Which of the following statements is correct anout a volatile liquid at constant temperature in a closed container before equilibrium is reached ?

Options

A. Rate of evaporation `(R_(E))` increases with time while rate of condensation `(R_(C ))` decreases with time.
B. `R_(E )` decreases with time while while `R_(C )` increases with time.
C. `R_(E )` is constant while `R_(C )` increases with time.
D. `R_(E )` is constant of vapor while `R_(C )` decreases with time.

Answer

C

Explanation

The rate of evaporation (which depends upon the kinetic energy of molecules) is constant at any given temperature , while the rate of condensation increases with increasing concentration of molecules in the vapor phase. Vapor molecules cannot escape when vaporization of a liquid occurs in a closed container. As more molecules leave the liquid, more gaseous molecules collide with the walls of the container with one another and with the liquid surface, so more condensation occurs . This is responsible for the formation of liquid droplets that adhere to the sides of the vessel above a liquid surface and for the eventual establishment of equilibrium between liquid and vapor.
However , if the vessel were left open to the air , this equilibrium would not be reached. Molecules would diffuse away, and slight air currents would also sweep some gas molecules away from the liquid surface. This would allow more evaporation to occur to replace the lost vapor molecules. Consequently , a liquid can eventually evaporate entirely if it is left uncovered. THis situation illustrates Le Chatelier's principle : A system at equilibrium , or changing toward equilibrium , responds in a way that tends to relieve or ''undo'' any stress placed on it .
Here the stress is the removal of molecules in the vapor phase. The response is the continued evaporation of the liquid.