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CIE iGCSE Co-Ordinated Science C6.3 Redox Exam Style Questions Paper 4

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Question

(a) Aqueous iron(III) sulfate contains iron(III) ions, \( \text{Fe}^{3+} \), and sulfate ions, \( \text{SO}_4^{2-} \).
Deduce the formula of iron(III) sulfate.
(b) Aqueous sodium hydroxide is used to test for iron(III) ions, \( \text{Fe}^{3+} \).
The iron(III) ions react with the hydroxide ions, \( \text{OH}^- \), from the aqueous sodium hydroxide.
A precipitate of iron(III) hydroxide, \( \text{Fe(OH)}_3 \), is made.
(i) State the colour of the precipitate.
(ii) Construct the balanced ionic equation for the reaction. Include state symbols.
(c) Iron is obtained from iron(III) oxide in a blast furnace. The equation for the reaction is shown:
\( \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \)
(i) State if iron(III) oxide is oxidised or reduced in this reaction. Explain your answer.
(ii) The iron(III) oxide reacts with the carbon monoxide in a blast furnace to make iron.
Calculate the minimum mass of iron(III) oxide required to make 28,000 g of iron.
\( [A_r: \text{C}, 12; \text{O}, 16; \text{Fe}, 56] \)

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C3.1 — Formulas (Part (a))
• Topic C12.5 — Qualitative analysis (Part (b)(i), (b)(ii))
• Topic C6.3 — Redox (Part (c)(i))
• Topic C3.2 — Relative masses of atoms and molecules (Part (c)(ii))
• Topic C9.6 — Extraction of metals (Part (c)(i), (c)(ii))

▶️ Answer/Explanation

(a) \( \text{Fe}_2(\text{SO}_4)_3 \)

Iron(III) ions have a charge of \( 3+ \) (\( \text{Fe}^{3+} \)) and sulfate ions have a charge of \( 2- \) (\( \text{SO}_4^{2-} \)). To form a neutral compound, the total positive charge must balance the total negative charge. The lowest common multiple of 3 and 2 is 6. Therefore, we need 2 iron(III) ions (total charge \( 2 \times 3+ = 6+ \)) and 3 sulfate ions (total charge \( 3 \times 2- = 6- \)). Hence, the formula is \( \text{Fe}_2(\text{SO}_4)_3 \).

(b)(i) Red-brown.

When aqueous sodium hydroxide is added to a solution containing iron(III) ions, a red-brown precipitate of iron(III) hydroxide is formed. This is a characteristic test for \( \text{Fe}^{3+} \) ions.

(b)(ii) \( \text{Fe}^{3+}(\text{aq}) + 3\text{OH}^-(\text{aq}) \rightarrow \text{Fe(OH)}_3(\text{s}) \)

The ionic equation shows only the ions that participate in the reaction. The \( \text{Fe}^{3+} \) ions from the iron(III) sulfate solution react with \( \text{OH}^- \) ions from the sodium hydroxide solution to form solid iron(III) hydroxide precipitate. The sodium and sulfate ions are spectator ions and are not included in the ionic equation. The state symbols are (aq) for aqueous ions and (s) for the solid precipitate.

(c)(i) Iron(III) oxide is reduced because it loses oxygen / \( \text{Fe}_2\text{O}_3 \) loses oxygen.

In the blast furnace reaction, iron(III) oxide (\( \text{Fe}_2\text{O}_3 \)) is converted to iron (\( \text{Fe} \)). The iron(III) oxide loses oxygen atoms (it is reduced from \( \text{Fe}_2\text{O}_3 \) to \( \text{Fe} \)). Loss of oxygen is reduction. Carbon monoxide (\( \text{CO} \)) gains oxygen to become carbon dioxide (\( \text{CO}_2 \)), so carbon monoxide is oxidised. This is a redox reaction where reduction and oxidation occur simultaneously.

