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

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