1. 1
   A sports injury cold pack contains solid ammonium nitrate and a separate pouch of water. When the pouch is burst, the ammonium nitrate dissolves and the temperature of the pack falls from 22.0 °C to 5.0 °C. A hand warmer contains iron powder, which is oxidised by oxygen from the air when the packet is opened, and its temperature rises to about 50 °C.

   (a)

   Which statement describes the dissolving of ammonium nitrate in the cold pack?

   [1 mark]

   • AExothermic; energy is transferred from the system to the surroundings; ΔH is negative.
   • BEndothermic; energy is transferred from the system to the surroundings; ΔH is negative.
   • CExothermic; the temperature falls because heat is released by the system.
   • DEndothermic; energy is transferred from the surroundings to the system; ΔH is positive.

   (b)

   Which statement about heat and temperature is correct?

   [1 mark]

   • ATemperature is a measure of the average kinetic energy of the particles; heat is energy transferred because of a temperature difference.
   • BHeat and temperature are both measured in joules.
   • CTemperature is the total energy of all the particles in a substance.
   • DA bath of water at 40 °C has a higher temperature than a cup of water at 60 °C because it contains more energy.

   (c)

   Explain, in terms of the relative stability of the reactants and products, why the reaction in the hand warmer releases heat.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a polystyrene-cup calorimeter, 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid is mixed with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution. Both solutions start at 21.0 °C and the highest temperature reached is 27.6 °C. Assume the mixture has a density of 1.00 g cm⁻³ and a specific heat capacity of 4.18 J g⁻¹ K⁻¹. The accepted value for the enthalpy change of neutralisation is −57.1 kJ mol⁻¹.

   (a)

   How much heat energy is transferred to the mixture?

   [1 mark]

   • A1.38 kJ
   • B2.76 kJ
   • C11.5 kJ
   • D2.76 × 10³ kJ

   (b)

   What is the enthalpy change of neutralisation calculated from these results?

   [1 mark]

   • A+55.2 kJ mol⁻¹
   • B−27.6 kJ mol⁻¹
   • C−55.2 kJ mol⁻¹
   • D−2.76 kJ mol⁻¹

   (c)

   Calculate the percentage error in the experimental value, and suggest the main reason why it is less exothermic than the accepted value.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds 1.50 g of zinc powder (an excess) to 25.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution in an insulated cup, stirring continuously. The temperature rises from 20.4 °C to a maximum of 30.1 °C. The reaction is Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Assume the solution has a density of 1.00 g cm⁻³ and a specific heat capacity of 4.18 J g⁻¹ K⁻¹.

   (a)

   Calculate the enthalpy change, in kJ mol⁻¹, for the reaction between zinc and copper(II) sulfate.

   [3 marks]

   (b)

   The student repeats the experiment with 50.0 cm³ of the same copper(II) sulfate solution and the same mass of zinc. Predict and explain the maximum temperature rise, and discuss how one assumption in the calculation in (a) affects the value of ΔH obtained.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student burns ethanol (C₂H₅OH, M = 46.08 g mol⁻¹) in a spirit burner beneath a copper can containing 200.0 g of water. The mass of the burner falls from 85.42 g to 84.97 g, and the water temperature rises from 19.5 °C to 34.0 °C. A layer of soot forms on the bottom of the can. The data booklet value of the standard enthalpy of combustion of ethanol is −1367 kJ mol⁻¹. For comparison, the thermal decomposition of calcium carbonate, CaCO₃(s) → CaO(s) + CO₂(g), has ΔH⦵ = +178 kJ mol⁻¹. Take the specific heat capacity of water as 4.18 J g⁻¹ K⁻¹.

   (a)

   Calculate the enthalpy of combustion of ethanol from the student's results, and evaluate the result by comparing it with the data booklet value.

   [6 marks]

   (b)

   Compare the combustion of ethanol with the thermal decomposition of calcium carbonate in terms of the direction of energy transfer, the relative stability of reactants and products, and their energy profiles. Outline what the symbol ⦵ in ΔH⦵ means.

   [6 marks]

   Total for question 4: 12 marks

End of questions