Reversible reactions and equilibria

Chemistry· 3.17–3.22 (S3 sub-topic 3(c))· 15 min read

1. Reversible Reactions (Core Content, All Tiers)★★☆☆☆⏱ 5 min

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📘 Definition

Reversible Reaction

A chemical reaction that can proceed in both the forward (reactants → products) and reverse (products → reactants) directions, indicated by the ⇌ symbol in balanced equations.

You must recall two key reversible reactions for the exam: the dehydration of hydrated copper(II) sulfate and the effect of heat on ammonium chloride.

📐 Worked Example

Write the balanced equation for the reversible dehydration of hydrated copper(II) sulfate, including state symbols.

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1. Identify the reactant: hydrated copper(II) sulfate is blue crystalline $C u S O_{4} \cdot 5 H_{2} O (s)$
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1. Identify products on heating: white anhydrous copper(II) sulfate $C u S O_{4} (s)$ and water vapour $H_{2} O (g)$
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1. Add the reversible symbol (the equation is already balanced):
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$CuSO_4 \cdot 5H_2O(s) \rightleftharpoons CuSO_4(s) + 5H_2O(g)$

📐 Worked Example

Describe the reversible change observed when ammonium chloride is heated in a sealed test tube.

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1. Heating white solid ammonium chloride causes it to thermally decompose into colourless ammonia and hydrogen chloride gases
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1. As the gases cool at the top of the sealed tube, they react to reform solid white ammonium chloride, which deposits on the glass
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$NH_4Cl(s) \rightleftharpoons NH_3(g) + HCl(g)$

Exam tip:

You may be asked to link copper sulfate dehydration to the test for water: adding water to white anhydrous copper sulfate turns it blue, confirming water is present.

2. Dynamic Equilibrium (Higher/Chemistry Only)★★★☆☆HL only⏱ 4 min

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📘 Definition

Dynamic Equilibrium

State reached in a closed (sealed) system where the rate of the forward reaction equals the rate of the reverse reaction, so concentrations of reactants and products remain constant over time.

Note that 'dynamic' means both reactions are still occurring constantly, not that the reaction has stopped. The equal reaction rates mean no overall observable change is seen at equilibrium.

📐 Worked Example

A student heats hydrated copper sulfate in an open beaker and claims equilibrium is reached. Explain why they are incorrect.

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1. Dynamic equilibrium requires a closed system, where no substances can enter or leave.
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1. In an open beaker, water vapour produced from dehydration escapes into the air, so the reverse reaction (reforming hydrated copper sulfate) cannot occur at the same rate as the forward reaction.
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1. Equilibrium is therefore never achieved in an open container for this reaction.

3. Factors Affecting Equilibrium Position (Higher/Chemistry Only)★★★★☆HL only⏱ 6 min

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The position of equilibrium describes whether the mixture contains more reactants or products at equilibrium. Changes to temperature or pressure can shift this position, while catalysts have no effect.

📘 Definition

Catalyst effect on equilibrium

A catalyst speeds up both the forward and reverse reaction rates equally, so it does not change the position of equilibrium, only reduces the time taken to reach equilibrium.

Temperature change: An increase in temperature shifts equilibrium in the direction of the endothermic reaction (absorbs heat). A decrease in temperature shifts equilibrium in the direction of the exothermic reaction (releases heat).
Pressure change (only applies to gas-phase reactions): An increase in pressure shifts equilibrium towards the side with fewer moles of gas. A decrease in pressure shifts equilibrium towards the side with more moles of gas.

📐 Worked Example

For the reversible reaction below, predict the effect of (a) increasing temperature, (b) increasing pressure: $N_{2} (g) + 3 H_{2} (g) ⇌ 2 N H_{3} (g)$ ΔH = -92 kJ/mol (forward reaction exothermic)

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(a) Increasing temperature: Shift occurs in the endothermic direction. Since forward is exothermic, reverse is endothermic, so equilibrium shifts left, forming more $N_{2}$ and $H_{2}$ .
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(b) Count moles of gas on each side: Left side = 1 + 3 = 4 moles of gas, right side = 2 moles of gas. Increasing pressure shifts to fewer moles, so equilibrium shifts right, forming more $N H_{3}$ .

