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InkFlaskCambridge IGCSE
Contains the answers

Answers · Redox

Oxygen in, oxygen out — answer key

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

1. ZQ1 · Practice

Model answer. Carbon becomes carbon dioxide: it gains oxygen, so it is oxidised. Tin(IV) oxide becomes tin: it loses oxygen, so it is reduced. The two happen at once, so the reaction is redox. Row C.

A full answer includes

  • I chose row C.
  • I took carbon to be oxidised: it gains oxygen and becomes carbon dioxide.
  • I took tin(IV) oxide to be reduced: it loses oxygen and becomes tin.
  • I did not choose row B, which names the products.

Also accept

  • Row C

Watch for

  • “Row A.” — That swaps them. Tin(IV) oxide loses oxygen, so it is reduced; carbon gains oxygen, so it is oxidised.
  • “Row B.” — Carbon dioxide and tin are the products. The substances oxidised and reduced are the reactants that gain and lose the oxygen.
  • “Row D.” — The oxygen that tin(IV) oxide loses goes to the carbon, so tin(IV) oxide is reduced at the same time as carbon is oxidised.

2. ZQ2 · Practice

Model answer. Magnesium becomes magnesium oxide: it gains oxygen, so it is oxidised. Water becomes hydrogen: it loses its oxygen, so it is reduced. Oxidation and reduction happen together, so the reaction is redox.

A full answer includes

  • (1) gains
  • (2) oxidised
  • (3) loses
  • (4) reduced
  • (5) redox

Also accept

  • gains; oxidised; loses; reduced; redox

Watch for

  • “Water is oxidised, because it contains oxygen.” — What matters is the change: water loses its oxygen to the magnesium, so water is reduced.
  • “Hydrogen is reduced, because it is set free.” — Hydrogen is a product. The reactant that lost the oxygen is the water, so water is reduced.

3. ZQ3 · Practice

Model answer. The Roman numeral gives the oxidation number of the metal: manganese is +4. The formula agrees: two oxide ions, O2−, give −4, so the one manganese ion must be +4 to balance them. It is not a count of atoms: MnO2 has one manganese atom and two oxygen atoms.

A full answer includes

  • The oxidation number of manganese in the compound is +4.

Also accept

  • The oxidation number of manganese is +4

Watch for

  • “The Roman numeral counts atoms, so copper(II) oxide has two copper atoms.” — The numeral gives the oxidation number of the element, here the charge on the metal ion: copper(II) means Cu2+, and copper(II) oxide, CuO, has one copper for each oxygen.
  • “The (IV) counts the oxygen atoms.” — MnO2 has two oxygen atoms, not four. The numeral gives the oxidation number of manganese.
  • “Manganese has four electrons in its outer shell.” — The numeral in a name is about the element in that compound, not about its atom: manganese is +4 in MnO2.

4. ZQ4 · Practice

Model answer. (a) In PbO2 two oxide ions give −4, so lead is +4: lead(IV) oxide. (b) In CuCl one chloride ion gives −1, so copper is +1: copper(I) chloride. (c) In CrCl3 three chloride ions give −3, so chromium is +3: chromium(III) chloride. (d) Magnesium only ever forms Mg2+ ions, so no numeral is needed: magnesium chloride.

A full answer includes

  • (a) Lead(IV) oxide: two O2− give −4, so lead is +4.
  • (b) Copper(I) chloride: one Cl−, so copper is +1.
  • (c) Chromium(III) chloride: three Cl−, so chromium is +3.
  • (d) Magnesium chloride, with no numeral: magnesium only forms Mg2+ ions.
  • I did not name PbO2 lead(II) oxide by counting the oxygen atoms.

Also accept

  • lead(IV) oxide; copper(I) chloride; chromium(III) chloride; magnesium chloride

Watch for

  • “PbO2 is lead(II) oxide, because there are two oxygen atoms.” — The numeral is the oxidation number of lead, not a count of oxygen atoms. Two O2− ions give −4, so lead is +4: lead(IV) oxide.
  • “MgCl2 is magnesium(II) chloride.” — Magnesium only forms Mg2+ ions, so its compounds need no numeral, like those of sodium.

