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?

Question

Philoid · Accepted Answer

A. When volume is suddenly increased, the vapour pressure decreases.

Explanation: When volume is increased suddenly there is no sudden increase in number of vapour substance. Thus, same number of particle occupies larger volume and thus have lower pressure. (After some time, the number of vapour molecules increases and vapour pressure slowly rises from the decreased value)

B. At equilibrium state,

Rate of evaporation = Rate of condensation

When volume is increased, the vapour pressure decreases and so, the rate of evaporation will be greater than rate of condensation.

This can be understood by the Le Chatelier’s principle, which states that, when an equilibrium state experiences a disturbance (here pressure decrease) the system will try to cancel the effect of change in the system.

C. When equilibrium is finally restored, the system will have initial vapour pressure. Vapour pressure does not depend on volume of container, it only depends on nature of substance and temperature. Thus, the sudden decrease in partial pressure is slowly raised so that a new equilibrium is obtained. Partial pressure of the vapour will be same as the initial.