The Effect of Changing Conditions on EquilibriumOxford AQA IGCSE Chemistry: Revision notes
Section 1
What happens when conditions change at equilibrium?
When a reversible reaction is at equilibrium, the relative amounts of all the reacting substances present depend on the conditions of the reaction. If those conditions are changed — for example, temperature, pressure, or concentration — the position of equilibrium can shift, changing how much product is formed.
This is important industrially: manufacturers choose conditions carefully to maximise the yield (amount of useful product) of a reaction.
Section 2
How does temperature affect the equilibrium position?
Every reversible reaction has an exothermic direction and an endothermic direction.
- Raising the temperature increases the yield from the endothermic reaction and decreases the yield from the exothermic reaction
- Lowering the temperature increases the yield from the exothermic reaction and decreases the yield from the endothermic reaction
This is because the equilibrium shifts in the direction that opposes the change — if you add heat, the system shifts in the endothermic direction to absorb some of that extra heat.
Always identify which direction of the specific reaction is exothermic and which is endothermic before applying the rule — this is usually given or can be worked out from ΔH.
Section 3
How does pressure affect the equilibrium position (gas reactions)?
For reactions involving gases, changing pressure can shift the equilibrium:
- Increasing pressure favours the reaction that produces the smaller number of gas molecules (as shown by the symbol equation)
- Decreasing pressure favours the reaction that produces the larger number of gas molecules
This is why the Haber process (4 molecules of gas → 2 molecules of gas) uses high pressure to favour ammonia formation.
In N2 + 3H2 ⇌ 2NH3, there are 4 moles of gas on the left and 2 on the right. Increasing pressure favours the forward reaction, which produces fewer gas molecules, increasing ammonia yield.
Section 4
Why do industrial processes use a compromise of conditions?
Choosing conditions that maximise yield is not always the best overall choice. Very high pressure or very low temperature can be expensive to achieve and maintain, and can slow the rate of reaction (if temperature is too low). Industrial processes therefore use compromise conditions that balance:
- a good yield of product
- a fast enough rate of reaction
- reasonable running costs and energy use
Must Know
- The equilibrium position depends on the conditions of the reaction
- Raising temperature favours the endothermic direction; lowering temperature favours the exothermic direction
- Increasing pressure favours the side with fewer gas molecules; decreasing pressure favours the side with more gas molecules
- These effects apply only to reversible reactions at equilibrium
- Industrial processes use compromise conditions balancing yield, rate and cost
- Concentration changes can also shift equilibrium position
That's the notes covered.
Carry on to the next subtopic.