CC4.1 — Equilibrium (Edexcel 1SC0, Higher tier)
Reversible reactions
Some reactions are reversible — products can react to re-form reactants: $$A + B \rightleftharpoons C + D$$
Dynamic equilibrium
In a closed system, a reversible reaction reaches dynamic equilibrium when:
- The rate of the forward reaction = rate of the reverse reaction.
- Concentrations of all species remain constant (but not necessarily equal).
Le Chatelier's principle
If the conditions of a system at equilibrium are changed, the equilibrium shifts to oppose the change.
| Change | Effect on equilibrium |
|---|---|
| Increase concentration of reactant | Shifts right (toward products) |
| Increase concentration of product | Shifts left (toward reactants) |
| Increase temperature | Shifts in the endothermic direction |
| Increase pressure | Shifts to the side with fewer moles of gas |
| Add catalyst | No shift — reaches equilibrium faster |
Haber process example
N₂ + 3H₂ ⇌ 2NH₃ (forward reaction is exothermic; 4 moles → 2 moles of gas)
- High pressure → shifts right (fewer gas moles) → more NH₃.
- Low temperature → shifts right (exothermic) → more NH₃.
- But low temperature = slow rate → compromise ~450 °C.
- Catalyst (iron) → reaches equilibrium faster, no change to yield.
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