun is:


texts: Can you have addition polymerisation with monomers other than alkenes?

texts:
Every example of addition polymerisation I have seen so far has used alkenes as the monomers. I was wondering if it is possible to have some other molecule. I don't think the following molecule can exist due to stability reasons, but as an example could you not use something like

$$\ce{H-N=N-H}\,?$$

Surely all you need is the presence of a double bond?


texts: Products of radical bromination and mechanism

texts:
My organic chemistry final is next Tuesday, and I have worked through the practice final. The last question is the following: Give and name all the compounds resulting from the radical monobromination 2,3-dimethylpentane. Indicate the major and minor products.

I drew:

My prof says that I am missing my R/S. I can clearly see my chiral centres, but how do I determine the orientation of the H? Red is the major product, blue is the minor product, and magenta is the very minor product.

The second question is: write the mechanism of this reaction identifying the initiation, propagation, and termination steps for the formation of one of the products. This is what I drew:

My prof says that I am missing a propagation step with another bromine-bromine molecule.

EDIT:


texts: Yellowish tinge during titration of oxalic acid with potassium permanganate

texts:
One of the our chemistry practicals involve finding the molarity of an oxalic acid solution by titrating it with standard $\ce{KMnO4}$ solution.

The oxalic acid solution is heated and little sulfuric acid is added to titration flask before starting the titration.

It was my personal observation that adding the $\ce{KMnO4}$ solution rapidly causes a temporary appearance of a light yellow colour. Slow and gradual addition of $\ce{KMnO4}$ does not lead to this observation.

If sulfuric acid is not added to the titration flask, then also a yellowish tinge appears as we get closer to the end point.

What is the for this yellowish tinge? Is there any intermediate species formed (maybe a complex one) that leads to this observation?


texts: Difference between oxidation and formal charge

texts: What is the difference between oxidation number and formal charge? I am confused about the two.


texts:
How to prepare $\mathrm{0.1M}$ $\ce{I_2}$ solution.
I have found in books that it should be prepared in $\ce{KI}$ solution but none of them explains why? Please explain in detail not just the procedure.


texts:
I was in a chemistry lecture about the upcoming laboratory practical on esters that I would have to take. My lecturer had mentioned that we ought not worry about "the pink coloration that will occur" during the esterification process. However, when we asked why a color change would even occur, his reply was very vague. According to him, there is a general consensus that no-one knows why.

We will be asked to produce an ester using ethanoic acid, butan-1-ol, and concentrated sulfuric acid, in the presence of boiling chips.

So is there a known answer to the mysterious pink coloration? I think it's due to some contaminants that might be present in the sulfuric acid, but my lecturer says  the pink coloration is completely random and that everyone in past few years have always used the same solutions.


texts:
Is there a name for the class of stand alone molecules that contains oxygen, water, hydrogen peroxide, and ozone? ($\ce{O2}$, $\ce{H2O}$, $\ce{H2O2}$, $\ce{O3}$) I would not want to include hydroxyls $\ce{OH}$ because they aren't neutral molecules that I can buy.

I think oxide and oxygenate is too generic.

Thanks for your help!


texts: Esterification of racemic acid with optically active alcohol

texts:
An excess of racemic acid ($\ce{CH3CHClCOOH}$) is allowed to react with (S)-2-methyl-1-butanol to form the ester, $\ce{CH3CHClCOOCH2CH(CH3)CH2CH3}$, and the reaction mixture is carefully distilled. Three fractions are obtained, each of which is optically active. Draw stereochemical formulas of the compound or compounds making up each fraction.

I know that two fractions obtained will be of the diastereomers (R,S) and (S,S), what is the other fraction?

