Factors affecting equilibria

What happens when we disturb an equilibrium? Le Chatelier's principle: if a change is made to a system at equilibrium, the system shifts so as to counteract the change. It pushes back.

• Concentration: adding a reactant, or removing a product, shifts the equilibrium forward (to use up the added reactant or replace the lost product). Adding a product shifts it backward. The value of K does not change; only the amounts readjust.
• Pressure (changed by changing the volume): increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, which takes up less room. In N₂ + 3H₂ ⇌ 2NH₃, four molecules become two, so high pressure favours ammonia. If both sides have the same number of gas molecules, pressure has no effect.
• Inert gas: adding an unreactive gas (such as argon) at constant volume does not change the partial pressures of the reacting gases, so it has no effect.
• Temperature: this is the only change that alters K itself. Raising the temperature favours the endothermic direction (which absorbs the added heat). So for an endothermic reaction, K increases with temperature; for an exothermic one, it decreases.
• Catalyst: speeds up the forward and backward reactions equally. Equilibrium is reached sooner, but K and the equilibrium composition are unchanged.

Industry uses these ideas. In the Haber process for ammonia (exothermic, fewer gas molecules on the product side), high pressure helps, removing ammonia as it forms helps, and a catalyst makes the reaction fast enough at a moderate temperature.