## Wednesday, July 20, 2011

### Equilibrium in Physical Processes

·                    Solid – Liquid Equilibrium
When a solid – liquid system at melting point is taken in a well – insulated container, then this system constitute a system in which solid is in dynamic equilibrium with liquid.
For Example:
Let us consider ice and water at 273K (melting point of ice) taken in a perfectly insulated thermos flask. It may be noted that temperature as well as masses of ice and water remain constant. This represents a dynamic equilibrium between ice and water.
Ice       Water
Since there is no change in mass of ice and water, the number of molecules going from ice into water is equal to number of molecules going from water into ice. Thus, at equilibrium,
Rate of melting   =    Rate of freezing.
·                    Liquid – Gas Equilibrium
Let us consider evaporation of water in a closed vessel. When a small amount of water is taken in an evacuated vessel at room temperature it starts evaporating. This process continues for some time as indicated by the gradual decrease in level of water.
After some time it is observed that level of water becomes constant indicating that no more water is evaporating. This indicates that a state of equilibrium has attained between water and water vapours.
H2O (l)            H2O (g).
The equilibrium seems to be static, but in actual practice it is dynamic. In beginning rate of evaporation is more and hence water vapours concentration increases, which in turn condenses back into the liquid. As the concentration of water vapours increases, rate of condensation also increases. At equilibrium, rate of evaporation is equal to rate of condensation.
\    Rate of evaporation   =   Rate of condensation.