Simple molecular and giant covalent structuresIB MYP Chemistry: Subtopic test

1. 1
   A technician in Dubai gently warms a few crystals of iodine, I₂, in a fume cupboard. The grey-black solid easily turns into a purple vapour at a temperature far below 200 °C. Solid iodine does not conduct electricity.

   (a)

   Which statement describes the structure of iodine?

   [1 mark]

   • ASimple molecules, each made of two atoms joined by a covalent bond
   • BA giant lattice of positive and negative ions
   • CA giant network of atoms joined by covalent bonds
   • DPositive ions in a sea of delocalised electrons

   (b)

   What must be overcome when solid iodine changes into vapour?

   [1 mark]

   • AThe strong covalent bonds within each iodine molecule
   • BThe electrostatic attraction between ions
   • CWeak forces between the molecules
   • DThe attraction between positive ions and delocalised electrons

   (c)

   Explain why iodine changes into a vapour at such a low temperature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A jeweller in Antwerp compares a diamond with a piece of graphite from a pencil. Both are made of carbon atoms only, but diamond is the hardest natural material and graphite is soft and slippery.

   (a)

   Which statement describes the bonding in diamond?

   [1 mark]

   • AEach carbon atom is joined to three others by covalent bonds, in layers
   • BEach carbon atom is joined to four others by covalent bonds
   • CCarbon atoms are in separate small molecules
   • DCarbon ions are held in a lattice by electrostatic attraction

   (b)

   Why can graphite conduct electricity?

   [1 mark]

   • AIt contains ions that can move
   • BIt is made of small molecules
   • CEach carbon atom uses all four outer electrons in bonds
   • DEach carbon atom has one delocalised electron that can move

   (c)

   Explain why diamond is so hard.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student tests three unknown solids, P, Q and R. P melts at 114 °C and does not conduct electricity when solid or molten. Q is very hard, melts at about 1700 °C and does not conduct electricity in any state. R is a soft black solid that does not melt even above 3000 °C and conducts electricity when solid. The student predicts that any solid with a high melting point will conduct electricity.

   (a)

   Identify the type of structure of each of P, Q and R, choosing from simple molecular, a giant covalent structure like silicon dioxide, and a giant covalent structure like graphite.

   [3 marks]

   (b)

   Evaluate the student's prediction using Q and R, and explain in terms of structure why Q and R differ in conductivity.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineering firm in Pune makes drill bits tipped with diamond and electrodes made of graphite. It is deciding whether to buy mined diamond for the tips or to use diamond made in a laboratory at very high temperature and pressure.

   (a)

   Explain, in terms of structure and bonding, why diamond is used on drill bits and graphite is used for electrodes.

   [6 marks]

   (b)

   Discuss the social, economic and environmental implications of buying laboratory-made diamond rather than mined diamond for the drill bits, and evaluate which the firm should choose.

   [6 marks]

   Total for question 4: 12 marks

End of questions