Source: Morison and Boyd


texts:
A question from a grade 9 admission exam:
When 1 kg of salt is added to a solution of salt
and water, the solution becomes 33 1/3% salt by
mass. When 1 kg of water is added to the new solution, the resulting solution is 30% by mass. The percentage of salt in the original solution is...
I tried:
(Salt orig +1)/(Water orig + salt orig + 1) = 33 1/3. (S + 1)/(W+S+2) = 30. Which means 33 1/3*(W+S+1)=30*(W+S+2); 33 1/3 W+33 1/3 S +33 1/3 = 30W + 30S + 60; 3 1/3 W + 3 1/3 S = 26 1/3; S+W = 79. Putting that into the 1st eq gives S+1 / 80 = 33 1/3 meaning S= 26 2/3. Solving for W = 79-26 2/3 = 52 1/3. So the original concentration = 26 2/3 / (52 1/3 + 26 2/3) = 33 3/4%. But that's not one of the multiple choice answers (I did get further this time!).


texts: Free Energy vs. Common Ion Effect in Ultrapurification

texts:
I'm wondering which of the two phenomenae grows in importance and which one becomes irrelevant when the proportion of impurities in a product being purified become increasingly small:
The common ion effect will favor the solubility of the purified substance to the detriment of impurities that have lower solubility values if these impurities share a common ion with the substance being purified.
The higher free energy of impure crystals vs pure crystals on the other hand, will favor the dissolution of impurities over that of the pure product.
Is it reasonable to say that, as the purity of a more soluble desired substance increases the common ion effect loses relevance vs the effect imparted by the difference in free energy of pure vs impure precipitated particles?
My basic reasoning comes from the idea that the common ion effect decreasing the solubility of a lower solubility impurity with a common ion will almost double going from 50% to 99% purity, but has a plateauing influence as the purity grows, while on the other hand I can't see why the other effect would plateau with increasing purity.
In other words: Is the common ion effect of interest for the purpose of purification only in early (bulk) stages, while recursive recrystalisation would forever decrease a stable proportion of insolubilities (provided ideally pure solvent & glassware)?


texts: Is there a more direct way to show this identity for the projection operator?

texts:
The following question I jotted down from an online quantum chemistry pdf.

Show that
$$i\hbar \frac{\partial P_j}{\partial t}= [H,P_j]$$

in which $P_j$ is the projection operator and $H$ is the Hamiltonian. We are given the usual time-dependent form of Schrodinger's equation.
What I did: I supposed that this was an application of Ehrenfest's theorem because it involved a commutator and derivatives. $P_j$ does not explicitly involve time and so Ehrenfest's theorem reduces to$^*$
$$\frac{d}{dt}\langle\psi|P_j|\psi\rangle=\frac{1}{i\hbar}\langle\psi|[H,P]|\psi\rangle $$
But assuming I think w.l.o.g. that $\psi$ is normalized we can pre-multiplyby $|\psi\rangle$ to get (using $|\psi\rangle\langle\psi|=1$)
$$i\hbar\frac{d}{dt}P_j|\psi\rangle~= [H,P_j]|\psi\rangle$$
If this works (I don't assume that), my question is whether there is a direct way to do this, using the definition of $P_j$, and if so whether it would not also be an appeal to this theorem?
$^*$ Using eq. (9) from this note.


texts: I am looking for a specification for pharmaceutical grade phosphoric acid.


texts: Internal energy change during phase change

texts:
Ice at $\mathrm{0\ ^\circ C}$ is converted to water at $\mathrm{0\ ^\circ C}$. If $\Delta H$ for the transition of ice to water is $\mathrm{1440~cal}$, calculate the change in internal energy.

Since internal energy $\Delta U=nC_V\Delta T$ and $\Delta T=0$, shouldn't $\Delta U=0$?

But if I use $
\Delta H=\Delta U +\Delta nRT$, I get $\Delta U=\Delta H\neq 0$.


texts: How to dilute a stock solution when the resulting solution has a smaller volume?

texts:
Describe how you would dilute $2.0~\mathrm{L}$ of a $0.85~\mathrm{mol/L}$ magnesium hydroxide stock solution to make $200.0~\mathrm{mL}$ of $0.30~\mathrm{mol/L}$ magnesium hydroxide.

I know the $$c_1 V_1 = c_2 V_2$$ formula and that magnesium hydroxide's molar mass is $58.32~\mathrm{g/mol}$. However, I am confused because $V_2$ is lower than $V_1$.


texts: why does lithium fluoride have a higher lattice energy than lithium iodide?

texts:
Based on my knowledge, lattice energy is proportional to the multiplcation of the charge of the ions, divided by the sum of the radius of ions, as follows.

