Chemistry questions, chapter 1

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Redox equilibrium

45 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first. Use the standard electrode potentials in the chapter notes where needed.

Section A: objective questions

12 marks

Choose the best answer for each question.

Question 1

In terms of electron transfer, what happens to a substance that is oxidised?

It gains electrons
It loses electrons
It gains hydrogen
It loses oxygen

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Oxidation is loss of electrons (OIL RIG). Gaining hydrogen and losing oxygen describe reduction.

For the teacher: Remembering, low difficulty, standard 1.1.1

Textbook p. 7

Question 2

Magnesium burns in carbon dioxide: 2Mg + CO₂ → 2MgO + C. Which substance is the reducing agent?

Carbon dioxide
Magnesium oxide
Magnesium
Carbon

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Magnesium gains oxygen, so it is oxidised. The substance that is oxidised is the reducing agent.

For the teacher: Applying, low difficulty, standard 1.1.1

Textbook p. 5

Question 3

What is the oxidation number of manganese in potassium manganate(VII), KMnO₄?

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(+1) + x + 4(-2) = 0, so x = +7. The roman numeral (VII) in the name also gives +7.

For the teacher: Applying, low difficulty, standard 1.1.2

Textbook p. 13

Question 4

What is the oxidation number of chromium in the dichromate(VI) ion, Cr₂O₇²⁻?

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2x + 7(-2) = -2, so 2x = +12 and x = +6 for each chromium atom.

For the teacher: Applying, medium difficulty, standard 1.1.2

Textbook p. 12

Question 5

Hydrogen sulphide reacts with chlorine: H₂S + Cl₂ → S + 2HCl. Which substance is the reducing agent?

Hydrogen sulphide, H₂S
Chlorine, Cl₂
Sulphur, S
Hydrogen chloride, HCl

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H₂S loses hydrogen (and sulphur goes from -2 to 0), so it is oxidised and acts as the reducing agent. Cl₂ gains hydrogen and is the oxidising agent.

For the teacher: Analysing, medium difficulty, standard 1.1.1

Textbook p. 6

Question 6

What is the IUPAC name of Cu₂O?

Copper oxide
Copper(II) oxide
Copper(I) oxide
Dicopper oxide

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2x + (-2) = 0, so copper is +1. The oxidation number is written as a roman numeral: copper(I) oxide. CuO is copper(II) oxide.

For the teacher: Understanding, low difficulty, standard 1.1.2

Textbook p. 14

Question 7

E° values: Mg²⁺/Mg = -2.38 V, Zn²⁺/Zn = -0.76 V, Cu²⁺/Cu = +0.34 V, Ag⁺/Ag = +0.80 V. Which mixture will react?

Cu metal and Zn²⁺ solution
Cu metal and Mg²⁺ solution
Ag metal and Cu²⁺ solution
Mg metal and Zn²⁺ solution

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Magnesium has the more negative E°, so it is the stronger reducing agent and displaces zinc. In the other mixtures the metal is the weaker reducing agent.

For the teacher: Analysing, medium difficulty, standard 1.2.2

Textbook p. 25

Question 8

A voltaic cell uses zinc (E° = -0.76 V) and silver (E° = +0.80 V) half-cells. What is its cell voltage?

+1.56 V
+0.04 V
+1.10 V
-1.56 V

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E°cell = E°(cathode) - E°(anode) = (+0.80) - (-0.76) = +1.56 V.

For the teacher: Applying, low difficulty, standard 1.3.1

Textbook p. 29

Question 9

Copper(II) sulphate solution is electrolysed with carbon electrodes. Which ion is discharged at the cathode?

Hydrogen ion, H⁺
Hydroxide ion, OH⁻
Copper(II) ion, Cu²⁺
Sulphate ion, SO₄²⁻

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Cations go to the cathode. Cu²⁺ has a more positive E° (+0.34 V) than H⁺ (0.00 V), so it is discharged: Cu²⁺ + 2e⁻ → Cu.

For the teacher: Understanding, low difficulty, standard 1.4.3

Textbook p. 40

Question 10

Which arrangement can be used to electroplate an iron spoon with copper?

Spoon as anode, copper(II) sulphate solution
Spoon as cathode, copper(II) sulphate solution
Spoon as cathode, sodium chloride solution
Spoon as anode, iron(II) sulphate solution

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The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.

For the teacher: Understanding, low difficulty, standard 1.4.5

Textbook p. 44

Question 11

What is the purpose of adding cryolite, Na₃AlF₆, in the extraction of aluminium?

To act as a reducing agent
To remove impurities as slag
To increase the density of molten aluminium
To lower the melting point of aluminium oxide

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Aluminium oxide melts at about 2000 °C. Cryolite lowers the melting point and so saves energy.

For the teacher: Remembering, low difficulty, standard 1.5.1

Textbook p. 48

Question 12

An iron nail is coiled with a different metal and left in agar containing hexacyanoferrate(III). With which metal will the iron rust fastest?

Copper
Magnesium
Zinc
Aluminium

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Copper is less electropositive than iron, so iron releases electrons more easily and rusts faster. Mg, Zn and Al are more electropositive and slow rusting down.

For the teacher: Analysing, high difficulty, standard 1.6.2

Textbook p. 55

Section B: structured questions

33 marks

Answer all questions. Write your answers in the spaces provided.

Question 13

Copper(II) oxide is heated in a stream of hydrogen: CuO + H₂ → Cu + H₂O.

