6.2

Reversible reactions and equilibrium

Cambridge IGCSE Chemistry (0620)  · Unit 6: Chemical reactions  · 12 flashcards

Reversible reactions and equilibrium is topic 6.2 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 6 — Chemical reactions , alongside Rate of reaction and Redox.  In one line: Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products are constant. The system is closed, meaning no matter can enter or leave.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 show this topic across undefined questions worth 237 marks (around 3.7% of all Chemistry marks in those years).

The deck below contains 12 flashcards — 3 definitions and 9 key concepts — covering the precise wording mark schemes reward.  Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

What condition defines dynamic equilibrium

Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products are constant. The system is closed, meaning no matter can enter or leave.

What the Cambridge 0620 syllabus says

Official 2026-2028 spec

These are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.

  1. Describe Describe the effect on the rate of reaction of: (a) changing the concentration of solutions (b) changing the pressure of gases (c) changing the surface area of solids (d) changing the temperature (e) adding or removing a catalyst, including enzymes
  2. State State that a catalyst increases the rate of a reaction and is unchanged at the end of a reaction
  3. Describe Describe practical methods for investigating the rate of a reaction including change in mass of a reactant or a product and the formation of a gas
  4. Interpret Interpret data, including graphs, from rate of reaction experiments
  5. Describe Describe collision theory in terms of: (a) number of particles per unit volume (b) frequency of collisions between particles (c) kinetic energy of particles (d) activation energy, Ea Supplement
  6. Describe Describe and explain the effect on the rate of reaction of: (a) changing the concentration of solutions (b) changing the pressure of gases (c) changing the surface area of solids (d) changing the temperature (e) adding or removing a catalyst, including enzymes using collision theory Supplement
  7. State State that a catalyst decreases the activation energy, E, of a reaction Supplement
  8. Evaluate Evaluate practical methods for investigating the rate of a reaction including change in mass of a reactant or a product and the formation of a gas Supplement
Definition Flip

What condition defines dynamic equilibrium?

Answer Flip

Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products are constant. The system is closed, meaning no matter can enter or leave.

Definition Flip

What is a closed system?

Answer Flip

A closed system is one where matter cannot enter or leave. Energy can still be exchanged with the surroundings, but the amount of reactants/products remains constant if equilibrium is achieved.

Key Concept Flip

What does a negative ΔH value indicate?

Answer Flip

A negative ΔH value indicates that the forward reaction is exothermic, meaning it releases heat to the surroundings.

Example: N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = -92 kJ/mol
Key Concept Flip

What is the effect of increasing temperature on the equilibrium of an exothermic reaction?

Answer Flip

Increasing the temperature decreases the yield of the products in an exothermic reaction. According to Le Chatelier's principle, the equilibrium will shift to favour the endothermic (reverse) reaction to counteract the increase in temperature.

Key Concept Flip

What is the effect of adding a reactant (e.g., CO) on the equilibrium position?

Answer Flip

Adding a reactant increases the yield of the products. The equilibrium will shift to the right to consume the added reactant (CO) and re-establish equilibrium.

Key Concept Flip

What effect does increasing the pressure have on an equilibrium where the number of moles of gas is different on each side?

Answer Flip

Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas. For

Example: N2(g) + 3H2(g) ⇌ 2NH3(g). Increasing pressure favors the formation of ammonia (NH3).
Key Concept Flip

What effect does a catalyst have on the position of equilibrium?

Answer Flip

A catalyst has no effect on the position of equilibrium. It increases the rate of both the forward and reverse reactions equally, so the equilibrium is reached faster, but the final concentrations of reactants and products remain the same.

Key Concept Flip

How does increasing the concentration of a solution affect the rate of reaction?

Answer Flip

Increasing the concentration of a solution increases the rate of reaction. According to collision theory, a higher concentration means more particles per unit volume, leading to a higher frequency of collisions.

Key Concept Flip

How does increasing the surface area of a solid affect the rate of reaction?

Answer Flip

Increasing the surface area of a solid increases the rate of reaction. A larger surface area provides more area for the reactants to collide, leading to a higher frequency of successful collisions.

Example: powdered zinc reacts faster than a zinc strip.
Key Concept Flip

How does increasing temperature affect the rate of reaction according to collision theory?

Answer Flip

Increasing temperature increases the rate of reaction. Higher temperature means particles have more kinetic energy, leading to more frequent and more energetic collisions. More collisions exceed the activation energy, Ea, resulting in more successful reactions.

Key Concept Flip

How does adding a catalyst affect the rate of reaction according to collision theory?

Answer Flip

Adding a catalyst increases the rate of reaction by decreasing the activation energy (Ea). This means that a greater proportion of collisions have enough energy to overcome the energy barrier and react. The catalyst itself is unchanged.

Definition Flip

What is activation energy (Ea)?

Answer Flip

Activation energy (Ea) is the minimum amount of energy required for a reaction to occur during a collision between particles. If colliding particles do not possess at least this much kinetic energy, the collision will not result in a reaction.

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6.1 Rate of reaction 6.3 Redox

Key Questions: Reversible reactions and equilibrium

What condition defines dynamic equilibrium?

Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products are constant. The system is closed, meaning no matter can enter or leave.

What is a closed system?

A closed system is one where matter cannot enter or leave. Energy can still be exchanged with the surroundings, but the amount of reactants/products remains constant if equilibrium is achieved.

What is activation energy (Ea)?

Activation energy (Ea) is the minimum amount of energy required for a reaction to occur during a collision between particles. If colliding particles do not possess at least this much kinetic energy, the collision will not result in a reaction.

Tips to avoid common mistakes in Reversible reactions and equilibrium

More topics in Unit 6 — Chemical reactions

Reversible reactions and equilibrium sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

6.1 Rate of reaction

9 flashcards
6.3 Redox

12 flashcards

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

reversible reaction equilibrium dynamic equilibrium forward reaction backward reaction Le Chatelier Le Chatelier's principle position of equilibrium pressure temperature concentration Haber process ammonia Contact process sulfuric acid

Key terms covered in this Reversible reactions and equilibrium deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

What condition defines dynamic equilibrium
Closed system
Activation energy (Ea)

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