FreeScienceLessons · AQA ChemistryReversible Reactions and Equilibrium
Cognito · AQA ChemistryLe Chatelier's Principle and the Haber process
Key facts
Dynamic equilibrium — fill in the table
Term
Definition
Reversible reaction
Dynamic equilibrium
Position of equilibrium
Closed system
Le Chatelier's Principle — fill in the table
If a system at equilibrium is disturbed, it will shift to oppose the change.
Change
Effect on equilibrium
Temperature ↑
Concentration ↑ of a reactant
Pressure ↑ (gases)
Catalyst
The Haber Process — industrial application of equilibrium — fill in the table
Condition
Detail
Equation
Temperature
Pressure
Catalyst
Recycling
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.