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Equilibrium in Chemical Processes – Dynamic Equilibrium
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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