Question 7.1 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?
Class XI Equilibrium Page 224
Answers
low pressure high pressure
we know,
if volume increases at constant temperature , pressure decreases
(a) volume of container suddenly increase at constant temperature , so, pressure decreases . and decrease pressure shift the equilibrium in the direction of high pressure.e.g., more vapor are formed hence, vapor pressure increases.
(b) rate of evaporation increase so, rate of condensation decreases.
(c) when equilibrium is restored finally the rate of evaporation =rate of condensation and final vapor pressure will be equal to the vapor pressure that was before the sudden increase the volume of the container.
Answer:
(a) If the volume of the container is suddenly increased, then the vapour pressure would decrease initially.
This is because the amount of vapour remains the same, but the volume increases suddenly.
As a result, the same amount of vapour is distributed in a larger volume.
(b) Since the temperature is constant, the rate of evaporation also remains constant.
When the volume of the container is increased, the density of the vapour phase decreases.
As a result, the rate of collisions of the vapour particles also decreases.
Hence, the rate of condensation decreases initially.
(c) When equilibrium is restored finally, the rate of evaporation becomes equal to the rate of condensation.
In this case, only the volume changes while the temperature remains constant. The vapour pressure depends on temperature and not on volume.
Hence, the final vapour pressure will be equal to the original vapour pressure of the system.