Equilibrium

Chemical equilibria are important in numerous biological and environmental processes.

For example:

When a liquid evaporates in a closed container, molecules with relatively higher kinetic energy escape the liquid surface into the vapour phase and a number of liquid molecules from the vapour phase striking the liquid surface are retained in the liquid phase. It gives rise to a constant vapour pressure because of an equilibrium in which the number of molecules leaving the liquid equals the number returning to liquid from the vapour. It is called as equilibrium state.

At equilibrium, the rate of evaporation is equal to the rate of condensation. It may be represented by

H₂O (I) ⇌ H₂O (vap)

The double half arrows indicate that the processes in both the directions are going on simultaneously. The mixture of reactants and products in the equilibrium state is called an equilibrium mixture.

Dynamic nature of equilibrium

The most important matter in the case of equilibrium is that there is a continuous transformation of reactant to product and product to reactant. This state appears to be steady but it is not so. This type of reaction which takes place in both the directions is called reversible reaction and it is expressed by the symbol of two half – arrows (⇌). This symbol indicates that such reaction occurs simultaneously in both (forward and reverse) directions.

Generally, the change of reactant to product is called forward reaction and the change of product to reactant is called the reverse reaction. Thus, in reversible reactions, forward and reverse reactions continuously occur and we find it as equilibrium state. The decomposition reaction of solid calcium carbonate in a closed vessel, at high temperature can be shown as below:

CaCO_{3(S) ⇋} CaO_(s) + CO_2(g)

The equilibrium is dynamic and it is neither steady nor static as the forward and the reversible reactions occur with the same velocity in closed vessels. In the above reaction obtaining CaO and CO₂ by decomposition of CaCO₄ and obtaining CaCO₃ by combination of CaO and CO₂ takes place continuously.

Suppose, if we deposit some amount in our bank account and withdraw the same amount from our bank account, then the balance in the account appears steady or static. But this can be considered operative or dynamic and not closed or static. It is very difficult to determine the dynamic nature of equilibrium, even then with the help of radioactive isotope, it can be proved ¹⁴CO₂ gas containing radioactive isotope ¹⁴C and CaCO₃ are taken in two different flasks and CO₂ obtained by decomposition is connected with vessel containing ¹⁴CO₂ gas, after some time. Ca¹⁴CO₃ will be formed in the vessel of CaCO₃ and ¹⁴CO₂ will be obtained in the vessel containing ¹⁴CO₂.

Thus, if the equilibrium would have been steady, there must not be exchange of ¹²C and ¹⁴C. With the help of suitable counter, the radioactivity can be measured and the proof for the dynamic nature of equilibrium can be obtained through the proportions of concentrations of reactants and products remain constant. The reaction can be fast or slow depending upon the nature of the reactant and the experimental conditions.

(i) Reactions which are almost at the extent of completion and concentration of reactants may be negligible.

(ii) Reactions in which the products are formed in very less proportions and most part of the reactant remains unchanged at the equilibrium.

(iii) Reactions in which the concentrations of reactants and products are in comparable proportions at equilibrium.