Redox
Cambridge IGCSE Chemistry (0620) · Unit 6: Chemical reactions · 12 flashcards
Redox is topic 6.3 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 6 — Chemical reactions , alongside Rate of reaction and Reversible reactions and equilibrium. In one line: A reversible reaction is shown by the symbol ⇌, indicating that the reaction can proceed in both the forward and reverse directions.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).
The deck below contains 12 flashcards — 2 definitions, 5 key concepts and 5 identification cards — covering the precise wording mark schemes reward. Use the 2 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 symbol is used to indicate a reversible reaction
A reversible reaction is shown by the symbol ⇌, indicating that the reaction can proceed in both the forward and reverse directions.
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).
- Use Use Roman numerals to show an element's oxidation number in a compound.
- Define Define a redox reaction as one in which oxidation and reduction happen at the same time.
- Define Define oxidation as gaining oxygen, and reduction as losing oxygen.
- Identify Identify a reaction as redox when one substance gains oxygen and another loses it.
- Identify Identify which substance is oxidised and which is reduced in a redox reaction.
- Define Define oxidation in two ways: as a loss of electrons, and as a rise in oxidation number. Supplement
- Define Define reduction in two ways: as a gain of electrons, and as a fall in oxidation number. Supplement
- Identify Identify a reaction as redox when electrons are lost by one substance and gained by another. Supplement
- Identify Use changes in oxidation number to identify redox reactions, applying these rules: an uncombined element has an oxidation number of zero; a monatomic ion's oxidation number equals its charge; the oxidation numbers in a compound add up to zero; and in an ion they add up to the ion's charge. Supplement
- Identify Identify a redox reaction from the colour change seen when testing with acidified potassium manganate(VII) solution or with potassium iodide solution. Supplement
- Define Define an oxidising agent: a substance that is reduced itself while it oxidises something else. Supplement
- Define Define a reducing agent: a substance that is oxidised itself while it reduces something else. Supplement
- Identify Identify which substance acts as the oxidising agent and which as the reducing agent in a redox reaction. Supplement
What symbol is used to indicate a reversible reaction?
A reversible reaction is shown by the symbol ⇌, indicating that the reaction can proceed in both the forward and reverse directions.
How does heat affect the direction of the reversible reaction involving hydrated copper(II) sulfate?
Heating hydrated copper(II) sulfate (CuSO₄·5H₂O) drives off water, changing it from blue to anhydrous white copper(II) sulfate (CuSO₄). This favors the reverse reaction. Cooling and adding water reverses this, returning it to the blue hydrated form.
What happens to anhydrous cobalt(II) chloride when water is added?
Anhydrous cobalt(II) chloride (CoCl₂) is blue. Adding water causes it to become hydrated cobalt(II) chloride (CoCl₂·6H₂O), which is pink. The forward reaction (anhydrous to hydrated) is favored.
What two conditions define equilibrium in a closed system?
A reversible reaction in a closed system is at equilibrium when: (a) the rate of the forward reaction is equal to the rate of the reverse reaction, AND (b) the concentrations of reactants and products are no longer changing.
How does increasing temperature affect the equilibrium position of an exothermic reversible reaction?
Increasing the temperature shifts the equilibrium position to favor the endothermic reaction. For an exothermic forward reaction, this means the equilibrium will shift to the left, favoring reactants and reducing product yield.
How does increasing pressure affect the equilibrium position of a reversible reaction with more moles of gas on the reactant side?
Increasing the pressure shifts the equilibrium position to favor the side with fewer moles of gas.
What is the effect of a catalyst on the position of equilibrium?
A catalyst does NOT affect the position of equilibrium. It only speeds up the rate at which equilibrium is reached, by lowering the activation energy for both forward and reverse reactions equally.
What is the balanced symbol equation for the Haber process?
The symbol equation for the Haber process is N₂(g) + 3H₂(g) ⇌ 2NH₃(g). This represents the reversible reaction between nitrogen and hydrogen to produce ammonia.
What are the sources of hydrogen and nitrogen used in the Haber process?
The hydrogen in the Haber process comes from methane (natural gas). The nitrogen comes directly from the air.
What are the typical conditions used in the Haber process?
The Haber process typically uses the following conditions: 450°C, 20,000 kPa (200 atm) pressure, and an iron catalyst.
What is the balanced symbol equation for the key reaction in the Contact process?
The key reaction is the conversion of sulfur dioxide to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).
What are the sources of sulfur dioxide and oxygen in the Contact process?
The sulfur dioxide (SO₂) comes from burning sulfur or roasting sulfide ores. The oxygen (O₂) is obtained from the air.
Key Questions: Redox
What symbol is used to indicate a reversible reaction?
A reversible reaction is shown by the symbol ⇌, indicating that the reaction can proceed in both the forward and reverse directions.
What two conditions define equilibrium in a closed system?
A reversible reaction in a closed system is at equilibrium when: (a) the rate of the forward reaction is equal to the rate of the reverse reaction, AND (b) the concentrations of reactants and products are no longer changing.
More topics in Unit 6 — Chemical reactions
Redox 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 Redox 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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