NJC 2020 Prelim Paper 4 Suggested Answer

1 NJC SH2 Preliminary Examination 9729/04/20 NATIONAL JUNIOR COLLEGE SH 2 Year − End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidate answer on the Question paper. 9729/04 Wednesday 19 August 2020 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 19 and 20. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 18 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 16 2 / 13 3 / 13 4 / 12 Presentation / 1 Total / 55

2 NJC SH2 Preliminary Examination 9729/04/20 1 Determination of a value for the number of water of crystallization, x, in an iron(II) salt, (NH4)2Fe(SO4)2. x H2O. The iron (II) salt, (NH4)2Fe(SO4)2. x H2O, can be titrated against potassium manganate (VII) solution. The two half–equations for the reaction are shown below. MnO4 + 8H+ + 5e− Mn2+ + 4H2O Fe2+ Fe3+ + e− FA 1 is potassium manganate(VII), KMnO4 solution, of concentration around 0.02 mol dm–3 FA 2 is 0.0780 mol dm–3 ethanedioate ions C2O42– FA 3 is a solution of the iron(II) salt containing 38.4 g dm–3 of (NH4)2Fe(SO4)2. x H2O FA 4 is dilute sulfuric acid, H2SO4 The exact concentration of FA 1 can be determined when FA 2 is titrated against it. C2O42 reacts with MnO4 in an acidic medium to form Mn2+ and CO2 gas. 2MnO4 + 16H+ + 5C2O42 2Mn2+ + 8H2O + 10CO2 As this reaction is very slow at the start , the C2O42 solution is heated prior to titration against MnO4. However, it is not necessary to continuously heat the solution as the rate of reaction will increase as the reaction proceeds. Method (a) (i) Standardisation of FA 1 solution 1. Fill the burette with FA 1. 2. Using a pipette, transfer 10.0 cm3 of FA 2 into the conical flask. 3. Using an appropriate measuring cylinder, transfer 20 cm 3 of FA 4 to the same conical flask. 4. Place a 1 oC thermometer into this conical flask. 5. Heat the contents of the conical flask using the bunsen burner. 6. Turn off the Bunsen burner once a temperature of 60 oC is reached. Caution: The conical flask should be comfortable to the touch. However, you may also use the paper towels to handle the neck of the heated flask if it is too hot for you. 7. Using a small volume of deionised water, wash down the sides of the thermometer and remove it from the conical flask. 8. Add FA 1 from the burette into this flask until a permanent pale pink colour is obtained. Initially, the colour of FA 1 will take some time to disappear, continue swirling . Subsequently, as the reaction proceeds, the rate of reaction will increase.      

3 NJC SH2 Preliminary Examination 9729/04/20 [Turn over 9. Record your titration results in the space provided on page 3. 10. Repeat steps 1 to 7 as necessary until consistent results are obtained. Results: 1 2 Final burette reading / cm3 15.10 29.90 Initial burette reading / cm3 0.30 15.10 Volume of FA 1 / cm3 14.80 14.80 [1]: Both titration tables with correct headings and units (Do not award if any final and initial burette readings are inverted / if 50.00 is used as initial burette reading / burette reading is > 50.00) [1]: Both titration tables with all burette readings recorded to the nearest 0.05 cm 3 + correct computation of titres [1]: Both tables with two uncorrected titres for endpoint within 0.10 cm3. [3] (ii) From your titration results, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 1 used = (14.80 + 14.80) / 2 = 14.80 cm3 (2 d.p.)  Quote the selected titres in the working for average volume  Give correct final value to 2 decimal places  No need write statement, can just show working Volume of FA 1 used = 14.80 cm3 [1] (iii) Hence, determine the concentration of MnO4– in FA 1. Amount of C2O42 used in 10.0 cm3 = 10.00.0780 1000 = 7.80  10–4 mol 2MnO4 + 16H+ + 5C2O42 2Mn2+ + 8H2O + 10CO2 2MnO4 ≡ 5C2O4 2 Amount of MnO4 reacted in 14.80 cm3 = 42 × 7.80 × 105 = 3.12  104 mol [1] [MnO4] = 3.12 × 104 / 14.80 1000 = 0.0211 mol dm3 (3 s.f.) [1] (ecf from 1st point) Concentration of MnO4– in FA 1 = 0.0211 mol dm−3 [2]  

4 NJC SH2 Preliminary Examination 9729/04/20 (iv) For the titration in (a)(i), the C2O42 solution needs to be at about 60 C at the start. During the titration, as FA 1 is added, the temperature of the mixture decreases. However, this decrease in temperature did not cause the rate of reaction between C2O42– and MnO4– to decrease. (i) Suggest a reason for the slow reaction at the start. The rate of reaction between MnO 4– and C 2O42– is slow initially due to the electrostatic repulsion between the two negatively charged ions, resulting in high activation energy. [1] (ii) Explain why the rate of reaction did not decrease even though the temperature of the mixture decreased. As the reaction proceeds, Mn2+, a catalyst is generated which increases the rate of the reaction OR autocatalysis reaction has taken place. [1] (v) Calculate the maximum total percentage uncertainty for your titration in (a)(i) if the uncertainty associated with each reading using a 10.0 cm3 pipette and a burette are ±0.03 cm3 and ±0.05 cm3 respectively. % uncertainty due to pipette = 0.03 10.0  x 100 % = ± 0.300 % % uncertainty due to burette used in titration = 0.05 × 2 14.80  x 100 % = ± 0.676 % Maximum total percentage uncertainty = 0.300 + 0.676 = ± 0.976 % (3 s.f.) (0 mark if + missing) Percentage uncertainty = ± 0.976 % [1] (b) (i) Titration of FA3 against FA1 1. Fill the burette with FA 1. 2. Pipette 25.0 cm 3 of FA3 into a conical flask and add 20 cm 3 of FA 4 using a n appropriate measuring cylinder. 3. Titrate FA 3 with FA 1 until a pale orange colour is obtained. 4. Carry out as many titrations as you think necessary to obtain consistent results. Record your titration results in the space on the next page.

5 NJC SH2 Preliminary Examination 9729/04/20 [Turn over Results: 1 2 Final burette reading / cm3 26.55 35.90 Initial burette reading / cm3 2.90 12.20 Volume of FA 1 / cm3 23.65 23.70 Accuracy: ± 0.20 cm3 of teacher’s reading: [2] ± 0.40 cm3 of teacher’s reading: [1] [2] (ii) From your titration results, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 1 used = (23.65 + 23.70) / 2 = 23.68 cm3 (2 d.p.)  Quote the selected titres in the working for average volume  Give correct final value to 2 decimal places  No need write stat