Question

Vapor Pressure and Heat of Vaporization

a. The vapor pressure of acetone at 20° C is 185.5 mm Hg, and the vapor pressure of water at this same temperature is 17.5 mm Hg. Predict which of the two liquids will have a higher normal boiling point. Use a molecular point of view to explain your prediction.

b. Use data from the table below and make a graph of the lnP versus the reciprocal of absolute temperature. Use the slope from a linear regression fit and determine the heat of vaporization. It should be about +42.8 kJ per mole.

P in mm Hg	1	10	40	100	400
T in °C	-31.3	-2.3	19.0	34.9	63.5

Answer 1

The normal boiling point of a liquid is that temperature at which its vapor pressure becomes equal to the atmospheric pressure.

Acetone with a higher vapor pressure will require a lesser amount of energy than water for its vapor presure to become equal to the atmospheric pressure.(See Figure)

Hence the boiling point of acetone will be lesser than that of water.

REASON FOR THE LOWER VAPOR PRESSUR OF WATER.

At a given temperature the vapor presure of a liquid depends on ihe tenedency of its molecules to escape from the liquid phase and come into the vapor phase.

The escaping tendency of the molecules will in turn depend on the intermolecular attaction

The greater the inter molecular attraction the lesser the escaping tendency

In water,the intermolecular attaction is very strong because of the hydrogen bonding between the molecules. The  intermolecular attaction is less in acetone( There is only dipole-dipole attraction but no hydrogen bonding )

This explains the lesser vapor pressure and hence the higher boiling point of water