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Pearson Edexcel ChemistryRates and equilibrium

Reversible reactions and equilibrium

Apply Le Chatelier's principle to predict the effect of changing conditions on equilibrium position.

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The key idea

A reversible reaction reaches dynamic equilibrium when the forward and reverse rates are equal.Le Chatelier's principle states that the system shifts to oppose any imposed change.

Reversible Reactions
reactantsproductsforward rate = reverse rate

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Keep the idea tight, then use the worked example to practise the exact exam wording.

1

Reversible reactions and the ⇌ symbol

A reversible reaction can proceed in both directions.The double arrow ⇌ indicates that the reaction does not go to completion but reaches a state of equilibrium.At equilibrium, both forward and reverse reactions still occur but at equal rates, so the concentrations of reactants and products remain constant.This is called dynamic equilibrium.

2

Le Chatelier's principle

Le Chatelier's principle allows us to predict how a system at equilibrium responds to a change.Increasing concentration of a reactant: equilibrium shifts to the right (towards products).Increasing temperature: equilibrium shifts towards the endothermic direction (if the forward reaction is exothermic, it shifts left).Increasing pressure: equilibrium shifts towards the side with fewer moles of gas.

3

The Haber process

The Haber process makes ammonia from nitrogen and hydrogen: N₂ + 3H₂ ⇌ 2NH₃ (exothermic).The industrial conditions are: 450°C (compromise between rate and yield), 200 atm pressure (increases yield and rate) and an iron catalyst (increases rate without affecting yield).The ammonia produced is continuously removed to shift the equilibrium to the right.

4

Catalysts and equilibrium

A catalyst speeds up both the forward and reverse reactions by the same factor (it provides a lower activation energy pathway for both).Therefore, it does not change the equilibrium position — the ratio of products to reactants at equilibrium is unchanged.It only helps the system reach equilibrium more quickly, which is valuable industrially to maintain productivity.

Key terms

Definitions to learn

Reversible reaction

A reaction that can proceed in both the forward and reverse directions, shown by ⇌.

Dynamic equilibrium

The state where the forward and reverse reaction rates are equal and concentrations remain constant.

Le Chatelier's principle

If a change is made to a system at equilibrium, the system shifts to oppose that change.

Equilibrium position

The relative amounts of reactants and products at equilibrium; shifts left or right depending on conditions.

Contact process

The industrial process for making sulfuric acid via the catalytic oxidation of SO₂ to SO₃.

Worked example

The reaction N₂ + 3H₂ ⇌ 2NH₃ is exothermic. Predict the effect of increasing temperature on the yield of ammonia.

1

The forward reaction is exothermic.

2

Increasing temperature provides more energy.

3

Le Chatelier's principle: the system opposes the change by favouring the endothermic reverse reaction.

4

Equilibrium shifts to the left, decreasing the yield of ammonia.

Final answer

Yield of ammonia decreases because equilibrium shifts to the left.

Exam habit

For every Le Chatelier answer, identify the imposed change, state which direction the equilibrium shifts, and explain how this change opposes the imposed change.

Watch out

Do not say the reaction 'speeds up' when temperature increases. State that the equilibrium position shifts.

Examiner tips

How to score full marks

  • 1Always name the direction of equilibrium shift (left/right) and state why that direction OPPOSES the change.
  • 2Pressure only affects equilibrium position if the number of moles of gas differs on each side.
  • 3A catalyst speeds up reaching equilibrium but does NOT change the position of equilibrium — examiners test this directly.
Practice questions

Try these yourself

Open each answer only after you have explained the full chemical process.

1Define dynamic equilibrium.[2 marks]
Mark scheme
  1. 1.Include both directions of reaction and rates.
Dynamic equilibrium exists when the rate of the forward reaction equals the rate of the reverse reaction (1) and the concentrations of reactants and products remain constant (1).
2State Le Chatelier's principle.[1 mark]
Mark scheme
  1. 1.Quote the principle precisely.
If a system at equilibrium is subjected to a change, the equilibrium shifts in the direction that opposes the change (1).
3For the reaction H₂(g) + I₂(g) ⇌ 2HI(g), predict the effect of increasing pressure.[2 marks]
Mark scheme
  1. 1.Count moles of gas on each side.
The number of moles of gas is equal on both sides (1 + 1 = 2) (1); increasing pressure has no effect on the equilibrium position (1).
4For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), predict the effect of increasing pressure on the yield of SO₃.[3 marks]
Mark scheme
  1. 1.Compare moles of gas on each side.
Left has 3 moles of gas; right has 2 moles (1); increasing pressure shifts equilibrium to the side with fewer gas moles (1), i.e. to the right, increasing the yield of SO₃ (1).
5Explain why the Haber process uses a temperature of about 450°C rather than a higher temperature.[3 marks]
Mark scheme
  1. 1.Link temperature to yield and rate.
The forward reaction is exothermic, so a higher temperature shifts equilibrium left, reducing yield (1); 450°C is a compromise that gives a reasonable rate of reaction (1) without too great a reduction in yield (1).
6Explain the effect of increasing the concentration of SO₂ in the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).[3 marks]
Mark scheme
  1. 1.Apply Le Chatelier's principle to a concentration change.
Increasing [SO₂] disturbs the equilibrium (1); the system shifts to the right to remove the extra SO₂ (1), increasing the yield of SO₃ (1).
7Explain why a catalyst does not change the equilibrium position but does change how quickly equilibrium is reached.[4 marks]
Mark scheme
  1. 1.Link catalyst to activation energy of both reactions.
A catalyst lowers the activation energy of both the forward and reverse reactions equally (1); both rates increase by the same factor (1); equilibrium is reached faster (1) but the equilibrium position is unchanged (1).
8Describe the conditions used in the Contact process for making sulfuric acid and explain why these conditions are chosen.[3 marks]
Mark scheme
  1. 1.State temperature, pressure and catalyst and justify each.
Temperature: 450°C — a compromise between yield (lower temperature favours product) and reaction rate (1); Pressure: 1–2 atm — higher pressure increases yield but is too expensive/dangerous for industrial use (1); Catalyst: vanadium(V) oxide (V₂O₅) — increases rate without affecting yield (1).
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