Introduction

The chemical reactions can occur both in forward and backward  directions. When the rates of the forward and reverse reactions become equal, this  stage is called chemical equilibrium. This equilibrium is dynamicin nature as it consists of a forward reaction in which the reactants give product(s) and reverse reaction in which product(s) gives the original reactants.

In general, a chemical equilibrium is represented as:

aA + bB ---------à  xX + yY

Concept of Chemical Equilibrium

Consider a general reaction,

A + B ------> à C + D

In the beginning (at time t=0) the concentration of A and B are  maximum and the concentrations of C and D are minimum (equal to zero).  With passage of time, there is accumulation of  the products C and D and depletion of the reactants A and B (Figure). As a result, the rate of forward reaction decreases while the rate of backward reaction goes on increasing.

Ultimately, a stage comes when the rate of forward reaction becomes equal to  rate of backward reaction. The reaction is then said to be in equilibrium.
 

Following examples illustrate how equilibrium  is attained.
 

Decomposition of calcium carbonate in a closed vessel

If calcium carbonate is heated in a closed  vessel, the following equilibrium is achieved:

CaCO₃(s) → CaO (s) + CO₂ (g)
 

Decomposition of N₂O₄ in a Closed Vessel

The decomposition of N2O4  in a closed vessel  

 

Combination of H₂ and I₂ to form HI

This is a reversible reaction as shown below:

 
If H₂ and I₂ (in  equimolar ratio) are enclosed in glass bulb at 500°C, the colour in the  beginning is purple but as the time passes, the intensity of the colour  decreases and then no further change in colour is observed. This again  indicates that the reaction has attained a state of equilibrium.
 

Reaction of Ferric  Nitrate and Potassium Thiocyanate Solution

This reaction is also a reversible reaction  and may be represented as follows:
 
 

Dynamic Nature of Equilibrium

When chemical  equilibrium is reached, the observable property is that the concentration of  each of the reactants becomes constant.

In Haber’s process, known amounts of dinitrogen and dihydrogen mixed at high  temperature and pressure, when the equilibrium is attained, the concentrations  nitrogen, hydrogen and ammonia becomes constant.
 

To know the dynamic nature of the equilibrium, ammonia is synthesised  at completely similar initial conditions, only difference is H2 is  replaced by D2 (deuterium). Irrespective of the starting components H2  or D2, at equilibrium the reaction mixture has same composition; only  difference is that D2 and ND3 are found instead of H2  and NH3. On mixing both the reaction mixtures, the equilibrium is disturbed  and after reestablishing the equilibrium, the reaction mixture is found to have  same concentration of ammonia as in earlier cases. Now, the mixture has NH3,  NH2D, NHD2, ND3 and H2, HD and D2  present in it. This confirms the equilibrium  is dynamic in nature.
 

Similarly, the reactioncan be attained from


either side as shown in the figure. 
 

What do you understand by dynamic equilibrium?

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