Why balance electrons first?

Electrons cannot pile up in solution, so the number lost in oxidation must equal the number gained in reduction; atoms and charge then fall into place.

How does alkaline balancing differ?

Acidic solutions are balanced with H+ and water, alkaline ones with OH- and water, and the two must not be mixed.

S4 Chemistry: Redox and Electrolysis (Paper 2)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

This paper focuses on half-equations: balance the electrons first, then atoms and finally charge.

Direct answer　Three steps: equal electrons, equal atoms, equal charge.

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 oxygen in hydrogen peroxide, H2O2.
FoundationFind the oxidation number of sulphur in sodium thiosulphate, Na2S2O3.
FoundationIs Cu2+ plus two electrons giving Cu an oxidation or a reduction?
FoundationWrite the half-equation for iron(II) ions being oxidised to iron(III) ions.
IntermediateIn 2Fe3+ plus Sn2+ giving 2Fe2+ plus Sn4+, are the electrons balanced?
IntermediateFind the oxidation number of chromium in the chromate ion, CrO42-.
IntermediateCombine Na giving Na+ plus an electron with Cl2 plus two electrons giving 2Cl- into one equation.
ChallengeBalance MnO4- with C2O42- in acidic solution and give the full ionic equation.
ChallengeTitrating iron(II) solution with 25.0 cm3 of 0.0200 mol dm-3 KMnO4 needs how many moles of iron(II)?
Error redoError repair: a student balances MnO4- to Mn2+ with H+ in alkaline solution. What is wrong and what forms instead?

03　Answers and one-line notes

#	Answer	One-line note
1	-1	Two hydrogens give plus two and the molecule is neutral, so the two oxygens share minus two.
2	+2	Two sodiums give plus two and three oxygens minus six, so the two sulphurs share plus four.
3	Reduction	Copper falls from plus two to zero by gaining electrons, which is reduction.
4	Fe2+ gives Fe3+ + e-	Iron rises from plus two to plus three, so one electron is released.
5	yes: two irons gain one electron each and tin releases two	The left carries plus eight and so does the right, so both electrons and charge balance.
6	+6	Four oxygens give minus eight and the ion carries minus two, leaving chromium at plus six.
7	2Na + Cl2 gives 2NaCl	The sodium half-equation is doubled so that its two electrons are taken by one chlorine molecule.
8	2MnO4- + 16H+ + 5C2O42- gives 2Mn2+ + 8H2O + 10CO2	Each manganese takes five electrons for ten in total, so five oxalates release two each, and both sides carry plus four.
9	0.00250 mol	The manganate(VII) amount is 0.000500 mol and each ion takes five irons, giving 0.00250 mol.
10	H+ cannot be used in alkali; manganate(VII) is reduced to MnO2	An alkaline solution has no spare H+, so OH- and water are used and manganese only falls to plus four.

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.

Book an S4 Chemistry tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.