Equilibrium constant is useful in:

• Predicting the extent of a reaction
• Predicting the direction of a reaction
• Calculating equilibrium concentrations

The magnitude of equilibrium constant provides a useful indication of the extent of a chemical reaction. The equilibrium constant for a reversible reaction is equal to the ratio of concentrations of product and the concentrations of the reactants.

           A + B ⇌ C + D
    
          K_c or K_p = $\frac{\text{[products]}}{\text{[reactants]}}$

The magnitude of equilibrium constant K_c and K_p indicates the extent to which a reaction will proceed.

If the value of Kp>10³, then the reaction proceeds to completion.

If the value of Kp <10^-3 then the reaction proceeds rarely.

If the value of Kpis between10^-3 to 10³, then appreciable quantities of the reactants as well as the products are present.

At any point in a reversible reaction, the ratio of the concentration of the products to that of the reactants is known as the reaction quotient Q (Q_c with molar concentrations and Q_p with partial pressures).

Q_c = $\frac{\text{[products]}}{\text{[reactants]}}$

Q_c =   $\frac{{\text{[C]}}^{\text{c}}{\left[\text{D}\right]}^{\text{d}}}{{\text{[A]}}^{\text{a}}{\text{[B]}}^{\text{b}}}$

The reaction quotient expression of a reaction has the same equation as the equilibrium constant expression for that reaction. Reaction quotient is calculated using the concentrations or pressure at a given point of time in the reaction, and not its equilibrium concentrations or pressures.

If Q_c > K_c, backward reaction takes place.

If Q_c < K_c, forward reaction takes place.

If Q_c = K_c, the reaction is already at equilibrium.