APPLICATION OF EQUILIBRIUM CONSTANT
 
 
 
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Knowledge of equilibrium constant for a given reaction is very helpful aid in laboratory analysis as well as in industry. Equilibrium constant of a reaction is used for two purposes:
Value of Kc is used to predict the direction of the reaction.
Value of Kc is also used to predict the extent to which a reaction occurs.
To predict the direction of reaction:
The value of Kc is helpful in determining the direction in which a reaction will shift in order to achieve the equilibrium.
Consider a reaction
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Reactants ======== Products
First we determined the ratio of initial concentrations of reactants and products.
There are three possible values of this ratio when it is compared with the value of Kc.
When ratio=Kc
According to the law of mass action, there is no shifting of reaction and there will be no change in the concentration of reactants and products and the system is already at equilibrium.
When ratio> Kc
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In this condition the reaction will shift in the backward direction to achieve equilibrium state. At equilibrium quantity of product will decrease and the quantity of reactants will increase.
When ratio<Kc
In this condition the reaction will shift in forward direction to achieve equilibrium state. At equilibrium quantity of product will increase and the quantity of reactants will decrease.
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To predict the extent of a reaction :
From the magnitude of equilibrium constant, we can predict not only the direction of a reaction but also the extent to which a reaction proceeds.
There may be three values of
equilibrium constant:
A very high value of equilibrium constant
A very small value of equilibrium constant
A moderate value of equilibrium constant
When the value of equilibrium constant is very high:
A very high value of equilibrium constant indicates that the forward is almost complete and in other words we can say that the reactants are very unstable and they react spontaneously.
For example the equilibrium constant for the reaction 2O3 3O2 is very large i.e. 1 x 1055.
This indicates that the forward is almost complete and ozone (O3) is very unstable.
When the value of equilibrium constant is very small:
A very small value of equilibrium constant indicates that there is very little tendency for the reaction to occur in the forward direction and in other words we can say that the reactants are very stable.
For example the equilibrium constant for the reaction 2HF H2 + F2 is very small i.e.
1 x 10-13.
This indicates that the forward occurs with negligible speed and hydrogen fluoride (HF) is very stable.
When the value of equilibrium constant moderate:
When the value of equilibrium constant is neither very high nor very small, we conclude that the reaction occurs both in forward and backward direction and equilibrium will be attained after certain period of time.
For example the equilibrium constant for the reaction N2 +3H2 NH3 is 10, which is a moderate value.
 
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