ADSS Prelim P3 Ans

Admiralty Secondary School Marking Scheme 4E Pure Chemistry (Paper 3) PRELIMINARY EXAMINATION 2024 PAPER 3 (overall deduct 1 mark of consistently did not round off to 3 sf) Qn. Answers Mark Remarks Marker’s Report 1(a) Results e x p t volume of P /cm3 volume of Q /cm3 initial temperature of P /°C highest temperature of mixture /°C temperature rise /°C 1 40.00 10 29.5 32.5 3.0 2 35.00 15 29.5 34.0 4.5 3 30.00 20 29.0 36.0 7 4 25.00 25 29.0 37.0 8 5 20.00 30 29.0 35.5 6.5 6 15.00 35 29.0 34.5 5.5 7 10.00 40 29.5 33.0 3.5 1m: volumes must be between 15 & 35 cm3, and must be at least 5 cm3 apart. 5 Well done. Some students miscalculated the temperature rise.

1m: total volume of P and Q must add up to 50 cm3 1m: all volumes of P to nearest 0.05 cm3, all volumes of Q to nearest cm3 or nearest 0.5 cm3) 1m: temp should be recorded to 1 d.p., to the nearest 0.5 °C 1m: correct calculation for temperature rise (b) 1m: correct labels & units: x-axis (vol of Q/cm3); y-axis (temperature rise/°C) 1m: appropriate scale 1m: correct plotted points, within half a small square accuracy 1m: 2 smooth lines that intersect, passing through origin (0,0) 4 Generally okay. Most students were able to plot the points correctly except for a handful. Some students did not draw the line passing through the origin. Only a small number of students drew a curve instead of a line. 0 5 10 15 20 25 30 35 40 2 4 6 8 10 Volume of Q/cm3

(c) 26 cm3 1 Based on graph, at point of intersection of the 2 straight lines Ok (d) 50 – 26 = 24.00 cm3 (to 2 d.p.) 1 Must show working A few students did not show working (e) H2SO4 + 2NaOH  Na2SO4 + 2H2O No. of moles of Q = (26/1000) x 1.20 = 0.0312 mol [M1] Mole ratio of P:Q = 1:2 No. of mole of P in 24 cm3 = 0.0312/2 = 0.0156 mol [M1] Concentration of P = 0.0156 mol / (24/1000) = 0.650 mol/dm3 (to 3 s.f.) [A1] (to 3 s.f.) 1 1 1 allow ecf minus 1m overall if not to 3 s.f. Not very well done. Quite a number of students assumed that the mole ratio of P:Q was 1:1 as they did not write out the chemical equation, wrote the chemical formula for Na2SO4 wrongly or did not balance the equation correctly. (f) To better prevent heat loss as polystyrene is an insulator of heat 1 Reject prevent temperature loss Some students did not explain clearly and just stated that polystyrene reduces heat loss, without explaining why. (g) Maximum temperature will be higher because the same amount of energy was given out but the total volume of mixture was lesser than 50 cm3, hence the mixture will be heated to a higher temperature. OR there are more moles of reacting particles per unit volume, hence there is a higher frequency of collisions and higher frequency of effective collisions, (therefore a higher rate of reaction.) 1 1 1 Max 2 marks