Reversible vs. Irreversible
Most everyday chemical changes are irreversible — cooking, or neutralising hydrochloric acid with sodium hydroxide (HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)) — the products don’t spontaneously turn back into the reactants. A reversible reaction can run in either direction, depending on conditions, and is written with a “⇌” symbol between reactants and products rather than a one-way arrow. Example: heating ammonium chloride decomposes it into ammonia and hydrogen chloride gas, which recombine into ammonium chloride again on cooling: NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
Le Chatelier’s Principle
Left alone, a reversible reaction settles into equilibrium — a state where the concentrations of reactants and products stop changing (though both reactions are still happening, at matched rates). Le Chatelier’s Principle: if a system at equilibrium is disturbed — by a change in temperature, concentration, or pressure — the equilibrium shifts in whatever direction reduces that disturbance. This lets you predict which way an equilibrium will shift without having to reason through the forward and backward reaction rates directly.
Effect of Temperature
Raising the temperature shifts an equilibrium toward whichever side absorbs heat (the endothermic direction); lowering it shifts things the other way, toward the exothermic direction.
- N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ (exothermic forward)
- 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197 kJ (exothermic forward)
- CH₄(g) + H₂O(l) ⇌ CO(g) + 3H₂(g), ΔH = +210 kJ (endothermic forward)
Exercise: describe what raising the temperature does to each of these three equilibria.
Effect of Concentration
Changing how much of any one species is present shifts the equilibrium to counteract that change. For A + B ⇌ C + D: adding a reactant, or removing a product, pushes the equilibrium forward; removing a reactant, or adding a product, pushes it backward.
Exercise — for the equilibrium 2BrCl(g) ⇌ Br₂(g) + Cl₂(g): (i) what happens if chlorine is removed as it forms? (ii) what happens if bromine is added? (iii) what happens if chlorine is added?
Effect of Pressure
Pressure only matters for equilibria where every reactant and product is a gas. To work out the effect: balance the equation, count the total moles of gas on each side, and increasing pressure will shift the equilibrium toward whichever side has fewer total moles (since that side takes up less volume).
N₂(g) + 3H₂(g) ⇌ 2NH₃(g): 4 moles of gas on the left, 2 on the right — so higher pressure favours the forward (ammonia-producing) direction, which is exactly why the industrial Haber process is run under high pressure.
By contrast, A + 3B ⇌ 2C also has unequal totals (4 moles left, 2 right) — so, just as with the Haber process, increasing pressure here favours the forward reaction (fewer moles, the product side). Pressure only has no effect when the total moles of gas are equal on both sides.
Effect of a Catalyst
A catalyst has no effect on the equilibrium position at all — it doesn’t change how far a reaction goes, only how fast it gets there, by speeding up both the forward and backward reactions equally. The amount of catalyst used doesn’t change this.
