The equilibrium law

The Equilibrium Law

The equilibrium law allows us to express the concept of equilibrium in a mathematical way.
It is based on the idea that at equilibrium, the forwards and reverse reactions occur at the same rate, so the concentration of the reactants and products remain constant.
This constant relationship is represented by the equilibrium law equation.
The equation is: Kc = [Products] / [Reactants].
In the equilibrium expression, concentrations of products are multiplied together and divided by the multiply multiplied concentrations of reactants.
Square brackets are used to denote the concentration of a substance in mol dm^-3.
Each concentration is raised to the power (given an exponent) of its stoichiometric coefficient in the balanced chemical equation.
Pure solids and liquids do not appear in the equilibrium expression.
The equilibrium constant Kc is specific for a given reaction at a given temperature and pressure. The value of Kc will only change if we change the temperature.
A large value of Kc (greater than 1) indicates the equilibrium position is on the side of the products. In other words, the reaction goes pretty much to completion.
A small value of Kc (less than 1) indicates the equilibrium position is on the side of the reactants. In other words, the reaction hardly proceeds at all.

If Kc > 1, products are favoured over reactants at equilibrium.
If Kc < 1, reactants are favoured over products at equilibrium.
If Kc = 1, neither reactants nor products are favoured; the system is at perfect equilibrium.
Remember changes in concentration, pressure or the addition of a catalyst do not affect the value of Kc. Only temperature changes can do this.
A value of Kc raises the importance of calculating the extent of a reaction at equilibrium and also making predictions.