Question 2
A value for enthalpy of solution can be determined in two ways:
- from a cycle, using lattice enthalpy and enthalpies of hydration
- from the results of a calorimetry experiment.
(c) A student does an experiment to determine a value for the enthalpy of solution for ammonium nitrate.
The student uses this method.
- Measure 25.0 cm3 of distilled water in a measuring cylinder.
- Pour the water into a beaker.
- Record the temperature of the water in the beaker.
- Add 4.00 g of solid NH4NO3 to the water in the beaker.
- Stir the solution and record the lowest temperature reached.
Table 2 shows the student’s results.
Calculate the enthalpy of solution, in kJ mol−1, for ammonium nitrate in this experiment.
Assume that the specific heat capacity of the solution, c = 4.18 J K−1 g−1
Assume that the density of the solution = 1.00 g cm−3
(3)
(d) The uncertainty in each of the temperature readings from the thermometer used in this experiment is ±0.1°C
Calculate the percentage uncertainty in the temperature change in this experiment.
(1)
(e) Suggest a change to the student’s method, using the same apparatus, that would reduce the percentage uncertainty in the temperature change.
Give a reason for your answer.
(2)
(f) Another student obtained a value of +15 kJ mol−1 using the same method.
Suggest the main reason for the difference between this experimental value for the enthalpy of solution and the correct value of +26 kJ mol−1
(1)
(Total 7 marks)