Since iodide has a larger radius than fluoride, by this equation lithium iodide would have a less exothermic lattice energy (lower lattice energy).
However, I also learned that an ionic bond with higher covalent character is stronger (i.e. more exothermic lattice energy). Since the electron cloud of iodide ion is larger and is more polarisable, lithium iodide should have greater covalent character than lithium fluoride and thus its lattice energy should be more exothermic (higher lattice energy)


texts: How is o-Fluorophenol more acidic than phenol even after having Hydrogen bonding?

texts:
Source: Concepts of Organic Chemistry by Dr OP Tandon, Himanshu Pandey, Dr AK Virmani
Page no: 241


texts: Combustion of hydrocarbons reveals hydrogen, but not sure at what efficiency. Is there a reaction to get hydrogen from hydrocarbons?


texts: MO-Scheme of SCN- and its bonding properties when used as a ligand

texts:
I tried to figure out the MO-scheme of the tetragonal-bipyramidal complex trans-$\ce{[Co(en)2(NCS)2]SCN}$ in which the isothiocyanate ligands are bound to the $\ce{Co^3+}$-Ion in $\eta^{1}$-mode (en = ethylenediamine). While undertaking this task I noticed that I don't know the MO-scheme of the isothiocyanate ion, which I need to determine whether it acts as a $\sigma$ and/or $\pi$ acceptor or donor.

I tried to construct the MO via a symmetry-based treatment but ran into the problem that the isothiocyanate ion - belonging to thet $C_{\infty v}$ point group - lacks symmetry elements perpendicular to its principal axis. Thus, I wasn't able to construct the bonding and antibonding sigma-orbitals with this method.

I could construct those $\sigma$-interactions by hand or use $\ce{CO2}$ as a model but I don't know whether this will give me the right results. I haven't found the MO-scheme in the internet so I would be happy if someone could advise me on how to construct a qualitative MO-scheme of $\ce{SCN-}$ or simply show it to me. I would also be very grateful for getting some information on how exactly the isothiocyanate ion acts as a ligand (which $\pi$-interactions and how strong are they, is there also $\sigma$-donation as with carbonyl ligands, etc.).

Edit:
I forgot to mention explicitly that the $\ce{SCN-}$ ion is bound to the metal center via its nitrogen atom.


texts: Is there a reason why we add (CH3CSNH2) but not H2S in qualitative group analysis?

texts: Brief question: In $\ce{Cu}$-$\ce{As}$ group analysis we are expected to add thinoacetamide to make a precipitate with sulfide. Can we use $\ce{H2S}$ instead?


texts: H1 NMR Spectra of inorganic complex, Cu4OCl6(2-mepy)4

texts:
I'm having difficult interpreting the following H1 NMR I have been given for Cu4OCl6(2-mepy)4.

The NMR spectra obtained with integration trace beneath. I'm not too sure how to rationalise the 4.5:1 ratio between the peaks nor the cause of the small broad peak at 115ppm (well to the left of the spectra).

I'm also not too sure why the H1 NMR range is so vast. I appreciate being bonded to metals the electron density is changed across the relevant proton environments but by that much?

Thanks a lot


texts: How can decreasing in temperature indicate an endothermic reaction?

texts:
I’m really wondering how can decreasing the temperature refer to an endothermic reaction.

I faced this when I was calculating the heat of a reaction Q in a constant-pressure calorimeter.

What I believe is that when something becomes cooler, then it has released some amount of heat.


texts: Why is (1E,3Z,5E,7E,9Z,11E)-cyclotetradeca-1,3,5,7,9,11-hexaen-13-yne not antiaromatic?

texts:
The compound shown below was synthesized by Professor I M Dimm who was astonished to find that it was exceedingly stable. Prof. Dimm had expected world renown for making what he thought would be an unstable compound. Dimm reasoned that with six double bonds and a triple bond in the molecule there would be 16 π electrons and thus, no aromatic character ($4n, n=4$, not $4n+2$). Where was his reasoning wrong?