(c)(ii) Minimum mass of iron(III) oxide required = 40,000 g

Calculation steps:

  • \( M_r \) of \( \text{Fe}_2\text{O}_3 = (2 \times 56) + (3 \times 16) = 112 + 48 = 160 \)
  • From the equation, \( 2 \text{Fe} \) atoms are produced from 1 \( \text{Fe}_2\text{O}_3 \) molecule.
  • Mass of Fe produced from 160 g of \( \text{Fe}_2\text{O}_3 = 112 \text{g} \)
  • Mass of \( \text{Fe}_2\text{O}_3 \) required = \( \frac{160}{112} \times 28000 = 40,000 \text{g} \)

OR using moles:

  • Moles of Fe = \( 28000 \div 56 = 500 \) moles
  • Mole ratio \( \text{Fe}_2\text{O}_3 : \text{Fe} = 1 : 2 \)
  • Moles of \( \text{Fe}_2\text{O}_3 = 500 \div 2 = 250 \) moles
  • Mass of \( \text{Fe}_2\text{O}_3 = 250 \times 160 = 40,000 \text{g} \)

Question

The metal iron is extracted from hematite in a blast furnace.
The extraction happens in several stages.
(a) In the first stage, carbon (coke) is burnt to provide heat and produce carbon dioxide. State the type of reaction that transfers thermal (heat) energy to the surroundings.
(b) In the second stage, carbon reacts with carbon dioxide to make carbon monoxide.
\(\text{C} + \text{CO}_2 \rightarrow 2\text{CO}\)
State what happens to the carbon dioxide in this reaction. Choose from the list.
combustion       oxidation       reduction       thermal decomposition
(c) In the third stage, iron(III) oxide, \(\text{Fe}_2\text{O}_3\), reacts with carbon monoxide. Iron and carbon dioxide are made. Construct the balanced symbol equation for this reaction.
(d) Calcium carbonate (limestone) is added to the blast furnace to remove impurities from the hematite. The calcium carbonate thermally decomposes to make calcium oxide and carbon dioxide.
\(\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\)
Calculate the mass of calcium carbonate needed to make 7 tonnes of calcium oxide. [A: C, 12; Ca, 40; O, 16]
(e) Iron is protected from rusting by coating the iron with a layer of zinc.
This is called sacrificial protection.
Explain how sacrificial protection protects iron.
Use ideas about the reactivity series and loss of electrons.
(f) Fig. 7.1 shows the metallic bonding in zinc.
Use Fig. 7.1 to describe the metallic bonding in zinc.

Topic codes:

• Topic C9.6 — Extraction of metals (Part (a), (b), (c), (d))
• Topic C9.5 — Corrosion of metals / Sacrificial protection (Part (e))
• Topic C2.7 — Metallic bonding (Part (f))
• Topic C5.1 — Exothermic and endothermic reactions (Part (a))
• Topic C6.3 — Redox (Part (b))

▶️ Answer/Explanation

(a) Exothermic — the reaction transfers thermal energy to the surroundings.

(b) The carbon dioxide undergoes reduction (it loses oxygen).

In this reaction: \(\text{C} + \text{CO}_2 \rightarrow 2\text{CO}\), carbon dioxide (\(\text{CO}_2\)) is reduced to carbon monoxide (\(\text{CO}\)) as it loses one oxygen atom.

(c) \(\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2\)

Check: Iron: 2 atoms on each side; Carbon: 3 atoms on each side; Oxygen: 3 + 3 = 6 on each side.

(d) \(M_r\) of CaCO₃ = 40 + 12 + (3 × 16) = 100

\(M_r\) of CaO = 40 + 16 = 56

Mass of CaCO₃ needed = \(100 \times 7 \div 56 = 12.5\) tonnes

(e) Sacrificial protection works because zinc is more reactive than iron (higher in the reactivity series).

Zinc loses electrons more easily than iron, so zinc is oxidised preferentially (it acts as the anode). This means the iron is protected from losing electrons and therefore from rusting.

(f) Metallic bonding in zinc involves electrostatic attraction between the positive zinc (metal) ions and the ‘sea’ of delocalised electrons.

In the metallic lattice, the outer electrons of zinc atoms become delocalised and are free to move throughout the structure. These mobile electrons hold the positively charged zinc ions together.

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