Exam tip:

Always state your reasoning when predicting shifts: e.g., 'shift left because the reverse reaction is endothermic' rather than just writing 'shifts left' to secure full marks.

4. Key Recap and Concept Check★★★☆☆⏱ 2 min

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✓ Quick check

Which symbol indicates a reversible reaction?
- →
- ↔
- ⇌
- =
Reveal answer
2 —
Correct. The ⇌ symbol is used exclusively for reversible reactions in Edexcel IGCSE Chemistry.
(Higher only) Which statement about dynamic equilibrium is true?
- Reactions stop when equilibrium is reached
- Forward reaction rate is higher than reverse rate
- Concentrations of reactants and products are equal
- Equilibrium only occurs in closed systems
Reveal answer
3 —
Correct. A closed sealed system is required so no reactants or products can escape, allowing rates to equalise.

5. Common Pitfalls

Wrong move:

Using → instead of ⇌ for reversible reactions

Why:

The single arrow indicates a reaction that goes to completion, not reversible, so you lose the mark for correct symbol use.

Correct move:

Always use ⇌ for all reversible reaction equations, including the two named core examples.

Wrong move:

Stating that reactions stop at dynamic equilibrium

Why:

Equilibrium is dynamic, meaning both forward and reverse reactions are still occurring at equal rates, with no net change.

Correct move:

Explicitly state that both reactions continue at the same rate at equilibrium.

Wrong move:

Claiming catalysts shift equilibrium position

Why:

Catalysts speed up forward and reverse rates equally, so there is no change to equilibrium position or product yield.

Correct move:

State that catalysts only reduce the time taken to reach equilibrium, with no effect on the equilibrium position.

Wrong move:

Forgetting to state a closed system is required for equilibrium

Why:

Exam answers require explicit mention of a closed/sealed system to award full marks for equilibrium conditions.

Correct move:

Always include reference to a closed system when explaining how equilibrium is achieved.

Wrong move:

Applying pressure shift rules to reactions with no gaseous reactants/products

Why:

Pressure only affects equilibrium of gas-phase reactions, not reactions with only solid or liquid reactants/products.

Correct move:

First check for gas state symbols before applying pressure shift rules, and count only moles of gas on each side of the equation.

6. Quick Reference Cheatsheet

Content Area	Core Tier Requirement	Higher Tier Requirement
Reversible reaction symbol	Recall ⇌ means reversible	Same as core
Named reactions	Describe copper sulfate dehydration and ammonium chloride heating reversibility	Same as core
Dynamic equilibrium	Not assessed	Closed system, equal forward/reverse rates, constant reactant/product concentrations
Catalyst effect	Not assessed	No change to equilibrium position, only speeds up reaching equilibrium
Temperature shift	Not assessed	Increase temp → shift to endothermic direction; decrease temp → shift to exothermic direction
Pressure shift	Not assessed	Increase pressure → shift to side with fewer gas moles; decrease pressure → shift to side with more gas moles

7. Frequently Asked

Do I need to learn Le Chatelier's principle for this exam?

No, the Edexcel 4CH1 specification explicitly states Le Chatelier's principle is not required. You only need to apply the given temperature and pressure shift rules to answer questions.

Can equilibrium be achieved in an open container?

No, dynamic equilibrium requires a sealed (closed) system so no reactants or products can escape, allowing forward and reverse reaction rates to equalise.

Going deeper

official_specEdexcel IGCSE Chemistry 4CH1 SpecificationFull official syllabus for reference

What's Next

Now that you have mastered reversible reactions and equilibria, you can move on to applying these rules to context-based exam questions, and link this content to other physical chemistry topics such as reaction rates and energetics. This knowledge will also form a strong foundation if you progress to A-Level Chemistry, where you will learn about quantitative equilibrium calculations. Be sure to practice past paper questions specifically for this topic to test your ability to apply shift rules correctly under timed exam conditions.

→Energetics (S3_T04)