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InkFlaskCambridge IGCSE
Contains the answers

Answers · Redox

Electrons on the move — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. ZQ8 · Practice

Model answer. A copper atom has no charge; the Cu2+ ion has a charge of 2+. Electrons are negative, so the atom has lost two electrons. Loss of electrons is oxidation: copper is oxidised. Each silver ion, Ag+, gains one electron and is reduced to a silver atom.

A full answer includes

  • It loses two electrons, so it is oxidised.

Also accept

  • It loses two electrons and is oxidised

Watch for

  • “It gains two electrons, so it is oxidised.” — Electrons are negative: losing two of them leaves the ion with two positive charges, Cu2+. And a gain of electrons would be reduction.
  • “It loses two electrons, so it is reduced.” — Loss of electrons is oxidation; gain of electrons is reduction.
  • “It gains two electrons, so it is reduced.” — The charge goes from 0 to 2+: the atom has lost negative electrons, not gained them.

2. ZQ9 · Practice

Model answer. (1) Br2 is an element on its own: 0. (2) Al3+ is an ion of one atom, so its oxidation number is its charge: +3. (3) NH3 is a compound, so the total is 0; three hydrogens at +1 give +3, so nitrogen is −3. (4) In CO32− three oxygens at −2 give −6; the total must be the charge on the ion, −2, so carbon is +4.

A full answer includes

  • (1) 0
  • (2) +3
  • (3) −3
  • (4) +4
  • I wrote the sign before the number.

Also accept

  • 0; +3; −3; +4
  • 0; 3+; −3; +4

Watch for

  • “Bromine in Br2 is −1, because bromine forms Br− ions.” — An element on its own, not combined with another element, is 0.
  • “Carbon in CO32− is +6, so that it balances the oxygens.” — In an ion of several atoms the total is the charge on the ion, −2, not 0.

3. ZQ10 · Practice

Options as numbered on the pupil sheet: 1 — 0 → −2; 2 — 0 → +2; 3 — +2 → 0; 4 — gains 2; 5 — loses 2.

Model answer. Magnesium atoms (0) become Mg2+ ions (+2): each loses two electrons, an increase in oxidation number, so magnesium is oxidised. Oxygen atoms in O2 (0) become O2− ions (−2): each gains two electrons, a decrease in oxidation number, so oxygen is reduced. The electrons and the oxygen tell the same story: magnesium is oxidised.

A full answer includes

  • 1. Loses 2.
  • 2. Gains 2.
  • 3. 0 → +2.
  • 4. 0 → −2.

Also accept

  • loses 2; gains 2; 0 → +2; 0 → −2

Watch for

  • “Oxygen is oxidised, because this is a reaction with oxygen.” — Oxygen gains electrons and its oxidation number goes down from 0 to −2, so oxygen is reduced.
  • “Magnesium goes from +2 to 0.” — Magnesium starts as the element, 0, and ends as Mg2+, +2: it goes up.

4. ZQ11 · Practice

Options as numbered on the pupil sheet: 1 — SO3; 2 — SO2; 3 — H2S; 4 — S.

Model answer. H2S: two hydrogens at +1 give +2, so sulfur is −2. S: an element on its own, 0. SO2: two oxygens give −4, so sulfur is +4. SO3: three oxygens give −6, so sulfur is +6. Lowest first: H2S, S, SO2, SO3.

A full answer includes

  • H2S (−2), S (0), SO2 (+4), SO3 (+6).

Also accept

  • H2S, S, SO2, SO3

Watch for

  • “Sulfur in H2S is +2, because there are two hydrogens.” — The two hydrogens together are +2, and the compound totals 0, so sulfur is −2.
  • “S is the lowest, at 0.” — In H2S sulfur is below 0.

5. ZQ12 · Practice

Model answer. Each bromide ion goes from −1 to 0: it loses an electron, so it is oxidised, and it is the reducing agent. Each chlorine atom goes from 0 to −1: it gains an electron, so chlorine is reduced, and it is the oxidising agent. The potassium ions are spectators. Row B.

A full answer includes

  • I chose row B.
  • I took Br− to be oxidised: it loses electrons, −1 → 0.
  • I took Cl2 to be the oxidising agent: it gains electrons and is reduced.
  • I did not choose row C, where the oxidising agent is also the substance oxidised.

Also accept

  • Row B

Watch for

  • “Row A.” — That swaps the agents. Chlorine takes the electrons, so it is the oxidising agent; bromide gives them, so it is the reducing agent.
  • “Row C.” — The oxidising agent is not the substance oxidised: it takes electrons and is reduced. Chlorine is reduced; bromide is oxidised.
  • “Row D.” — Bromide ions are oxidised, but by giving electrons away: that makes them the reducing agent.

