What is an oxidising agent?

A species that gains electrons (and is reduced), oxidising another species.

What is a reducing agent?

A species that loses electrons (and is oxidised), reducing another species.

In 2Mg + O₂ → 2MgO, which is the reducing agent?

Magnesium (0 to +2, loses electrons).

How do metals generally form ions?

By electron loss, forming positive ions and increasing their oxidation number.

How do non-metals generally form ions?

By electron gain, forming negative ions and decreasing their oxidation number.

Define disproportionation.

A redox reaction in which an element in a single species is simultaneously oxidised and reduced.

Show Cl₂ + H₂O → HCl + HClO is disproportionation.

Cl goes from 0 to −1 (reduced) and from 0 to +1 (oxidised) in the same species, Cl₂.

What are the oxidation numbers of chlorine in cold NaOH disproportionation?

0 in Cl₂, −1 in NaCl and +1 in NaClO.

What are the oxidation numbers of chlorine in hot concentrated NaOH disproportionation?

0 in Cl₂, −1 in NaCl and +5 in NaClO₃.

Oxidation, reduction and disproportionationEdexcel A-Level Chemistry: Flashcards

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Define oxidation in terms of electrons.

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Define oxidation in terms of electrons.: Loss of electrons.
Define reduction in terms of electrons.: Gain of electrons.
What happens to oxidation number on oxidation?: It increases.
What is an oxidising agent?: A species that gains electrons (and is reduced), oxidising another species.
What is a reducing agent?: A species that loses electrons (and is oxidised), reducing another species.
In 2Mg + O₂ → 2MgO, which is the reducing agent?: Magnesium (0 to +2, loses electrons).
How do metals generally form ions?: By electron loss, forming positive ions and increasing their oxidation number.
How do non-metals generally form ions?: By electron gain, forming negative ions and decreasing their oxidation number.
Define disproportionation.: A redox reaction in which an element in a single species is simultaneously oxidised and reduced.
Show Cl₂ + H₂O → HCl + HClO is disproportionation.: Cl goes from 0 to −1 (reduced) and from 0 to +1 (oxidised) in the same species, Cl₂.
What are the oxidation numbers of chlorine in cold NaOH disproportionation?: 0 in Cl₂, −1 in NaCl and +1 in NaClO.
What are the oxidation numbers of chlorine in hot concentrated NaOH disproportionation?: 0 in Cl₂, −1 in NaCl and +5 in NaClO₃.

Exam questions on Oxidation, reduction and disproportionation

A teacher demonstrates two reactions of s- and p-block elements: magnesium ribbon burning in oxygen, 2Mg + O₂ → 2MgO, and sodium burning in chlorine, 2Na + Cl₂ → 2NaCl.
Explain, in terms of electron transfer, why magnesium acts as a reducing agent in its reaction with oxygen.2 marks
Chlorine gas is bubbled through aqueous potassium bromide. The solution turns orange as bromine forms. The ionic equation is Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
Use oxidation numbers to show that this is a redox reaction.2 marks
Chlorine is used to disinfect swimming pool water. When it dissolves in water it forms chloride ions and chlorate(I) ions, ClO⁻, which kill bacteria.
The reaction of chlorine with water is Cl₂ + H₂O → HCl + HClO. Use oxidation numbers to explain why this is a disproportionation.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).