30 votes

Why is the boiling point of fluorine lower than that of oxygen?

It would be tempting to argue that fluorine is so electronegative and holds its electrons so tightly that their polarizability is reduced, thus so are the dispersion forces in $\ce{F2}$. But upon ...
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29 votes
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What is the exchange interaction?

In quantum chemistry, probably the easiest way to understand the "exchange interaction" is within the context of the Hartree-Fock model. $ \newcommand{\op}{\hat} \newcommand{\el}{_\mathrm{e}} \...
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29 votes
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Why is the melting point of PCl3 less than that of PCl5?

Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions ...
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23 votes
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Alkane, alkene, alkyne boiling point comparison

Disclaimer: All of this "jazz" will be about reaching a mere rule-of-thumb. You can't just compare whole families of organic compounds with each other. There are more factors to consider than below, ...
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22 votes

Why does graphite have a high melting point even though its intermolecular forces are weak?

It's not usual to consider graphite as a material composed of "molecules" in the typical sense, though it could be viewed as a kind of polymer with two-dimensional macromolecules. Regardless, it is ...
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21 votes
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Why doesn't the viscosity of water change much with temperature like it does for other substances?

This is something I spent a lot of time thinking about during my PhD, so let me see if I can explain it. What you are talking about is the relationship between macroscopically observable properties (...
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19 votes
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Can nonpolar molecules exhibit dipole-dipole forces?

Short answer: there are many electrostatic interactions between two non-polar molecules. Beyond monopole (full charges) and permanent dipole moments (polar molecules), there is a full multipole ...
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17 votes
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Help understanding how "steric effects" are distinct from "electronic effects"?

The normal distinction between "steric" and "electronic" is based on whether the effect is transmitted through space or through bonds All the normal physical interactions we ...
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14 votes
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Why is tetrafluoromethane non-polar and fluoroform polar?

Draw the structures in 3D and then you will see why one is polar and the other not. $\ce{CF4}$: As you can see this molecules adopts a tetrahedral geometry which is perfectly symmetrical in every ...
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14 votes
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Why does HCN boil at a higher temperature than NH3?

The enthalpy of vaporization of $\ce{HCN}$ is higher than for $\ce{NH3}$, which suggests that $\ce{HCN}$ molecules interact more strongly than $\ce{NH3}$ molecules. $\ce{C-H}$ bonds are not usually ...
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14 votes
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Is Hydrogen Bonding a Type of Dipole Dipole Interaction?

Well, it turns out that this is a very active area of research. I will only summarize what I understand to be true about the covalent nature of the hydrogen bond, so I'm sure the explanation could be ...
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14 votes
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How to calculate Lennard-Jones potential with quantum mechanical methods

Yes, this is technically possible. A basic tutorial for this is in the excellent Psi4Numpy project, which I'll reproduce here with minor modifications. Their example fits the counterpoise-corrected ...
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Differences between formulae for dipole–dipole interaction energy

They are both dipole-dipole energies as from the link but their contexts are different. Eq. 3 (as numbered in the LibreTexts link), $$V(r) = - \frac{\mu_{1}\mu_{2}}{4\pi\epsilon_{0}r^{3}} \tag{3}$$ is ...
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13 votes
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Why are fluoroalkyl chains hydrophobic/oleophobic?

You have identified a fairly common but counter-intuitive result. At least it's counter-intuitive based on the way the properties of fluorine are described in undergraduate chemistry classes. ...
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13 votes
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How to identify hydrogen bonds and other non-covalent interactions from structure considerations?

It is safe to say that there will always be intermolecular forces at play. At the time where you will consider these you should already have a good idea about the molecules involved in your system. ...
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13 votes

Can a long polymer chain interact with itself via van der Waals forces?

You have a possible answer to your question in proteins, an example which includes some long polymer chains. Intramolecular interactions - while not necessarily the driving force for formation of a ...
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12 votes
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How to calculate which ionic compound has the highest melting point?

The strength of the ionic bond depends on Coulomb's law for the force acting between two charged particles where larger force translates to a stronger ionic bond. The equation is $$F = \frac{-k\cdot ...
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12 votes
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Natural Bond Orbital analysis: Significance of stabilization energy determined by 2nd order perturbation

TL;DR: Lewis $\to$ Non-Lewis $\mathbf{E(2)}$ values have no direct physical significance, are intrinsically un-measurable, and serve only to quantify the extent to which the "real" wavefunction for a ...
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12 votes
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Significance of single point energy when calculating interaction energies

Single point energy arises in the framework of the Born–Oppenheimer approximation and corresponds to just one point on the potential energy surface. Physically it is the total energy of the molecular ...
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12 votes

Is an ionic bond more like a covalent bond or an intermolecular force?

An ionic bond could maybe be described as an inter-ionic force. All electron interactions are most accurately described by wavefunctions and quantum mechanics, but in practice we use successively more ...
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12 votes

Sublimation of Iodine

Iodine sublimes for the same reasons that all solids do: because it has some equilibrium vapor pressure an normal conditions. Now, the value of that pressure varies greatly in different solids. For ...
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12 votes
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Why does graphite have a high melting point even though its intermolecular forces are weak?

Graphite has got a structure similar to books stacked on top of each other. Multiple layers on top of each other and each layer going by the name graphene. Atoms in each individual layer is ...
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  • 144
11 votes
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Arrange these compounds: CO2, CH3OH, RbF, CH3Br in order of increasing boiling points

You know $\ce{CO_2}$ is gaseous at room temperature, so let's put that at the bottom. Methanol forms hydrogen bonds, so that will be above bromomethane which does not. At last we have rubidium ...
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  • 6,053
11 votes

Is viscosity proportional to the number of hydroxyl groups?

It is a reasonable rule of thumb, but certainly not always true. Compare for example the viscosities of Dodecane ($\ce{C12H26}$, $\mu=1.374 \ce{\; mPa\cdot s}$, no $\ce{OH}$ groups) and Ethanol ($\...
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  • 6,590
11 votes

How does intramolecular hydrogen bonding cause the molecules to be separated from each other?

Molecules with inter-molecular hydrogen bonds tend to associate with one another, while molecules with intra-molecular hydrogen bonds tend to associate with themselves. Ortho- and para-nitrophenol ...
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11 votes
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Why do dianions (such as malonate) bind cations more strongly than anions?

This is known as the chelate effect. The main reason why you observe this is that cations in solution have an ordered solvent shell around them, especially in polar solvents where there will be ...
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11 votes

Why are they called ‘dispersion forces’ if they are an attractice force?

Although Danny Rodriguez has already excellently exposed what the dispersion force is in simple terms, the word dispersion still demands a better explanation in my opinion. According to Wikipedia: ...
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11 votes
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How DFT-D3 incorporates coordination number (CN) into C6

I will quote the paper quite a bit, but I'll try and summarize a bit after the quotes and equations. You might want to start at the bottom and work backward, a lot of this is just for later reference. ...
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11 votes

Is a temperature decrease required for hydrogen bonds to form in water?

You are right, in a sense. A more accurate statement would be that as the temperature decreases, the average lifetime of a hydrogen bond increases. Thus, on average the liquid/solid/molecule will be ...
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  • 12.6k
11 votes

Differences between formulae for dipole–dipole interaction energy

An equivalent and easier formula when a molecule's coordinates are known is to use vectors. The energy is then $$V=\frac{1}{4\pi\epsilon_0}\left(\frac{\vec\mu_1\cdot\vec\mu_2}{r^3}-3\frac{(\vec\mu_1\...
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