A-LevelChemistryChemical energetics9701/32 · May/June 2017 · Q212 Marks

You are to determine the enthalpy change of neutralisation for a different acid from that used in Question 1. The acid is represented by HB where B⁻ represents the anion. HB(aq) + NaOH(aq) → NaB(aq) + H₂O(l) FB 3 is 2.00 mol dm⁻³ acid, HB. FB 4 is 72.00 g dm⁻³ sodium hydroxide, NaOH.

(a)Method Read through the method before starting your practical work and prepare a table below for recording your results. Experiment 1 • Place the plastic cup in the 250 cm³ beaker. • Pour 25 cm³ of FB 3 into the larger measuring cylinder. • Measure and record the temperature of FB 3. • Rinse and dry the thermometer. • Use the smaller measuring cylinder to transfer 25 cm³ of FB 4 into the plastic cup. • Measure and record the temperature of FB 4. • Add the 25 cm³ of FB 3 to FB 4 in the plastic cup and stir the mixture. Measure and record the highest temperature reached. • Calculate and record the average initial temperature of the solutions. • Calculate and record the temperature rise. • Empty the plastic cup, rinse it with water and shake it to remove excess water. Experiment 2 • Repeat the method given for Experiment 1 using 50 cm³ of each solution. • Use the larger measuring cylinder for FB 3 and the smaller measuring cylinder for FB 4. Results I II III IV[4]
(b)Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations.
(b)(i)Show by calculation that in Experiment 1, the number of moles of acid was in excess of the number of moles of sodium hydroxide.[1]
(b)(ii)Calculate the heat energy evolved in Experiment 1. [Assume that 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0 °C.] heat energy evolved = .............................................. J[1]
(b)(iii)Calculate the enthalpy change, in kJ mol⁻¹, for Experiment 1. enthalpy change = .............................................. kJ mol⁻¹ (sign) (value)[1]
(b)(iv)Calculate the number of moles of sodium hydroxide neutralised in Experiment 2. moles of NaOH = .............................................. mol[1]
(b)(v)Calculate the enthalpy change, in kJ mol⁻¹, for Experiment 2. enthalpy change = .............................................. kJ mol⁻¹ (sign) (value)[1]
(c)(i)The accuracy of the larger measuring cylinder is ± 0.5 cm³. The accuracy of the smaller measuring cylinder is ± 0.25 cm³. Calculate the maximum percentage error in the measurement of the volume of FB 3 used in Experiment 2 and the measurement of the volume of FB 4 used in Experiment 2. Show your working. maximum % error in volume of FB 3 .............................................. maximum % error in volume of FB 4 ..............................................[3]
(c)(ii)Suggest a change to the method used in (a) that would improve the accuracy of your results.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] I Unambiguous headings and correct units tabulated for all 6 thermometer readings, mean temps, and ΔTs
[1] II All thermometer readings recorded to 0.5 °C and ΔTs correctly calculated and Mean temperatures correctly calculated to nearest .5 °C or to 1 or 2 dp
[1] Award III if candidate ΔTs within 1.5 °C
[1] Award III and IV if candidate ΔTs within 1.0 °C
[1] Correctly calculates n acid = 0.05(0) mol and n NaOH = 0.045 mol
[1] Correctly calculates 50 × 4.2 × ΔT₁ to minimum 2 sf
[1] Correct expression (ii) / 1000 x 0.045
[1] 2 x mol NaOH in (i) or 0.09(0) in (iv) and Correctly uses 100 x 4.2 x ΔT (2) / 1000 x(iv)
[1] Negative signs shown in (iii) and (v) and final answers to 2–4 sf in (ii), (iii) & (v)
[1] % error in vol of FB 3 = 0.5 x 100 / 50 = 1.(0)%
[1] % error in vol of FB 4 = 2 x 0.25 x 100 / 25 = 2.(0)%
[1] Use a burette / pipette for volume measurements / instead of a measuring cylinder or Add a lid to reduce heat loss (by convection) / to reduce convection or Use thermometer reading to 0.2 °C / smaller divisions / calibrations / more sensitive