Le Chatelier's Principle: Concentration, Pressure, Temperature and Catalysts

Le Chatelier's principle: if a system at equilibrium is disturbed, the position of equilibrium shifts in the direction that opposes the change.

• Adding a reactant, or removing a product, shifts the equilibrium to the right.
• Adding a product, or removing a reactant, shifts it to the left.
• A species can be removed indirectly by adding something that reacts with it: an acid removes $\mathrm{OH^-}$, an alkali removes $\mathrm{H^+}$.
• Adding or removing a pure solid or liquid has no effect, since it is not in Kc.
• A concentration change does not change Kc.

• Increasing the pressure (by compressing) shifts the equilibrium towards the side with fewer moles of gas. Decreasing it shifts towards more moles of gas.
• Count only gases; ignore solids, liquids and dissolved species.
• Equal moles of gas on both sides: no shift.
• Adding an unreactive gas at constant volume changes no partial pressures: no shift.
• A pressure change does not change Kc. It also speeds up a gas reaction, so equilibrium is reached sooner.

• Raising the temperature shifts the equilibrium in the endothermic direction; lowering it shifts it in the exothermic direction.
• Forward reaction exothermic ($\Delta H < 0$): heating decreases K; cooling increases K.
• Forward reaction endothermic ($\Delta H > 0$): heating increases K.
• Temperature is the only factor that changes K. Concentration, pressure and catalysts leave it unchanged.
• Example: the self-ionisation of water, $\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$, is endothermic. On heating, $K_w$ rises (about $5.5 \times 10^{-14}$ at 50 °C) and the pH of pure water falls to about 6.6, but the water stays neutral because $[\mathrm{H^+}] = [\mathrm{OH^-}]$.

The Haber process: $\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$, $\Delta H = -92\ \text{kJ/mol}$.

• High pressure (about 200 atm) favours the side with fewer gas moles and raises the rate; higher still would cost too much in equipment and safety.
• Moderate temperature (about 450 °C): lower would give a higher yield but too slow a rate.
• Iron catalyst: faster, with no change in yield.
• Ammonia is condensed and removed, and unreacted gases are recycled.

The Contact process ($\mathrm{2SO_2 + O_2 \rightleftharpoons 2SO_3}$, exothermic) uses the same reasoning with a vanadium(V) oxide catalyst at about 450 °C and low pressure, because the conversion is already high.