Factors Affecting Equilibrium Constant

Ionic And Chemical Equilibrium of Class 11

Factors Affecting Equilibrium Constant

(1) Nature of Reactants And/or Products

The value of equilibrium constant depends on the nature of reactants and/or products. By changing the reactant(s) or product(s) of a reaction, its equilibrium constant will change. For example

N2(g) + O2(g) Factors Affecting Equilibrium Constant 2NO(g)

N2(g) + 2O2(g) Factors Affecting Equilibrium Constant 2NO2(g)

Although the reactants are same but products in the two reactions being different, the value of equilibrium constant for the two reactions will be different.

Similarly for reactions,

H2(g) + I2(g) Factors Affecting Equilibrium Constant 2HI(g)

H2(g) + Cl2(g) Factors Affecting Equilibrium Constant 2HCl(g)

the values of the equilibrium constant for the two reactions will be completely different.

(2) Temperature

The variation of equilibrium constant with temperature is given by the relation logFactors Affecting Equilibrium Constant.

This can be simply obtained by Arrhenius equation.

Starting with Arrhenius equation of rate constant

Factors Affecting Equilibrium Constant….(2)

where, kf = rate constant for forward reaction, Af = Arrhenius constant of forward reaction,

andFactors Affecting Equilibrium Constant = Energy of activation of forward reaction.

Factors Affecting Equilibrium Constant….(3)

where, kr = rate constant for backward reaction, Ar = Arrhenius constant of backward reaction and Factors Affecting Equilibrium Constant = Energy of activation of backward reaction.

Dividing (2) by (3) we get,

Factors Affecting Equilibrium Constant

We know that Factors Affecting Equilibrium Constant (equilibrium constant )

∴  Factors Affecting Equilibrium Constant

At temperature T1,

Factors Affecting Equilibrium Constant ….(4)

At temperature T2,

Factors Affecting Equilibrium Constant….(5)

Dividing (5) by (4) we get

Factors Affecting Equilibrium Constant

Taking log of both the sides,

 Factors Affecting Equilibrium Constant

The enthalpy of a reaction is defined in terms of activation energies of forward and reverse reactions as Factors Affecting Equilibrium Constant = ΔH

Factors Affecting Equilibrium Constant

∴logFactors Affecting Equilibrium Constant….(6)

(a) If ΔH is +ve (endothermic) an increase in temperature (T2 > T1) will make K2 > K1, i.e. the reaction goes more towards the forward direction and vice-versa.

(b) If ΔH is −ve (exothermic) an increase in temperature (T2 > T1), will take K2 < K1 i.e., the reaction goes in the reverse direction.

(3) Stoichiometry of Reaction

The value of Kp and Kc depends upon the stoichiometry of reaction as

N2(g) + 3H2(g) Factors Affecting Equilibrium Constant 2NH3(g)

Kc = Factors Affecting Equilibrium Constant….(i)

1/2 N2(g) + 3/2 H2(g) Factors Affecting Equilibrium Constant NH3(g)

Factors Affecting Equilibrium Constant….(ii)

From (i) and (ii), Factors Affecting Equilibrium Constant

(4) Mode of Writing a Chemical Equation

The value of Kp and Kc also depend on the method of representing a chemical equation.

N2(g) + 3H2(g) Factors Affecting Equilibrium Constant 2NH3(g)

  Factors Affecting Equilibrium Constant

If the equilibrium reaction is reversed,

2NH3(g) Factors Affecting Equilibrium Constant N2(g) + 3H2(g)

Factors Affecting Equilibrium Constant

Now, if we write the equilibrium reaction as,

NH3(g) Factors Affecting Equilibrium Constant 1/2 N2(g) + 3/2 H2(g)

Factors Affecting Equilibrium Constant

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