For a balanced reaction of the type,
aA + bB ⇌ cC + dD
According to the law of mass action, the constant value obtained by relating equilibrium concentrations of reactants and products is called the equilibrium constant. For the forward reaction, this is given by
K c=[ C ] c [ D ] d / [ A ] a [ B ] b
The equilibrium constant for the reverse reaction is the inverse of the forward reaction and is given by:
K c ′=1 / K c
=[ A ] a [ B ] b / [ C ] c [ D ] d
If the coefficients of the chemical equation are multiplied by a factor ‘n’ then the equilibrium constant is raised by the power ‘n’ i.e. the constant becomes
K c n
.
| Equilibrium Constant Representation | Expressed in terms of | Expressed as |
| Kc | Concentrations of reactants and products | [C]c[D]d / [A]a[B]b |
| Kp | Partial pressures of reactants and products. (only for the substances which are in gaseous state) | PccPdD / PaAPbB |
| Kx | Mole fractions of reactants and products | [XC]c[XD]d[XA]a[XB]b |