Redox — final check key

1. ZK1 · Final check

Model answer. (a) Three oxide ions give −6, so tungsten is +6: tungsten(VI) oxide. (b) Tungsten(VI) oxide loses its oxygen and becomes tungsten, so it is reduced. (c) The hydrogen gains that oxygen and becomes water, so it is oxidised. Oxidation and reduction happen together: a redox reaction.

A full answer includes

• (a) Tungsten(VI) oxide: three O²⁻ give −6, so tungsten is +6.
• (b) Tungsten(VI) oxide: it loses oxygen and becomes tungsten.
• (c) Hydrogen gains the oxygen and is oxidised at the same time.
• I did not name WO₃ tungsten(III) oxide by counting the oxygen atoms.

Also accept

• Tungsten(VI) oxide; tungsten(VI) oxide, it loses oxygen; hydrogen is oxidised at the same time

Watch for

• “WO₃ is tungsten(III) oxide.” — The numeral is the oxidation number of tungsten, not a count of oxygen atoms: three O²⁻ give −6, so tungsten is +6.
• “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.

2. ZK2 · Final check

Options as numbered on the pupil sheet: 1 — neither; 2 — oxidised; 3 — reduced.

Model answer. 1. Calcium becomes calcium oxide: it gains oxygen, so it is oxidised. 2. Silver oxide becomes silver: it loses oxygen, so it is reduced. 3. The oxygen atoms stay inside the carbonate group, which moves from sodium to calcium: neither. 4. Copper(I) oxide becomes copper: it loses oxygen, so it is reduced.

A full answer includes

• 1. Oxidised: calcium gains oxygen.
• 2. Reduced: silver oxide loses oxygen.
• 3. Neither: no substance gains or loses oxygen.
• 4. Reduced: copper(I) oxide loses oxygen.

Also accept

• oxidised; reduced; neither; reduced

Watch for

• “Calcium chloride is oxidised, because calcium carbonate contains oxygen.” — The oxygen atoms arrive inside the carbonate group, which was already there in sodium carbonate. No element gains oxygen and no oxide loses it, so this is not redox.
• “Silver oxide is oxidised, because it contains oxygen.” — Silver oxide loses its oxygen to the hydrogen and becomes silver, so it is reduced.

3. ZK3 · Final check

Model answer. (a) Aluminium goes from 0 to +3, an increase; lead goes from +2 to 0, a decrease. (b) Each aluminium atom loses three electrons and each Pb²⁺ ion gains two: two aluminium atoms give six electrons, which three lead(II) ions take. (c) The lead(II) ion takes electrons and is reduced: it is the oxidising agent. Aluminium gives electrons and is oxidised: it is the reducing agent. (d) The nitrate ions are in the solution before and after, unchanged: they are spectator ions.

A full answer includes

• (a) Aluminium: 0 → +3.
• (a) Lead: +2 → 0.
• (b) Each aluminium atom loses 3 electrons.
• (b) Each lead(II) ion gains 2 electrons.
• (c) Oxidising agent: Pb²⁺.
• (c) Reducing agent: aluminium.
• (d) They are spectator ions: in the solution before and after, unchanged.
• I did not call aluminium the oxidising agent because it is oxidised.

Also accept

• Al 0 → +3, Pb +2 → 0; 3 and 2; Pb²⁺ oxidising, Al reducing; spectator ions

Watch for

• “The oxidising agent is the substance that is oxidised.” — It is the other way round. The oxidising agent oxidises the other substance: it takes electrons, so it is itself reduced. The substance oxidised is the reducing agent.
• “The nitrate ions take part, because they are in the solution.” — They are there before and after, unchanged: spectator ions, left out of the ionic equation.

