A-level Chemistry/OCR (Salters)/Equilibrium constants

Mathematical definition of an equilibrium constant

For a reversible reaction,

$a\text{A}+b\text{B}\rightleftharpoons c\text{C}+d\text{D}$

the equilibrium constant K is defined as

$K=\frac{[\text{C}{]}^c[\text{D}{]}^d}{[\text{A}{]}^a[\text{B}{]}^b}$

where [X] denotes the concentration of chemical species X.

For example, for the Haber process reaction,

${\text{N}}_2(\text{g})+3{\text{H}}_2(\text{g})\rightleftharpoons 2{\text{NH}}_3(\text{g})$

the equilibrium constant can be calculated by

$K=\frac{[{\text{NH}}_3(\text{g}){]}^2}{[{\text{N}}_2(\text{g})][{\text{H}}_2(\text{g}){]}^3}$