A-LevelChemistryChemical energetics9701/33 · Oct/Nov 2025 · Q211 Marks

When hydrated sodium thiosulfate is dissolved in water the temperature of the liquid changes. You will carry out an experiment to determine the enthalpy change, ΔΗ, when one mole of hydrated sodium thiosulfate dissolves in water. FA 3 is hydrated sodium thiosulfate, Na2S2O3 5H2O. (a) Method • Support the cup in the 250 cm³ beaker. • Use the 50 cm³ measuring cylinder to transfer 30.0 cm³ of distilled water into the cup. • Measure the temperature of the water in the cup. Record this in the space for results. • Weigh the container with FA 3. Record the mass. • Tip all the FA 3 into the cup. • Stir the mixture until the minimum temperature is obtained. Record this temperature. • Weigh the container with any residual FA 3. Record the mass. • Calculate and record the mass of FA 3 added. • Calculate and record the temperature change. Results

(b)(i)Calculate the amount, in mol, of hydrated sodium thiosulfate added. amount of Na2S2O3•5H2O = ...................................... mol[1]
(b)(ii)Calculate the energy change, in J, in your experiment. energy change = ...................................... J[1]
(b)(iii)Calculate the enthalpy change, ΔH, in kJ mol¯¹, when 1.00 mol of hydrated sodium thiosulfate dissolves in water. Show your working. ΔH = .......................... sign .......................... value kJ mol⁻¹[1]
(c)The value calculated in (b)(iii) can be used to determine the enthalpy change, ΔHᵣ, for the following reaction. Na2S2O3(s) + 5H2O(l) → Na2S2O3.5H2O(s) Outline the method of one further experiment you would need to carry out to obtain the data necessary to calculate the value of ΔHᵣ. Show how you would use your results from this experiment and (b)(iii) to calculate ΔHᵣ. Do not carry out your experiment. method calculation[4]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[1] 6 correct unambiguous headings shown, with units, in list / table • (mass of) container + FA 3 / solid / Na2S2O3•5H2O • (mass of) container (empty OR with residual FA 3) • (mass of) FA 3 / solid / Na2S2O3•5H2O OR mass used OR mass added • initial temperature / thermometer reading • lowest / minimum/final temperature / thermometer reading • temperature change / decrease Mass units: /g OR in g OR (g) OR g by every reading Temperature units: / °C OR in °C OR (°C) OR °C by every reading
[1] All readings shown to appropriate precision • all four balance / thermometer readings shown • both thermometer readings to .0 OR .5 • both weighings consistently to either 2 d.p. OR to 3 d.p.
[1] Subtractions correctly calculated • mass of solid correct AND temperature change correct
[1] ΔT is within 1.0 °C of expected temperature change for the mass used
[1] Correctly calculates amount of FA 3 = mass FA 3 in 2(a) / 248.2 mol AND answer given to 2–4 s.f.
[1] Correctly calculates energy change = 30 × 4.18 × ΔT J AND answer given to 2 – 4 sf
[1] Correctly uses (b)(ii) / (b)(i) × 1000 AND plus sign shown AND answer given to 2 – 4 sf
[4] M1 repeating experiment with Na2S2O3 OR adding Na2S2O3 in water M2 any two from: • known volume of water • known mass of solid • measuring the temperature change. M3 indicating that use of Hess's law is needed M4 ΔHr = ΔH from their additional experiment – (b)(iii)

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