# Factors Affecting Equilibrium Constant

## 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) 2NO(g)

N2(g) + 2O2(g) 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) 2HI(g)

H2(g) + Cl2(g) 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 log .

This can be simply obtained by Arrhenius equation.

Starting with Arrhenius equation of rate constant ….(2)

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

and = Energy of activation of forward reaction. ….(3)

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

Dividing (2) by (3) we get, We know that (equilibrium constant )

∴ At temperature T1, ….(4)

At temperature T2, ….(5)

Dividing (5) by (4) we get Taking log of both the sides, The enthalpy of a reaction is defined in terms of activation energies of forward and reverse reactions as = ΔH ∴log ….(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) 2NH3(g)

Kc = ….(i)

1/2 N2(g) + 3/2 H2(g) NH3(g) ….(ii)

From (i) and (ii), ### (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) 2NH3(g) If the equilibrium reaction is reversed,

2NH3(g) N2(g) + 3H2(g) Now, if we write the equilibrium reaction as,

NH3(g) 1/2 N2(g) + 3/2 H2(g) 