CalorimetryEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student measures the enthalpy change of neutralisation. She mixes 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in a polystyrene cup with a lid, stirring gently. Both solutions start at the same temperature and the highest temperature reached is 6.8 °C higher. Assume that the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ °C⁻¹.

   (a)

   Why is a polystyrene cup with a lid used rather than a glass beaker?

   [1 mark]

   • AGlass reacts with the acid and alkali.
   • BPolystyrene absorbs the heat released by the reaction.
   • CPolystyrene is a poor conductor of heat, so less heat is lost to the surroundings.
   • DPolystyrene increases the specific heat capacity of the solution.

   (b)

   What is the enthalpy change of neutralisation, in kJ mol⁻¹, calculated from the student's results?

   [1 mark]

   • A+56.8
   • B–2.84
   • C–28.4
   • D–56.8

   (c)

   Calculate the energy transferred to the solution, in J, and state one assumption made in your calculation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student determines the standard enthalpy change of combustion of ethanol, C₂H₅OH (Mr = 46.0). She heats 200.0 g of water in a copper can using a spirit burner and measures a rise in temperature of 25.0 °C. The mass of the burner falls by 0.92 g. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹. The data-book value for ethanol is –1367 kJ mol⁻¹.

   (a)

   Which change would make the student's result closer to the data-book value?

   [1 mark]

   • APlace the burner further below the can so that the flame spreads out.
   • BSurround the apparatus with a draught shield and place the flame close to the can.
   • CStir the water less often.
   • DUse a thermometer with a wider range.

   (b)

   What is the experimental value of the enthalpy change of combustion of ethanol, in kJ mol⁻¹?

   [1 mark]

   • A–1045
   • B–1.05
   • C–10 450
   • D+1045

   (c)

   The experimental value is less exothermic than the data-book value. Suggest two reasons for this.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student measures the enthalpy change for the reaction Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). She puts 50.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup and records its temperature every 30 seconds for 2 minutes. At 2.5 minutes she adds an excess of zinc powder (without taking a reading) and carries on recording the temperature every 30 seconds until 8 minutes, stirring throughout. She plots temperature against time.

   (a)

   Explain why a cooling-curve correction is needed and describe how the student uses her graph to find the correct temperature change.

   [3 marks]

   (b)

   The corrected temperature rise is 9.8 °C. Assume that the solution has a mass of 50.0 g and a specific heat capacity of 4.18 J g⁻¹ °C⁻¹. Calculate the enthalpy change of the reaction in kJ mol⁻¹, with its sign.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student finds the enthalpy change of combustion of propan-1-ol, C₃H₇OH (Mr = 60.0), using a spirit burner. The burner heats 100 cm³ of water (taken as 100.0 g, measured in a measuring cylinder) in a copper can. The thermometer is graduated in 1 °C divisions and the temperature rises by 24.0 °C. The balance reads to ±0.01 g, and the mass of the burner falls from 152.40 g to 151.85 g. The data-book value is –2021 kJ mol⁻¹. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹.

   (a)

   Calculate the experimental enthalpy change of combustion of propan-1-ol in kJ mol⁻¹, and express it as a percentage of the data-book value.

   [6 marks]

   (b)

   Evaluate the reliability of this experiment, referring to sources of error, the uncertainties in the measurements and assumptions, and suggest improvements.

   [6 marks]

   Total for question 4: 12 marks

End of questions