Equilibrium Constant: Applications

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
    
          Kc or Kp = [products] [reactants]

The magnitude of equilibrium constant Kc and Kp 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 (Qc with molar concentrations and Qp with partial pressures).

Qc = [products] [reactants]

Qc =   [C] c D d [A] a [B] 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 Qc > Kc, backward reaction takes place.

If Qc < Kc, forward reaction takes place.

If Qc = Kc, the reaction is already at equilibrium.

Summary

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
    
          Kc or Kp = [products] [reactants]

The magnitude of equilibrium constant Kc and Kp 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 (Qc with molar concentrations and Qp with partial pressures).

Qc = [products] [reactants]

Qc =   [C] c D d [A] a [B] 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 Qc > Kc, backward reaction takes place.

If Qc < Kc, forward reaction takes place.

If Qc = Kc, the reaction is already at equilibrium.

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