TJC H2 CHEM P2 Qn and ans Prelim

1 2016 TJC H2 Chemistry Preliminary Exam [Turn over a s CANDIDATE NAME CIVICS GROUP / CENTER NUMBER S INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions 29th August 2016 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Civics Group, centre number, index number and name on all the work you hand in. Write in dark 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. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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. This document consists of 21 printed pages. For Examiner’s Use 1 / 12 2 / 15 3 / 12 4 / 9 5 / 24 Paper 1 / 40 Paper 3 / 80 Total PRELIMINARY EXAMINATIONS HIGHER 2

2 2016 TJC H2 Chemistry Preliminary Exam [Turn over Answer all the questions 1 Planning (P) Potentiometric titration is a technique similar to direct titration of a redox reaction and can be used to determine the concentration of a reducing ion such as Fe 2+ without the use of an indicator. It involves measurement of E cell at intervals as an oxidising titrant is added. The titration is carried out on one side of a cell whose other half is a reference electrode which can be a standard hydrogen electrode or a calomel electrode. Consider the titration of Fe 2+ with standard Ce 4+, the course of which is monitored potentiometrically as shown in Figure 1 below. The reference electrode used is the calomel electrode shown in Figure 2. To follow the course of the reaction, a pair of electrodes is inserted into the reaction mixture. At the calomel reference electrode, Reference half reaction is: Hg2Cl2(s) + 2e 2Hg(l) + 2Cl- E = + 0.24 V At the suitable electrode, there are two reactions that will come to equilibrium: Fe3+ + e Fe2+ E = + 0.77 V Ce4+ + e Ce3+ E = + 1.70 V As the Ce4+ titrant is added, Fe2+ is oxidised to Fe3+ and the cell potential is controlled by the ratio of reduced and oxidised iron according to the Nerst equation which causes the potential to rise as more iron becomes oxidised. Ecell =  E (Fe3+/Fe2+) - 0.05916 log ][Fe ][Fe 3 2    – 0.24 ………….(1) Figure 2: A calomel electrode Figure 1: Set-up for potentiometric titration Titration reaction: Fe2+ + Ce4+  Fe3+ + Ce3+

3 2016 TJC H2 Chemistry Preliminary Exam [Turn over When the equivalence point is reached, the Fe 2+ will have been totally consumed, and the potential will then be controlled by the concentration ratio of Ce3+/ Ce4+. Thus beyond the end- point the potential rises rapidly to that of a Ce4+/Ce3+ cell with excess Ce4+. Ecell =  E (Ce4+/Ce3+) - 0.05916 log ][Ce ][Ce 4 3    – 0.24 ………..(2) A graph of Ecell against volume of titrant added can be drawn and the resulting titration curve looks much like the familiar acid-base titration curve. The end point is found not by measuring a particular cell voltage, but by finding what volume of titrant gives the steepest part of the curve. Solution FA 1 was prepared by diss olving 0.960 g of ‘iron tablets’ in 100 cm 3 of 1 mol dm -3 HClO4 acid. By titration method using KMnO4, the percentage of iron(II) ions in the iron tablets used to make FA 1 was found to be 12.5%. A student wishes to verify the percentage of iron(II) ions p resent in FA 1 by potentiometric titration using standard 0.100 mol dm -3 Ce4+ solution. (a) Suggest a suitable electrode for the potentiometric titration between Fe 2+ and Ce 4+ solution. Suitable indicator electrode: Pt [1] (b) Perform a pre-calculation to estimate the end-point volume for the potentiometric titration of 100 cm3 of FA 1 with 0.100 mol dm-3 Ce4+ solution.  No of moles of Fe2+ in 100 cm3 of FA 1 = (0.125 x 0.960) ÷ 55.8 = 0.00215 mol  Vol of Ce4+ required to reach the end-point = (0.00215 ÷ 0.100) dm3 = 21.50 cm3 [1] (c) Using all the above information including the estimated end -point volume , y ou are required to write a plan to verify the percentage of Fe2+ present in FA 1 by potentiometric titration. You may assume that you are provided with the following: 100 cm3 of FA 1 solution prepared by dissolving 0.960 g of ‘iron tablets’ in 1 mol dm-3 HClO4 acid.  1.25 mol dm-3 Ce4+ solution  a calomel reference electrode  a suitable electrode  a voltmeter  the apparatus normally found in a school laboratory. Your plan should include details of  the preparation of 0.100 mol dm-3 Ce4+solution  how the electrochemical cell set-up in Figure 1 is assembled  how you would recognize that the end-point has been reached

4 2016 TJC H2 Chemistry Preliminary Exam [Turn over  a table for the recording of readings needed for the plotting of titration curve, showing suitable volumes of titrant added  the calculation to verify the percentage of Fe 2+ in FA 1 in terms of the end-point volume, Vo cm3 of Ce4+ Preparation of the standard 0.100 mol dm-3 Ce4+solution 1. Using a burette, transfer 20.00 cm3 of 1.25 mol dm-3 Ce4+ solution directly into a 250 cm3 standard flask. 2. Make up to the mark with distilled water. Stopper and shake well to obtain a homogeneous solution. 1m–correct volume for dilution using burette/pipette for measurement. 1m- using a standard & logical procedure in the preparation Procedure to record cell potential in the potentiometric titration 1. Transfer the given 100.0 cm3 of FA 1 solution into a 500 ml beaker. 2. Fill a burette with the prepared 0.100 mol dm-3 Ce4+ solution. 3. Assemble the apparatus as shown in Figure 1. 4. Titrate FA 1 solution with Ce4+solution. Read and record the cell voltage registered on the voltmeter as each aliquot of Ce4+solution is introduced from the burette . Carry out a more careful titration by adding titrant in s mall increments as the volume gets close to the equivalence point. 5. The end -point volume is reached when the E cell value switches from cell potential based on Fe3+/Fe2+ to one based on Ce4+/Ce3+. Table of readings Volume of Ce4+ solution added / cm3 Ecell / V 5.00 10.00 15.00 17.00 19.00 21.00 23.00 25.00 30.00 35.00 40.00 45.00 1m – Procedure on the assembly of set-up as in figure 1 and titrating carefully by adding titrant in small increments nearing the end-point 1m - how to recognize that the end-point had been passed 1m – table of readings with volume exceeding 21.50 cm3 of Ce4+solution Calculation of % of Fe2+ in FA 1 in terms of end-point volume, Vo cm3 of Ce4+soluton  No of moles of Ce4+ used = 0.100Vo ÷ 1000 mol = 10-4 Vo