Chapter 5, Problem 5A.1AST

Interpretation Introduction

Interpretation:

Among tetrabromomethane and tetra chloromethane, the one which is having higher vapor pressure at room temperature has to be explained.

Concept Introduction:

Vapor pressure:

The vapour pressure of a substance is the pressure exerted by its vapor, when the vapor is in dynamic equilibrium with the condensed phase. At equilibrium, the rate of vaporization is equal to the rate of condensation and neither vaporization nor condensation is spontaneous.

Explanation of Solution

The attractive forces between molecules are known as intermolecular forces.  Stronger the force, stronger will be the attraction between the molecules.  Boiling of a compound breaks the intermolecular forces, so stronger the force higher will be the boiling point and lower will be the vapor pressure.  That is higher the vapor pressure, lower the boiling point and weaker the intermolecular force.

The force that exist in tetrabromomethane and tetra chloromethane is London dispersion force.  But in ${\text{CCl}}_{\text{4}}$, there exist weak London force, therefore force of attraction is less and the vapor pressure will be high.