5. ZQ5 · Practice

Model answer. In 2NiO + C → 2Ni + CO2, nickel(II) oxide loses oxygen (reduced) and carbon gains it (oxidised). In 2Al + Cr2O3 → Al2O3 + 2Cr, chromium(III) oxide loses oxygen (reduced) and aluminium gains it (oxidised). Both are redox. In NH3 + HCl → NH4Cl no oxygen moves at all. In Pb(NO3)2 + 2KI → PbI2 + 2KNO3 the oxygen atoms stay inside the nitrate groups, which only change partner. Neither of these two is redox.

A full answer includes

  • I chose 2NiO + C → 2Ni + CO2.
  • I chose 2Al + Cr2O3 → Al2O3 + 2Cr.
  • I did not choose the two reactions in which no substance gains or loses oxygen.

Also accept

  • The nickel(II) oxide and the chromium(III) oxide reactions

Watch for

  • “Lead(II) nitrate and potassium iodide is redox, because there is oxygen in the equation.” — The oxygen atoms stay inside the nitrate groups, NO3, from start to finish. No substance gains or loses oxygen, so it is not redox.
  • “Aluminium and chromium(III) oxide is not redox, because two metals only swap places.” — Chromium(III) oxide loses oxygen and aluminium gains it: one is reduced and the other oxidised at the same time, so it is redox.
  • “NH3 + HCl → NH4Cl is redox, because two substances join.” — Joining is not the test. No substance gains or loses oxygen here, so it is not redox.

6. ZQ6 · Practice

Model answer. (a) Carbon monoxide, CO, becomes carbon dioxide, CO2: it gains oxygen, so it is oxidised. (b) Nitrogen monoxide, NO, becomes nitrogen, N2: it loses oxygen, so it is reduced. (c) The oxygen that nitrogen monoxide loses is gained by carbon monoxide, so oxidation and reduction happen together: a redox reaction.

A full answer includes

  • (a) Carbon monoxide: it gains oxygen and becomes carbon dioxide.
  • (b) Nitrogen monoxide: it loses oxygen and becomes nitrogen.
  • (c) Oxidation and reduction happen at the same time, in the same reaction.
  • I did not name a product, such as carbon dioxide, as the substance oxidised.

Also accept

  • CO oxidised (gains oxygen); NO reduced (loses oxygen); both at once

Watch for

  • “Nitrogen monoxide is oxidised, because it contains oxygen.” — Containing oxygen is not the test: nitrogen monoxide loses its oxygen and becomes nitrogen, so it is reduced.
  • “Carbon dioxide is oxidised.” — Carbon dioxide is a product. The substance oxidised is the reactant that gained the oxygen: carbon monoxide.

7. ZQ7 · Practice

Model answer. The pupil is right that zinc oxide is reduced: it loses oxygen and becomes zinc. But the oxygen goes to the carbon, which becomes carbon monoxide: carbon gains oxygen, so it is oxidised. Reduction and oxidation happen at the same time, so the reaction is redox.

A full answer includes

  • Carbon gains oxygen and becomes carbon monoxide, so carbon is oxidised.
  • Oxidation and reduction happen at the same time, so the reaction is redox.
  • I did not only repeat that zinc oxide is reduced.

Also accept

  • Carbon is oxidised at the same time, so it is redox

Watch for

  • “A reaction can be only a reduction, with nothing oxidised.” — Whatever one substance loses, another gains at the same moment: the substance that takes the oxygen is oxidised. Reduction and oxidation always happen together, in a redox reaction.

8. ZT1 · Apply

Model answer. The lead in the oxide is all left in the tube; the oxygen joined the hydrogen and left as water. Mass of oxygen lost = 4.46 − 4.14 = 0.32 g. Lead(II) oxide lost oxygen, so it was reduced; the hydrogen gained that oxygen, so it was oxidised.

Checked automatically

  • Mass of oxygen lost: 0.32 g (2 significant figures)
    • Common wrong answer 8.6 : That adds the two masses. The oxygen lost is the difference: the mass before minus the mass after.

A full answer includes

  • 0.32 g
  • I subtracted the mass of lead from the mass of lead(II) oxide.

Also accept

  • 0.32 g

Watch for

  • “The solid was oxidised, because it was heated.” — The solid lost mass: lead(II) oxide lost its oxygen, so it was reduced. The hydrogen took the oxygen and was oxidised.
  • “Add the two masses.” — The oxygen lost is the difference: the mass before minus the mass after.