# Tag Info

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Mathew Mahindaratne has provided analysis based on experimental values of the boiling points of the two compounds. I would like to offer a different view using bonding analysis. Before I begin tackling the question, we shall first clarify the concept of the hydrogen bond. While the popular view of the hydrogen bond is as a particularly strong type of "...

9

The boiling point of non-ionic compounds are highly depend on their H-bonding abilities. For example, boiling point of water (molar mass: $\pu{18.02 g/mol}$) is $\pu{100 ^\circ C}$ at $\pu{1 atm}$ while that of ethanol (molar mass: $\pu{46.07 g/mol}$) is $\pu{78.4 ^\circ C}$ at $\pu{1 atm}$, even though ethanol is heavier and have more other intermolecular ...

9

In Principles of Nucleic Acid Structure, W. Saenger argues that hydrogen-bonded bases contain at least two hydrogen bonds (forming a "cyclic" pattern). Often, there is a tautomeric form possible that also makes two hydrogen bonds, with two covalent bonds turning into hydrogen bonds, two hydrogen bonds turning into covalent bonds, and double bonds moving ...

7

I think you have the causality backwards here. It's not as if we have the vibrations to start with and they happen to be matched by the eigenvectors of the Hessian. We choose to look at these particular vibrations precisely because they are the eigenvectors of the Hessian. Consider the example of water. We know that it has it has $9$ degrees of freedom, ...

7

To start off, you definitely need to use dispersion-corrected density functional methods if you want to obtain reasonable non-bonded interaction parameters. I've seen many cases where B3LYP (with no dispersion) yields a fully-repulsive potential energy scan. I also agree with your assertion about doing a rigid scan. In principal, a relaxed scan is a good ...

6

It does not have much to do with entropy, rather with the way and place of ozone creation. Stratospheric ozone is produced typically at altitude 20-30 km by UVC radiation with $\lambda \lt \pu{280 nm}$: $$\ce{O2 + \nu -> 2 O}$$ $$\ce{O + O2 -> O3}$$ See ozone cycle as courtesy of @M. Farooq. Ozone in lower troposphere troposphere is created by UVB (...

6

The three reasons that you present are not sufficient to explain the high boiling point of pure $\ce{HF}$. There is a fourth reason, that you have not mentioned : the Hydrogen bond. Molecules $\ce{HF}$ are interacting with one another through $\ce{H}$-bonds, exactly like $\ce{H_2O}$ molecules attract one another because of $\ce{H}$-bonds. In the liquid state,...

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The upper limit are typically optical light frequencies. That is where the very last plateau is reached in dielectric spectroscopy. The electron shell becomes purely elastic. I wrote a bit more about DES in my answer here: Difference between relaxation and resonance leading to an absorption spectral feature?

3

You are correct in your ideas. The effect is important in the van-der-waals interaction and appears as an additional $1/r^7$ term compared to the usual $1/r^6$ term. (This is sometimes called a retardation effect). It is present only at larger distances (a few nm) and arises only in induced-dipole interactions (dispersion interaction) because of the time it ...

3

Can anyone explain why the exchange contribution to the total energy is negative? I find it misleading that exchange interaction is treated as something that changes total energy of the system. This lowering of energy is actually due to the Hartree-Fock scheme being in principle inexact, and is not really specific to indistinguishable particles. Let me ...

3

I'll try to give my interpretation of the "physical" explanation of why exchange would lower the energy. For the true wavefunction, the motion of all the electrons should be correlated, with the classical view being that the electrons are avoiding each other to minimize repulsion. With Hartree-Fock, we find an approximate wavefunction by solving for 1-...

3

I have been watching this question with keen interest, but wanted to let someone else go for the bounty -- However it expires tomorrow and no one has answered, so I will give my perspective. Why does the exchange interaction in Hartree-Fock theory lower the total energy? The answer is given in the question: This integral is always positive, and so the ...

