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?
What is the initial effect of thechange on vapour pressure?
When the volume of the container is suddenly increased, the vapour pressure initially decreases.
The rate of evaporation remains constant as the temperature is fixed. However, the rate of condensation decreases initially because the vapour molecules are spread over a larger volume, reducing the number of molecules striking the liquid surface.
To restore equilibrium, more liquid evaporates until the rate of evaporation equals the rate of condensation again. Finally, the vapour pressure is restored to its original constant value, as for a liquid ⇌ vapour equilibrium, the vapour pressure is constant at a given temperature.
Explanation
The textbook states that for a liquid-vapour equilibrium, the rate of evaporation equals the rate of condensation, and the vapour pressure remains constant at a given temperature. When volume increases, the initial vapour pressure drops, lowering the condensation rate while evaporation stays constant. The system shifts to evaporate more liquid until the rates balance again, bringing the vapour pressure back to its fixed equilibrium value for that temperature.