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CIE iGCSE Co-Ordinated Science C2.7 Metallic bonding Exam Style Questions Paper 4

CIE iGCSE Co-Ordinated Science Paper 4 - All Chapters

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.

Question

Aluminium is extracted by electrolysis from the ore bauxite that contains aluminium oxide, \(Al_2O_3\).
The equation for the overall reaction is
\(2Al_2O_3(l) \rightarrow 4Al(l) + 3O_2(g)\).
(a) A scientist electrolyses 81.6g of aluminium oxide.
Calculate the maximum mass of aluminium extracted from the aluminium oxide.
Show your working.
[\(A_r\): Al, 27; O, 16]
(b) At the anode, oxide ions, \(O^{2-}\), form oxygen molecules.
\(2O^{2-} \rightarrow O_2 + 4e^-\)
State if this reaction is oxidation or reduction. Explain your answer.
(c) Construct the ionic half-equation for the reaction at the cathode.
(d) Iron can be extracted from iron oxide by heating the iron oxide with carbon.
Explain why aluminium cannot be extracted from aluminium oxide using this method.
(e) Fig. 11.1 shows metallic bonding.
Use Fig. 11.1 to explain why metals conduct electricity.

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

• Topic C9.6 — Extraction of metals (Parts (a), (b), (c), (d))
• Topic C2.7 — Metallic bonding (Part (e))

▶️ Answer/Explanation

(a) 43.2 g

\(M_r\) of \(Al_2O_3 = (2\times27)+(3\times16) = 102\).
Using the mole ratio \(2Al_2O_3 : 4Al\), mass of Al \(= \dfrac{54 \times 81.6}{102} = 43.2\) g.

(b) Oxidation; electrons are lost

Oxide ions lose electrons (\(4e^-\)) to form neutral oxygen molecules.
Loss of electrons is, by definition, oxidation.

(c) \(Al^{3+} + 3e^- \rightarrow Al\)

At the cathode, positive aluminium ions gain electrons (reduction) to form neutral aluminium metal.
The equation must be balanced for both charge and atoms: three electrons reduce each \(Al^{3+}\) ion.

(d) Aluminium is more reactive than carbon

Carbon can only displace/reduce metals that are less reactive than itself.
Since aluminium is higher than carbon in the reactivity series, carbon cannot reduce aluminium oxide, so electrolysis must be used instead.

(e) Metals have free (delocalised) electrons that can move throughout the structure

As Fig. 11.1 shows, the positive metal ions are fixed in a lattice, but the outer-shell electrons are delocalised and free to move between them.
These mobile electrons act as charge carriers, allowing current to flow when a potential difference is applied — this is why metals conduct electricity.

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