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I don't know if anyone is still looking for the answer but here I go anyway because I spent 40 minutes researching this for an assignment TL;DR - The better solubility of potassium salts is the key factor, but not in the way one would initially suspect. Industrial processes and the efficiency of large-scale soap making explains the choice. The reason why ...

3

I found a nice figure and the relevant statement in a paper by Frenking and Krapp (Unicorns in the world of chemical bonding models, 2006, https://doi.org/10.1002/jcc.20543): The crucial term which is responsible for repulsive interactions in chemical bonds except in two‐electron systems such as H2 is the Pauli repulsion. The three terms (a) ...

2

Anton and its successor Anton2 are highly specialized pieces of machinery. In fact, the computer, the chip, and the only code that can run on the chip are all called Anton. There's a reason why the only Anton computer outside of D.E. Shaw is located in the Pittsburgh Supercomputing Center (Anton now deprecated there, replaced by Anton2): this highly ...

2

Let me crash the party here. TL;DR: Dispersion interactions are not due to fluctuating induced-dipole attractions, even if everyone tries to explain it that way. (I know that this must seem like an outrageous statement, and surely downvote fingers are itching now. Bear with me.) Based on the Hellmann-Feynman theorem, it is known that the forces acting on ...

2

You are correct that simply stating that the Schrodinger equation is time dependent does not really answer the question. In fact, time-dependence is not a component of the derivation of the dispersion forces. At a surface level, they just represent how much the ground state energy of a system of polarizable molecules is lowered by their interactions. It's ...

2

Assuming that by air bubble you're referring to the pocket of air now sealed in the bottle above the water surface: Immediately after sealing the bottle, the air pocket is not at all compressed: it's at the same pressure as the surrounding environment (the air outside the bottle). However, because you have a closed container, some water will evaporate, ...

1

Technically, the concentration and positioning of ozone in the atmosphere likely relates directly to both ozone formation, stabilization and a destruction cycle. While formation creation cycles has already been outlined, I would like to add more detail on the chemistry based on this source: Creation: $\ce{O2 + ℎν → 2 O• }$ $\ce{O• + O2 → O3 }$ which is ...

1

The primary thing to realize is that, although the ozone molecule has a bent structure, it is a homonuclear molecule. This means that between the individual oxygens, the bond moments being formed are quite less, as inherently, both the oxygens have the same Pauling electronegativity values. Only a slight difference in the same will occur, due to the ...

1

You may consider the vdW equation as the deviation from the ideal gas state equation $pV_\mathrm{m} = RT$. If you look at the vdW equation $$\left(P+\frac a{V_m^2}\right)(V_m-b)=R T$$ you can easily see why the corrections have their signs. The pressure term correction has the positive sign, as the real gas has due molecule cohesion smaller volume ...

1

The Hamiltonian for the collection of charged particles (electrons and nuclei) comprising a pair of molecules or ions has terms describing the kinetic energy of individual particles and pair-potential terms describing Coulombic interactions, that is, all interactions between particles are Coulombic. Where a Pauli repulsion term can crop up is when ...

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Reason 1: For example if you have an porous ore and you melt it, the surface area will decrease dramatically. If you just roast it, the surface area will stay about the same. A higher surface area will aid calcination proceses as more oxygen gets into contact with the surface and will eventually react. Reason 2: A higher temperature means more energy to ...

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Some general comments on the current use of the VDW equation in both teaching and research (added as an answer at the OP's suggestion): The OP writes: "[VDW] is only still around for historical purposes, as it is largely inaccurate." That's not quite right—it's like concluding that, since the ideal gas law is inaccurate, it must only persist for historical ...

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I think we are thinking too chemically, i.e., too basically. From my (limited) experience, soft soaps contain more water. The correspondence of viscosity or some other measure of strength or hardness with water content is not exactly identical for the sodium and potassium soaps, but it is fairly close. If you make a potassium soap with low water content, it ...

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