Who this is for: Cambridge IGCSE Chemistry (0620) students who want reversible reactions and equilibrium — the ⇌ symbol, closed systems and predicting position shifts — to become reliable marks instead of definitions they cannot apply to unfamiliar scenarios.
What query it owns: how to understand and revise reversible reactions and equilibrium in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the reversible reactions and equilibrium revision-guide angle, while Tutopiya’s Reversible Reactions and Equilibrium subtopic page owns the learning resource and the free Reversible Reactions and Equilibrium quiz owns the practice.

Reversible reactions and equilibrium are tested every year in Cambridge IGCSE Chemistry (0620). Examiners expect you to write reversible equations with the ⇌ symbol, explain why a closed system is needed for equilibrium, and predict how changing conditions shifts the equilibrium position. This guide covers the syllabus content, the explain chains that earn marks, and the question types that appear in Paper 2 and Paper 4.

Key takeaways

• A reversible reaction can proceed in both directions — products can reform reactants.
• Equilibrium is reached in a closed system when the forward and reverse rates are equal.
• At equilibrium, concentrations stay constant but the reaction has not stopped.
• Le Chatelier’s principle: a system at equilibrium shifts to oppose a change in conditions.

What are reversible reactions and equilibrium in Cambridge IGCSE Chemistry?

A reversible reaction is one where the products can react to re-form the original reactants. The symbol ⇌ is used in equations. When a reversible reaction occurs in a closed system (no substances escape), the forward and reverse reactions eventually reach equal rates — this is dynamic equilibrium. The position of equilibrium describes whether there are more reactants or products at equilibrium, and it shifts when conditions change.

You can read the full explanation, worked examples and notes on Tutopiya’s Reversible Reactions and Equilibrium subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Reversible reaction	Products reform reactants	”Write a reversible equation for the Haber process.”
Closed system	No reactants or products escape	”Explain why equilibrium needs a closed system.”
Dynamic equilibrium	Forward rate = reverse rate	”State what happens to rates at equilibrium.”
Equilibrium position	Relative amounts of reactants/products	”Predict the effect of increasing pressure.”
Le Chatelier	System opposes the change	”Explain why yield decreases when temperature rises (exothermic).”

How conditions shift equilibrium — the patterns to learn

Change	Effect on equilibrium (general)	Example
Increase concentration of reactant	Shifts right (more products)	More N₂ → more NH₃ in Haber process
Increase pressure (gases)	Shifts to side with fewer gas moles	3 moles → 2 moles: shifts right for N₂ + 3H₂ ⇌ 2NH₃
Increase temperature	Shifts in endothermic direction	Exothermic forward reaction: shifts left
Add catalyst	No shift in position — reaches equilibrium faster	Catalyst does not change yield at equilibrium

How to answer equilibrium questions — step by step

1. Write the reversible equation with ⇌ and state symbols where given.
2. Identify whether the forward reaction is exothermic or endothermic (often stated in the question).
3. State the change (e.g. pressure increased, temperature decreased).
4. Apply Le Chatelier: system shifts to oppose the change.
5. Conclude the effect on yield or equilibrium position.

Once you have worked through a few, test yourself with the free Reversible Reactions and Equilibrium quiz — it tells you fast whether the shift predictions have actually stuck.

Reversible reactions in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical equilibrium stem
Explain	Full Le Chatelier chain	”Explain the effect of increasing pressure on yield.”
Predict	State direction of shift	”Predict the effect of adding a catalyst.”
State	Short factual answer	”State two characteristics of dynamic equilibrium.”
Describe	What happens at equilibrium	”Describe what happens in a closed system at equilibrium.”
Write an equation	Reversible equation with ⇌	“Write a reversible equation for the decomposition of NH₃.”

Worked exam-style stems (how to answer the wording)

1. “N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol. Explain the effect of increasing temperature on the yield of ammonia.” Forward reaction is exothermic. Increasing temperature → system shifts in the endothermic direction (left) to oppose the temperature rise → less NH₃ produced → yield decreases. Mark-scheme reward: exothermic forward + shift left + lower yield.
2. “Explain why a catalyst does not change the position of equilibrium.” A catalyst speeds up both forward and reverse reactions equally → equilibrium is reached faster but the relative amounts of reactants and products at equilibrium are unchanged. Reward: both directions equally + no position change.
3. “State two characteristics of dynamic equilibrium.” (Any two from:) rates of forward and reverse reactions are equal; concentrations of reactants and products remain constant; reaction occurs in a closed system; macroscopic properties show no change. Reward: any two valid characteristics.

When you can recognise the wording instantly, work the full set on the Chemical Reactions topical past paper questions and the Reversible Reactions and Equilibrium quiz to lock the method in.

How reversible reactions connect to the rest of the syllabus

Reversible reactions link to Chemical Energetics (exothermic/endothermic direction) and Rate of Reaction (catalysts and collision theory). The Haber process and Contact process are classic industrial examples. When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub links every Chemical Reactions subtopic.

Common mistakes students make

• Saying the reaction stops at equilibrium (it is dynamic — both directions continue).
• Thinking a catalyst increases yield (it only speeds up attainment of equilibrium).
• Applying pressure changes to reactions with equal moles of gas on both sides (no shift).
• Forgetting to state the forward reaction is exothermic or endothermic before predicting temperature effects.
• Using an open system when explaining equilibrium (products must not escape).

When you need more support

If equilibrium questions keep costing marks — especially Le Chatelier explain chains — work through the Chemical Reactions topical past paper questions and the Reversible Reactions and Equilibrium quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

Is equilibrium hard in Cambridge IGCSE Chemistry? The principles are consistent once you learn Le Chatelier’s patterns. Marks are lost when students say reactions stop at equilibrium or think catalysts change yield.

What is a closed system? A system where no reactants or products can escape — essential for equilibrium to be established.

How does temperature affect equilibrium? The system shifts in the endothermic direction to oppose the temperature change. For an exothermic forward reaction, increasing temperature shifts left.

How do I revise reversible reactions effectively? Learn the Haber process equation, practise shift predictions for pressure and temperature, then take the Reversible Reactions and Equilibrium quiz.

Ready to master Cambridge IGCSE Chemistry reversible reactions and equilibrium?

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