My reasoning so far:
Two of the electrons in the triple bond are not part of the conjugated π system. They are in a π orbital that is perpendicular to the rest of the p orbitals that make the conjugated π system. The conjugated π system thus only contains 14 electrons. $14 = 4n + 2$ for $n= 3$. So this molecule is aromatic.


texts: Why shouldn't Uranium come in contact with water?

texts: In the documentary about Chernobyl, it was mentioned that Uranium should not  come in contact with water, otherwise an explosion occurs. What is the reason for that? What kind of reaction makes it explode?


texts:
What is the explanation of barreled spirits (at 62.5% ethanol) that undergo changes in concentration of ethanol. Whereas the volume after aging is typically reduced, the proof varies between low 100 to low 140 (50-70%). Does the presence or formation of azeoptopes play a role?
I contend that the barrel interface is important and not so much the differences in evaporation, that the industry states: Angel's Share and Devil's Cut.


texts: How to disperse salicylic acid in sodium lauryl sulfate solution?

texts:
Anti-dandruff shampoo contains salicylic acid as an active ingredient, but when salicylic acid powder comes in contact with a base solution such as sodium laurylsulfate and water (1:3 wt/wt) caking of the particle occurs, forming a non-dispersible aggregate.

I know from the study of pharmaceutics that decreasing the contact angle between particles and solution leads to a decrease in surface tension and thereby dispersion of the particles.

How can one disperse the salicylic acid particles specifically in this base solution?


texts: How to calculate the enthalpy change from the reaction of silver nitrate and sodium iodide?

texts:
When $\pu{20.00 mL}$ of $\pu{1.00 M}$ $\ce{AgNO3}$ solution is added to $\pu{20.00 mL}$ of $\pu{1.00 M}$ of $\ce{NaI}$ at $\pu{25 ^\circ C}$ in a calorimeter, a white precipitate of $\ce{AgI}$ is formed. The temperature of the aqueous mixture increases to $\pu{40 ^\circ C}$.

I am trying to calculate the $\Delta H$ for the reaction per mole of $\ce{AgI}$.
The specific heat of the aqueous mixture is $\pu{4.184 J//g K}$, the density of the mixture $\rho = \pu{1.00 g//ml}$. And I assume that the calorimeter absorbs a negligible amount of heat.

How am I suppose to go about finding the $\Delta H$ per mole of $\ce{AgI}$?
I don't really know where to start here as I am unclear about the whole problem here. In order to use the formula
$$Q = m C_p \Delta T$$
I need to find the mass. But which mass do I take? $\ce{AgI}$, or $\ce{NaNO3}$,  or is it the mass of the entire product of the reaction?

This is the balanced equation
$$\ce{AgNO3 + NaI -> AgI + NaNO3}$$


texts:
With regard to the above professor's claim, I disagree. Bent's rule, which states that atomic p-character is concentrated in orbitals directed toward more electronegative substituents, implies that the $\ce{Br-C-Br}$ bond angle is going to be smaller than 109.5 degrees since the "$\ce{sp^3}$" hybrid orbitals involved in C-Br bonding have more p-character than 75%.

Therefore, the $\ce{H-C-H}$ bond angle should be bigger than 109.5 degrees.

Questions

1) Does Bent's rule work here (i.e. am I correct?)

2) Does Bent's rule always work?

Revision

HCH bond angle is ~110 degrees.
HCBr bond angle is ~108 degrees.
Br-C-Br bond angle is 112 degrees.

Rationales

vdW repulsions play a role in Br-C-Br bond angle expansion, although the amount of p-character in the C-Br bonds might be >75%.
HCH angle is expanded because of Bent's rule; s-character in the H-C bond exceeds 25%.
The expansion of the Br-C-Br bond angle and the expansion of the H-C-H bond angle squeezes together the remaining H-C-Br bond angle.


texts:
I am in the process of seasoning a cast iron pan. (Creating the black non-stick coating typical of such pans.)

This is done by causing thin layers of flaxseed oil to polymerize and bond to the iron.

In the literature there is the recommendation that the polymerized oil bonds better to the pan, if the pan is covered with magnetite rather than blank iron, in part due to the porous nature of the oxide layer.

My question is: is there a DIY method that can either create $\ce{Fe3O4}$ on a polished cast iron pan, or alternatively convert a layer of $\ce{Fe2O3}$ into $\ce{Fe3O4}$?

In the gun manufacturing trade a process called "rust bluing" is used. In it the gun barrel is encouraged to form a thin layer of $\ce{Fe2O3}$ before being immersed in boiling distilled water. This causes the $\ce{Fe2O3}$ layer to convert into $\ce{Fe3O4}$ within minutes. Apparently due to the fact that boiling water lacks dissolved oxygen.

