NJC 2020 Prelim Paper 4 Question Paper

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. 9. Record your titration results in the space provided on page 3.      

3 NJC SH2 Preliminary Examination 9729/04/20 [Turn over 10. Repeat steps 1 to 7 as necessary until consistent results are obtained. Results: [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. Volume of FA 1 used = ………………………………. [1] (iii) Hence, determine the concentration of MnO4– in FA 1. Concentration of MnO4– in FA 1 = ………………………………. [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. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. [1] (ii) Explain why the rate of reaction did not decrease even though the temperature of the mixture decreased. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. [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. Percentage uncertainty = ………………....……. [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 an orange-pink 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: [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. Volume of FA 1 used = ………………………………. [1] (c) Calculations (i) Calculate the amount of iron(II) ions present in 1.00 dm3 of FA 3. Amount of iron(II) ions in 1.00 dm3 of FA 3 = ………………………………. [2]

6 NJC SH2 Preliminary Examination 9729/04/20 (ii) Using your answer in (c)(i), determine the value of x, the number of water of crystallisation in the iron(II) salt, (NH4)2Fe(SO4)2. x H2O. [Ar: Fe, 55.8; H, 1.0; N, 14.0; O, 16.0; S, 32.1] x = ………………………………. [2] [Total: 16]

7 NJC SH2 Preliminary Examination 9729/04/20 [Turn over 2 Determination of a value for the enthalpy change, ∆H, of the reaction between magnesium and hydrochloric acid. Magnesium reacts with an excess of hydrochloric acid according to the following equation. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) FA 5 is 2 mol dm−3 hydrochloric acid, HCl FA 6 is magnesium ribbon, Mg You are also provided with sandpaper (a) Method 1. Place a Styrofoam cup in a 250 cm3 beaker to prevent it from tipping over. 2. Sand a piece of FA6, weigh and record its mass in the space below. 3. Using a measuring cylinder, transfer 25 cm3 of FA 5 into the Styrofoam cup. 4. Measure the temperature of FA 5 in the Styrofoam cup using the 0.2 oC thermometer. This is the temperature at time = 0 minute. 5. Start the stopwatch and leave it running for the whole experiment. 6. Record the temperature of the acid every half a minute for 2 minutes. 7. At time = 2.5 minutes, carefully drop the piece of FA 6 into the acid and stir the mi