In chemistry, equilibrium is a state where the rate of forward reaction is equal to the rate of backward reaction. At this state, the concentrations of reactants and products are called equilibrium concentrations. The relation between them is given by the law of chemical equilibrium or law of mass action.
Consider a general balanced reversible reaction where A and B are the reactants, C and D are products such that a, b, c and d are stoichiometric coefficients.
a A + b B ⇌ c C + d D
According to the law of chemical equilibrium, equilibrium concentrations can be related by as:
\( K_c \)
Where Kc is known as the equilibrium constant (quantity of substances are expressed regarding concentration) and [C] represents the concentration of the chemical entity C. Equilibrium constant can also be expressed in terms of partial pressures and mole fraction; they are represented as Kp and Kc respectively. The relation between Kp and Kc is given by:
\( K_p \)
Applications of Equilibrium Constant:
- ✤ Predicting the extent of reaction:
Kc can be used to predict the extent of a reaction, i.e. the degree of disappearance of the reactants.
- If Kc> 103 (very large) the products predominate over reactants leading to near completion of the forward reaction. The reverse reaction is negligible.
- If Kc< 10-3 (very low), the reaction proceeds very rarely. Here the reactants dominate over the products, backward reaction is more prevalent.
- If 10-3<Kc< 103, there will be an appreciable amount of both products and reactants.
- ✤ Predicting the direction of reaction:
Equilibrium constant can be used to predict the direction of the reaction. We need a term, reaction quotient (Qc expressed in terms of concentrations or Qp in terms of partial pressures) similar to the equilibrium constant except that the conditions are not at equilibrium, i.e. reaction quotient is ratio of concentration of products raised to their respective stoichiometric coefficients to concentration of reactants raised to their respective stoichiometric coefficients at any time in the reaction.
For a balanced reaction, a A + b B ⇌ c C + d D
Reaction quotient (Qc or Qp) is given as:
\( Q_C \)
\( Q_p \)
|Comparison with Kc||Direction of Reaction|
|Qc> Kc||The result will move backwards i.e. in the direction of reactants (reverse reaction).|
|Qc< Kc||The reaction will move forward i.e. in the directions of products (forward reaction).|
|Qc = Kc||The reaction is at equilibrium.|
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