Reversible reactions and equilibrium
Reversible reactions
Some reactions can go both ways:
A + B ⇌ C + D
The double-headed arrow ⇌ shows the reaction is reversible. Example: hydrated copper sulfate ⇌ anhydrous copper sulfate + water (blue ⇌ white). Heating drives water off; adding water restores the blue.
Dynamic equilibrium
In a closed system, a reversible reaction reaches dynamic equilibrium when:
- The forward and reverse rates are equal.
- Concentrations of reactants and products stay constant (but not zero, and not equal to each other).
It is dynamic because both forward and reverse reactions are still happening — they're balanced, not stopped.
Energy changes are mirrored
If the forward reaction is exothermic, the reverse is endothermic by the same amount. This matters for predicting the effect of temperature changes.
Le Chatelier's principle
When a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.
| Change | Position shifts | Why |
|---|---|---|
| Increase reactant concentration | Towards products | Uses up the extra reactant |
| Increase pressure | Towards fewer gas moles | Reduces pressure |
| Increase temperature | Endothermic direction | Absorbs the extra heat |
| Add a catalyst | No shift | Speeds both directions equally |
✦Worked example— Worked example — Haber process
N₂ + 3H₂ ⇌ 2NH₃ (forward exothermic; 4 moles gas → 2 moles gas)
To maximise NH₃ yield: high pressure (favours fewer moles) and low temperature (favours exothermic). In practice: ~200 atm and 450 °C (a compromise — too low T and the rate is too slow). Iron catalyst speeds equilibrium without shifting it.
OCR exam tip
When asked "explain the effect of raising T on yield", the marks are:
- State whether forward is exo/endo.
- Apply Le Chatelier — equilibrium shifts in the endothermic direction.
- State the effect on yield (rises / falls).
Three steps, three marks.
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves