Quantitative Equilibrium

If you remember from the equilibrium page, this relates to a group of reactions we call equilibrium reactions. These occur simultaneously in bot directions, forward and reverse, at equal rates such that the overall concentrations of reactants and products remain the same. e.g.

Nitrogen + Hydrogen Ammonia
2N_2(g) + 3H_2(g) 2NH_3(g)
In such reactions there is a mathematical relationship between the concentrations of the reactants and products.

In this case it is:

K_c = [NH_3(g)] ² [N_2(g)]²[H_2(g)]³
Where K_c is called the equilibrium constant (this time where the concentrations are given in moldm^-3)

and

[ ] represents the equilibrium concentrations of the reactants and products (measured in moldm^-3).

Remember that the terms for the products form the numerator and the reactants the denominator. Each concentration is raised to a power which is the balancing number in the chemical equation. (Called the stoichiometric coefficient)

Some examples of Equilibrium Expressions

Below are some examples of equilibria and their K_c expressions:

a).N₂O_4(g) 2NO_2(g)

K_c = [NO_2(g)]² [N₂O_4(g)]
Units of K_c = moldm^-3

b).