EJC 2022 Prelims Paper 4 QP and Ans

© EJC 9729/04/J2Prelim/22 [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2022 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 1 – INDEX NUMBER CHEMISTRY Paper 4 Practical 9729/04 30 August 2022 2 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidental Instructions READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on the work you hand in. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use paper clips, highlighters, 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. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use 1 / 18 2 / 18 3 / 11 4 / 8 Total / 55 This document consists of 17 printed pages and 3 blank pages.

2 © EJC 9729/04/J2Prelim/22 Answer all the questions in the spaces provided. 1 To determine the percentage by mass of Fe(NO3)2 in a salt mixture FA 1 is a solution containing a salt mixture of two different ionic compounds. One of the compound is Fe(NO3)2. The other compound, X, contains a cation and an anion. In this question, you will determine the percentage by mass of Fe(NO3)2 in the mixture by preparing a diluted solution of FA 1 and carry out a titration using potassium manganate(VII). The iron(II) ions, Fe2+, are oxidised by the manganate(VII) ions, MnO4–. equation 1 5Fe2+(aq) + MnO4–(aq) + 8H+(aq) → 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) The end-point of the titration occurs when the presence of unreacted MnO 4− causes the colour of the solution to become pale pink. The following reagents are provided. FA 1 is a solution containing 90.0 g dm−3 of the salt mixture in sulfuric acid FA 2 is 0.010 mol dm–3 manganate(VII) ions, MnO4– FA 3 is 1 mol dm–3 sulfuric acid, H2SO4 (a) Determining the percentage by mass of Fe(NO3)2 1. Fill a burette with FA 2. 2. Use a pipette to transfer 25.0 cm3 of FA 1 into a 250 cm3 volumetric flask. 3. Make up to the mark with deionised water and label this solution FA 4. Shake to obtain a homogeneous solution. 4. Pipette 25.0 cm3 of FA 4 into a 250 cm3 conical flask. 5. Use the 25.0 cm3 measuring cylinder to add 25.0 cm3 of FA 3 into the conical flask. 6. Run FA 2 from the burette into the conical flask . The end-point is reached when the solution changes colour from yellow to pale pink. 7. Record your titration results in the space on the next page. Make certain that your recorded results show the precision of your working. 8. Repeat steps 4 to 7 as necessary until consistent results are obtained.

3 © EJC 9729/04/J2Prelim/22 [Turn Over Results [3] (b) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 = ................................ ......................... [3] (c) (i) Calculate the amount of MnO 4– ions present in the volume of FA 2 calculated in 1(b). amount of MnO4– ions = ................................ .......................... [1] (ii) Calculate the amount of Fe2+ ions present in 25.0 cm3 of FA 4. amount of Fe2+ ions = ................................ .......................... [1] (iii) Calculate the concentration of Fe2+ ions present in 25.0 cm3 of FA 1. concentration of Fe2+ ions = ................................ .......................... [2] (iv) Hence, calculate the percentage by mass of Fe(NO3)2 in the salt mixture. [Ar: Fe, 55.8; N, 14.0; O, 16.0] percentage by mass of Fe(NO3)2 in salt mixture = ................................ ......................... [2] final burette reading / cm3 20.75 20.80 initial burette reading / cm3 0.00 0.00 volume of FA 2 added / cm3 20.75 20.80

4 © EJC 9729/04/J2Prelim/22 (d) Suggest why a solution containing Fe 2+ ions readily turns yellow when it comes into contact with air. ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ [1] (e) A student suggested that FA 3 can be hydrochloric acid instead of sulfuric acid. Explain why hydrochloric acid cannot be used in the titration and how it would impact the accuracy of the titration result. ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ .... ................................ ................................ ................................ ................................ [2] (f) Calculate the percentage error for your average titre value in (b). percentage error = ................................ .......................... [1] (g) Hydrogen peroxide can act as an oxidising agent and reducing agent as shown by the following half-equations: H2O2 + 2H+ + 2e−  2H2O O2 + 2H+ + 2e−  H2O2 (i) Hydrogen peroxide can also oxidise the Fe 2+ ions. Write a balanced equation for the reaction of Fe2+ with H2O2 in an acidic medium. ................................ ................................ ................................ ......................... [1] (ii) Explain why hydrogen peroxide is not a suitable replacement for KMnO 4 in this experiment. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ............................. ................................ ................................ ................................ ......................... [1] [Total: 18]

5 © EJC 9729/04/J2Prelim/22 [Turn Over 2 Determination of the kinetics of the reaction between iron(III) and iodide ions FA 5 is aqueous potassium iodide, KI. FA 6 is aqueous iron(III) chloride, FeCl3. FA 7 is 0.0065 mol dm−3 sodium thiosulfate, Na2S2O3. You are also provided with a starch indicator and deionised water. A solution containing potassium iodide, sodium thiosulfate and starch is mixed with iron(III) chloride solution. After a few seconds a dark blue colour suddenly appears. This is one of a number of reactions referred to as iodine clock reaction. equation 2 2Fe3+(aq) + 2I−(aq) → 2Fe2+(aq) + I2(aq) equation 3 2S2O32−(aq) + I2(aq) → S4O62−(aq) + 2I−(aq) When the above solutions are mixed, iodine is generated according to equation 2. The iodine is being consumed as they are generated, according to equation 3. Therefore, only a small amount of iodine is present in the mixture. The r eaction in equation 3 stops once all the thiosulfate ions have reacted. The concentration of iodine now increases and the dark blue colour of the iodine-starch complex appears. The reaction in equation 2 is first order with respect to the iodide ion concentration, [I–]. You are to perform a series of experiments to determine the rate order for the reaction in equ