Physical Equilibrium

Equilibrium

Equilibrium is a state  in which the system macroscopic properties of the system is not changed.
Example
The water in a closed container is in equilibrium state. The number of molecules escaping the water surface is equal to the number of water molecule entering the surface. The rate of evaporation and rate of condensation is same and the water and vapour is in equilibrium state.
The mixture of products(water in liquid form) and reactant (water in vapour form) in this state is called equilibrium mixture.  This is a dynamic equilibrium

Types of Equilibrium

1. Physical Equilibrium
2. Chemical Equilibrium

Physical Equilibrium

Equilibrium can be seen in the physical processes also. It is equilibrium between the same chemical species in the different phases
Equilibrium in physical processes: -
a. Solid liquid Equilibrium-
Water and ice is kept in a insulated container where no heat transfer happens between surrounding and container. We can observer water and ice mass amount remains constant with time and they are in equilibrium
$H_2O(s) \rightleftharpoons H_2O(l)$
$ice \rightleftharpoons water$
Rate of melting of Ice = Rate of freezing H2 O

b. Liquid – Vapour Equilibrium
$H_2O(l) \rightleftharpoons H_2O(g)$
Rate of Vaporization of H2 O = rate of Condensation of H2 O

c. Solid – Vapour Equilibrium
$I_2(s) \rightleftharpoons I_2(g)$
$CO_2(s) \rightleftharpoons CO_2(g)$
Rate of sublimation of solid I2 = Rate of condensation of I2 vapour

d. Dissolution of Solid in liquid
In equilibrium state, the concentration of solid in liquid remains constant
Eg:
$Sugar (solid) \rightleftharpoons Sugar (in \; solution)$
Rate of Dissolution of sugar in water = rate of precipitation of sugar

e. Solubility of Gas in a liquid
In equilibrium state, the solubility of gas remains constant. carbon dioxide gas dissolved in the soda water solution in the sealed bottle
$CO_2(g) \rightleftharpoons CO_2 (in \; solution)$
The solubility of gas is governed by the Henry Law

Henry’s Law:-

The mass of the gas dissolved in the given mass of the solvent at any temperature  is directly proportional to the partial pressure of gas above the solvent
Mathematically,
$m \propto p$
$m =K_Hp$
KH = Henry’s constant
Applications of Henry’s Law:-

Why the Soda water turns flat when left open to the air for some time
Explanation
The soda water bottle is sealed under pressure of gas when its solubility in water is high. As soon as the bottle is opened,some of the dissolved carbon dioxide gas escapes to reach a new equilibrium condition required for the lower pressure, namely its partial pressure in the atmosphere.

Characteristics of Equilibrium:-

1. Equilibrium is dynamic in nature.
2. At equilibrium state, the concentration of substances becomes constant at constant temperature.
3. At equilibrium state, the measurable properties of a systems remains constant.
4. Equilibrium  is achieved only in closed system at given temperature.
5. The value of Equilibrium  Constant represents the extent to which the reaction is proceeded before equilibrium is reached.