Vapor Pressure and BP

[September24]

So good ol Berkeley Review talks about vapor pressure in a way being independent from boiling point. "Boiling point depends on atmospheric pressure, but do not confuse boiling point and vapor pressure."

I know that vapor pressure depends on temperature and delta H of vaporization. However, where does BP come into play with VP. Berkeley says vapor pressure decreases as BP increases but I'm having a bit of confusion figuring out why. It seems intuitive but I'd like to know for sure.

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[monkeyvokes]

boiling point is a colligative property and as such depends on the amount of solutes added. Adding solutes raises the boiling point. This makes sense because if you add solutes they spread out evenly, some even on the surface of the solvent. If you have less molecules of solvent on the surface then they are less likely to float off into the air (evaporate), lowing your vapor pressure. (the solution is also stabilized for other reasons which also decreases the likelihood for evaporation.)

 

[eudovcic]

In EK they define (not exact words) BP as P(vapor)=P(atm).

The way I think of it is simply if vapor P is increasing then more molecules from the surface of the liquid have evaporated thus increasing the frequency of molecular collisions with the wall of the container or the v(rms) of the molecules has increased also increasing the frequency of molecular collision. In both cases it would take less heat in order to change the state of the liquid thus the BP is lowered. Conversely, if vapor P is lowered then that means there are less molecular collisions with the wall and some of the molecules have returned to the surface of the liquid so more heat is required to give these molecules enough KE to escape so BP is increased.