6. ZQ13 · Practice

Model answer. (a) Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s). (b) Each magnesium atom (0) becomes Mg2+ (+2): it loses two electrons and is oxidised. (c) Each Fe2+ ion gains those two electrons and becomes an iron atom: it is reduced, so the iron(II) ion is the oxidising agent. (d) The sulfate ions are in the solution at the start and at the end, unchanged: they are spectator ions, so the ionic equation leaves them out.

A full answer includes

  • (a) Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s).
  • (b) Two.
  • (c) The iron(II) ion, Fe2+: it gains electrons and is reduced.
  • (d) They are spectator ions: in the solution before and after, unchanged.
  • I did not name magnesium as the oxidising agent.

Also accept

  • Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s); 2; Fe2+; spectator ions

Watch for

  • “The sulfate ions take part, because they are in the solution.” — They are in the solution before and after, unchanged: they are spectator ions, so the ionic equation leaves them out.
  • “Magnesium is the oxidising agent, because it is oxidised.” — The substance oxidised is the reducing agent. The iron(II) ions take the electrons, so they are the oxidising agent.

7. ZQ14 · Practice

Model answer. (a) Iron goes from 0, the element, to +2 in Fe2+. Hydrogen goes from +1 in the acid to 0 in H2. Sulfur stays +6 and oxygen −2: the sulfate is unchanged. (b) An increase in oxidation number is oxidation and a decrease is reduction. Iron is oxidised and hydrogen is reduced at the same time, so the reaction is redox, though the oxygen atoms stay in the sulfate. (c) Iron gives electrons to the hydrogen ions, so iron is the reducing agent.

A full answer includes

  • (a) Iron: 0 → +2.
  • (a) Hydrogen: +1 → 0.
  • (b) The oxidation number of iron increases (oxidised) and that of hydrogen decreases (reduced), at the same time.
  • (c) Iron.
  • I did not say it cannot be redox because no oxygen moves.

Also accept

  • Fe 0 → +2, H +1 → 0; one goes up and one goes down; iron

Watch for

  • “Nothing can be oxidised here, because no oxygen is gained or lost.” — Oxidation is also loss of electrons and an increase in oxidation number. A substance that loses electrons is oxidised even when no oxygen moves.
  • “Sulfuric acid is the reducing agent, because it reacts with the iron.” — Hydrogen in the acid goes down from +1 to 0: it is reduced, so the acid is the oxidising agent. Iron, which goes up, is the reducing agent.

8. ZQ15 · Practice

Model answer. Calcium is +2 in CaO and +2 in CaCl2. Oxygen is −2 in CaO and −2 in H2O. Hydrogen is +1 in HCl and in H2O, and chlorine is −1 in HCl and in CaCl2. No oxidation number goes up or down, so nothing is oxidised and nothing is reduced. The oxygen only changes partner: this is neutralisation, not redox.

A full answer includes

  • Calcium stays +2.
  • Oxygen stays −2.
  • Hydrogen stays +1.
  • Chlorine stays −1.
  • No oxidation number changes, so the reaction is not redox.
  • The oxygen only moves from calcium to hydrogen: this is neutralisation.
  • I did not call it redox just because calcium oxide loses its oxygen.

Also accept

  • No oxidation number changes, so it is not redox

Watch for

  • “Any reaction in which an oxide loses its oxygen is redox.” — Only if an oxidation number changes. Here the oxygen moves from calcium to hydrogen, but every element keeps its oxidation number.

9. ZT2 · Apply

Model answer. (a) Both elements start at 0. In HCl hydrogen is +1, so chlorine is −1 for the total to be 0. (b) Hydrogen goes up (oxidised) and chlorine goes down (reduced) at the same time, so the reaction is redox. No oxygen moves and no ions form, so the oxidation numbers are the test to use here. (c) Chlorine is reduced, and it oxidises the hydrogen: chlorine is the oxidising agent.

A full answer includes

  • (a) Hydrogen: 0 → +1.
  • (a) Chlorine: 0 → −1.
  • (b) Yes: the oxidation number of hydrogen increases and that of chlorine decreases.
  • (c) Chlorine.
  • I did not rule out redox because no ions form and no oxygen moves.

Also accept

  • H 0 → +1, Cl 0 → −1; redox; chlorine

Watch for

  • “It is not redox, because no oxygen is gained or lost and no ions form.” — The oxidation numbers still change: hydrogen goes up and chlorine goes down, so it is redox.
  • “Hydrogen is the oxidising agent, because it is oxidised.” — The substance oxidised is the reducing agent. Chlorine, which is reduced, is the oxidising agent.