1. 1
   A pencil contains a core of graphite, which leaves a dark mark on paper as the pencil is dragged across it, because thin layers of graphite rub off onto the page. A student also notices that a graphite pencil core conducts electricity when tested in a simple circuit.

   (a)

   Which statement explains why layers of graphite can be rubbed off so easily?

   [1 mark]

   • AGraphite consists of simple molecules held together by strong covalent bonds
   • BEach carbon atom in graphite forms only three covalent bonds, creating layers with only weak forces between them, which allows the layers to slide apart
   • CGraphite is an ionic compound with weak electrostatic forces between its ions
   • DGraphite atoms are held together by delocalised electrons alone, with no covalent bonds at all

   (b)

   The student tests the graphite pencil core in a circuit with a battery and ammeter and finds that it conducts electricity. Which statement explains why graphite conducts electricity?

   [1 mark]

   • AGraphite conducts because it is a simple molecular substance with weak intermolecular forces
   • BGraphite contains free-moving positive and negative ions
   • CEach carbon atom in graphite forms only three covalent bonds, leaving one outer electron per atom delocalised and free to move along the layers
   • DGraphite conducts only because it is shaped into a pencil core

   (c)

   Explain, in terms of structure and bonding, why graphite is soft enough to leave a mark on paper and also conducts electricity.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An engineer applies powdered graphite as a dry lubricant between two moving metal surfaces in a machine, instead of using oil. The same machine uses graphite rods as electrodes to carry current in an electrolysis cell elsewhere in the factory.

   (a)

   Which statement explains why graphite works well as a lubricant?

   [1 mark]

   • AGraphite contains delocalised electrons that push the two surfaces apart
   • BGraphite melts at low temperatures, coating the surfaces with a liquid film
   • CGraphite reacts chemically with the metal to form a smooth new surface
   • DThe weak forces between the layers of carbon atoms in graphite allow the layers to slide over each other easily, reducing friction between the surfaces

   (b)

   Elsewhere in the factory, graphite rods are used as electrodes to carry electric current into an electrolysis cell. Which statement explains why graphite is suitable for use as an electrode?

   [1 mark]

   • AGraphite is chosen because it dissolves easily in the electrolyte
   • BGraphite contains delocalised electrons, one per carbon atom, that are free to move through the structure and carry electric current
   • CGraphite is an ionic compound that conducts electricity when solid
   • DGraphite conducts electricity because its layers are held together by strong covalent bonds

   (c)

   Explain, in terms of structure and bonding, why graphite is effective both as a dry lubricant between moving metal surfaces and as an electrode carrying electric current.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A materials scientist compares graphite with diamond, both forms of carbon. She measures the hardness and electrical conductivity of each sample.

   (a)

   She measures the hardness of each and finds graphite is soft while diamond is extremely hard. Explain, in terms of the number of covalent bonds formed by each carbon atom, why graphite is much softer than diamond.

   [3 marks]

   (b)

   The scientist also tests the electrical conductivity of both samples and finds that graphite conducts electricity but diamond does not. Explain, referring to the number of covalent bonds and delocalised electrons in each structure, why graphite conducts electricity while diamond does not, even though both are made of only carbon atoms.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A factory engineer is designing electrodes for an industrial electrolysis cell that must conduct electricity efficiently while also being inserted and removed from the cell without special tools, relying on graphite's softness for easy handling.

   (a)

   Describe, in terms of structure and bonding, why graphite is well suited to both of these requirements, referring to the arrangement of carbon atoms within its layers and the forces between layers.

   [6 marks]

   (b)

   The engineer considers using diamond instead of graphite for the same electrode, since diamond is also a form of carbon, but rejects this idea. Explain, in terms of structure and bonding, why diamond would be unsuitable as an electrode for the electrolysis cell, even though it is an extremely strong and durable material.

   [6 marks]

   Total for question 4: 12 marks

End of questions