CalorimetryAQA A-Level Chemistry: Subtopic test

1. 1
   A student mixes 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 a polystyrene cup. The initial temperature of both solutions is 20.5 °C and the highest temperature reached is 27.3 °C. Assume that the density of the solution is 1.00 g cm⁻³ and that its specific heat capacity is 4.18 J g⁻¹ K⁻¹.

   (a)

   What mass of solution should be used as m in the equation q = mcΔT?

   [1 mark]

   • A25.0 g
   • B50.0 g
   • C0.0500 g
   • D100 g

   (b)

   What is the heat energy released by the reaction?

   [1 mark]

   • A1.42 kJ
   • B0.71 kJ
   • C2.84 kJ
   • D5.71 kJ

   (c)

   Calculate the molar enthalpy change of neutralisation, in kJ mol⁻¹, for this reaction. Include the sign.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the enthalpy of combustion of ethanol using a spirit burner to heat 100 g of water in a copper calorimeter. The mass of the burner and ethanol falls from 215.30 g to 214.84 g during the experiment, and the temperature of the water rises from 19.0 °C to 38.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹ and the data book value for the enthalpy of combustion of ethanol is −1367 kJ mol⁻¹. The relative formula mass, Mr, of ethanol is 46.0.

   (a)

   Which mass is used as m when calculating q = mcΔT in this experiment?

   [1 mark]

   • A0.46 g
   • B46.0 g
   • C215.30 g
   • D100 g

   (b)

   Which value is the experimental enthalpy of combustion of ethanol?

   [1 mark]

   • A−7.94 kJ mol⁻¹
   • B+794 kJ mol⁻¹
   • C−794 kJ mol⁻¹
   • D−79.4 kJ mol⁻¹

   (c)

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

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student adds excess zinc powder to 50.0 cm³ of 0.500 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup fitted with a lid. The temperature rises from 19.5 °C to 45.0 °C. The thermometer has an uncertainty of ±0.1 °C on each reading. Assume that the solution has a density of 1.00 g cm⁻³ and a specific heat capacity of 4.18 J g⁻¹ K⁻¹. The equation for the reaction is Zn + CuSO₄ → ZnSO₄ + Cu.

   (a)

   Calculate the molar enthalpy change for this reaction, in kJ mol⁻¹, per mole of copper(II) sulfate.

   [3 marks]

   (b)

   Calculate the percentage uncertainty in the temperature change. State and explain one graphical method that would improve the accuracy of the temperature change measured.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher wants a class to determine the enthalpy change when solid ammonium nitrate, NH₄NO₃ (Mr = 80.0), dissolves in water, which is an endothermic process. Each group has a polystyrene cup with a lid, a beaker, a balance, a thermometer with an uncertainty of ±0.1 °C on each reading, a stopwatch, and 50.0 cm³ of water. One group adds 4.00 g of ammonium nitrate, and the temperature falls by 5.4 °C. They calculate the enthalpy change using a mass of 50.0 g and a specific heat capacity of 4.18 J g⁻¹ K⁻¹, and obtain +22.6 kJ mol⁻¹. The data book value is +25.7 kJ mol⁻¹.

   (a)

   Describe how the students should carry out the experiment to obtain a reliable value for the temperature change.

   [6 marks]

   (b)

   Evaluate whether the thermometer uncertainty alone can explain the difference between the group's value and the data book value, and discuss the most likely cause of the difference.

   [6 marks]

   Total for question 4: 12 marks

End of questions