1. 1
   A technician heats a pure sample of solid naphthalene slowly in a water bath, recording its temperature every minute as it melts completely into a colourless liquid at a constant temperature of 80°C. The technician then repeats the experiment with a mixture of naphthalene and an impurity, and finds that this sample begins melting below 80°C and continues melting gradually over a range of several degrees.

   (a)

   While the pure naphthalene is melting, its temperature stays constant even though it is still being heated. What is happening to the energy supplied during this time?

   [1 mark]

   • AIt is being used to overcome the forces of attraction between particles, changing their arrangement
   • BIt is increasing the kinetic energy of the particles
   • CIt is being released to the surroundings as the particles form a lattice
   • DIt is decomposing the naphthalene into simpler substances

   (b)

   Which observation from the two experiments shows that the second sample was a mixture rather than a pure substance?

   [1 mark]

   • AIt melted at a temperature below 80°C
   • BIt melted completely into a liquid
   • CIt melted over a range of temperatures rather than at one fixed temperature
   • DIt was heated in the same water bath as the pure sample

   (c)

   State one difference between the melting behaviour of a pure substance and a mixture, and explain what this shows about the strength and uniformity of the forces holding the particles together in a mixture compared with a pure substance.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is given information about an atom of the element silicon: it has atomic number 14 and mass number 28, and is located in Period 3, Group 4 of the Periodic Table.

   (a)

   How many neutrons are there in an atom of this silicon isotope?

   [1 mark]

   • A28
   • B14
   • C42
   • D16

   (b)

   Which statement correctly explains why silicon is placed in Group 4 of the Periodic Table?

   [1 mark]

   • AIt has a mass number of 28
   • BIt is the 4th element in Period 3
   • CIt has 4 electron shells
   • DIt has 4 electrons in its outer shell

   (c)

   Deduce the electronic configuration of this silicon atom, and use it to explain why silicon is placed in Period 3 of the Periodic Table.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds 4.0 g of solid calcium hydroxide to 250 cm3 of dilute hydrochloric acid, and the two substances react completely with no acid left over, forming calcium chloride solution and water. (Ar: Ca = 40, O = 16, H = 1.) The equation for the reaction is: Ca(OH)2 + 2HCl → CaCl2 + 2H2O

   (a)

   Calculate the relative formula mass (Mr) of calcium hydroxide, Ca(OH)2, and use it to calculate the number of moles of calcium hydroxide in 4.0 g.

   [3 marks]

   (b)

   Using the balanced equation, calculate the number of moles, and hence the concentration in mol/dm3, of hydrochloric acid that exactly reacted with the 4.0 g of calcium hydroxide in the 250 cm3 of acid used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist compares two compounds: potassium chloride, which is a solid at room temperature with a very high melting point and conducts electricity when molten or dissolved in water but not when solid; and tetrachloromethane (CCl4), which is a liquid at room temperature with a low boiling point and does not conduct electricity in any state.

   (a)

   Explain, in terms of structure and bonding, why potassium chloride has a very high melting point, and why it conducts electricity when molten or in aqueous solution but not when solid.

   [6 marks]

   (b)

   Explain, in terms of structure and bonding, why tetrachloromethane has a low boiling point and does not conduct electricity in any state.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A metalworker compares a bar of pure copper, which bends easily and can be drawn into thin wire, with a sample of bronze (an alloy of copper and tin), which is much harder and more resistant to bending.

   (a)

   Why can the pure copper be bent and drawn into wire relatively easily?

   [1 mark]

   • AIts ions are held in a fixed lattice by strong covalent bonds
   • BIts atoms are arranged in layers of identical-sized atoms that can slide over each other
   • CIt contains no delocalised electrons to hold the structure together
   • DIts atoms are different sizes, which distorts the regular layers

   (b)

   Why is the bronze alloy harder and more resistant to bending than pure copper?

   [1 mark]

   • AThe alloy has weaker metallic bonding than pure copper
   • BThe alloy contains covalent bonds that pure copper does not have
   • CThe different-sized tin atoms distort the regular layers of copper atoms, making it harder for the layers to slide over each other
   • DThe tin atoms remove all the delocalised electrons from the structure

   (c)

   Using the idea of electrostatic attraction, explain why both pure copper and the bronze alloy are good conductors of electricity.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A technician melts solid zinc chloride and electrolyses the molten liquid using inert graphite electrodes connected to a direct current supply. A grey liquid metal collects at one electrode, and a pale green gas is given off at the other.

   (a)

   At which electrode does the grey liquid metal (zinc) form, and why?

   [1 mark]

   • AThe anode, because zinc ions are negatively charged and are attracted there
   • BThe anode, because zinc ions gain electrons there
   • CThe cathode, because zinc ions lose electrons there
   • DThe cathode, because zinc ions are positively charged and are attracted there

   (b)

   The pale green gas produced at the other electrode is chlorine. Why can this compound only be electrolysed when it is molten (or in aqueous solution), and not when it is solid?

   [1 mark]

   • AIn the solid, the ions are held in fixed positions and are not free to move and carry charge
   • BIn the solid, there are no ions present at all
   • CThe solid does not contain enough energy to form ions
   • DThe solid conducts electricity just as well as the molten liquid

   (c)

   Write the half-equation for the reaction at the cathode, and state whether this reaction is oxidation or reduction.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A student reacts 5.0 g of calcium carbonate with excess dilute hydrochloric acid in a conical flask connected to a gas syringe, and collects all the carbon dioxide gas produced at room temperature and pressure (rtp). (Ar: Ca = 40, C = 12, O = 16. Molar volume of a gas = 24 dm3 at rtp.) The equation for the reaction is: CaCO3 + 2HCl → CaCl2 + H2O + CO2

   (a)

   Calculate the number of moles of calcium carbonate in 5.0 g, and use the balanced equation to calculate the number of moles of carbon dioxide gas produced.

   [3 marks]

   (b)

   Calculate the volume, in dm3, of carbon dioxide gas produced at rtp, and state one change to the experiment that the student could make to increase the rate at which the gas is produced, without changing the total volume of gas collected.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A school technician sets up an electrolysis experiment using two rods of graphite as inert electrodes, connected to a power supply and dipped into molten zinc bromide, contained in a heatproof crucible. The graphite rods themselves do not react or dissolve during the electrolysis, and a grey solid metal forms at one electrode while an orange-brown gas is given off at the other.

   (a)

   Explain, in terms of its structure and bonding, why graphite is able to conduct electricity and can therefore be used to make the electrodes for this experiment, even though it is a giant covalent structure.

   [6 marks]

   (b)

   Explain, in terms of ions and electron transfer, what happens at each electrode during the electrolysis of molten zinc bromide, identifying the products formed and stating whether each electrode reaction is oxidation or reduction.

   [6 marks]

   Total for question 8: 12 marks

End of questions