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

## Mathematical definition of an equilibrium constant

For a reversible reaction,

$a\mbox{A} + b\mbox{B} \rightleftharpoons c\mbox{C} + d\mbox{D} \,\!$

the equilibrium constant K is defined as

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

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

For example, for the Haber process reaction,

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

the equilibrium constant can be calculated by

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