| Q = | p(NO2)2 |
| p(NO)2p(O2) |
2NO2(g) ΔH = -114 kJ mol-1| direction reaction proceeds | ||||
| Q | K | to reach equilibrium | ||
| Increasing temperature | decreases as ΔH negative | reverse | ||
| adding a catalyst | no change | no change | Q = K | no change |
| increasing p(NO) | decreases | no change | Q < K | forward |
| doubling total pressure | decreases | no change | Q < K | forward |
| halving total volume | decreases | no change | Q < K | forward |
Changes in overall pressure affect the equilibrium composition only if there is a change in volume as the reaction proceeds. This happens if the total number of moles of gases in reactants (3 above) is different to the total number of moles of gases in products (2 above).
An increase in pressure (or a decrease in volume) results in reaction to reduce volume in order to restore equilibrium. In the example above, this results in higher partial pressures of products in the new equilibrium mixture.