In this experiment you will determine the enthalpy change of solution, ΔHsol, for hydrated sodium thiosulfate, Na₂S₂O₃•5H₂O. To do this you will measure the temperature change when a known mass of hydrated sodium thiosulfate is dissolved in a known volume of water. FB 5 is hydrated sodium thiosulfate, Na₂S₂O₃•5H₂O.

1. (a)Method • Support the cup in the 250 cm³ beaker. • Use the 25 cm³ measuring cylinder to transfer 20.0 cm³ of distilled water into the cup. • Weigh the stoppered container of FB 5 and record the mass. • Measure and record the initial temperature of the water in the cup. • Add all the FB 5 to the water in the cup. • Stir the mixture and record the minimum temperature that is reached. • Reweigh the stoppered container. Record the mass. • Calculate and record the mass of FB 5 added to the water and the change in temperature. I II III IV[4]
2. (b)(i)Calculations Calculate the energy change of the reaction. (Assume that 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0 °C.) Show your working. energy change = ................................. J[1]
3. (b)(ii)Calculate the enthalpy change of solution, ∆Hsol, for hydrated sodium thiosulfate. ΔHsol for Na₂S₂O₃•5H₂O = ................................. kJ mol⁻¹ sign value[2]
4. (b)(iii)Assume that under the same conditions, the enthalpy change of solution, ΔHsol, for anhydrous sodium thiosulfate, Na₂S₂O₃, is −7.7 kJ mol⁻¹. Construct a Hess's cycle and determine the enthalpy change for the following reaction. (If you were unable to calculate an answer to (b)(ii), assume a value of +32.2 kJ mol⁻¹. Note this is not the correct value.) Na₂S₂O₃(s) + 5H₂O(l) → Na₂S₂O₃•5H₂O(s) ΔH = ................................. kJ mol⁻¹ sign value[2]
5. (c)How would your temperature change in (a) be affected if your sample of FB 5 contained a small amount of anhydrous sodium thiosulfate? Explain your answer.[1]