Equilibrium of a reversible reaction is achieved when the concentrations of both reactants and products do not change.
At equilibrium, the rate of the forward reaction is the same as the reverse reaction.
How much the forward reaction or reverse reaction is favored can be shown mathematically by comparing the concentrations of reactants to products at equilibrium.
The equilibrium constant, Kc, is a constant that describes the ratio between reactants and products at equilibrium. It is calculated by dividing the concentrations of products by the concentrations of reactants (raised to the power of their molar ratios).
Kc values are only for a specific temperature.
Equilibrium Constant, Kc
When the concentrations of reactants and products reacting in a reversible reaction no longer change, equilibrium is reached. The position of equilibrium describes how much either the forward or reverse reaction is favoured. Equilibrium constants (Kc) can be used to show this position.
The basic premise is simple – if you have more A + B at equilibrium, the equilibrium must favour the backward reaction.
If you have more C + D at equilibrium, the equilibrium must favour the forward reaction.
Concentrations are used to describe how much we have of A, B, C and D. To calculate the Kc value, we just need to use the concentrations of each in the expression.
Where a, b, c and d are the molar ratios of each in the reaction.
If the amounts of A + B and C + D are the same, the top of the fraction will be the same as the bottom and Kc will have a value of exactly 1. This would show an equilibrium that is perfectly in the middle, so it favours the forward and backward reactions equally.
If there is more C + D at equilibrium than A + B, the top of the fraction will be larger than the bottom and Kc will have a value greater than 1. This would show an equilibrium that is in favour of the forward reaction. The larger the value of Kc over 1, the further the equilibrium favours the forward reaction.
If there is more A + B at equilibrium than C + D, the bottom of the fraction will be larger than the top and Kc will have a value smaller than 1. This would show an equilibrium that is in favour of the reverse reaction. The smaller the value of Kc (under 1), the further the equilibrium favours the reverse reaction.
It is important to understand that Kc does not give any information about how fast the reaction(s) happen, it just gives information about the balance between forward and reverse reactions in an equilibrium.
Changing the concentrations of the reactants and products does not change the value of Kc for a reaction. Only temperature changes the value of Kc.