Introduction

Equilibrium is a general term which applies to physical processes and chemical reactions. It represents the state of a process in which the properties like temperature, pressure, concentration of the system do not show any change with passage of time.

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Equilibria in Physical Processes

The common examples of equilibria involving physical process are the phase transformations. These include:

(1) Solid -liquid equilibrium (Melting of ice)

Solid ↔ Liquid

When ice and water kept in a perfectly insulated thermos flask so that there is no exchange of heat between its contents and the surroundings at 0°C and the atmospheric pressure are in equilibrium state. Under these conditions, it is observed that the mass of ice and water do not change with time and the temperature remains constant.

For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance.

(2) Liquid - Gas equilibrium (Evaporation of water in a closed vessel)

When a watch glass containing water is kept in a box (in dry atmosphere), the mercury level in the right limb of the manometer slowly increases and finally attains a constant value, that is, the pressure inside the box increases and reaches a constant value. Also the volume of water in the watch glass decreases.

This implies that the more and more water molecules change into vapour till the equilibrium is attained. This process is called evaporation. Ultimately when the amount of water vapours become constant, the same water vapour changes back to liquid water called condensation. At equilibrium:
 

Rate of evaporation = Rate of condensation


The equilibrium is represented as

H2O(liquid) ↔ H2O (vapour)

3. Solid - Gas equilibrium

Solid ↔ Gas

Example: When solid iodine is kept in a closed vessel, violet vapours appear in the vessel whose intensity increases with time and ultimately becomes constant.

I2(solid) ↔ I2(vapour)
 

Other examples:
Camphor (solid)  ↔ Camphor (vapour)

NH4Cl (solid) ↔ NH4Cl (vapour)

4. Solid - solution equilibrium (Dissolution of sugar in water)

Solid  ↔ Solution

Example: Dissolution of sugar in water
At equilibrium,

Rate of dissolution of sugar = Rate of precipitation of sugar

Sugar (solid) ↔ Sugar (in solution)

5. Gas - Solution equilibrium (Dissolution of a gas in a liquid under pressure in a closed vessel)

Gas  Solution

Example: Equilibrium in a soda bottle

CO2 (gas)   CO2 (in solution)

This equilibrium is governed by Henry’s law, which states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent.

General Characteristics of Equilibria in Physical Processes

For the physical processes, following characteristics are common to the system at equilibrium:

Why do we say equilibrium is dynamic in nature?

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