Factors affecting equilibria: Le Chatelier's principle

Le Chatelier's principle: disturb an equilibrium by altering concentration, pressure or temperature, and the system responds by moving in whichever direction partly cancels that disturbance.

Adding a reactant or removing a product shifts the equilibrium forward; the value of K does not change.

Raising pressure by reducing volume shifts a gaseous equilibrium towards the side with fewer gas moles; if Δn = 0 pressure has no effect.

Adding an inert gas at constant volume leaves the equilibrium unchanged, because the partial pressures and molar concentrations of the reacting gases stay the same.

Temperature is the only factor that changes the value of K: for an exothermic reaction K falls as temperature rises; for an endothermic reaction K rises.

For N₂O₄(g) ⇌ 2NO₂(g), which is endothermic, heating increases the brown NO₂ and cooling favours colourless N₂O₄.

In the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g) with ΔH = −92.38 kJ mol⁻¹, high pressure favours ammonia, and a moderate temperature with an iron catalyst balances yield against rate.

A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but its composition and K do not change.