A-LevelChemistryChemical energetics9701/52 · May/June 2021 · Q114 Marks

A student is asked to find the enthalpy change for the reaction between anhydrous magnesium sulfate and water. MgSO4(s) + 7H2O(l) → MgSO4•7H2O(s) This enthalpy change cannot be measured directly.

(a)Predict whether the enthalpy change for this reaction is positive or negative. Explain the reason for your prediction. prediction explanation[1]
(b)(i)Draw and extrapolate the cooling curve back to 180 seconds. Determine the temperature change during the reaction. temperature change = .............................................. °C[1]
(b)(ii)The anhydrous magnesium sulfate was not added when the timing started. Explain why.[1]
(b)(iii)3.01 g (0.0250 mol) of anhydrous magnesium sulfate is weighed. Outline the next steps that should be taken in order to obtain the results in Experiment 1. Write your answer using a series of numbered steps.[4]
(c)(i)The student realised that when dissolving 0.0250 moles MgSO4.7H2O(s), the amount of water present in the compound alters the total volume of water used in Experiment 2. Calculate the volume of distilled water needed to make the total volume of water 50.00 cm³ in Experiment 2. Give your answer to the nearest 0.05 cm³. Assume that 1.00 cm³ of distilled water has a mass of 1.00 g. [Aʳ: O, 16.0; H, 1.0] volume of distilled water = ........................................ cm³[1]
(c)(ii)State which piece of apparatus should be used to measure the volume of distilled water in (c)(i). Explain your answer. apparatus explanation[2]
(d)The temperature change when 0.0250 moles of MgSO4•7H2O(s) is added to the water is very small. Suggest how the student should modify the experimental procedure to make the temperature change larger.[1]
(e)(i)The energy released by 0.0250 moles of MgSO₄(s), in Experiment 1, is 2125J. The energy absorbed by 0.0250 moles of MgSO4•7H2O(s), in Experiment 2, is 477.5J. Calculate the enthalpy change, ΔH, for the reaction. Include a sign in your answer. Give your answer to one decimal place. MgSO4(s) + 7H2O(l) → MgSO4.7H2O(s) enthalpy change, ΔH = ..................................... kJ mol⁻¹[2]
(e)(ii)The student noticed that some MgSO₄(s) in Experiment 1 was left undissolved. State and explain the effect this would have on the value of the enthalpy change for the reaction in Experiment 1. effect explanation[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] negative AND bond formation is happening (between MgSO4 and H2O)
[1] 8.05(°C)
[1] to allow the temperature of the water to equilibrate with room temperature
[1] M1: (measure) 50 cm³ (distilled) water and transfer into a polystyrene / polyfoam / plastic cup
[1] M2: measure the temperature of the water at (30 second) intervals (until constant / before adding MgSO4)
[1] M3: MgSO4 added at 180s / 3 minutes
[1] M4: stir and measure the temperature of the water at (30 second) intervals (until at least 8 mins have elapsed since time 0)
[1] 0.025 x 7 x 18 = 3.15 AND 50.00 – 3.15 = 46.85 (cm³)
[1] M1: burette
[1] M2: (apparatus) measures to 0.05 (cm³)
[1] use a larger number of moles (of hydrated magnesium sulfate/solid) OR use a smaller volume of distilled water
[1] M1: ∆Hexp1 = -2125 / 1000 × 0.025 = -85.0 and ΔΗexp2 = 477.5 / 1000 × 0.025 = 19.1
[1] M2: ΔΗ = ΔΗexp1 – ΔΗexp2 -85.0 - (+)19.1 = –104.1 kJ mol-1
[1] less exothermic AND fewer moles reacted