FreeScienceLessons · AQA ChemistryReversible Reactions and Equilibrium
Cognito · AQA ChemistryLe Chatelier's Principle and the Haber process
Key facts
Dynamic equilibrium
A reversible reaction can proceed in both directions: A + B ⇌ C + D.
Dynamic equilibrium — in a closed system, the rate of forward reaction = rate of reverse reaction. Concentrations of all species remain constant but both reactions still occur.
The position of equilibrium describes where the balance lies — closer to products or reactants.
Equilibrium can only be established in a closed system.
Le Chatelier's Principle
If a system at equilibrium is disturbed, it will shift to oppose the change.
Temperature ↑ → equilibrium shifts in the endothermic direction (absorbs heat).
Concentration ↑ of a reactant → equilibrium shifts towards products.
Pressure ↑ (gases) → equilibrium shifts towards the side with fewer moles of gas.
Catalyst → speeds up both directions equally → NO change in equilibrium position. Reaches equilibrium faster.
The Haber Process — industrial application of equilibrium
N₂ + 3H₂ ⇌ 2NH₃ (forward reaction is exothermic).
Temperature: 450°C. Higher temp → faster rate BUT equilibrium shifts towards reactants (less yield). Compromise chosen.
Pressure: 200 atmospheres. Higher pressure → shifts towards products (fewer moles of gas on right: 2 vs 4). Higher pressure costs more.
Catalyst: iron catalyst. Speeds up reaching equilibrium. Does NOT change yield.
Unreacted N₂ and H₂ are recycled — nothing is wasted.
Exam questions — 4 questions · 12 marks
⚡ Extended questions included. Read the hint. Show all working. AI marks against the real AQA mark scheme.
1 markLe Chatelier — temperatureAQA 8462 P2 style
The forward reaction A + B ⇌ C is exothermic. What happens to the yield of C when temperature is increased?
A Yield of C increases — higher temperature always shifts equilibrium towards products
B Yield of C decreases — equilibrium shifts in the endothermic direction (the reverse reaction) to oppose the temperature increase
C Yield of C is unaffected — temperature only affects rate, not equilibrium position
D Yield of C increases — more particles have sufficient energy to react
4 marksHaber process conditionsAQA 8462 P2 style
For the Haber process (N₂ + 3H₂ ⇌ 2NH₃), explain why (a) a temperature of 450°C is used rather than a higher temperature, and (b) a pressure of 200 atm is used. (4 marks)
Hint: (a) exothermic → high temp reduces yield. Compromise rate vs yield. (b) fewer moles of gas on right → high pressure increases yield. Compromise yield vs cost/safety.
+40 XP
1 markCatalyst and equilibriumAQA 8462 P2 style
A catalyst is added to a reaction at equilibrium. What is the effect on the equilibrium position and the yield of products?
A The equilibrium position shifts towards products increasing the yield
B The equilibrium position is unchanged and yield stays the same, but equilibrium is reached more quickly
C The catalyst increases the activation energy of the reverse reaction only
D The yield of products increases because the forward reaction is faster
3 marksClosed system explanationAQA 8462 P2 style
Explain why a dynamic equilibrium can only be established in a closed system. (3 marks)
Hint: Open system: products escape → reverse reaction cannot keep up → equilibrium not reached. Closed system: nothing escapes → both directions continue → equilibrium reached.