How does vapor pressure relate to intermolecular forces?

1 Answer
Mar 17, 2018

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At any given temperature, molecules in a liquid are bumping into each other with a lot of kinetic energy. If those molecules have enough kinetic energy, they'll escape the liquid and go into the gas phase above the liquid. This gas exerts a pressure, and this pressure is called the vapor pressure.

The only thing that prevents these molecules from escaping (assuming approximately the same molecular weight) is the intermolecular forces the molecules possess.

Acetone is a compound with no hydrogen bonding, and only polarity (along with London forces). At 25C, acetone has a vapor pressure of 0.3atm.

Isopropanol is a compound with almost exactly the same molecular weight at acetone, but it has Hydrogen bonding, polarity, and london forces. At 25C, isopropanol has a vapor pressure of 0.057 atm.

Water is a compound with extensive hydrogen bonding (as well as polarity and london forces). It is smaller in mass than the other two. At 25C, water has a vapor pressure of 0.031 atm.

Vapor pressure is strongly related to intermolecular forces.