Equilibrium
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Dynamic Equilibrium
A reversible reaction in a closed system reaches equilibrium when:
- the rate of the forward reaction equals the rate of the reverse reaction, and
- the concentrations of reactants and products are no longer changing
It is called dynamic because both reactions are still happening - they haven't stopped, they are just occurring at equal rates, so there is no overall change.
Analogy: imagine two escalators between the same two floors, one going up and one going down. If people board each at the same rate, the number of people on each floor stays constant - but people are still moving the whole time.
Closed system required: if substances can enter or leave (an open system, e.g. an unstoppered flask losing gas), the rates can never balance and equilibrium cannot be reached.
Common Mistakes
- ❌ "At equilibrium, concentrations of reactants and products are equal"
- ✓ concentrations are constant, not necessarily equal - there can be far more product than reactant (or vice versa) at equilibrium
- ❌ "At equilibrium, the reaction has stopped"
- ✓ both the forward and reverse reactions are still happening, just at equal rates
Predicting Shifts in Equilibrium
If conditions change, the position of equilibrium shifts to oppose the change.
| Change | Effect on position of equilibrium |
|---|---|
| Increase temperature | shifts towards the endothermic direction |
| Decrease temperature | shifts towards the exothermic direction |
| Increase pressure (gases) | shifts towards the side with fewer gas molecules |
| Decrease pressure (gases) | shifts towards the side with more gas molecules |
| Increase concentration of a substance | shifts away from that substance (uses it up) |
| Decrease concentration of a substance | shifts towards that substance (replaces it) |
| Add a catalyst | no effect on position - equilibrium is just reached faster |
A catalyst speeds up the forward and reverse reactions equally, so it does not change the position of equilibrium or the final yield - it only gets there sooner.
Worked Example: Methanol Synthesis
3H₂(g) + CO₂(g) ⇌ CH₃OH(g) + H₂O(g) ΔH = −40 kJ/mol, Ea(forward) = +220 kJ/mol
- Question (a): state how the information shows this reaction is reversible and exothermic. [2]
- Answer: reversible - shown by the ⇌ symbol [1]; exothermic - ΔH is negative (−40 kJ/mol) [1]
- Question (b): complete the table below using "increases", "decreases" or "no change". [4]
| Change of condition | Effect on [methanol] at equilibrium | Effect on rate of forward reaction |
|---|---|---|
| Temperature decreased | increases | decreases |
| Pressure decreased | decreases | decreases (given) |
| Catalyst removed | no change (given) | decreases |
- Temperature decreased: the forward reaction is exothermic, so a lower temperature favours it (Le Chatelier) - equilibrium shifts right, [methanol] increases. But fewer particles now have energy ≥ Ea, so the rate of the forward reaction decreases.
- Pressure decreased: the reactant side has 4 mol of gas (3H₂ + CO₂), the product side has 2 mol - lower pressure favours the side with more gas molecules (reactants), so [methanol] decreases.
- Catalyst removed: a catalyst never shifts equilibrium position, so [methanol] is unaffected. But without the catalyst's lower Ea, fewer collisions succeed, so the rate decreases.
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