Law of Chemical Equilibrium Definition

Chemistry Glossary Definition of Law of Chemical Equilibrium

At equilibrium, the concentration of reactants and products remains the same over time.
At equilibrium, the concentration of reactants and products remains the same over time. Westend61 / Getty Images

When a chemical reaction is at equilibrium, the concentration of the reactants and the products remains the same over time. In other words, the forward and reverse chemical reaction are the same. Note: this does not mean the concentration of reactants and products is the same. There is a law that relates the concentration of reactants and products to the equilibrium constant:

Law of Chemical Equilibrium Definition

The Law of Chemical Equilibrium is a relation stating that in a reaction mixture at equilibrium, there is a condition (given by the equilibrium constant, Kc) relating the concentrations of the reactants and products.

For the reaction

aA(g) + bB(g) ↔ cC(g) + dD(g),
Kc = [ C ]c·[ D ]d / [ A ]a·[ B ]b

Equilibrium Constant Example

For example, for the chemical reaction:

2HI(g) ⇆ H2 + I2(g)

The equilibrium constant would be calculated by:

Kc = ([H2][I2] )/ [HI]2