4. ZK4 · Final check

Model answer. In CuCO₃ → CuO + CO₂, copper stays +2, carbon stays +4 and oxygen stays −2: no oxidation number changes, so it is not redox (it is thermal decomposition). In 2Mg + CO₂ → 2MgO + C, magnesium goes from 0 to +2 and carbon from +4 to 0. In 2SO₂ + O₂ → 2SO₃, sulfur goes from +4 to +6 and oxygen from 0 to −2. In 4Na + O₂ → 2Na₂O, sodium goes from 0 to +1 and oxygen from 0 to −2. Those three are redox.

A full answer includes

• CuCO₃ → CuO + CO₂: no oxidation number changes.

Also accept

• CuCO₃ → CuO + CO₂

Watch for

• “Copper(II) carbonate losing carbon dioxide is redox, because it loses oxygen.” — Copper is +2 in CuCO₃ and in CuO, carbon is +4 in the carbonate and in CO₂, and oxygen is −2 throughout. No oxidation number changes.
• “Magnesium and carbon dioxide is not redox, because carbon dioxide is not oxygen gas.” — Carbon dioxide loses oxygen to the magnesium: carbon goes from +4 to 0 and is reduced, while magnesium goes from 0 to +2 and is oxidised.
• “2SO₂ + O₂ → 2SO₃ is not redox, because sulfur dioxide is already an oxide.” — Sulfur dioxide gains more oxygen: sulfur goes from +4 to +6 and is oxidised, while oxygen goes from 0 to −2 and is reduced.
• “4Na + O₂ → 2Na₂O is not redox, because no oxide loses oxygen.” — Oxygen gas is reduced, from 0 to −2, while sodium is oxidised, from 0 to +1.

5. ZK5 · Final check

Model answer. (a) Iodine, I₂. (b) Each iodide ion loses an electron: its oxidation number goes up from −1 to 0, so the iodide ions are oxidised. (c) Oxygen is −2 and the oxidation numbers in the ion add up to its charge, −1, so chlorine is +1; in Cl⁻ it is −1, so the oxidation number of chlorine goes down by 2: it is reduced. (d) The bleach takes electrons from the iodide ions, so the ClO⁻ ion is the oxidising agent.

A full answer includes

• (a) Iodine.
• (b) Each iodide ion loses an electron.
• (b) Iodine goes from −1 to 0, so the iodide ions are oxidised.
• (c) In ClO⁻, chlorine + (−2) = −1, so chlorine is +1.
• (c) Chlorine goes from +1 to −1.
• (d) The oxidising agent is the bleach (the ClO⁻ ion).
• I did not say the brown is the colour of the bleach.

Also accept

• Iodine; they lose electrons, −1 → 0, oxidised; +1 → −1; the bleach (ClO⁻)

Watch for

• “The brown is the colour of the substance added.” — The substance added was colourless, and so was the potassium iodide. The brown is iodine, made in the tube from the iodide ions.
• “Chlorine in ClO⁻ is −1, as in a chloride.” — Oxygen is −2, and the oxidation numbers in the ion add up to its charge, −1. So chlorine is +1.

6. ZK6 · Final check

Model answer. The (II) in tin(II) chloride means tin starts at +2; it ends as Sn⁴⁺, +4. Tin goes up, so it loses electrons and is oxidised. Those electrons go to the manganese, which goes down from +7 to +2: manganese is reduced. Tin(II) chloride gives the electrons, so it is a reducing agent. Row D.

A full answer includes

• I chose row D.
• I took tin to go up from +2 to +4.
• I took manganese to be reduced, from +7 to +2.
• I took tin(II) chloride to be a reducing agent: it gives the electrons.
• I did not choose row B, which calls tin(II) chloride an oxidising agent.

Also accept

• Row D

Watch for

• “Row A.” — Tin(II) means tin starts at +2, and it ends as Sn⁴⁺, +4: it goes up, not down.
• “Row B.” — Manganese is reduced, but tin(II) chloride is what reduces it: it gives electrons, so it is a reducing agent.
• “Row C.” — The purple turned colourless, so manganese went down from +7 to +2: a decrease in oxidation number is reduction, not oxidation.