Question

A liquid is in equilibrium with its vapour in a sealed container at a fixed temperature. The volume of the container is suddenly increased.
a. what is the initial effect of the change on vapour pressure?
b. How do rates of evaporation and condensation change initially?
c. What happens when equilibrium is restored finally and what will be the final vapour pressure?

Answer 1

(i) On increasing the volume of the container, the vapour pressure will initially decrease because the same amount of vapours are now distributed over a larger space.
(ii) On increasing the volume of the container, the rate of evaporation will increase initially because now more space is availble. Since hte amount of the vapours per unit volume decrease on increasing the volume, therefore , the rate of condensation will decrease initially.
(iii) Finally, equilibrium will be restored when the rates of the forward and backward processes become equal. However, the vapours pressure will remain unchanged because it depends upon the temperature and not upon the volume of the container.