Measuring enthalpy changes of reactions
4 marks · NormalA student measures the temperature change when potassium nitrate dissolves in water in a calorimeter. The data is shown below:
| Time (min) | Temperature (°C) |
|---|---|
| 0 | 20.0 |
| 1 | 19.1 |
| 2 | 19.1 |
| 3 | 19.2 |
Describe how the temperature of the solution changes during the experiment.
Calculate the overall temperature change from 0 to 1 minute.
What type of reaction is occurring? Explain using the data.
Suggest why the temperature remains almost constant after 1 minute.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Temperature decreases then remains steady
(b) -0.9 °C
(c) Endothermic; temperature falls as heat is absorbed
(d) Reaction is complete or temperature has reached thermal equilibrium
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes the initial decrease then constant (plateau) temperature
A1 (b) · 1 mark · Correct temperature change is -0.9 °C (19.1 - 20.0)
B2 (c) · 1 mark · States 'endothermic', with explanation related to absorption of heat/temperature decrease
B3 (d) · 1 mark · Suggests reaction completed or equilibrium reached
Worked solution
(a) The temperature decreases at first, then stays almost constant.
(b) Overall temperature change from 0 to 1 min: 19.1 °C - 20.0 °C = -0.9 °C
(c) The reaction is endothermic as the temperature falls, showing heat is absorbed.
(d) The temperature remains steady because the dissolution (reaction) is complete or has reached equilibrium.