Equilibria and the equilibrium constant


Dynamic equilibrium occurs when the forwards and backwards reactions occur at the same time and same rate, so that the concentrations of reactants and products remain constant.

For this to be achieved we need the system to be closed .

We describe the equilibrium of a reversible reaction:


using the equilibrium constant, Kc:


where [A] is the concentration of A etc.

The units for Kc must be worked out using the units for concentration. Eg, for:


the units are given by:

scan0004         scan0006

If Kc is very large (> x105), the reaction is said to have gone to completion. If Kc is very small (< x10-5), the reaction is said not to have happened under those conditions.

Kc is not affected by concentration or pressure, as this will affect every concentration by the same amount, so the equation can cancel back down to the original. Kc is affected by temperature. To work out the effect of a change in temperature, we consider whether the forward reaction is exothermic or endothermic. Using Le Chatelier’s principle, if the forward reaction is exothermic, increasing the pressure moves the position of equilibrium to the left, so Kc decreases etc.


For a reaction:


the equilibrium constant can also be written in terms of partial pressures, pA, pB etc.:


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