When I attempt to apply this "rust bluing" process to a cast iron pan, the $\ce{Fe2O3}$ layer stays red rather then turning black: Clearly the conversion is not taking place.

I am not clear why this is the case. Is it due to difference in materials, i.e. cast iron vs. gun steel?

Is there any way I can promote creation of/conversion to a magnetite layer on cast iron with DIY-compatible means? The industrial solution of caustic soda solution at 150$^{\circ}$C does not sound like a good idea for home implementation.


texts: How to prepare a Gaussian input file for a binary solvent?

texts:
The UV absorption energy spectrum of a molecule has been recorded experimentally using binary solvent (DMSO:Water=9:1 v/v). I am trying to calculate the absorption energy of the same molecule using computational methods. For that I want to give the same solvent mixture (DMSO:Water=9:1 v/v) in my DFT calculation.

How can I prepare the input file for Gaussian? What are the keywords that are used in Gaussian for binary solvents?

I have calculated the dielectric constant of the binary solvent (DMSO:Water=9:1 v/v) using the following expression
$$\epsilon_\mathrm{m}= x_1 \epsilon_1 + x_2 \epsilon_2,$$
where $x_1$, $x_2$ are the mole fractions of each solvent in the binary mixture, subscripts $1$ and $2$ denote the different solvents, and $\epsilon_1$/$\epsilon_2$  their dielectric constants.


texts: Molar heat of reaction for water in a neutralization reaction

texts:
I have a simple lab problem:

$30\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{H2SO4}$ reacts with $50\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{NaOH}$ in a neutralization reaction as per the following equation:

$$\ce{H2SO4 + 2NaOH -> 2H2O + Na2SO4}$$

Initial temperature of reactants was $22.0\ \mathrm{^\circ C}$ and final solution temperature was $30.5\ \mathrm{^\circ C}$.

What I CAN solve is the molar heat of reaction for $\ce{NaOH}$, which I found to be $-56.8\ \mathrm{kJ/mol}$. I found that by using:  $\mathrm dHm = Q/n = mc\left(\mathrm dT\right) / n$, where $n$ = moles of $\ce{NaOH}$.

$n = 1\ \mathrm{mol/L} \times 0.05\ \mathrm L = 0.05\ \mathrm{mol}$

Then $\mathrm dHm = \left[ \left( 50\ \mathrm{ml} + 30\ \mathrm{ml} \right) \times 4.19\ \mathrm{J/\left(g\ ^\circ C\right)} \times 8.5\ \mathrm{^\circ C} \right] / 0.05\ \mathrm{mol} = -56.8\ \mathrm{kJ/mol}$ since the reaction is exothermic.

The trickier part is when it asks me what the molar heat of reaction for water is in this neutralization reaction. I know the general enthalpy is just $mc\left(\mathrm dT\right) = 2.84\ \mathrm{kJ}$. But then what do I do with this value to find the molar heat of neutralization for water? I have to divide by the number of mols of water as per the equation above, but I don't know which number to plug in? I need explanation, please.


texts: I'm trying to figure out the isomeric products of the radical monobromination of bicyclo[2.2.1] heptane. I'm not sure at which site the bromination would happen and what makes one site better than the other (how can one determine this without given the reaction enthalpy ?). I was also given the hint that there are stereoisomers but I can't think of one.


texts:
This reaction supposedly passes through an epoxide intermediate. Can someone please explain why this is necessary?

Source


texts: Brown stains on skin after working with Tollens' reagent

texts:
I was working in the lab preparing Tollens’ reagent. When I came back home, I observed that my fingertips were brown, and I tried to wash it with soap several times but it didn't work.

It has been a day and they don't seem to go.

What may have been the reason? Is it permanent and how can I get rid of it?


texts: Which has one monochlorination possibility?

texts:
Which would have one monochlorination possibility?

