A-LevelChemistryReaction kinetics9701/23 · May/June 2025 · Q411 Marks

The reaction between Na₂S₂O₃(aq) and HCl(aq) is monitored at constant temperature. Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O Fig. 4.1 shows how the concentration of HCl(aq) varies with time. [Fig. 4.1]

(a)(i)Use Fig. 4.1 to find the average rate of change of concentration of HCl(aq) in this reaction between 0-100 seconds and between 400-500 seconds. Include units in your answers. 0-100 seconds ..................................................... units ..................................................... 400-500 seconds ..................................................... units .....................................................[2]
(a)(ii)Use Fig. 4.1 to identify the limiting reagent. Explain your answer.[1]
(a)(iii)Explain why the rate of reaction changes with time.[1]
(a)(iv)The reaction between Na₂S₂O₃(aq) and HCl(aq) is repeated in a second experiment. In this second experiment, 25.0 cm³ of 0.050 mol dm⁻³ Na₂S₂O₃(aq) reacts with 0.0020 mol of HCl(aq). Calculate the number of sulfur atoms produced. number of sulfur atoms produced = ..............................................[2]
(a)(v)Explain why the rate of reaction cannot be monitored accurately by measuring the volume of SO₂(g) produced in this reaction.[1]
(b)(i)Use Fig. 4.2 to predict the shape and bond angle of a molecule of SO₂. shape ................................................... bond angle ......................................... °[2]
(b)(ii)Use Table 4.1 to predict the strength of the dipole moment of SO₂, if any, compared to that of H₂O. Explain your answer.[2]

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Mark scheme

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

[2] Check final graph for range of acceptable answers • ±(1-0.5)/100 = ±0.005 • 0 • mol dm⁻³s⁻¹ for both Any 2 points = 1 mark, all 3 points = 2 marks
[1] Na₂S₂O₃ because reaction stops before all the HCl is used up. OR Na₂S₂O₃ because there is still HCl remaining (when the reaction has stopped)
[1] the frequency of effective collisions decreases as concentration of HCl(aq) and / or Na₂S₂O₃ decreases
[2] M1 0.001(00) mol sulfur produced M2 M1 × 6.02 × 10²³ = 6.02 x 10²⁰
[1] It reacts with water (to produce H₂SO₃)
[2] M1 non-linear M2 bond angle – 120 > x > 116°
[2] M1 weaker dipole moment (in SO₂) M2 electronegativity (difference) is larger(/ 1.4) for H and O than for S and O (/0.9) OR S=O bond is less polar than H-O

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