Applications of Equilibrium Constant


Predicting the Extent of a Reaction

If the value of Kc>103 will be high, then the equilibrium reaction is forward dominant.
If the value of Kc<10−3, then the equilibrium reaction is backward dominant.
The moderate value of Kc (between 103 and 10−3 ) dominates equilibrium in neither direction.


Predicting the Direction of the reaction

We can predict the direction in which the reaction will proceed at any stage with the help of the equilibrium constant. Reaction Quotient Q will calculate for this purpose. In the same way as the equilibrium constant Kc, the Reaction Quotient will not necessarily have equilibrium values.


·         the reaction proceeds in the direction of reactants when Qc>Kc

·         the reaction proceeds in the direction of products when Qc<Kc

·         the reaction mixture is already at equilibrium when Qc=Kc

reaction quotient

The reaction quotient (Q) is a mathematical expression that is similar to the equilibrium constant (K), but it is used to describe the relationship between the concentrations (or pressures) of reactants and products at any point during a reaction, rather than at equilibrium. The reaction quotient is important because it helps us predict the direction in which a reaction will proceed under non-equilibrium conditions.

Calculating Equilibrium concentrations

In case of a problem in which we know the initial concentrations but do not know any of the equilibrium concentrations, the following three steps shall be followed:
Step 1. Write the balanced equation for the reaction.
Step 2. Under the balanced equation, make a table that lists each substance involved in the reaction:
(a) The initial concentration,
(b) The change in concentration on going to equilibrium, and
(c) The equilibrium concentration.
In constructing the table, define x as the concentration (mol/L) of one of the substances that react on going to equilibrium, then use the stoichiometry of the reaction to determine the concentrations of the other substances in terms of x.
Step 3. Substitute the equilibrium concentrations into the equilibrium equation for the reaction and solve for x.
Step 4. Calculate the equilibrium concentrations from the calculated value of x.
Step 5. Check your results by substituting them into the equilibrium equation.