How is an oxidation number different from an ionic charge?

The oxidation number is the value assigned to one element inside a compound, while the charge belongs to the whole ion: manganese is plus seven in MnO4- though the ion carries minus one.

How do I spot a redox reaction quickly?

Compare oxidation numbers: a reaction is redox when one element rises and another falls.

S4 Chemistry: Redox and Electrolysis (Paper 1)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

This paper starts from oxidation numbers, then asks which species is oxidised and which is reduced.

Direct answer　Write the oxidation number of each element first: a rise means oxidation and a fall means reduction.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry: Redox and Electrolysis
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationFind the oxidation number of manganese in potassium manganate(VII), KMnO4.
FoundationFind the oxidation number of iron in iron(III) oxide, Fe2O3.
FoundationFind the oxidation number of sulphur in sulphuric acid, H2SO4.
FoundationIs the half-equation Zn to Zn2+ plus two electrons oxidation or reduction?
IntermediateIn 2Mg plus O2 giving 2MgO, which substance is oxidised and which is reduced?
IntermediateFind the oxidation number of chromium in the dichromate ion, Cr2O72-.
IntermediateIn Cl2 plus 2KBr giving 2KCl plus Br2, which species is the oxidising agent?
ChallengeIn MnO4- plus 8H+ plus 5 electrons giving Mn2+ plus 4H2O, how does manganese change and is the charge balanced?
ChallengeCombine Fe2+ giving Fe3+ plus an electron with the manganate(VII) half-equation into one balanced ionic equation.
Error redoError repair: a student writes that oxidation means losing oxygen. Correct the statement using oxidation numbers.

03　Answers and one-line notes

#	Answer	One-line note
1	+7	Potassium is plus one and four oxygens total minus eight, so manganese must be plus seven.
2	+3	Three oxygens give minus six, so two irons share plus six and each is plus three.
3	+6	Two hydrogens give plus two and four oxygens minus eight, so sulphur is plus six.
4	Oxidation	Zinc rises from zero to plus two and loses electrons, which is oxidation.
5	magnesium is oxidised from 0 to +2, oxygen is reduced from 0 to -2	Magnesium loses electrons and rises in number while oxygen gains electrons and falls, both at once.
6	+6	Seven oxygens give minus fourteen and the ion carries minus two, so two chromiums share plus twelve.
7	Cl2	Chlorine falls from zero to minus one, and the species that is reduced acts as the oxidising agent.
8	from +7 to +2 by gaining 5 electrons; both sides carry +2	Manganese falls from plus seven to plus two by gaining five electrons, and both sides carry plus two.
9	5Fe2+ + MnO4- + 8H+ gives 5Fe3+ + Mn2+ + 4H2O	Manganese needs five electrons so the iron half-equation is multiplied by five, and both sides carry plus seventeen.
10	oxidation is loss of electrons with the oxidation number rising; losing oxygen is only one case	For example zinc turns into Zn2+ with no oxygen involved, so the modern definition follows electrons and oxidation numbers.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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