A-LevelChemistryChemical energetics9701/33 · May/June 2019 · Q210 Marks

2 In this experiment you will determine the enthalpy change of solution for hydrated sodium thiosulfate. FA 3 is hydrated sodium thiosulfate, Na2S2O3.5H2O.

(a)Method • Support the plastic cup in the 250 cm³ beaker. • Rinse the measuring cylinder. • Using the measuring cylinder, pour 25.0 cm³ of distilled water into the plastic cup. • Measure the temperature of the water in the cup. • Weigh the container with FA 3. • Add all the FA 3 to the distilled water in the cup. • Use the thermometer to stir the mixture gently until all the solid has dissolved. • Measure the lowest temperature that is reached. • Reweigh the container with any remaining FA 3. • Record all your measurements. • Calculate and record the mass of FA 3 added and the change in temperature.[4]
(b)(i)Calculate how many moles of FA 3 were added to the water. moles of FA 3 = ...................................... mol[1]
(b)(ii)Calculate the energy change when the sample of FA 3 was added to the distilled water. [Assume that 4.2J of heat energy changes the temperature of 1.0cm³ of solution by 1.0°C.] energy change = ...................................... J[1]
(b)(iii)Calculate the enthalpy change of solution of FA 3. enthalpy change of solution = sign value units[1]
(c)One way to improve this experiment would be to use a balance that reads to more decimal places. Suggest two other ways in which this experiment could be altered to give a more accurate value for the enthalpy change. Explain how each would improve the accuracy. Suggestion 1 Suggestion 2[2]
(d)A student carrying out the experiment in Question 1 used all the FA 1. The student made up a fresh sample of FA 1 of the correct concentration by dissolving some FA 3 in water. This solution was then used immediately to repeat one of the experiments in Question 1 but the time was then much greater than had been measured previously. Explain why.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] I Unambiguous headings for 3 masses and 3 temperatures AND Correct units.
[1] II Both balance readings recorded to the same dp AND Both thermometer readings recorded to either 0.0 or 0.5 °C
[2] III and IV Compare ΔT / m for candidate with that of supervisor. Award 2 marks if δ is within 0.15 of supervisor Award 1 mark if δ is within 0.30 of supervisor
[1] Correctly calculates mass used AND Correctly calculates mass/248.2 AND answer displayed to 3 or 4 sf
[1] Correctly calculates temperature change AND Correctly calculates ΔT × 4.2 × 25 to a minimum of 2 sf
[1] Correctly uses ans(ii) / ans(i) AND answer given to a minimum of 2 sf AND + sign AND appropriate units of kJ mol⁻¹ or J mol⁻¹
[2] Each correct suggestion = * Each correct explanation = * (Explanation must follow from available suggestion) 2* = 1 mark, 3* = 2 marks Insulate the cup (or use a vacuum flask or use lid) * to prevent heat energy entering * Use a thermometer with smaller scale divisions / finer calibration / reads to more dp* to record the temperature with greater precision / increase accuracy (of temperature reading) * Use more FA 3 or less water * to make ΔT greater * Use burette or pipette (instead of measuring cylinder) * to record the volume with greater precision / increase accuracy (of volume reading) * Allow decrease in % error or increase accuracy only once as an explanation. If more than two suggestions given mark the first two only.
[1] Concludes: Solution was colder / lower temperature (so longer time/ lower rate)