Higher BP = Lower Pv

[MedPR]

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Pv = vapor pressure.

I've had this memorized for a while now, but after doing a passage I realized that I don't know why. And I would like to know, just in case it becomes necessary.

If compound A has a lower BP than compound B, then you know that compound A is also more volatile, has a higher Pv, and has a lower heat of vaporization. I understand the volatility and heat of vaporization, but I don't understand why the vapor pressure is lower.

Vapor pressure is the pressure exerted on the liquid by the evaporated vapor directly above it, right? So if the BP is lower, that means that at any given temperature, there are more moles of gas compared to something with a higher BP. Wouldn't more moles of gas exert a greater pressure (more molecules = more collisions per unit area = more pressure) than less moles of gas?

 

[mcloaf]

If compound A has a HIGHER BP, then it must be LESS volatile and have a LOWER Vp and HIGHER heat of vaporization.

Your last paragraph is right on, so I'm not sure from where you got the stuff above. You can also think about it terms of boiling being defined as when a liquid's Vp equals atmospheric pressure. If a liquid boils at a lower temperature, then it must be producing higher vapor pressure at a given temperature, which allows it to be equal to Patm at a lower temp.

 

[MedPR]

If compound A has a HIGHER BP, then it must be LESS volatile and have a LOWER Vp and HIGHER heat of vaporization.

Your last paragraph is right on, so I'm not sure from where you got the stuff above. You can also think about it terms of boiling being defined as when a liquid's Vp equals atmospheric pressure. If a liquid boils at a lower temperature, then it must be producing higher vapor pressure at a given temperature, which allows it to be equal to Patm at a lower temp.

Thanks, I think I confused myself with my example.

I just reread that last paragraph and I answered my own question.. Not sure what I was thinking 🙂

Thank you.