R2.3 How far? The extent of chemical changeIB Chemistry SL: Revision notes

In a closed system, a reversible reaction reaches dynamic equilibrium when the rates of the forward and backward reactions are equal. Both reactions continue, but concentrations and macroscopic properties (colour, pressure, density) stay constant. Equilibrium can be approached from either direction.

The same idea applies to physical systems: in a sealed flask of bromine, the rate of evaporation equals the rate of condensation, so the amount of vapour is constant.

For a homogeneous reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ), using equilibrium concentrations. Coefficients become powers, and products go on top.

Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) gives Kc = [NH₃]² / ([N₂][H₂]³).

K ≫ 1: position of equilibrium lies to the right (mostly products). K ≪ 1: lies to the left (mostly reactants). K ≈ 1: significant amounts of both.

K is temperature dependent: it changes only when the temperature changes. For the reverse reaction at the same temperature, K′ = 1/K.

Le Châtelier's principle: if a system at equilibrium is disturbed, the position of equilibrium shifts to partly oppose the change.

• Concentration: adding a reactant or removing a product shifts right. K unchanged.
• Pressure (gases): higher pressure shifts towards fewer gas molecules; no effect if both sides have equal numbers. K unchanged.
• Temperature: higher temperature favours the endothermic direction. K increases for an endothermic forward reaction and decreases for an exothermic one.
• Catalyst: speeds up forward and reverse reactions equally; equilibrium is reached faster, position and K unchanged.

Haber process (N₂ + 3H₂ ⇌ 2NH₃, exothermic, 4 → 2 mol gas): high pressure and low temperature favour yield, but low temperature is too slow, so about 450 °C, 200 atm, iron catalyst.

Contact process (2SO₂ + O₂ ⇌ 2SO₃, exothermic, 3 → 2 mol gas): K is very large, so conversion is high even at 1–2 atm; about 450 °C, V₂O₅ catalyst, excess air.