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CIE iGCSE Co-Ordinated Science C2.6 Giant covalent structures Exam Style Questions Paper 4

CIE iGCSE Co-Ordinated Science Paper 4 - All Chapters

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

Fig. 6.1 shows part of the Periodic Table.
(a)(i) State the symbol of an element with 8 electrons in its outer shell. Choose your answer from Fig. 6.1.
(a)(ii) State the symbol of an element that forms a positive ion. Choose your answer from Fig. 6.1.
(a)(iii) State the symbol of the element with the electronic configuration 2,8,3. Choose your answer from Fig. 6.1.
(b) Sodium reacts with chlorine to make the ionic compound sodium chloride, NaCl.
Draw a dot-and-cross diagram to show the ions in sodium chloride.
You only need to show the outer-shell electrons.
Include the charges on the ions.
(c) Fig. 6.2 shows a carbon atom.
(i) Carbon is in Period 2 of the Periodic Table. Use Fig. 6.2 to explain why carbon is in Period 2.
(ii) Carbon exists as isotopes, carbon-12 and carbon-13. Explain why both isotopes have the same chemical properties.
(iii) Graphite is one form of the element carbon. Fig. 6.3 shows the structure of graphite.
Graphite is used to make electrodes because it conducts electricity.
Explain why graphite conducts electricity.
(iv) Graphite electrodes are used in the electrolysis of aqueous copper(II) sulfate.
State the name of the product made at each electrode.

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

• Topic C2.2 — Atomic structure and the Periodic Table (Part (a)(i), (a)(ii), (a)(iii), (c)(i), (c)(ii))
• Topic C2.4 — Ions and ionic bonds (Part (b))
• Topic C2.6 — Giant covalent structures (Part (c)(iii))
• Topic C4.1 — Electrolysis (Part (c)(iv))

▶️ Answer/Explanation

(a)(i) Ar (argon).

Argon (Ar) is a noble gas with the electronic configuration 2,8,8. It has 8 electrons in its outer shell (a full outer shell), making it chemically stable and unreactive.

(a)(ii) Na, K, or Al (sodium, potassium, or aluminium).

Sodium (Na), potassium (K), and aluminium (Al) are metals. Metals tend to lose electrons from their outer shells to form positive ions (cations). Sodium loses 1 electron to form Na+, potassium loses 1 to form K+, and aluminium loses 3 to form Al3+.

(a)(iii) Al (aluminium).

The electronic configuration 2,8,3 means there are 13 electrons in total. The element with 13 electrons is aluminium (Al), which has the atomic number 13.

(b)

Sodium chloride is an ionic compound. Sodium (Na) has 1 electron in its outer shell and chlorine (Cl) has 7 electrons in its outer shell. Sodium transfers its single outer electron to chlorine. Sodium becomes a positive ion (Na+) with a full outer shell (2,8), and chlorine becomes a negative ion (Cl−) with a full outer shell (2,8,8). The dot-and-cross diagram shows the electron transfer with crosses representing sodium’s electron and dots representing chlorine’s electrons.

(c)(i) Carbon has 2 occupied electron shells.

The period number of an element in the Periodic Table corresponds to the number of occupied electron shells. Fig. 6.2 shows a carbon atom with 2 electron shells (the inner shell and the outer shell). Therefore, carbon is in Period 2.

(c)(ii) Both isotopes have the same electronic configuration.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons, while carbon-13 has 6 protons and 7 neutrons. Chemical properties are determined by the number and arrangement of electrons in the atom. Since both isotopes have the same number of electrons (6) and the same electronic configuration (2,4), they have identical chemical properties.

(c)(iii) Graphite has electrons that can move.

Graphite has a giant covalent structure where each carbon atom is bonded to three other carbon atoms in a hexagonal ring structure. This leaves one delocalised electron per carbon atom that is not used in bonding. These delocalised electrons are free to move throughout the structure, allowing graphite to conduct electricity.

(c)(iv) Anode: oxygen.
Cathode: copper.

During the electrolysis of aqueous copper(II) sulfate using graphite (inert) electrodes:
• At the anode (positive electrode), hydroxide ions (OH−) from the water are discharged. They lose electrons and form oxygen gas: 4OH− → O2 + 2H2O + 4e−.
• At the cathode (negative electrode), copper(II) ions (Cu2+) are discharged. They gain electrons and form copper metal: Cu2+ + 2e− → Cu.

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