[1 mark] (a)Name the substance that is oxidised.

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Hydrogen, H₂1

Tick each point you got.

[1 mark] (b)Name the substance that is reduced.

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Copper(II) oxide, CuO1

Tick each point you got.

[1 mark] (c)Name the oxidising agent and give a reason.

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Copper(II) oxide: it is reduced (it loses oxygen / its oxidation number decreases)1

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[1 mark] (d)State the change in the oxidation number of copper.

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+2 to 01

Tick each point you got.

For the teacher: Remembering, low difficulty, standard 1.1.1

Textbook p. 11

Question 14

Bromine water is added to potassium iodide solution. The solution turns from orange to dark brown.

[1 mark] (a)Write the half equation for the oxidation reaction.

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2I⁻ → I₂ + 2e⁻1

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[1 mark] (b)Write the half equation for the reduction reaction.

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Br₂ + 2e⁻ → 2Br⁻1

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[1 mark] (c)Write the overall ionic equation.

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2I⁻ + Br₂ → I₂ + 2Br⁻1

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[1 mark] (d)Explain why bromine can displace iodine from potassium iodide.

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Bromine is higher in Group 17 / a stronger oxidising agent (more reactive) than iodine, so it accepts electrons from I⁻1

Tick each point you got.

For the teacher: Applying, low difficulty, standard 1.1.3

Textbook p. 19

Question 15

A voltaic cell is made from an iron electrode in iron(II) nitrate solution and a silver electrode in silver nitrate solution, joined by a salt bridge. E°: Fe²⁺/Fe = -0.44 V, Ag⁺/Ag = +0.80 V.

[1 mark] (a)Which electrode is the negative terminal? Give a reason.

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Iron, because it has the more negative E° (it releases electrons more easily)1

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[2 marks] (b)Write the half equations at the negative and positive terminals.

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Fe → Fe²⁺ + 2e⁻1
Ag⁺ + e⁻ → Ag1

Tick each point you got.

[1 mark] (c)Write the overall ionic equation.

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Fe + 2Ag⁺ → Fe²⁺ + 2Ag1

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[1 mark] (d)Calculate the cell voltage.

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(+0.80) - (-0.44) = +1.24 V1

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[1 mark] (e)State the direction of electron flow in the external wire.

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From the iron electrode to the silver electrode1

Tick each point you got.

For the teacher: Applying, low difficulty, standard 1.3.1

Textbook p. 30

Question 16

Copper(II) chloride solution, CuCl₂, is electrolysed in three cells: Cell I (0.0001 mol dm⁻³, carbon electrodes), Cell II (1.0 mol dm⁻³, carbon electrodes) and Cell III (0.0001 mol dm⁻³, copper electrodes).

[1 mark] (a)Name the anions present in copper(II) chloride solution.

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Chloride ion, Cl⁻ and hydroxide ion, OH⁻1

Tick each point you got.

[2 marks] (b)Cell I: 0.0001 mol dm⁻³ solution, carbon electrodes. Name the gas released at the anode and explain.

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Oxygen [1]1
Chloride ion concentration is very low, so OH⁻ (less positive E°) is discharged [1]1

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[2 marks] (c)Cell II: 1.0 mol dm⁻³ solution, carbon electrodes. Name the gas released at the anode and explain.

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Chlorine [1]1
Cl⁻ is at high concentration, so it is discharged in preference to OH⁻ [1]1

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[1 mark] (d)Cell III: 0.0001 mol dm⁻³ solution, copper electrodes. Write the half equation at the anode.

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Cu → Cu²⁺ + 2e⁻1

Tick each point you got.

For the teacher: Analysing, high difficulty, standard 1.4.3

Textbook p. 41

Question 17

Zinc blocks are attached to the iron hull of a ship.

[2 marks] (a)State two conditions needed for iron to rust.

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Water [1]1
Oxygen [1]1

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[1 mark] (b)Write the half equation for the oxidation of iron in rusting.

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Fe → Fe²⁺ + 2e⁻1

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[1 mark] (c)Name the method of rust prevention used.

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Sacrificial protection1

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[2 marks] (d)Explain how the zinc prevents the hull from rusting.

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Zinc is more electropositive than iron, so it releases electrons more easily and is oxidised (corrodes) instead [1]1
Iron receives electrons and is not oxidised [1]1

Tick each point you got.

For the teacher: Understanding, medium difficulty, standard 1.6.2

Textbook p. 56

Question 18

A scratched tin-plated can and a scratched galvanised sheet are both left in damp air.

[2 marks] (a)Which one is protected from rusting? Explain.

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Galvanised sheet (zinc) [1]1
Zinc is more electropositive than iron so it corrodes first; tin is less electropositive so iron rusts faster [1]1

Tick each point you got.

For the teacher: Evaluating, high difficulty, standard 1.6.2

Textbook p. 56

Question 19

Plan the electroplating of an iron ring with nickel.

[2 marks] (a)State the anode and the cathode.

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Anode: nickel plate [1]1
Cathode: iron ring [1]1

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[1 mark] (b)Suggest a suitable electrolyte.

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Nickel(II) sulphate (or nitrate) solution1

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[1 mark] (c)Write the half equation at the cathode.

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Ni²⁺ + 2e⁻ → Ni1

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[1 mark] (d)State the change observed at the anode.

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The nickel plate becomes thinner (it dissolves)1

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For the teacher: Creating, medium difficulty, standard 1.4.5

Textbook p. 45

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