ASRJC 2021 J2Prelims H2Chem P4 soln

2021 H2 Chemistry Paper 4 Suggested Solution Qn Teaching points Marks 1(a) mass of capped bottle and FA 1 / g 7.378 mass of capped bottle and residual FA 1 / g 3.488 mass of FA 1 used / g 3.490 initial temperature / oC 28.8 lowest / minimum temperature reached / oC 24.8 decrease in temperature / maximum change in temperature / oC 4.0 • May record data in a single table or have one table for mass and one table for temperature; Tabulation may be vertical or horizontal; lines are not essential • For “temperature drop” allow “temperature change” sign not essential as it will be accounted for later in the sign for the enthalpy change. [1]: correct headers and units. [1]: mass readings to 3 d.p. and temperature readings to 1 d.p. [1]: correctly determined maximum temperature change and mass of FA 1 used [3] • Accuracy marks compare student’s and teacher’s [2] (b)(i) Calculate heat change using result from 1(a) Heat change (q1) = mcDT = (50 x 1.00) x 4.18 x (temp drop) = ________ J [1] (b)(ii) Determine value of DHsol(KHCO3) with correct sign DHsol(KHCO3) = + (q1) / n(KHCO3) = + _______ J mol-1 [1] (b)(iii) Calculate correctly initial Tav = 28.6 oC [1] (b)(iv) Heat change (q2) = mcDT = (25+50) x 1.00 x 4.18 x (28.6 – 28.2) = 125.4 J DHr(KHCO3(aq)) = + (125.4) / (3.450/100.1) = +3640 J mol-1 Final answer to 3 s.f. or 4 s.f. and appropriate units for (b)(i), (b)(ii), (b)(iii) and (b)(iv). [1] [1] [1] T m D

(c) 2 x [1(b)(iv)] 2KHCO3(aq) + H2SO4(aq) K2SO4(aq) + 2H2O(l) + 2CO2(g) 2 x [1(b)(ii)] DHr(KHCO3(s)) 2KHCO3(s) + H2SO4(aq) 2 x [1(b)(ii)] + 2 x [1(b)(iv)] = DHr(KHCO3(s)) = ________ J mol-1 [1]: correct application of Hess’ Law [1]: correct answer; awarded only if (b)(ii) and (b)(iv) are correct and applied correctly (ignore units) [2]

Qn Teaching points Marks 2(a) Preliminary Calculations - Calculate the mass of MgSO4 to use for the experiment. - Assuming that 100 cm3 of water was used in the experiment and a temperature change of 5 oC is measured and no heat loss to surroundings, nsalt x 78.9 x 103 = 100 x 4.3 x 5 Þ nsalt = 0.02724 mol minimum mass of MgSO4 to use = 0.02724 x 120.4 = 3.28 g Given the solubility of MgSO4 at 20 oC = 0.292 mol per 100 cm3 maximum mass that can dissolve in 100 cm3 of water = 0.292 x 120.4 = 35.2 g Hence a mass of about 10 g of MgSO4 can be used for the experiment. (10 g of MgSO4 is a suitable mass as it can be easily measure and from the above preliminary calculations, we know that this mass chosen will be able to give a temperature rise of about 5°C and will completely dissolve in 100 cm3 of water.) Marking consideration • for min and max temp • for proposed mass Expt Assumed volume of water/cm3 heat needed/ J nsalt Min Mass of solid/g Max Mass of solid/g 1 50 1075 0.01362 1.6404 17.5784 2 100 2150 0.02725 3.2809 35.1568 3 150 3225 0.04087 4.9213 52.7352 4 200 4300 0.05450 6.5617 70.3136 5 25 537.5 0.00681 0.8202 8.7892 6 75 1612.5 0.02044 2.4606 26.3676 7 125 2687.5 0.03406 4.1011 43.946 No marks will be deducted for wrong/no determination of max mass of MgSO4 used since students will be penalized in (b) No marks awarded if only the volume of water used is quoted with NO workings to determine mass of solid. [1] [1]

