What is an oxidising agent?

An electron acceptor, which is itself reduced.

What is a reducing agent?

An electron donor, which is itself oxidised.

What is the usual oxidation state of oxygen, and when is it different?

−2, but −1 in peroxides (such as H₂O₂) and +2 in OF₂.

What is the usual oxidation state of hydrogen, and when is it different?

+1, but −1 in metal hydrides (such as NaH).

Oxidation states and redoxAQA A-Level Chemistry: Flashcards

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What is oxidation?

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What is oxidation?: The process of electron loss (an increase in oxidation state).
What is reduction?: The process of electron gain (a decrease in oxidation state).
What is an oxidising agent?: An electron acceptor, which is itself reduced.
What is a reducing agent?: An electron donor, which is itself oxidised.
What is the oxidation state of an uncombined element?: Zero.
What do the oxidation states in a neutral compound add up to?: Zero.
What do the oxidation states in an ion add up to?: The charge on the ion.
What is the usual oxidation state of oxygen, and when is it different?: −2, but −1 in peroxides (such as H₂O₂) and +2 in OF₂.
What is the usual oxidation state of hydrogen, and when is it different?: +1, but −1 in metal hydrides (such as NaH).
What is the oxidation state of Mn in MnO₄⁻?: +7
What is the oxidation state of Cr in Cr₂O₇²⁻?: +6
What is the oxidation state of S in SO₄²⁻?: +6
What happens to oxidation state during oxidation?: It increases.

Exam questions on Oxidation states and redox

In a titration, a solution of iron(II) ions is oxidised by acidified potassium manganate(VII). The manganate(VII) ion, MnO₄⁻, is reduced to Mn²⁺, and the iron(II) ions are oxidised to iron(III) ions, Fe³⁺.
State the changes in oxidation state of manganese and of iron, and identify the reducing agent.2 marks
A technician is checking the oxidation states of an element in a range of compounds: sodium hydride, NaH; hydrogen peroxide, H₂O₂; potassium dichromate(VI), K₂Cr₂O₇; and sodium thiosulfate, Na₂S₂O₃.
Deduce the oxidation state of chromium in K₂Cr₂O₇ and of sulfur in Na₂S₂O₃. Show your working.2 marks
Solid sodium iodide reacts with concentrated sulfuric acid. Iodide ions are oxidised to iodine, and sulfuric acid is reduced to hydrogen sulfide. The ionic equation is 8I⁻ + 10H⁺ + SO₄²⁻ → 4I₂ + H₂S + 4H₂O.
Explain, in terms of electrons and oxidation states, why iodide ions are oxidised in this reaction and identify the oxidising agent.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).