VJC 2021 Prelim H2P4 Qns [Final]

1 Ó VJC 2021 9729/04/PRELIM [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729//04 Paper 4 Practical 30 August 2021 2 hours 30 minutes Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name and CT group on all 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 a 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 the last page. 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 2 3 4 Total This document consists of 19 printed pages (excluding the QA notes).

2 Ó VJC 2021 9729/04/PRELIM [Turn over Answer all the questions in the spaces provided. 1 Determination the extent of oxidation of iron(II) solution exposed to air Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided. Show your working and appropriate significant figures in the final answer to each step of your calculations. It is essential to dissolve solid samples of iron(II) compounds in acids such as dilute sulfuric acid instead of water. This is to prevent excessive oxidation of iron(II) ions to iron(III) ions in the presence of oxygen over a period of time. FA 1 is 0.020 mol dm–3 acidified potassium manganate(VII), KMnO4. FA 2 is 1.0 mol dm–3 sulfuric acid, H2SO4. FA 3 is iron(II) salt dissolved in water and left standing for an extended period of time. Using the apparatus and chemicals provided, you are required to determine the extent of oxidation of iron(II) salt in FA 3. (a) Dilution of FA 3 1. Pipette 25.0 cm3 of FA 3 into a 100 cm3 volumetric flask. 2. Make up to the contents of the flask to the 100 cm3 mark with deionised water. Shake to obtain a homogeneous solution. 3. Label this solution diluted FA 3 which is to be used for (b) and (c). (b) Determination of amount of iron(II) ions originally present in FA 3. 1. Add 50 cm3 of diluted FA 3 using a measuring cylinder into a 100 cm3 beaker. 2. Add about 2 spatula of zinc powder into the beaker and stir. 3. Filter the mixture into a boiling tube. 4. Pipette 10.0 cm3 of the filtrate into a conical flask. 5. Add 10 cm3 of FA 2 into the conical flask using a measuring cylinder. 6. Titrate this solution against FA1 from a burette until the first permanent pale pink colour is obtained. 7. Record your titration results, to an appropriate level of precision, in the space below. You are to perform this titration only once. Results: [2]

3 Ó VJC 2021 9729/04/PRELIM [Turn over (c) Determination of amount of iron(II) remaining in FA 3. 1. Pipette 10.0 cm3 of diluted FA 3 into a conical flask. 2. Add 10 cm3 of FA 2 into the conical flask using a measuring cylinder. 3. Titrate this solution against FA1 from a burette until the first permanent pale pink colour is obtained. 4. Record your titration results, to an appropriate level of precision, in the space below. 5. Repeat the titration as many times as necessary to obtain consistent results. Results: [3] (d) Calculations (i) From your titration results in (c), calculate the average volume of FA 1 used. [1] (ii) Calculate the amount of iron(II) in 10.0 cm3 of diluted FA 3. MnO4–(aq) + 8H+(aq) + 5Fe2+(aq) ® Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) amount of iron(II) in diluted FA 3 =……………… [1]

4 Ó VJC 2021 9729/04/PRELIM [Turn over (iii) Calculate the amount iron(II) in 25.0 cm3 of FA 3. amount of iron(II) in FA 3 =……………… [1] (iv) Using your titration results in (b), calculate the amount iron(II) originally present in 25.0 cm3 of FA 3. amount of iron(II) originally present in FA 3 =……………… [1] (v) Calculate the percentage of iron(II) in FA 3 oxidised upon standing for an extended period of time. percentage of iron(II) in FA 3 oxidised =……………… [2]

5 Ó VJC 2021 9729/04/PRELIM [Turn over (e) Evaluations (i) State one assumption made for the addition of zinc powder in (b). Explain how the titre volume would be affected if the assumption is not valid. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [2] (ii) With the help of relevant calculations, show that the iron(II) is more stable in air under acidic conditions compared to alkaline conditions. Fe3+ + e– ⇌ Fe2+ Eo = +0.77 V Fe(OH)3 + e– ⇌ Fe(OH)2 + OH– Eo = –0.56 V O2 + 4H+ + 4e– ⇌ 2H2O Eo = +1.23 V O2 + 2H2O + 4e– ⇌ 4OH– Eo = +0.40 V …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [2] [Total: 15]

6 Ó VJC 2021 9729/04/PRELIM [Turn over 2 Determination of the activation energy of the reaction between ammonium peroxodisulfate and potassium iodide. Equation 1 represents the reaction between peroxodisulfate ions and iodide ions. equation 1 S2O82–(aq) + 2I–(aq) ® I2(aq) + 2SO42– The reaction is first order with respect to both the concentration of peroxodisulfate ions and the concentration of iodide ions. rate = k[S2O82–][I–] where k = rate constant When starch is added to the reaction mixture, a blue colour is immediately seen due to the formation of an iodine–starch complex. If a small amount of sodium thiosulfate, Na2S2O3, is also present in the reaction mixture, the formation of the blue colour is delayed. The Na2S2O3 reacts with I2 as shown in equation 2. equation 2 I2(s) + 2S2O32–(aq) ® S4O62–(aq) + 2I–(aq) The activation energy, Ea, for the reaction can be obtained using the Arrhenius Equation: k = A𝑒–"#$% where k is the rate constant, A is the pre-exponential constant R is molar gas constant and T is the reaction temperature in Kelvin, The activation energy, Ea, of the reaction can be determined by performing a series of experiments where the reaction mixture is kept constant, but different reaction temperatures are used, and the same end-point is timed. Plotting ln k against 1/TK will give a best-fit straight line where the gradient of the line is – Ea/R. FA 4 is 0.200 mol dm–3 potassium iodide, KI. FA 5 is 0.00500 mol dm–3 sodium thiosulfate, Na2S2O3. FA 6 is 0.100 mol dm–3 ammonium peroxodisulfate, (NH4)2S2O8. Solution S is starch solution. You are to perform a series of five experiments, at different temperatures, in which the concentrations of FA 4, FA 5 and FA 6 are kept constant. The time taken for the dark blue colour to form allows the reaction rate to be determined. (a) Procedure You will attempt five experiments. In each experiment, solution 1 will be prepared as described below and a standard volume of FA 6 will then be added. For each experiment you will: • note the time in seconds, t, taken for the solution to turn