A-LevelChemistryChemical energetics9701/32 · May/June 2025 · Q212 Marks
You will determine the enthalpy change for the reaction of aqueous citric acid with aqueous sodium hydroxide to form aqueous sodium citrate. The procedure involves two experiments using solid citric acid. FB 4 is citric acid, C6H8O7. Use this for Experiment 1. FB 5 is citric acid, C6H8O7. Use this for Experiment 2. FB 6 is 2.00 mol dm⁻³ sodium hydroxide, NaOH.
- (a)Experiment 1 is the determination of the enthalpy change of solution, ΔH₁, for citric acid. This is the enthalpy change when one mole of citric acid dissolves in water. C6H8O7(s) + aq → C6H8O7(aq) ΔH₁ Method • Support one of the cups 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 temperature in the space for results. • Weigh the container with FB 4. Record the mass. • Tip all of the FB 4 into the water in the cup. • Stir the mixture until the minimum temperature is obtained. Record this temperature. • Weigh the container with any residual FB 4. Record the mass. • Calculate and record the mass of FB 4 used. • Calculate and record the temperature change. Results I II III [3][3]
- (b)(i)Calculate the energy change, in J, in your experiment. energy change = ................................. J [1][1]
- (b)(ii)Calculate the enthalpy change of solution, ∆H₁, in kJ mol⁻¹, for dissolving 1.00mol of solid citric acid, C6H8O7, in water. Show your working. ∆H₁ = ................................. kJ mol⁻¹ [2] sign value[2]
- (c)Experiment 2 is the determination of the enthalpy change, ∆H₂, for the reaction of one mole of solid citric acid with aqueous sodium hydroxide. In this experiment, aqueous sodium hydroxide, FB 6, is used in excess. Method • Support the second cup in the beaker. • Add 4.80–5.00 g of FB 5 to the cup. Record your weighings. • Measure and record the temperature of FB 6 in its container. • Use the 50 cm³ measuring cylinder to transfer 50.0 cm³ of FB 6 into the cup with FB 5. • Stir the mixture until the maximum temperature is obtained. Record the maximum temperature. • Calculate and record the mass of FB 5 used. • Calculate and record the temperature change. Results I II III [3][3]
- (d)Calculations Calculate the enthalpy change of reaction, ∆H₂, in kJ mol⁻¹, for the reaction of 1.00mol of solid citric acid, C6H8O7, with aqueous sodium hydroxide. Show your working. ∆H₂ = ................................. kJ mol⁻¹ [2] sign value[2]
- (e)Use your values for ∆H₁ and ∆H₂ to calculate the enthalpy change, ∆H₃, in kJ mol⁻¹, for the reaction of 1.00 mol of aqueous citric acid with aqueous sodium hydroxide. ∆H₃ = ................................. kJ mol⁻¹ [1] sign value[1]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [1] M1 Unambiguous headings for mass, with units, correctly displayed, in (a) AND (c) in (a): • (Mass of) container + FB 4/g • (Mass of) container empty (or, with residue)/g • (Mass of) FB 4 (used / added)/g AND value for mass of FB 5 used in (c) is between 4.80 and 5.00 g
- [1] M2 Unambiguous headings for temperature, with units, correctly displayed, in (a) AND (c) in (a) Initial temperature (or temperature of water) / °C Final/minimum temperature / °C Temperature change / fall / ΔT / °C
- [1] M3 Precision of readings in (a) AND (c) • Both balance readings in (a) given to the same number of decimal places (2 or 3). • Four thermometer readings recorded to .0 or .5 °C
- [1] Correctly calculates Energy change = 30 × 4.18 × temp decrease J AND answer correctly rounded to 2 – 4 sf
- [2] Correct use of (b)(i) M1: Correct display Amount of citric acid = mass used / 192 mol (no answer required) M2: Correctly uses Enthalpy change = (b)(i) / mol citric acid × 1/ 1000 kJ mol⁻¹ AND answer shows positive sign AND is quoted to 2 – 4 sf
- [1] M1 Correct subtractions in (a) AND (c) • Mass of FB 4 and FB 5 • Two temperature changes (ignore sign)
- [2] Accuracy (Q) marks Round thermometer readings to 0.5 °C if necessary before calculating temperature rise. If the candidate has a temp. increase in (a) OR a temp. decrease in (c), award zero Q marks. If temperatures are clearly a factor of 10 too small, multiply by 10 before assigning marks II and III. Calculate the candidate's mass ratio (to 2dp) = temp rise in (c) / mass of FB5 used Award accuracy (Q) marks as follows: Award II if 2.75 < ratio < 3.75 Award III if 3.00 < ratio < 3.50
- [2] Correct use of data in enthalpy calculation M1 Energy change = 50 × 4.18 × temp rise in (c) (J) M2 Correct use of data in remainder of calculation (all three bullets correct) • n = mass of FB 5 / 192 mol • ∆H₂ = energy change / n × 1/1000 kJ mol⁻¹ • Sign is negative AND answer given to 2–4 sig fig
- [1] Enthalpy change (with correct sign) ΔH₃ = (d) – (b)(ii) = ∆H₂ − ΔH₁
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