2,2-dimethylpropane:

or 2-methylpropane:

It seems I'm missing something because I thought both would only have one monochlorination possibility. (unless I am allowed to substitute the additional H on the 2-methylpropane with a Cl?)


texts: where will be highly electronegetive atom placed?

texts: consider a molecule PCl3F2 here F is more electronegetive than Cl, which of them should occupy axial position??when i searched on internet i found these two images , they increased my confusion instead of solving my problem.can anyone breifly explain me the which one would be on axial position(F or Cl).


texts: How do bonds form without violating the second law of thermodynamics

texts: It might be obvious that entropy related to reactions always increases and I understand (somewhat) why this happens. My question is why if you have two hydrogen atoms in a vacuum without being bonded to a molecule they would form a bond. Does that not show a movement from chaos to order and thus a decrease in entropy?


texts: How to set up an equation to solve a rate law computationally?

texts:
I am confused on how to set up my equations to use in the solver. I have an equation
$$\ce{A + 2B -> C}$$
As a given, it is first order in A and second order in B. What I need to do is to solve for the concentrations of all three of $\ce{A}$, $\ce{B}$ and $\ce{C}$ using an ordinary differential equation (computationally in Matlab). The rate constant is given as $\pu{8.5E-4}$. I am given all initial concentrations ($\pu{1.2 mol L-1}$ for both).

To start off I have
$$r = k\ce{[A][B]2}$$
I have no idea where I should take this next, I feel completely lost here.

The second part of this is to do the same thing, but instead of being irreversible, the reaction is given as reversible.
$$\ce{A + 2B <=> C}$$
The concentrations are all given as are the forward and backward rate constants. Its first order in $\ce{A}$, 2nd in $\ce{B}$, and first in $\ce{C}$. $\ce{[A]} = \pu{1.2 mol L-1}$, $\ce{[B]} = \pu{1.8 mol L-1}$, $\ce{[C]} = \pu{0.4 mol L-1}$, $k_\ce{for} = \pu{3.4E-2}$, $k_\ce{back} = \pu{1.2E-2}$.

Again my initial thoughts are
$$r = \frac{k_\ce{for}}{k_{back}} \times \frac{\ce{[A][B]^{2}}}{\ce{[C]}}$$
But again I just don't know where to go next.

I hope I made my question understandable, if not I'll try to clarify anything. Any and all help would be appreciated, I really need it!


texts: Why is cysteine more acidic than serine?

texts:
How is that possible that the -I effect of $\ce{-SH}$ is greater than $\ce{-OH}$ group?

In case of cysteine $\mathrm{p}K_\mathrm{a}$ value of $\ce{-COOH}$ is $1.96$, whereas in case of serine it is $2.1$; a similar trend can be observed in case of $\mathrm{p}K_\mathrm{a}$'s of $\ce{-NH3+}$ group.

$$
\begin{array}{lccc} \hline
\text{Amino acid} & \mathrm pK_{\mathrm{a}1} & \mathrm pK_{\mathrm{a}2} & \mathrm pK_{\mathrm{a}3} & \mathrm{pI} \\
\hline
\text{Serine} & \color{red}{2.21} & \color{red}{9.15} & - & 5.68 \\
\text{Cysteine} & \color{red}{1.96} & \color{red}{8.18} & - & 5.07 \\ \hline
\end{array}
$$


texts: What is the state of aggregation of iodine formed during electrolysis of molten nickel(II) iodide?

texts:
Assume the electrolysis of molten $\ce{NiI2(l)}$ (inert electrodes) in a cell. The cell with a heating element is in the room; the room is at $\pu{25 °C}$ and $\pu{101.3 kPa}.$

The half-reaction for oxidation at the anode would be

$$\ce{2 I^-(l) -> I2(\text{state}) + 2 e^-},$$

whereas the global reaction will be

$$\ce{NiI2(l) -> Ni(s) + I2(\text{state})}.$$

Given that iodine is solid at room temperature, should it be marked as a solid in the equations? However, given that the $\ce{NiI2}$ is liquid, I'd think that the newly synthesized iodine would be a gas for a while?

Also, what observations could one make during the electrolysis? I.e. will the iodine accumulate on the positive electrode as a solid, or will it exit the cell in the form of a gas?

Also, is the reduction potential for

$$\ce{I2(s) + 2 e^- -> 2 I^-(\color{red}{aq})}$$

the same as for

$$\ce{I2(s) + 2 e^- -> 2 I^-(\color{red}{l}) }?$$


texts: How to measure concentrations using emission spectrum?

texts:
Imagine I am using emission spectroscopy to identify the presence of ions in a solution.

Would it be possible to use the intensity of the spectrum to find the concentration of each ion in the solution?

Probably the intensity is directly proportional to the concentration but there might be some scaling factor that I would need to know to find precise values for the concentration of each ion?