CIE iGCSE Co-Ordinated Science C3.2 Relative masses of atoms and molecules Exam Style Questions Paper 4

Question

Iron is extracted from hematite by reduction of iron(III) oxide in a blast furnace.

(a) Iron(III) oxide reacts with carbon monoxide.

Complete the equation for the reduction of iron(III) oxide.

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

(b) The iron made in the blast furnace is an alloy containing about 90–95% iron.

Fig. 8.1 shows pure iron and the iron made in the blast furnace.

(i) Complete the sentences.

Pure iron is an element because ………………………………

Iron from the blast furnace is a mixture because ………………………………

(ii) Iron from the blast furnace is harder than pure iron.

Explain why. Use Fig. 8.1 to help you.

(iii) Pure iron is very malleable.

Explain why. Use Fig. 8.1 to help you.

(c) Calcium carbonate, \( \text{CaCO}_3 \), is added to the blast furnace to remove impurities.

(i) The calcium carbonate is heated to make calcium oxide, CaO.

The equation for the reaction is shown.

\( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)

State the name of the type of reaction that occurs.

(ii) The calcium oxide reacts with silicon dioxide, \( \text{SiO}_2 \), which is an impurity in the iron ore, to make calcium silicate.

\( \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \)

Calculate the minimum mass of calcium oxide needed to remove 720 tonnes of silicon dioxide.

\( [A_r: \text{Ca}, 40; \text{O}, 16; \text{Si}, 28] \)

(iii) Fig. 8.2 shows the structure and bonding in silicon dioxide.

The structure and bonding is similar to diamond.

State the type of structure and bonding in silicon dioxide.

Choose from the list.

• giant ionic

• giant covalent

• giant metallic

• simple covalent

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

• Topic C9.6 — Extraction of metals / Blast furnace (Part (a))
• Topic C9.3 — Alloys and their properties (Part (b)(i), (b)(ii) & (b)(iii))
• Topic C6.1 — Physical and chemical changes / Thermal decomposition (Part (c)(i))
• Topic C3.2 — Relative masses / Stoichiometry (Part (c)(ii))
• Topic C2.6 — Giant covalent structures (Part (c)(iii))

▶️ Answer/Explanation

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

Carbon monoxide reduces iron(III) oxide to iron metal, being oxidised to carbon dioxide. The equation is balanced with 2 Fe, 3 C, and 6 O atoms on both sides.

(b)(i) Pure iron is an element because it is made of only one type of atom. Iron from the blast furnace is a mixture because it contains two elements (iron and carbon/silicon) that are not chemically combined.

An element consists of identical atoms, while a mixture contains different substances that are not chemically bonded together.

(b)(ii) Iron from the blast furnace is harder because the atoms are different sizes, which prevents the layers of atoms from sliding over each other (unlike in pure iron where all atoms are the same size).

The different-sized atoms in the alloy disrupt the regular arrangement of layers, making it harder for them to slip past one another.

(b)(iii) Pure iron is malleable because layers of atoms can slide / move / slip over each other when a force is applied.

The regular, identical layers of atoms in pure iron can easily slide past one another, allowing the metal to be hammered or bent into shape.

(c)(i) Thermal decomposition

Calcium carbonate breaks down into calcium oxide and carbon dioxide when heated, which is a thermal decomposition reaction.

(c)(ii) Mass of CaO = 672 tonnes

\( M_r(\text{CaO}) = 40 + 16 = 56 \)

\( M_r(\text{SiO}_2) = 28 + (2 \times 16) = 60 \)

\( \text{Mass of CaO} = \frac{56}{60} \times 720 = 672 \text{ tonnes} \)

The mole ratio between CaO and SiO₂ is 1:1, so the mass of CaO needed is proportional to the ratio of their formula masses.

(c)(iii) Giant covalent

Silicon dioxide has a giant covalent structure similar to diamond, where each silicon atom is bonded to four oxygen atoms in a tetrahedral arrangement.

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 \( \text{Fe}_2\text{O}_3 = 250 \times 160 = 40,000 \text{g} \)

CIE iGCSE Co-Ordinated Science C3.2 Relative masses of atoms and molecules Exam Style Questions Paper 4

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