[youngsterjoey19]

Hi All! I was looking at some practice AP questions and found the following:

Neon, ammonia, carbon monoxide, and methylamine (CH3NH2) all have low molar masses. Place these substances in order of increasing boiling points.
Ne < CO < NH3 < CH3NH2
NH3 < CO < CH3NH2 < Ne
Ne < CO < CH3NH2 < NH3
CH3NH2 < CO < NH3 < Ne

The answer was given as A because Neon has only london dispersion forces, CO also has dipole-dipole interactions, and the other two have hydrogen bonding in addition to LDF and dipole. But the methylamine has more LDF because it has more electrons and is more polarizable. Great. I would have gotten that correct. But then I run across this question:

A solution of ethanol (C2H5OH) dissolved in water is left open to the atmosphere for one week. Which statement is most correct?
a. The concentration of ethanol does not change because the hydrogen bonds between ethanol and water are strong.
b. The concentration of ethanol increases because water is more volatile than ethanol.
c. The concentration of ethanol decreases because ethanol is more volatile than water.
d. The solution changes in volume, but not concentration, because the two substances evaporate at an equal rate.

Now using that same logic, both ethanol and water have hydrogen bonds, but ethanol has more electrons so shouldn't it have higher intermolecular forces? I know it doesn't, both because the answer was given as C and because I know that alcohol evaporates quickly and has a strong smell from life experience. And the justification for this answer was given as "water can hydrogen bond twice as much as alcohol." But how am I supposed to know that? On the first question, should I have taken into account how many sites for H-bonding there are? When do I use this "more electrons" justification and when should I not? Thanks!

[allyson10500]

London Dispersion forces are the weakest type of intermolecular interaction, so only consider them when all else is equal (as in the first case). Hydrogen bonding is a very strong interaction, as physical bonds are being made, so when there is a difference in hydrogen bonding ability, that will always win out.

[Enthalpy]

CO has only 0.122D dipole moment. Nearly the same boiling and melting points as N₂, which differs by the distribution of one proton. Why, I don't know.