8 4 When solid calcium sulfate dihydrate, CaSO4·2H2O, is heated in a crucible, it forms solid calcium sulfate hemihydrate, CaSO4·½H2O. (a) Write an equation, including state symbols, for this reaction. (1) (b) Which two terms could be used to describe this reaction? (1) Enthalpy change Type of process A endothermic hydration B exothermic hydration C exothermic dehydration D endothermic dehydration (c) When water is added to calcium sulfate hemihydrate, there is a rise in temperature. A student decided to investigate this reaction using the following procedure: Step 1    10 cm3 of distilled water is measured using a measuring cylinder having an uncertainty of ±0.5 cm3, and is placed in an insulated cup with a lid. Step 2    A thermometer with an uncertainty of ±0.5 °C is placed in the water. Step 3    Exactly 10.00 g of calcium sulfate hemihydrate is weighed out using a balance with an uncertainty of ±0.005 g. Step 4    The weighed quantity of calcium sulfate hemihydrate is added to the water in the insulated cup. Step 5    The mixture in the insulated cup is stirred until no further temperature change is observed. Results Temperature of the water before adding the solid = 23.5 °C Maximum temperature of the mixture after adding the solid = 26.3 °C Other data Molar mass of calcium sulfate hemihydrate, CaSO4·½H2O = 145.2 g mol–1 Density of water = 1.00 g cm–3 Turn over 9 (i) Calculate the minimum volume of water needed to convert 10.00 g of CaSO4·½H2O into CaSO4·2H2O. (2) (ii) Calculate the enthalpy change, in kJ mol–1, for this reaction. Include a sign in your answer and give your answer to an appropriate number of significant figures. Assume that the liquid has a mass of 10.00 g and a specific heat capacity of 4.18 J g–1 °C–1. (4) (iii) Deduce which measurement has the greatest uncertainty in this experiment. Justify your answer by calculating the percentage uncertainty of this piece of apparatus. (2) (Total for Question 4 = 10 marks)