What effect does changing the initial concentration of substances in a reaction have on the equilibrium constant?

A chemical equation is said to be at chemical equilibrium
when the rate of the forward reaction is equal to the rate of the reverse reaction. The
forward reaction is that of the reactants combining to give the products and the reverse
reaction is that of the products combining to give the
reactants.

Let the rate of the forward reaction be
represented by Kf and the rate of the reverse reaction by
kr.

For a reaction A + B <---> C + D, at
equilibrium kf[A][B] = kr[C][D]

The equilibrium constant Kc
= kf/kr = [C][D]/[A][B]. This is a constant value for each chemical
reaction.

Kc is the same irrespective of the initial
concentrations of the reactants. No matter what the initial concentration of the
reactants is, the concentrations at equilibrium are such that the ratio of the product
of the concentration of the products and the product of the concentration of the
reactants is equal to Kc.