[University - Chemistry] Intermolecular Forces

[sumzup]

Ok, so on a recent midterm we were asked to order 5 compounds by strength of intermolecular forces from least to greatest.

The compounds were: CCl4, CH3OH, CO, Ne, and CH4.

The way I ordered it when I took the exam was: Ne < CO < CH4 < CCl4 < CH3OH, and this turned out to be correct (and now I'm confused as to why it is; my logic at the time of the test left me right after I finished, it seems).

My reasoning: Ne only has Induced Dipole - Induced Dipole forces, so it's lowest, CH3OH has hydrogen bonds and therefore is highest, and CCl4 has a large molar mass and is therefore right below CH3OH.

What I can't figure out, though, is why CO has weaker intermolecular forces than CH4. CO has a higher molar mass, and has polar covalent bonding which lead to Dipole - Induced Dipole forces; CH4 only has Induced Dipole - Induced Dipole forces. Actually, for that matter, why is CH4 greater than Ne?

Crap, I've confused myself. Someone please enlighten me.

[tsugomaru]

You're 16 right?

Anyways, I'm stumped here too, I'm surprised they didn't ask you to give reasons why you ordered the way you had to, I know we had to when we took a test similar to this one.

Although you could logically conclude that Ne < CH4 < CO < ..., through experimentation, we may actually find CO will have weaker intermolecular forces than CH4. I do not know if this is true, but this was the reason my Chem teacher gave when we received an odd ball question like this.

~Tsugomaru

[sumzup]

I am 16, but I'm taking this course through a "do college credit while in high school" program.

And your reasoning is probably right. In real life, there are a lot of factors which ultimately result in a variation from theoretical chemistry.

[xjohnieboyx]

I may be mistaken, but I think CH4 has an intermolecular force stronger than that of CO due to the Hydrogen Bonds formed.

Sorry I'm a bit rusty on my chemistry. I'm not sure you are correct, however, that CH3OH is ranked 1 due to the hydrogen bonds. Rather, it's due to the OH at the end (called a hydroxide or hydroxyl?) Correct me if I'm wrong please.

[sumzup]

Well, the OH (hydroxyl) group at the end is what facilitates hydrogen bonds to exist. CH4 doesn't have any hydrogen bonds; hydrogen bonds form with H in the presence of either O, N, or F. But now that I think about it, CH3OH may also be #1 because the OH group makes it polar as well, which contribute to its strength; thus, even though CCl4 has a much higher molar mass (which is a factor in bond strength), CH3OH is able to easily overcome that.

Another previous chemistry teacher of mine said that she would put CH4 < CO, so I'm not sure about that.

[xjohnieboyx]

Thanks a lot for clearing that up for me sumzup. Not exactly clearing it up since i remember the NOF rule about hydrogen bonds now that you mention it . That's precisely what I meant about the hydroxyl group (polarity). I am pretty sure there's some "priority" because a hydroxyl group overpowers the strength of a high molar mass.

I'm still stumped about the CO and CH4 then.

Anybody got any ideas?

[tsugomaru]

Not a big reader of any other post but yours I guess. I already said that through real life experimentation, they may have found that CH4 had stronger bonds than CO, that's why CH4 would be > CO, but there's no way you would know that off-hand unless you walked into the test knowing it because you searched it up (who would do that), or your teacher told you so.

~Tsugomaru

[Master_of_the_Faster]

Originally Posted by xjohnieboyx

I may be mistaken, but I think CH4 has an intermolecular force stronger than that of CO due to the Hydrogen Bonds formed.

Sorry I'm a bit rusty on my chemistry. I'm not sure you are correct, however, that CH3OH is ranked 1 due to the hydrogen bonds. Rather, it's due to the OH at the end (called a hydroxide or hydroxyl?) Correct me if I'm wrong please.

http://www.newton.dep.anl.gov/askasc.../chem00933.htm

"Carbon monoxide does have a dipole moment but it is very small. C and O
are similar in electronegativity so the charge difference is quite small.

Methane has no dipole moment but has more electrons than CO does and is
more polarizable.

So even though CO molecules exhibit dipole-dipole and dispersion forces
and CH4 molecules only exhibit dispersion forces, CH4's dispersion forces
are apparently attractive enough to win the game."

[sumzup]

You are my hero! I'll remember this for the final.