對於可逆反應,
a A + b B ⇌ c C + d D {\displaystyle a{\mbox{A}}+b{\mbox{B}}\rightleftharpoons c{\mbox{C}}+d{\mbox{D}}\,\!}
平衡常數K 定義為
K = [ C ] c [ D ] d [ A ] a [ B ] b {\displaystyle K={\frac {[{\mbox{C}}]^{c}[{\mbox{D}}]^{d}}{[{\mbox{A}}]^{a}[{\mbox{B}}]^{b}}}}
其中 [X] 表示化學物質 X 的濃度。
例如，對於哈伯法反應,
N 2 ( g ) + 3 H 2 ( g ) ⇌ 2 NH 3 ( g ) {\displaystyle {\mbox{N}}_{2}({\mbox{g}})+3{\mbox{H}}_{2}({\mbox{g}})\rightleftharpoons 2{\mbox{NH}}_{3}({\mbox{g}})\,\!}
平衡常數可以透過以下方法計算
K = [ NH 3 ( g ) ] 2 [ N 2 ( g ) ] [ H 2 ( g ) ] 3 {\displaystyle K={\frac {[{\mbox{NH}}_{3}({\mbox{g}})]^{2}}{[{\mbox{N}}_{2}({\mbox{g}})][{\mbox{H}}_{2}({\mbox{g}})]^{3}}}}
![{\displaystyle K={\frac {[{\mbox{C}}]^{c}[{\mbox{D}}]^{d}}{[{\mbox{A}}]^{a}[{\mbox{B}}]^{b}}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4b495992a71b709753fac6ae1a81256d2d75890e)
![{\displaystyle K={\frac {[{\mbox{NH}}_{3}({\mbox{g}})]^{2}}{[{\mbox{N}}_{2}({\mbox{g}})][{\mbox{H}}_{2}({\mbox{g}})]^{3}}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/61edc067d1f01b881209c8977329d4b5c99be9eb)
