Boiling Point and Pressure on top of liquid

[77deuce]

Let us say that we have two compounds with different boiling points in a fractional vacuum distillation.

As the pressure above the liquid decreases, the BP of both compounds decrease and the difference in BP between two compounds in the solution becomes smaller.

As the pressure above the liquid increases, the BP of both compounds increase and the difference in BP between two compounds in the solution becomes greater.

Are the underlined parts of the above statements true?

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[PaulTownMD]

Generally this is the case, though not always. When looking at pressure/temperature diagrams, the pressure "exponentially" increases relative to temperature, so we'd expect small changes in BP temperature relative to changes in pressure. In a theoretical system of 0 pressure, these two compounds have identical boiling points (0 K, though is purely theoretical because we'd also have a 0-point-energy triple point, i.e. gas/liquid/solid equilibrium). As pressure increases, the difference between these BP temps becomes larger. However, many forces (London dispersion, electrostatic/conjugation, etc) have effects on the pressure/temperature curves, so it is possible to have ranges where the difference in BP temperature actually gets closer, though this is the exception and not the norm.

 

[77deuce]

Generally this is the case, though not always. When looking at pressure/temperature diagrams, the pressure "exponentially" increases relative to temperature, so we'd expect small changes in BP temperature relative to changes in pressure. In a theoretical system of 0 pressure, these two compounds have identical boiling points (0 K, though is purely theoretical because we'd also have a 0-point-energy triple point, i.e. gas/liquid/solid equilibrium). As pressure increases, the difference between these BP temps becomes larger. However, many forces (London dispersion, electrostatic/conjugation, etc) have effects on the pressure/temperature curves, so it is possible to have ranges where the difference in BP temperature actually gets closer, though this is the exception and not the norm.

Thank you for the answer!

Has anyone seen questions that test this concept come up in their prep/ actual MCAT?

 

[PaulTownMD]

It's fairly likely you'll get a question on this, being that it incorporates topics from general and organic chemistry. Know the properties of azeotropic mixtures too

 

[77deuce]

It's fairly likely you'll get a question on this, being that it incorporates topics from general and organic chemistry. Know the properties of azeotropic mixtures too

Thank you