What do oxidation numbers add up to in a neutral compound? In an ion?

Zero in a neutral compound; the charge on the ion in an ion.

What does a Roman numeral in a name show?

The oxidation number of the element, for example iron(III) is +3.

Define oxidation in terms of electrons and oxidation number.

Loss of electrons; an increase in oxidation number.

Define reduction in terms of electrons and oxidation number.

Gain of electrons; a decrease in oxidation number.

What is an oxidising agent?

A species that gains electrons (is reduced) and so oxidises another species.

What is a reducing agent?

A species that loses electrons (is oxidised) and so reduces another species.

Define disproportionation.

An element in a single species is simultaneously oxidised and reduced.

Give an example of disproportionation.

Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O (chlorine goes from 0 to −1 and +1).

Write the half-equation for MnO₄⁻ reduced to Mn²⁺ in acid.

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

Oxidation numbers and redoxEdexcel International A Level Chemistry: Flashcards

Card 1 of 140 of 14 known

Question

What is the oxidation number of an atom in an uncombined element?

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

What is the oxidation number of an atom in an uncombined element?: Zero.
What do oxidation numbers add up to in a neutral compound? In an ion?: Zero in a neutral compound; the charge on the ion in an ion.
What is the oxidation number of hydrogen in a metal hydride?: −1, for example in NaH.
What is the oxidation number of oxygen in a peroxide?: −1, for example in H₂O₂.
What is the oxidation number of sulfur in SO₄²⁻?: +6.
What does a Roman numeral in a name show?: The oxidation number of the element, for example iron(III) is +3.
Write the formula of vanadium(V) oxide.: V₂O₅.
Define oxidation in terms of electrons and oxidation number.: Loss of electrons; an increase in oxidation number.
Define reduction in terms of electrons and oxidation number.: Gain of electrons; a decrease in oxidation number.
What is an oxidising agent?: A species that gains electrons (is reduced) and so oxidises another species.
What is a reducing agent?: A species that loses electrons (is oxidised) and so reduces another species.
Define disproportionation.: An element in a single species is simultaneously oxidised and reduced.
Give an example of disproportionation.: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O (chlorine goes from 0 to −1 and +1).
Write the half-equation for MnO₄⁻ reduced to Mn²⁺ in acid.: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

Exam questions on Oxidation numbers and redox

Sulfur forms compounds and ions in which its oxidation number ranges from −2 to +6. Examples include hydrogen sulfide, H₂S, sulfur dioxide, SO₂, the sulfate ion, SO₄²⁻, and the sulfite ion, SO₃²⁻. In the contact process sulfur dioxide is oxidised to sulfur trioxide by oxygen.
In the reaction 2SO₂ + O₂ → 2SO₃, state which element is oxidised and which is reduced. Give the oxidation numbers to support your answer.2 marks
Hydrides and peroxides contain elements in unusual oxidation states. Sodium hydride, NaH, reacts with water: NaH + H₂O → NaOH + H₂. Hydrogen peroxide, H₂O₂, decomposes when a catalyst is added: 2H₂O₂ → 2H₂O + O₂.
Deduce the changes in oxidation number of hydrogen in the reaction NaH + H₂O → NaOH + H₂, and state which species is the reducing agent.2 marks
Acidified potassium manganate(VII) solution is purple. When it is added to a solution containing iron(II) ions, the purple colour disappears and iron(III) ions and manganese(II) ions form.
Write ionic half-equations for (i) the reduction of MnO₄⁻ to Mn²⁺ in acid solution and (ii) the oxidation of Fe²⁺ to Fe³⁺.3 marks

See the full worksheet

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).