(b) General outline of experiment 1. Using an electronic balance, weigh accurately 10.000 g of MgSO4 into a pre-weighed dry weighing bottle. 2. Record the mass of MgSO4 and the bottle in the table. 3. Using a 100 cm3 measuring cylinder, transfer 100 cm3 of water into a dry Styrofoam cup, placed in a 250 cm3 beaker. 4. Place the lid on the cup, slip the thermometer through the lid and stir the water gently using the thermometer. 5. Start the stop-watch and record the initial temperature of the water. Record the temperature of the water every 30s for 2.5 min. 6. At exactly 3 min, add the MgSO4 in the weighing bottle to the Styrofoam cup. Cover the cup with a lid and continue to stir the mixture gently with the thermometer and record the temperature every 30s from 3.5 min to 10.0 min. 7. Reweigh the weighing bottle and record the actual mass of MgSO4 that dissolved in water. 8. Plot graph of temperature against time 9. Extrapolate the graph to the third minute when MgSO4 was added to water to obtain the highest temperature reached Marking consideration: Correct mention of electronic balance, measuring cylinder, Styrofoam cup, beaker, thermometer Logical mass of MgSO4 used. (between 3.28 g and 35.2 g) Starting the stop-watch and measuring the temperature of water for a few minutes before adding MgSO4 into the water Showing understanding of reweighing the weighing bottle to obtain the accurate mass transferred Ensuring accurate and reliable value of DTmax is obtained Apparatus Procedure Graph Reliability A1 P1/ P2 G1 R1 1. Electronic/ weighing + balance/ scale/ machine 2. (weighing) bottle (reject: small beaker/ container/ bottle cap) 3. measuring cylinder 4. Styrofoam/ Polystyrene cup (Reject: cup) 5. Thermometer Any 4 will do. *capacity not needed 1. Corresponding mass of MgSO4 used and volume of water from (a) (Mass of solid must be < 50g) (If no mass value determined, accept any value <50g) 2. Starting the stop-watch. 3. Record temperature of water for a few minutes BEFORE adding MgSO4 into the water. 4. Add solid (at t = ?) 5. Record temperature of water for a few minutes AFTER adding MgSO4 into the water. 6. Reweigh the weighing bottle (All – P2, Only 3 points- P1) P=0 once solid is already in the cup. 1. Plot graph of temperature against time 2. Extrapolate the graph to the third minute (time of addition) when MgSO4 was added to water to obtain the highest temperature reached. (Both) 1. Dry weighing bottle 2. Stir the water 3. Styrofoam cup in beaker 4. Cup covered by lid. Any 2. Annotation A: apparatus, P: procedure R: Reliability

(c) Correct shape showing graph before and after addition of MgSO4. [1] Correct extrapolation of graph [1] (allow ecf if endo graph is shown) Indicate clearly, minimum temperature, maximum temperature and DT on graph [1] (allow ecf if endo graph is shown) time where reactant was added Tmax Tmin Ti Tf temperature / °C time / min

Qn Teaching points Marks 3(a) Table to include with correct headers and units: • Time of transfer (about 1 min, 2 min, 5 min, 8 min, 11 min and 14 min) • Decimal time • Initial and final burette readings • Volume of FA 3 added Table need not be populated. Table may be horizontal or vertical and lines are not essential but there should be no absence of headers. Appropriate unit for each entry in the table if no units in the header. Can be separate tables. [1] Record all • Burette readings and volumes added to 0.05 cm3 • Transfer times in minutes and seconds (to 1 s) • Correctly calculated values of decimal time • Decimal time to 0.1 min [1] 5 sets of result and are transferred within ± 30 seconds of the suggested times. [1] (b)(i) Correct axes, labels and units; scale uses over half the graph paper in both axes. Do not award this mark if an awkward scale (e.g recurring or each big square is 3, 6 etc) is used that makes plotting/reading difficult. Ignore dp of axes (e.g 5, 5.0, 5.00 all ok) Do NOT double penalize if the student has copied the same wrong units from the table. [1] Plotting within ± ½ small square. Check two points – the 2nd and 4th point; put ticks if correct. If less than 5 plotted points – do not award this mark If more than 5 plotted points – can award if 2nd and 4th points are correct Ignore if the plotted