Home / iGCSE Chemistry 0620 Extended – 2.6 Giant covalent structures- Exam Style Questions Paper 4

iGCSE Chemistry 0620 Extended - 2.6 Giant covalent structures- Exam Style Questions Paper 4- New Syllabus

iGCSE Chemistry Theory (Extended) Paper 4 - All Chapters

Question

This question is about covalent compounds.
(a) State what is meant by a covalent bond.
(b) Chlorine(I) oxide, $\mathrm{Cl}_2\mathrm{O}$, is a simple molecule with covalent bonds.
(i) State what is meant by (I) in the name chlorine(I) oxide.
(ii) Complete the dot-and-cross diagram in Fig. 2.1 of a molecule of chlorine(I) oxide.
Show outer electrons only.
(iii) Explain, in terms of structure and bonding, why $\mathrm{Cl_2O}$ boils at a low temperature and does not thermally decompose into its constituent elements, $\mathrm{Cl_2}$ and $\mathrm{O_2}$.
(iv) Give two reasons why liquid $\mathrm{Cl_2O}$ is a poor conductor of electricity.
(c) Carbon and silicon(IV) oxide both exist as giant covalent structures.
(i) Name a giant covalent structure of carbon which conducts electricity.
(ii) Identify the particles responsible for the conduction of electricity in this covalent structure of carbon.
(iii) Silicon(IV) oxide contains silicon atoms, $\mathrm{Si}$, and oxygen atoms, $\mathrm{O}$.
Fig. 2.2 shows part of the giant covalent structure of silicon(IV) oxide.
Complete the diagram in Fig. 2.2 by adding the symbol for each of the 9 atoms shown.

Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):

• Topic 2.5 — Simple molecules and covalent bonds (Parts (a), (b)(i), (b)(ii), (b)(iii), (b)(iv))
• Topic 2.6 — Giant covalent structures (Parts (c)(i), (c)(ii), (c)(iii))

▶️ Answer/Explanation

(a) A covalent bond is a pair of electrons shared between two atoms.
Both parts (“pair of electrons” and “shared between two atoms”) are required for the definition.

(b)(i) The oxidation number of chlorine / $\mathrm{Cl}$ is $+1$.
The Roman numeral (I) indicates the oxidation state of the chlorine atom in the compound.

(b)(ii) The diagram requires an $\mathrm{O}$ with one bonding pair (dot-cross) to each $\mathrm{Cl}$, $4$ non-bonding dots on $\mathrm{O}$, and $6$ non-bonding crosses on each $\mathrm{Cl}$.

(b)(iii) Low boiling point: weak (forces of) attraction between molecules are broken (during boiling). No thermal decomposition: covalent bonds (between $\mathrm{Cl}$ and $\mathrm{O}$ atoms) are strong / strong covalent bonds broken (during thermal decomposition).
Boiling is a physical change overcoming weak intermolecular forces, requiring little energy. Decomposition is a chemical change breaking strong intramolecular covalent bonds, requiring much more energy.

(b)(iv) Liquid $\mathrm{Cl_2O}$ is a poor conductor because:
$1$. no ions
$2$. no mobile electrons
$\mathrm{Cl_2O}$ is a simple molecular covalent compound with no free charged particles to carry current.

(c)(i) Graphite
Graphite is the only giant covalent structure of carbon that conducts electricity well.

(c)(ii) Electrons
Delocalised electrons between the layers of graphite act as mobile charge carriers.

(c)(iii) The diagram is completed with $5$ correct $\mathrm{Si}$ atoms and $4$ correct $\mathrm{O}$ atoms. In $\mathrm{SiO_2}$, each $\mathrm{Si}$ is bonded to four $\mathrm{O}$ atoms, and each $\mathrm{O}$ is bonded to two $\mathrm{Si}$ atoms.

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