Boiling/freezing point of solution

[brlin]

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The question was, which of the following have the highest boiling point? See the image below:

Edit: here's a direct link in case the inserted image isn't showing up: http://s14.postimage.org/m2cfri60x/Untitled.png

There were 2 ways I was thinking about this:
1) Greater molecular mass of the solute = higher boiling point
2) Stronger acid = more dissociation of ions = more # of solute = higher boiling point

It turns out the 2nd rational was correct and 1.0M trichloroacetic acid has the highest boiling point (because it is the most acidic).

Does acidity always have a greater impact on boiling point / freezing point than molecular weight? Or is this true only up until a certain point?

 

[SKaminski]

Hey,

While i may not be able to answer your question 100%, I do have some extra insight for you.

Carboxylic acids (like the first 4 options) are able to form dimers in the water, with two carboxylic acids double hydrogen bonding to each other. This VASTLY increases the boiling point. From their, the issue becomes between 1) and 2), and i can't tell you how to prioritize that.

 

[akim6890]

While that rationale works for this example especially since they give you the pKa's, it is better to base boiling point off attraction forces between molecules. Hydrogen bonding is the strongest intermolecular attraction, then dispersion forces (the bigger the molecule, the more dispersion forces), then polar groups (the more polar, the higher the boiling point), then symmetry (the more symmetrical, the less surface area for intermolecular attraction, the lower boiling point.

So for your example, the carboxylic acids have the most hydrogen bonding so eliminate the rest. From there, the molecule with the most polar bonds (think largest dipole moment) will have the highest boiling point. Thus, trichloroacetic acid is your answer.

 

[ChemTutor]

...
2) Stronger acid = more dissociation of ions = more # of solute = higher boiling point

...

This only works for a solution of a chemical. As far as I can tell, here, they ask about the boiling point of a pure chemical.

 

[meetsub]

.

 

[brlin]

Thanks for the input guys. Especially with the observation of the polarity. Usually I use bond strength (I default to H bonding) to look at BP and freezing point, but for some reason I completely neglected to consider bond strength at all when I was doing this.

 

[brlin]

This only works for a solution of a chemical. As far as I can tell, here, they ask about the boiling point of a pure chemical.

I should has posted the whole problem. I believe they were comparing 1M solutions of a few of them

 

[ChemTutor]

I should has posted the whole problem. I believe they were comparing 1M solutions of a few of them
....
1) Greater molecular mass of the solute = higher boiling point
2) Stronger acid = more dissociation of ions = more # of solute = higher boiling point
....

if all are solutions then 1) is irrelevant, we only should use 2) to decide
but it is actually more complicated with carboxylic acids because as somebody wrote above they form dimers.

are some of these solutions of different concentrations?

 

[brlin]

They had these 4 options for this question:

A. 1.0 M acetic acid has a higher boiling point than 1.0M p-nitro benzoic acid.
B. 1.0 M trichloroacetic acid has a higher boiling point than 1.0M p-nitrophenol (P-O2NC6H4OH).
C. 1.0 M trichloroacetic acid has a higher freezing point than 1.0 M benzoic acid.
D. 1.0 M acetic acid has a higher freezing point than 1.0 M phenol.

 

[ChemTutor]

I thought about this some more.

We should ignore the dimers because in water they won't exist to any considerable extent. So it boils down to which molecule creates more ions in water. Lower pKa wins so it is B

 

[drechie]

so why is "2) Stronger acid = more dissociation of ions = more # of solute = higher boiling point" true?