Oxidation and reduction always happen together: if one substance gains oxygen, another loses it; if one loses electrons, another picks them up. Spot which is which and you can name the oxidising and reducing agents in any redox reaction.
Oxidation is gain of oxygen and reduction is loss of oxygen; a reaction in which both happen is a redox reaction. A Roman numeral in a name gives the oxidation number of the element, as in iron(III) oxide. Extended: oxidation is also loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number. An oxidising agent oxidises another substance and is itself reduced; a reducing agent reduces another substance and is itself oxidised.
Fig. 1: electron transfer from zinc to copper(II) ions
Section 01
Oxygen in, oxygen out.
The simplest definitions use oxygen. Find the substance that gains oxygen and the one that loses it.
Extracting iron: Fe2O3 + 3CO → 2Fe + 3CO2. Iron(III) oxide loses oxygen: it is reduced. Carbon monoxide gains oxygen: it is oxidised.
Burning magnesium: 2Mg + O2 → 2MgO. Magnesium gains oxygen: it is oxidised.
Copper(II) oxide and hydrogen: CuO + H2 → Cu + H2O. Copper(II) oxide is reduced; hydrogen is oxidised.
Roman numerals: iron(II) contains Fe2+ and iron(III) contains Fe3+; copper(II) contains Cu2+. The numeral is the oxidation number of the metal.
Stage 1 · Learn
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In terms of oxygen, oxidation is:
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In CuO + H2 → Cu + H2O, which substance is reduced?
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What does the (III) in iron(III) oxide show?
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(Extended) In terms of electrons, reduction is:
Section 02
Electrons and oxidation numbers.
Extended. Watch zinc atoms hand electrons to copper(II) ions in the simulator. There is no oxygen anywhere, yet this is still redox.
OIL RIG: oxidation is loss of electrons, reduction is gain of electrons.
Zinc and copper(II) sulfate: Zn + Cu2+ → Zn2+ + Cu. Zinc loses two electrons (oxidised, 0 → +2); Cu2+ gains two electrons (reduced, +2 → 0).
Oxidation number rules: an element on its own is 0; a simple ion has its charge (Na+ is +1, O2− is −2); in a compound the numbers add up to zero.
Spotting redox: if any element's oxidation number changes, the reaction is redox. An increase is oxidation, a decrease is reduction.
Section 03
Oxidising and reducing agents.
Extended. Test yourself: for each reaction in the simulator, decide which species is oxidised and which is reduced, then name the agents.
Oxidising agent: takes electrons from another substance, so it is itself reduced. In Zn + Cu2+, Cu2+ is the oxidising agent.
Reducing agent: gives electrons to another substance, so it is itself oxidised. Zinc is the reducing agent.
Test for a reducing agent: acidified potassium manganate(VII) turns from purple to colourless.
Test for an oxidising agent: aqueous potassium iodide turns from colourless to brown, as iodide ions are oxidised to iodine.
Examiner trap
The oxidising agent is the substance that is reduced, not the one that is oxidised. It sounds backwards, so reason it out: it causes oxidation in something else by taking that thing's electrons, and taking electrons is reduction.
Stage 2 · Exam
Exam-style questions
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(Extended) In Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s), which statement is correct?
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(Extended) What is the oxidation number of chlorine in Cl2 and in Cl−?
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(Extended) Chlorine is bubbled into aqueous potassium iodide and the solution turns brown. Cl2 + 2I− → 2Cl− + I2. Which is the oxidising agent?
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(Extended) A colourless solution turns acidified potassium manganate(VII) from purple to colourless. What does this show?
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Redox, mastered.
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