2024 VJC H2 Chemistry Prelim P4 (Ans)

1 © VJC 2024 9729/04/PRELIM/24 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Paper 4 Practical Candidates answer on the Question Paper. 26 August 2024 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. 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 pages 19 and 20. 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 / 12 2 / 11 3 / 19 4 / 13 Total / 55 This document consists of 18 printed pages and 2 blank pages.

2 © VJC 2024 9729/04/PRELIM/24 Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of copper in an alloy Copper forms compounds containing Cu2+ and Cu+ ions. Those compounds containing Cu2+ ions tend to be relatively stable. The addition of an excess of potassium iodide, KI, to a solution of Cu2+ ions produces iodine, I2, and a stable precipitate of Cu I as shown in equation 1 . For a titration to be accurate, it is necessary that all the Cu2+ ions are reduced to Cu+ ions. The I2 turns the solution brown. equation 1 2Cu2+ + 4I− → 2CuI + I2 I2 has a relatively low solubility in water. However, the presence of an excess of I− ions in the reaction mixture allows the soluble tri–iodide ion, I3−, to form as shown in equation 2. This ensures that the I2 formed as shown in equation 1 is fully dissolved. equation 2 I2 + I− → I3− The I3− ions formed may be titrated against a standard solution of Na 2S2O3 as shown in equation 3. equation 3 I3− + 2S2O32− → S4O62− + 3I− In 1(a), you will perform titrations to determine the percentage by mass of copper present in an alloy. FA 2 is an aqueous solution containing 11.54 g dm−3 of the alloy in acid. You are also provided with • FA 1, 0.100 mol dm−3 sodium thiosulfate, Na2S2O3, • 0.600 mol dm−3 aqueous potassium iodide, KI, • aqueous sodium carbonate, Na2CO3, • aqueous ethanoic acid, CH3CO2H, • starch indicator. The FA 2 solution has been prepared using 11.54 g of the alloy containing copper and one other metal, in 1 dm3 of the solution. FA 2 contains residual traces of acid. The presence of acid in the titration mixture will affect the accuracy of the results. The procedure described is designed to reduce these errors. In this experiment, you will determine the percentage by mass of copper in the alloy used to prepare FA 2. You will titrate FA 2 against FA 1.

3 © VJC 2024 9729/04/PRELIM/24 [Turn over (a) (i) Titration of FA 2 against FA 1 1. Fill a burette with FA 1. This will be used for both Question 1 and Question 2. 2. Use a pipette to transfer 25.0 cm3 of FA 2 into a 250 cm3 conical flask. 3. Use a teat pipette to add Na2CO3(aq), slowly, with shaking, to FA 2 in the conical flask, until a slight permanent precipitate forms. 4. Use another teat pipette to add CH3CO2H(aq) slowly, with shaking, until this precipitate just dissolves. 5. Use a measuring cylinder to add about 20 cm3 of KI(aq) to this flask. A white precipitate forms in a brown solution. 6. Run FA 1 from the burette into this flask. Near the end–point, when the brown solution becomes pale, add about 1 cm3 of starch indicator. 7. Continue adding FA 1 slowly. The end –point is reached when the solution first becomes colourless. The white precipitate remains. 8. Record your titration results, to an appropriate level of precision, in the space provided. 9. Repeat points 2 to 8 until consistent results are obtained. Titration results Final burette reading / cm3 25.20 25.10 Initial burette reading / cm3 0.00 0.00 Volume of FA 1 used / cm3 25.20 25.10 • Correct headings and units for all recordings • Burette readings recorded to 0.05 cm3 precision • At least 2 consistent readings within ±0.10 cm3 [3] (ii) From your titrations, obtain a suitable volume of FA 1 , V FA 1, to be used in your calculations. Show clearly how you obtained this volume. • VFA1 = 25.20+25.10 2 (only consistent results should be used.) = 25.15 cm3 (2 d.p.; working is required) 2m for VFA 1 within ±0.20 cm3 of supervisor’s results 1m for VFA 1 within ±0.50 cm3 of supervisor’s results VFA 1 = ………………... [3] (b) Calculate the mass of copper in the 11.54 g of the alloy used to prepare 1.00 dm3 of FA 2, using your answer in 1(a)(ii). Hence, determine the percentage by mass of copper in the alloy. [Ar: Cu, 63.5] Amount of S2O32− = 25.15 1000 × 0.100 = 2.52 × 10−3 mol 25.15 cm3

4 © VJC 2024 9729/04/PRELIM/24 Amount of I3− = amount of I2 = (2.52 × 10−3) ÷ 2 = 1.26 × 10−3 mol Amount of Cu2+ in 25.0 cm3 of FA 2 = 2 × 1.26 × 10−3 = 2.52 × 10−3 mol Amount of Cu2+ in 1.00 dm3 of FA 2 = 2.52 × 10−3 ÷ 25.0 1000 = 0.101 mol • Mass of Cu2+ = 0.101 × 63.5 = 6.39 g (3 s.f.) mass of Cu in 11.54 g of the alloy = ……………………………………… • Percentage by mass of Cu in the alloy = 6.39 11.54 × 100 % = 55.4% (3 s.f.) percentage by mass of copper in the alloy = …………………………………… [2] (c) A large excess of potassium iodide is used in each titration. (i) Calculate the amount of iodide ions added in each titration. • Amount of iodide used in each titration = 20 1000 × 0.600 = 0.0120 mol amount of iodide ions = …………………………………. [1] (ii) Hence, determine the ratio of the amount of iodide ions added in each titration to the minimum amount of iodide ions required to reduce the copper(II) ions used. Amount of iodide needed to reduce Cu2+ = amount of Cu2+ × 2 = 2.52 × 10−3 × 2 = 5.03 × 10−3 mol • Ratio = 0.0120 ÷ 0.00503 = 2.39 (3 s.f.) (e.c.f) ratio = ……………………………. [1] (iii) Identify two different chemical processes that use iodide ions in the experiment. • Redox where Cu2+ is reduced to Cu+ while I– is oxidised to I2 in equation 1. • Precipitation where a CuI ppt is formed between Cu+ and I– ions in equation 1. • A soluble complex ion is formed between I2 and I– in equation 2. [Any 2 of the 3 points] [2] [Total: 12] 6.39 g 55.4% 0.0120 mol 2.39

5 © VJC 2024 9729/04/PRELIM/24 [Turn over 2 Investigation of rate between peroxydisulfate ions and iodide ions When oxidising agent peroxydisulfate ions, S2O82–(aq), reacts with aqueous iodide ions, I−(aq), a brown solution I2(aq) is formed as shown in equation 4. equation 4 S2O82–(aq) + 2I–(aq) → I2(aq) + 2SO42– The iodine, I2(aq), produced can be reacted immediately with thiosulfate ions, S 2O32−(aq), as