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.
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
2026-2028 syllabus, in our wordsWhat the syllabus expects you to be able to do in this topic, in plain English. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks. For the exact wording, see the official syllabus (PDF).
- State State that certain chemical reactions can be reversed, and that the ⇌ symbol is used to show this.
- Describe Describe how a reversible reaction can be pushed in either direction by changing conditions, for heating a hydrated compound and for adding water to an anhydrous compound. Only cobalt(II) chloride and copper(II) sulfate are required.
- State State that, in a closed system, a reversible reaction has reached equilibrium when the forward and reverse reactions go at equal rates and the reactant and product concentrations stop changing. Supplement
- Predict Using information provided to you, predict and explain what happens to the equilibrium position of a reversible reaction when the temperature, pressure or concentration is changed, or when a catalyst is used. Supplement
- State State the Haber process symbol equation for making ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Supplement
- State State where the raw materials for the Haber process come from: hydrogen from methane and nitrogen from the air. Supplement
- State State the usual conditions for the Haber process: an iron catalyst, 450 °C and 20 000 kPa (200 atm). Supplement
- State State the Contact process equation for turning sulfur dioxide into sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). Supplement
- State State where the Contact process gets its oxygen (from air) and its sulfur dioxide (from burning sulfur or from roasting sulfide ores). Supplement
- State State the usual Contact process conditions for turning sulfur dioxide into sulfur trioxide: a vanadium(V) oxide catalyst, 450 °C and 200 kPa (2 atm). Supplement
- Explain Explain why the usual conditions are chosen for both the Haber and Contact processes, using ideas about reaction rate and equilibrium position, and taking economic and safety factors into account. Supplement
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 does a negative ΔH value indicate?
A negative ΔH value indicates that the forward reaction is exothermic, meaning it releases heat to the surroundings.
What is the effect of increasing temperature on the equilibrium of an exothermic reaction?
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.
What is the effect of adding a reactant (e.g., CO) on the equilibrium position?
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.
What effect does increasing the pressure have on an equilibrium where the number of moles of gas is different on each side?
Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas. For
What effect does a catalyst have on the position of equilibrium?
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.
How does increasing the concentration of a solution affect the rate of reaction?
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.
How does increasing the surface area of a solid affect the rate of reaction?
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.
How does increasing temperature affect the rate of reaction according to collision theory?
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.
How does adding a catalyst affect the rate of reaction according to collision theory?
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.
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.
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
- ● Understand that concentration impacts reaction rate via collision frequency: more particles in the same volume, more collisions.
- ● Consider both what a metal does and does not react with to gauge its reactivity.
- ● Learn common applications: aluminium is used where lightness and corrosion resistance are key, stainless steel for its strength and resistance to rust.
- ● Create a table summarizing the reactions of common metals (Ca, Mg, Zn, Fe, Cu) with water, steam, and dilute acids, linking it to the metal reactivity series.
- ● When practising reactions, get used to writing the equations forwards AND backwards — note which way is exothermic/endothermic.
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.
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.
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.
Related Chemistry guides
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