Reversible Reactions and Equilibrium

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CognitoEquilibrium and Le Chatelier principle

Key facts & methods

Reversible reactions and equilibrium

A reversible reaction can proceed in both directions: A + B ⇌ C + D.
Dynamic equilibrium: rate of forward = rate of reverse. Concentrations remain constant.
Equilibrium only reached in a closed system.
Equilibrium shifted RIGHT = more product. Shifted LEFT = more reactant.

Le Chatelier principle

If conditions change, the equilibrium position shifts to oppose the change.
Temperature: increase temperature → equilibrium shifts in the endothermic direction.
Concentration: increase concentration → equilibrium shifts away from that substance.
Pressure (for gases): increase pressure → equilibrium shifts toward fewer moles of gas.

Haber process application

N2 + 3H2 ⇌ 2NH3. Forward reaction is exothermic.
High pressure (200 atm): favours right (2 moles < 4 moles).
Low temperature favours forward exothermic reaction — but too slow.
Compromise: 450°C — fast enough rate + reasonable yield.
Iron catalyst: increases rate only. Does not shift equilibrium position.

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markDynamic equilibriumAQA 8462

At dynamic equilibrium:

A The reaction has stopped completely

B Only the forward reaction occurs

C Forward and reverse reactions occur at equal rates; concentrations are constant

D All reactants have been converted to products

4 marksLe Chatelier — temperatureAQA 8462 Higher

N2 + 3H2 ⇌ 2NH3 (exothermic forward). (a) Effect of increasing temperature on equilibrium position. (b) Explain using Le Chatelier principle. (c) Why is 450°C used industrially rather than a much lower temperature? (4 marks)

Hint: (a) State direction. (b) Le Chatelier: shift endothermic way. (c) Rate vs yield compromise.

+40 XP

1 markPressure effectAQA 8462 Higher

N2 + 3H2 ⇌ 2NH3. Increasing pressure shifts equilibrium:

A Left, because fewer moles on right

B Right, because fewer moles on right (2 vs 4)

C Left, because NH3 is more stable at high pressure

D No effect

3 marksClosed system requirementAQA 8462

Explain why dynamic equilibrium can only be achieved in a closed system. Give an example. (3 marks)

+30 XP

Reversible Reactions and Equilibrium

Reversible reactions and equilibrium

Le Chatelier principle

Haber process application

Module complete! 🎉