A-LevelChemistryChemical energetics9701/31 · May/June 2020 · Q210 Marks

In this experiment you will determine the enthalpy change of solution for anhydrous sodium carbonate. FA 5 is anhydrous sodium carbonate, Na₂CO₃. (You are given approximately 11 g.)

(a)Method Experiment 1 • Weigh a cup. Record the mass. • Transfer 4.0–4.2 g of FA 5 from the container into the cup. • Reweigh and record the mass of the cup with FA 5. • Calculate and record the mass of FA 5 used. • Support the cup in the 250 cm³ beaker. • Pour 30 cm³ of distilled water into the 50 cm³ measuring cylinder. • Measure and record the temperature of the distilled water in the measuring cylinder. • Add the 30 cm³ of distilled water to the FA 5 in the cup. • Stir constantly until the maximum temperature is reached. • Measure and record the maximum temperature. • Calculate and record the temperature rise. Experiment 2 • Repeat Experiment 1 but this time use 5.0–5.2 g of FA 5 and the other cup. • Record all data from both experiments in one table.[4]
(b)(i)Calculate the energy produced during Experiment 1. (Assume that 4.2 J change the temperature of 1.0 cm³ of solution by 1.0°C.) energy produced = ................................... J[1]
(b)(ii)Calculate the number of moles of Na₂CO₃ used in Experiment 1. moles of Na2CO3 = ................................... mol[1]
(b)(iii)Use your answers to (b)(i) and (b)(ii) to calculate the enthalpy change, in kJ mol⁻¹, for the reaction below. Show your working. Na2CO3(s) + aq → Na2CO3(aq) enthalpy change = ................................... kJ mol⁻¹ sign value[1]
(c)(i)A student suggested that by using the same thermometer, quantities of FA 5, and water, a more accurate value for the temperature rise could be calculated. Suggest how the student could obtain a more accurate measurement.[1]
(c)(ii)State the maximum error in a single thermometer reading in your experiment in (a). maximum error = ................................... Hence calculate the maximum percentage error in the measurement of the temperature rise in Experiment 2. % error = ...................................[2]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] (a) I Unambiguous table of data • Mass of cup • Mass of cup + FA 5 • Mass of FA 5 used • Initial temperature or temperature of water / °C • Final / maximum temperature / °C • Temperature rise / °C Both experiments must be attempted
[1] (a) II Readings recorded appropriately • All four thermometer readings recorded to .0 or .5. • All four subtractions correct (two masses and two temp rises). • No thermometer reading is less than 10 °C • Mass of FA 5 used for first experiment is 4.0–4.2 g • Mass of FA 5 used for second experiment is 5.0–5.2 g
[1] (a) III Accuracy marks Check and correct temp subtractions for candidate and supervisor. Round each measured temperature to 0.5 °C if necessary Compare the temperature rises for the two experiments. Award III if candidate's rise is within 1.0 °C of supervisor in Expt 1.
[1] (a) IV Award IV if candidate's rise within 1.0 °C of supervisor in Expt 2.
[1] (b)(i) Correct calculation of energy change (2, 3 or 4 sf) Energy change = 30 × 4.2 × temp rise (J)
[1] (b)(ii) Correct calculation of no of moles of Na2CO3 used (2-4 sf) n = mass claimed / 106
[1] (b)(iii) Correct use of (i) and (ii) to calculate enthalpy change (2-4 sf) Enthalpy change = energy / 1000 x moles
[1] (c)(i) Accuracy improvement Plot graph to obtain better (estimate of) temp rise or plot a cooling curve
[1] (c)(ii) Error = 0.5
[1] (c)(ii) Correct calculation of % error to 2 or more sig fig % error = 2 × 0.5 / temp rise (expt 2) × 100