The pain of muscle strains and swellings can be eased by using heat packs. As a source of heat, some heat packs use the energy released when anhydrous calcium chloride dissolves in water. water CaCl₂(s) → Ca²⁺(aq) + 2Cl⁻(aq) A heat pack consists of a bag of water, inside which a smaller bag contains anhydrous calcium chloride. When pressure is applied to the heat pack, the smaller bag bursts releasing the anhydrous calcium chloride into the water. The heat pack is shaken to speed up dissolving. Energy is released which warms the heat pack. A student carried out an experiment to determine the enthalpy change when anhydrous calcium chloride dissolves in distilled water. The results the student obtained are plotted on the graph on page 4.

1. (a)By considering the graph of results, draw a labelled diagram of the experimental set-up that the student could have used to produce the graph shown. Label the apparatus and chemicals required to measure the two variables.[2]
2. (b)Explain why the student took readings between 0.0 minutes and 2.5 minutes.[1]
3. (c)Explain why the student did not take a reading at 3.0 minutes.[1]
4. (d)Explain why the temperature continued to increase between 3.5 minutes and 4.5 minutes.[1]
5. (e)Draw two straight lines of best fit on the grid. Extrapolate these lines to estimate the theoretical temperature rise at 3.0 minutes. Give your answer to one decimal place. theoretical temperature rise at 3.0 minutes = .............................................. °C[2]
6. (f)One of the results is anomalous. This occurred because the student took the thermometer out of the solution and then replaced it just before the reading was taken. The time at which the anomalous reading was taken was .............................................. minutes. Explain why these actions led to the anomalous point.[1]
7. (g)Explain why stirring the mixture would make this experiment more reliable.[1]
8. (h)Anhydrous calcium chloride is classified as a moderate health hazard. It is an irritant. Apart from wearing eye protection, state one other relevant safety precaution the student should have taken.[1]
9. (i)The student found the value for the enthalpy change of solution of anhydrous calcium chloride to be –82.5 kJ mol⁻¹. A manufacturer produces a heat pack that contains 75.0 g of water. Calculate the mass of anhydrous calcium chloride the manufacturer must use in the inner bag to produce a rise in temperature of 30.0 °C. The specific heat capacity of water, c = 4.18 J g⁻¹ K⁻¹. [Aᵣ: Ca, 40.1; Cl, 35.5] mass of anhydrous CaCl₂ = .............................................. g[2]

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