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

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Mathematical definition of an equilibrium constant[edit]

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 }