H bonding and boiling points

[pch]

Hello, I encountered this question in my practice:

Which of the following compounds has the highest boiling point?
a) CH4
b) NH3
c) H2O
d) HF

I understand that 3 things can influence boiling point. H bonding, molecular weight and branching of the molecule. In this question, I know (a) isn't correct because it has no H bond capacity but I'm wondering how you'd determine which is has the highest boiling point between b, c and d since they're all capable for H bonding.

Thank you!

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[laczlacylaci]

Yes, you are correct that Hbonding has to do with bp. (Can bind to FON) The answer would be C. And H2O>NH3>HF>CH4. You will need to analyze how much H binding can each molecule do. CH4=none; HF=2(H can bind to FON, F can receive one H) NH3 (potentially can bind 4) H2O can bind 4. Also H2Os molar mass is a gram heavier than NH3. So H2O would be the highest.

 

[laczlacylaci]

I am actually not sure if NH3 can do 4 H bonds.. any clarifications would be appreciated. I'm assuming it can bc H bonding is not really binding (sharing e-) and N can become NH4+ anyways

 

[aldol16]

I am actually not sure if NH3 can do 4 H bonds.. any clarifications would be appreciated. I'm assuming it can bc H bonding is not really binding (sharing e-) and N can become NH4+ anyways

NH3 has one lone pair and three N-H bonds, so it can donate three H-bonds and accept one. Water has two O-H bonds and two lone pairs, so it can donate two H-bonds and accept two.

 

[laczlacylaci]

NH3 has one lone pair and three N-H bonds, so it can donate three H-bonds and accept one. Water has two O-H bonds and two lone pairs, so it can donate two H-bonds and accept two.

So between NH3 and H2O. H2O has a higher bp because it has a higher molar mass?

 

[aldol16]

So between NH3 and H2O. H2O has a higher bp because it has a higher molar mass?

Well, the difference in molar mass (1 atomic mass unit) is insignificant. The key here is to look at what I said above - water can donate 2 hydrogen bonds whereas ammonia can only donate one. This means that each water molecule can donate a hydrogen bond to 2 other water molecules. In ammonia, there's only one lone pair so the nitrogen can only donate 1 hydrogen bond. That means that each ammonia molecule can donate a hydrogen bond to only one other ammonia molecule. In other words, take a group of 3 ammonia molecules. You can only ever have 3 hydrogen bonds formed because you need the same number of donors as you have acceptors. In a group of 3 water molecules, you can have many more hydrogen bonds than that depending on the different allowable spatial orientations of the water molecules (e.g. water 1 donates a H-bond to water 2 which donates a H-bond to water 3, which then accepts a H-bond from water 1, etc.).

Another way to look at it is simple math. You need to have the same number of donors as acceptors to have a hydrogen bond (you can't donate unless there's something to accept). Now, each water molecule has 2 donors and 2 acceptors. That's good because it maximizes the number of H-bonds you can have. However, ammonia has 1 donor and 3 acceptors per molecule. That means that the donor is the limiting factor and even if you have two ammonia molecules, you would only ever be able to have 1 hydrogen bond.

 

[pch]

Thank you!! I wish the answer key would explain as clearly as you have here. Very much appreciated!

 

[BerkReviewTeach]

I am actually not sure if NH3 can do 4 H bonds.. any clarifications would be appreciated. I'm assuming it can bc H bonding is not really binding (sharing e-) and N can become NH4+ anyways

You are correct to feel uncertainty here, but not because NH4+ forms (given that there is no source of H+ in pure NH3).

In a solution of pure NH3, there is one lone pair and three Hs per N, so there will be a shortage on lone pairs, resulting in 2/3s of the Hs not having a H-bond. So in a pure NH3 solution, there would be an average of 2 H-bonds per NH3 molecule.

Water, on the other hand, has two lone pairs and two Hs, so every H can form an H-bond. In a pure H2O solution, there would be an average of 4 H-bonds per H2O molecule. As a result, water has an extremely elevated boiling point relative to compounds of comparable mass (like NH3, HF, and CH4). The relationship is H2O >> NH3 > HF > CH4.

 

[wizzed101]

I am pretty sure that Bp of H20 > HF > NH3 ...

 

[BerkReviewTeach]

I am pretty sure that Bp of H20 > HF > NH3 ...

You are correct.