MI Prelim H2 Chem P4 Suggested answers for exchange

Class Adm No Candidate Name: 2025 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 28th Aug 2025 2 h 30 min Suggested answers Shift Laboratory H

2 1 Identification of anions FA 1 is an aqueous solution containing two anions. You will perform tests to identify the two anions in FA 1. You are not expected to identify the cations. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. Test and identify any gases evolved. For Examiners’ Use (a) Carry out the following tests. Carefully record your observations in Table 1.1. Table 1.1 tests observations 1 Test FA 1 solution using Universal Indicator paper. Universal Indicator paper turns blue/dark blue/dark violet (pH 14) 2 To a 1 cm depth of FA 1 in a clean test- tube, add 1 cm depth of aqueous silver nitrate. Add excess aqueous ammonia. Brown ppt forms Ppt dissolves in excess aqueous ammonia to give a colourless solution 3 To a 1 cm depth of FA 1 in a clean boiling tube, add a small piece of aluminium foil into the boiling tube, then heat gently over a Bunsen flame. Gas evolved upon warming turns damp red litmus blue. Gas is NH3. 4 To a 1 cm depth of FA 1 in a clean test- tube, add 2 cm depth of dilute hydrochloric acid. No effervescence observed [4]

3 [Turn over (b) Identify the two anions in FA 1 and state the evidence for each anion by completing Table 1.2. The evidence given must be sufficiently conclusive in identifying the anions. Table 1.2 anion evidence OH– Test 1: Solution is alkaline, indicating presence of OH– ions and Test 2: Solution contains OH− which forms brown ppt (Ag2O) when AgNO3 is added OR Test 3: NH3 gas is evolved when heated with Al, hence OH− ions are must be present NO3– Test 3: NH3 gas evolved indicates presence of NO3– or NO2– and Test 4: brown NO2 gas not evolved, indicating absence of NO2– [4] [Total: 8]

4 2 Investigation on a solution of oxalic acid Oxalic acid is a common organic acid containing the carboxylic acid functional group. You will perform two separate experiments to investigate its properties. For Examiners’ Use (a) Determination of concentration and enthalpy change of neutralisation by thermometric titration Oxalic acid is an organic acid that occurs naturally in many foods, such as quinoa and rhubarb leaves. It is known to be a dibasic acid, potentially donating two H+ ions per molecule of oxalic acid. FA 2 is aqueous oxalic acid of unknown concentration. FA 3 is 2.2 mol dm-3 sodium hydroxide, NaOH. Prepare a table in the space provided on the next page and record, to the appropriate level of precision: • all volumes of FA 3 added, V • the maximum temperature, T, after each addition of FA 3 It is important that the volume of FA 3 recorded is the total volume you have added up to the point when the temperature reading was made. Procedure 1. Place a polystyrene cup inside a second polystyrene cup and place both cups in a glass beaker. 2. Use a pipette to transfer 10.0 cm3 of FA 2 into the polystyrene cup. 3. Fill the burette with FA 3. 4. Stir the FA 2 in the cup gently with the thermometer. Read and record its temperature, tilting the cup to ensure that the bulb of thermometer is fully submerged in the solution. 5. Use the burette to add 2.00 cm3 of FA 3 to the cup and stir the mixture gently with the thermometer. Read and record both the maximum temperature and the actual total volume of FA 3 added. 6. Repeat step 5 until a total of 30.00 cm3 of FA 3 has been added. For each addition of FA 3, read and record both the maximum temperature, T, and the actual total volume of FA 3 added up to that point, V. Note: If you overshoot on an addition, record the actual total volume of FA 3 added up to that point.

5 [Turn over Results V / cm3 T / °C 0.00 31.2 2.00 34.2 4.00 37.2 6.00 42.0 8.00 43.6 10.00 43.8 12.00 43.2 14.00 42.8 16.00 41.8 18.00 40.8 20.00 40.0 22.00 39.2 24.00 38.2 26.00 37.8 28.00 37.2 30.00 37.0 [3]

6 (i) Plot a graph of temperature, T, on the y-axis, against total volume of FA 3, V, on the x-axis on the grid in Fig. 2.1. Your scale on the y-axis should allow for extrapolation above the highest temperature recorded. Draw two lines of best fit, taking into account the points when the temperature of the mixture was rising and the points when the temperature was falling. Each line should have a shape best suited to its plotted points. Extrapolate both lines until they intersect. [3]

7 [Turn over (ii) Explain the shape of your line of best-fit drawn in (a)(i), when the temperature is falling. The temperature starts to fall after the reaction is complete. As the t emperature of FA 3 is less than temperature of reaction mixture , the temperature falls when extra FA 3 is added. The temperature falls at a decreasing rate because the temperature difference between FA 3 and mixture decreases, hence the cooling effect of each FA 3 addition decreases. [2] (iii) Hence, f rom your graph, identify the total volume of FA 3 added at the point of neutralisation, Vneut. Vneut = 10.40 cm3 Using the initial temperature of FA 2, Tinitial, and the maximum temperature of the mixture from the graph, Tmax, calculate the change in temperature, ΔT. Tinitial = 31.2 °C Tmax = 44.7 °C ΔT = 13.5 °C [4] (iv) Determine the concentration of oxalic acid in FA 2. H2A + 2NaOH → Na2A + 2H2O Amount of NaOH used = 2.2 × 10.40 1000 = 0.02288 mol Amount of oxalic acid in 10.0 cm3 = 0.02288 2 = 0.01144 mol [oxalic acid] = 0.01144 10.0/1000 = 1.14 mol dm-3 Concentration of oxalic acid in FA 2 = ……………………………... [2] T = 44.7 – 31.2 = 13.5 °C

8 (v) Calculate the heat change, q, at the point of neutralisation in your experiment. You should assume that the specific heat capacity of the solution is 4.18 J g -1 K-1, and that the density of the solution is 1.0 g cm-3. q = mcT = (10.0+10.40)(4.18)(13.5) = 1151.17 J = 1150 J (3sf) q = ……………………………... [1] (vi) Calculate the enthalpy change of neutralisation, Hneut, for the reaction between oxalic acid and sodium hydroxide. Amount of H2O produced = Amount of NaOH used = 0.02365 mol ΔHneut = − 1151.17/1000 𝑘𝐽 0.02288 𝑚𝑜𝑙 = –50.3 kJ mol-1 (3sf) ΔHneut = ……………………………... [3] (vii) The enthalpy change of neutralisation for hydrochloric acid reacting with sodium hydroxide is -57.3 kJ mol-1. Comment on your value of ΔHneut obtained in (a)(vi). [2] The value obtained is less exothermic. This is because oxalic acid is a weak acid and some energ y released from the neutralisation reaction is used to complete its dissociation. •

9 [Turn over (b) Determination of concentration by acid-base titration The concentration of oxalic acid can also be determined via a simple acid-base titration. Procedure 1. Fill the burette with FA 3. 2. Use a pipette to transfer 10.0 cm3 of FA 2 into a 250 cm3 conical flask. 3. Add 2-3 drops of thymol blue indicator. 4. Titrate FA 2 against FA 3. The end-point is reached when the mixture first turns blue. 5. Record your titration results, to an appropriate level of precision, in the space provided below. 6. Repeat steps 2 to 5 until consistent results are obtained. Results 1 2 initial burette reading / cm3 0.00 0.10 final burette reading / cm3 11.00 11.10 volume of FA 3 used / cm3 11.00 11.00 ✓ ✓ [4] (i) From your titrations, obtain a suitable volume of FA 3 to be used in your calculations. Show clearly how you obtain