A-LevelChemistryChemical energetics9701/33 · Feb/Mar 2024 · Q210 Marks

In this experiment you will determine the enthalpy change, ΔH, for the reaction between aqueous copper(II) sulfate and magnesium. CuSO4(aq) + Mg(s) → Cu(s) + MgSO4(aq) FA 3 is 1.0 mol dm⁻³ copper(II) sulfate, CuSO4. FA 4 is magnesium powder, Mg.

(a)Method • Support the cup in the 250 cm³ beaker. • Use the 50 cm³ measuring cylinder to transfer 50.0 cm³ of FA 3 into the cup. • Weigh the stoppered container of FA 4. Record the mass. • Measure the temperature of FA 3 in the cup. Record the temperature. • Add the FA 4 to the FA 3 in the cup and stir the mixture constantly. • Measure and record the maximum temperature reached. • Reweigh the stoppered container and any residual FA 4. Record the mass. • Calculate and record the mass of FA 4 used. • Calculate and record the maximum temperature change that occurs during the reaction.[3]
(b)(i)Calculate the heat energy produced in the reaction. heat energy produced = ..................................................................... J[1]
(b)(ii)Determine which reactant, FA 3 or FA 4, is in excess for the reaction. Show your working.[1]
(b)(iii)Calculate the enthalpy change, ΔH, in kJ mol⁻¹, for the reaction. ΔH = .......................... (sign) ........................... (value) kJ mol⁻¹[2]
(c)A student suggests that the slow rate of the reaction using the method described in (a) means that heat energy is lost from the solution so the temperature change is inaccurate. Describe how you would change the method and processing of the results to improve the accuracy of the enthalpy change for this reaction. You should not change the quantities of FA 3 or FA 4 used. You may wish to illustrate your answer with a sketch graph.[3]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] (2a) M1 Unambiguous headings and correct units for: • (Mass of) container + FA 4 and g / grams • (Mass of) container (+ residue) and g / grams • Initial / FA 3 (temperature) and °C • Maximum (temperature) and °C • (Mass of) magnesium / FA 4 (added) and g / grams • (Temperature) change and °C
[1] (2a) M2 Readings are appropriately recorded against headings • Both temperature measurements recorded to the nearest 0.5 °C AND • Both balance readings recorded to a consistent number of decimal places (either 2 or 3) AND • Correct calculations of mass of FA 4 and temperature change
[1] (2a) M3 Examiner checks and corrects ΔT if necessary ΔT within x °C of supervisor value ΔTsup/°C <5.0 5.0–9.5 10.0–14.5 15.0–19.5 20.0–24.5 25.0–29.5 30.0–34.5 35.0–39.5 M3: ±0.5 ±1.0 1.5 2.0 2.5 3.0 3.5 4.0
[1] (2b)(i) Correctly calculates: 50 × 4.18 × ΔT AND answer to 2–4 sf
[1] (2b)(ii) Correctly calculates: amount of FA 3 = 0.05 × 1.0 = 0.050 mol AND amount of FA 4 = candidate's mass / 24.3 AND FA 3 / CuSO4 is in excess
[2] (2b)(iii) M1 Correctly uses: ΔH = (b)(i) / (1000 × n(FA 4) from 2(b)(ii)) M2 sign is negative AND answer to 2–4 sf
[3] (2c) M1 Measure the temperature of FA 3 (at regular intervals and) before adding FA 4 AND (Stir the mixture and) measure the temperature of the mixture at regular intervals after adding FA 4 until temperature is steady or decreasing M2 Plot a graph of temperature against time AND point of addition of FA 4 must be given in text or clearly shown/labelled on sketch graph M3 extrapolate lines of best fit to find ΔT(max) or ΔT at time of addition (of FA 4)