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InkFlaskCambridge IGCSE

Pupil sheet · Redox

Electrons on the move — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. identify

Supplement. A copper wire is hung in colourless silver nitrate solution. Silver crystals grow on the wire, and the solution turns blue: Cu + 2Ag+ → Cu2+ + 2Ag. Which statement about each copper atom is correct?

Circle one answer.

  • It loses two electrons, so it is reduced
  • It gains two electrons, so it is reduced
  • It gains two electrons, so it is oxidised
  • It loses two electrons, so it is oxidised

Nudge (only if you are stuck): Compare the charge on a copper atom with the charge on a Cu2+ ion. Electrons are negative.

2. deduce

Supplement. Give the oxidation number of: (1) bromine in Br2; (2) aluminium in Al3+; (3) nitrogen in NH3; (4) carbon in the carbonate ion, CO32−.

(1) Bromine in Br2

(2) Aluminium in Al3+

(3) Nitrogen in NH3

(4) Carbon in CO32−

Nudge (only if you are stuck): Use the rules in turn: an element on its own; an ion of one atom; the total for a compound; the total for an ion of several atoms.

3. identify

Supplement. Magnesium burns in oxygen: 2Mg + O2 → 2MgO. Magnesium oxide is made of Mg2+ and O2− ions. Match each description to what happens.

Write the number of the matching option.

  1. 0 → −2
  2. 0 → +2
  3. +2 → 0
  4. gains 2
  5. loses 2
  • 1. Electrons, for each magnesium atom: ______
  • 2. Electrons, for each oxygen atom: ______
  • 3. Oxidation number of magnesium: ______
  • 4. Oxidation number of oxygen: ______

Nudge (only if you are stuck): Start from the left: what is the oxidation number of an element on its own?

4. deduce

Supplement. Put these in order of the oxidation number of sulfur, lowest first: SO2, H2S, SO3, S.

Write the number of each step in order.

  1. SO3
  2. SO2
  3. H2S
  4. S
  • First: ______
  • Second: ______
  • Third: ______
  • Last: ______

Nudge (only if you are stuck): Hydrogen combined with a non-metal is +1, oxygen in a compound is −2, and a compound totals 0.

5. identify

Supplement. Chlorine water is added to colourless potassium bromide solution, and the solution turns orange: Cl2 + 2Br− → 2Cl− + Br2. Which row is correct?

RowOxidising agentReducing agentSubstance oxidised
ABr−Cl2Cl2
BCl2Br−Br−
CCl2Br−Cl2
DBr−Cl2Br−

Circle one answer.

  • A
  • B
  • C
  • D

Nudge (only if you are stuck): Find which particle loses electrons and which gains them.

6. construct / state / identify / explain

Supplement. A strip of magnesium ribbon is put into pale green iron(II) sulfate solution. A dark grey coat of iron forms on the magnesium, and the solution slowly turns colourless. (a) Construct the ionic equation, with state symbols. (b) How many electrons does each magnesium atom lose? (c) Identify the oxidising agent. (d) Explain why the sulfate ions are left out of the ionic equation.

(a) Ionic equation

(b) Electrons lost by each magnesium atom

(c) Oxidising agent

(d) Why no sulfate ions

Nudge (only if you are stuck): Magnesium forms Mg2+ ions. Which particles change, and which are the same before and after?

7. deduce / explain / identify

Supplement. Iron filings fizz in dilute sulfuric acid: Fe + H2SO4 → FeSO4 + H2. (a) Give the oxidation numbers of iron and of hydrogen on each side of the equation. (b) Explain why this is a redox reaction, although no substance gains or loses oxygen. (c) Identify the reducing agent.

(a) Oxidation numbers of iron and hydrogen

(b) Why redox

(c) Reducing agent

Nudge (only if you are stuck): FeSO4 is made of Fe2+ and SO42− ions. In the acid, hydrogen is combined with non-metals.

8. explain

Supplement. A pupil says: "CaO + 2HCl → CaCl2 + H2O is redox, because calcium oxide loses its oxygen." Explain why the pupil is incorrect.

Nudge (only if you are stuck): Deduce the oxidation number of each element on both sides.

9. deduce / explain / identify

Supplement. Hydrogen burns in chlorine to make hydrogen chloride: H2 + Cl2 → 2HCl. Hydrogen chloride is a covalent gas made of molecules, not ions. (a) Give the oxidation numbers of hydrogen and of chlorine on each side. (b) Is the reaction redox? Explain your answer. (c) Identify the oxidising agent.

(a) Oxidation numbers

(b) Redox or not, and why

(c) Oxidising agent

Nudge (only if you are stuck): Oxidation numbers work for covalent substances too: use the value for hydrogen combined with a non-metal.