Q. Illustrate Law of Equilibrium and Equilibrium Constants?

Ans.

The general equation for a reversible reaction is given by

aA + bB -> cC + dD

The law of equilibrium states that, at constant temperature, the rate of a reaction is proportional to the concentration of the reaction substances.

Chemists describe such equilibrium reactions with what is called an equilibrium expression, which they then equate to a constant, Keq. The Keq for the general reversible reaction expressed above is:

Keq = [C]c [D]d / [A]a [B]b

Note that [A] = concentration of A, [B] = concentration of B, [C] = concentration of C, and [D] = concentration of D.

By convention, the product of the concentrations of the products is divided by the product of the concentrations of the reactants.

Note the placement of coefficients from the chemical reaction as exponents in the chemical expression.

For example, consider the reaction:

2NH₃ (g) -> N₂ (g) + 3H₂ (g)

The equilibrium expression and constant are written:

Keq = [ N₂ ] [ H₂ ]3 / [NH₃]2

The equilibrium expression and constant can be used in calculations:

What is the concentration of N₂ if [H₂] = 0.1 M, [NH₃] = 0.2 M, and Keq = 0.0016 ?

Keq = [ N₂ ] [ H₂ ]3 / [NH₃ ]2

0.0016 = [N₂] (0.1)3 / (0.2)2

[N₂] = 0.064 M