Measuring enthalpy changesEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student measures the enthalpy change of combustion of propan-1-ol (Mr = 60.0) using a spirit burner to heat 150 g of water in a copper can. The temperature of the water rises by 28.0 K when 0.60 g of propan-1-ol is burned. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.

   (a)

   How much energy is transferred to the water?

   [1 mark]

   • A70.2 J
   • B117 J
   • C17.6 kJ
   • D17.6 J

   (b)

   What is the experimental value of the enthalpy change of combustion of propan-1-ol?

   [1 mark]

   • A−1760 kJ mol⁻¹
   • B+1760 kJ mol⁻¹
   • C−17.6 kJ mol⁻¹
   • D−176 kJ mol⁻¹

   (c)

   The data book value is −2021 kJ mol⁻¹. Give two reasons why the experimental value is less exothermic than this.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the enthalpy change of neutralisation by mixing 25.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 25.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in an insulated polystyrene cup. The temperature rises from 20.5 °C to 27.2 °C. Assume the solutions have density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.

   (a)

   Why is the reaction carried out in a polystyrene cup?

   [1 mark]

   • AIt is a good thermal insulator, so it reduces heat transfer to the surroundings
   • BIt is a good conductor of heat, so the temperature rise is measured faster
   • CIt increases the rate of reaction
   • DIt reacts with the acid to remove impurities

   (b)

   Which mass should be used as m in Q = mcΔT?

   [1 mark]

   • A25.0 g
   • B0.0250 g
   • C100 g
   • D50.0 g

   (c)

   Calculate the enthalpy change of neutralisation, in kJ mol⁻¹, including the sign.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student measures the enthalpy change for Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). She adds 2.00 g of zinc powder (Ar = 65.4) to 50.0 cm³ of 0.500 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup and stirs. The temperature rises from 19.8 °C to 45.6 °C. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.

   (a)

   Calculate the amount, in mol, of copper(II) ions in the solution and the energy transferred to the solution, in kJ.

   [3 marks]

   (b)

   Use your answers to part (a) to calculate the enthalpy change for the reaction in kJ mol⁻¹. Show by calculation which reagent is in excess, and explain the sign of your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student wants to measure enthalpy changes in a school laboratory using simple apparatus: a spirit burner and a metal can of water for combustion, and a polystyrene cup for reactions in solution.

   (a)

   Three students each use the spirit burner and metal can to find the enthalpy change of combustion of ethanol and obtain values between −600 and −800 kJ mol⁻¹. The data book value is −1367 kJ mol⁻¹. Explain why their values are much less exothermic than the data book value, and suggest improvements to the method.

   [6 marks]

   (b)

   Describe how the student should measure the enthalpy change of neutralisation of hydrochloric acid with sodium hydroxide solution using the polystyrene cup, and how the results should be processed to give the enthalpy change with its sign and units.

   [6 marks]

   Total for question 4: 12 marks

End of questions