DSE Chemistry Practice Paper: Chemical Bonding And Structure

DSE Chemistry Practice Paper: Chemical Bonding and Structure

2026-11-03

DSE Chemistry – Bonding and Structure Practice

DSE Chemistry practice paper on chemical bonding and structure. Twelve questions, 100 marks, 75 minutes. Covers ionic, covalent and metallic bonding models.

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Paper details　DSE Chemistry, ionic bonding, covalent bonding, metallic bonding, giant structure, hydrogen bonding, 12 questions, 100 marks

Section A (Questions 1 to 4, 26 marks)

Read each question and write the correct answer with an explanation. Show working and units for any calculation.

#	Question	Answer and marking notes
1	Describe how ionic bonding and covalent bonding differ in the way they are formed, giving one example of each.	Ionic bonding involves transfer of electrons: a metal atom loses electrons to become a cation and a non-metal atom gains them to become an anion, and the ions are held by electrostatic attraction, as in sodium chloride. Covalent bonding involves sharing: two non-metal atoms each contribute electrons to a shared pair, as in the oxygen to hydrogen bonds of a water molecule. Ionic bonding is non-directional while a covalent bond has a definite length and angle.
2	Explain why sodium chloride melts at about 801 degrees Celsius while water boils at 100 degrees Celsius, and name the structure type of each.	Sodium chloride has a giant ionic lattice in which sodium and chloride ions are arranged regularly in three dimensions; melting requires breaking many strong electrostatic attractions, so the melting point is high. Water is a simple molecular substance whose molecules are held only by hydrogen bonds and van der Waals forces, so much less energy is needed to separate them and the boiling point is low.
3	Magnesium oxide melts at about 2852 degrees Celsius, higher than sodium chloride. Explain this in terms of the charges on the ions.	Both are giant ionic lattices, but magnesium oxide contains Mg2+ and O2- ions with charges of 2+ and 2-, compared with 1+ and 1- in sodium chloride. The electrostatic attraction is proportional to the product of the charges, so the attraction in magnesium oxide is much stronger and a higher temperature is needed to break the lattice.
4	Carbon dioxide is a gas at room temperature while silicon dioxide (quartz) is a solid with a very high melting point. Both are covalent substances; explain the difference.	Carbon dioxide is a simple molecular substance with only weak van der Waals forces between molecules, which are easily overcome, so it is a gas at room temperature. Silicon dioxide is a giant covalent structure in which each silicon atom is bonded to four oxygen atoms in a three dimensional network; melting requires breaking many strong covalent bonds, so the melting point is very high.

Section B (Questions 5 to 8, 32 marks)

Answer the following questions with working, units and evidence drawn from the data given.

#	Question	Answer and marking notes
5	Use the model of metallic bonding to explain why metals conduct electricity and are malleable.	In the metallic model the metal atoms release their outer electrons, which become delocalised and form a sea of electrons around a lattice of positive ions. Conduction occurs because these electrons can move in an applied electric field, and malleability occurs because layers of ions can slide through the electron sea without the structure breaking.
6	Identify the structure type of each of the following and give a reason: potassium iodide, carbon tetrachloride, diamond.	Potassium iodide is a giant ionic structure of potassium and iodide ions held by electrostatic attraction, so it has a high melting point and conducts when molten. Carbon tetrachloride is a simple molecular substance with polar bonds but only van der Waals forces between molecules, so it is volatile. Diamond is a giant covalent structure of carbon atoms bonded tetrahedrally, very hard and a non-conductor.
7	The boiling points of the hydrogen halides HF, HCl, HBr and HI are about 20, -85, -67 and -35 degrees Celsius. Explain why HF is exceptional and why the other three rise in that order.	The HF molecule has hydrogen bonded to highly electronegative fluorine, so hydrogen bonds form between neighbouring molecules and the intermolecular attraction is far stronger than in the others, giving an unusually high boiling point. HCl, HBr and HI have only van der Waals forces; as the molecules get larger the number of electrons rises, they become more polarisable and the forces strengthen, so the boiling points rise in that order.
8	Explain why ice is less dense than water and state the role of hydrogen bonding.	Each oxygen atom in water has two lone pairs, each able to form a hydrogen bond with a hydrogen atom of another molecule. In ice every molecule is hydrogen bonded tetrahedrally to four neighbours, giving an open structure with cavities. The cavities make the volume larger for the same mass, so ice is less dense than water; on melting some hydrogen bonds break and the structure collapses into a closer packing, so the density increases.

Section C (Questions 9 to 12, 42 marks)

Write fuller answers using correct subject terms and support them with examples from Hong Kong or daily life.

#	Question	Answer and marking notes
9	Explain the electrical conductivity of solid sodium chloride, molten sodium chloride and graphite.	In solid sodium chloride the ions are fixed in the lattice; although charged, they cannot move, so it does not conduct. When molten or dissolved the ions become free to move and carry charge, so it conducts. In graphite each carbon atom bonds to three others in its layer and the fourth electron is delocalised between the layers, giving mobile electrons, so graphite conducts.
10	Compare the structure and properties of diamond and graphite, giving one use of each.	In diamond each carbon atom is covalently bonded to four others in a tetrahedral three dimensional network; the bonds are strong and there are no free electrons, so diamond is extremely hard, has a high melting point and does not conduct, and is used in drill bits and cutting tools. In graphite the carbon atoms form hexagonal layers with strong covalent bonds within a layer but only weak van der Waals forces between layers, so it is soft and used as a lubricant, and the delocalised electrons between layers let it conduct, so it is used for electrodes.
11	Given bond enthalpies H-H 436 kJ/mol, Cl-Cl 243 kJ/mol and H-Cl 431 kJ/mol, calculate the enthalpy change of H2 + Cl2 -> 2HCl and explain the sign.	Enthalpy change = energy absorbed in breaking bonds – energy released in forming bonds = (436 + 243) – 2 x 431 = 679 – 862 = -183 kJ/mol. The negative sign shows the reaction is exothermic: the products have lower enthalpy than the reactants and 183 kJ of heat is released per mole of reaction.
12	58.5 g of sodium chloride is dissolved completely in water. Given a relative formula mass of 58.5, find the total number of sodium and chloride ions using the Avogadro constant 6.02 x 10^23.	The molar mass of sodium chloride is 58.5 g/mol, so 58.5 g is 1 mol of sodium chloride. Each formula unit contains one sodium ion and one chloride ion, giving 2 mol of ions in total: 2 x 6.02 x 10^23 = 12.04 x 10^23, about 1.2 x 10^24 ions.

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