2021 SAJC Prelim P4 Answer

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 16 Aug 2021 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes Shift Laboratory For Examiner’s Use 1 2 3 4 Total This document consists of 21 printed pages including this page. READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, 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. The number of marks is given in the brackets [ ] at the end of each question or part question. 13 18 10 14 55

2 [Turn Over 1. Determination of the value of x in the oxyanion of iodine, IOx– Iodine is able to form more than one oxyanion, IOx–, polyatomic ions that contain oxygen, each containing a different number of oxygen atoms. In this experiment, you will determine the value of x in the oxyanion of iodine, IOx–. You will first react IOx– ions with an excess of iodide ions, I–, to form iodine, I2 as shown in equation 1. equation 1 IOx– + y I– + z H+ → !"𝒚$ I2 + 𝒛$ H2O where x, y and z are all integers The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2 + 2S2O32– → 2I– + S4O62– FA 1 is a solution containing 0.0150 mol dm–3 IOx– ions. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is 1.00 mol dm–3 potassium iodide, KI. FA 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Procedure 1. Fill the burette with FA 4. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 2 to the conical flask. 4. Use another measuring cylinder to add 10 cm3 of FA 3 to the conical flask. The solution will turn brown as iodine is produced. 5. Add FA 4 from the burette until the solution in the conical flask turns yellow. 6. Add 5 drops of starch indicator to the conical flask. The solution will turn blue-black. 7. Continue to add more FA 4 from the burette until the blue-black colour just disappears. This is the end-point of the titration. 8. Record your titration results, to an appropriate level of precision, in the space provided on page 3. 9. Repeat points 2 to 7 until consistent results are obtained.

3 [Turn Over Results Initial burette reading /cm3 0.00 0.00 Final burette reading /cm3 25.60 25.70 Volume of FA 4 /cm3 25.60 25.70 • Correct headers, units and final burette reading greater than initial burette reading • Correct precision to 2 d.p • 2 consistent values within ±0.10 cm3 [3] (b) From your titrations, obtain a suitable volume of FA 4, to be used in your calculations. Show clearly how you obtained this value. Volume of FA 4 = $&.() " $&.+)$ = 25.65 cm3 • Correct calculation of average titre volume to 2 d.p and correct calculation of volume of FA 4 in the titration table • Accuracy volume of FA 4 = ………..……………….. [2] (c) (i) Calculate the number of moles of iodine formed when FA 1 reacts with FA 3. Number of moles of I2 = ½ x (25.65 / 1000 x 0.1) = 1.28 x 10–3 number of moles of I2 = ………..……………….. [1] (ii) Calculate the number of moles of IOx– ions in 25.0 cm3 of FA 1. Number of moles of IOx– ions = 25 / 1000 x 0.015 = 3.75 x 10–4 number of moles of IOx– ions = ………..……………….. [1]

4 [Turn Over (iii) Using equation 1 and your answers in 1(c)(i) and 1(c)(ii), calculate the value of y. Show your working. (Note that y is an odd integer such as 1, 3, 5, 7 etc.) !.$, - !)!" ..+& - !)!# = !"/$ y = 5.83 ≈ 5 y = ……………… [1] (iv) Use your value of y in 1(c)(iii) and considering the number of electrons transferred, determine the oxidation number of I in IOx– ion. Hence, determine the value of x in IOx– ion. IOx– + 5I– + zH+ → 3I2 + 0$H2O 2I– à I2 + 2e– 2 moles of I– gives out 2 moles of electrons 5I– à &$ I2 + 5e– 5 moles of I– gives out 5 moles of electrons 1 mole of IOx– gains 5 moles of electrons to form I2 Hence, oxidation number of I in IOx– is +5 +5 + x(–2) = –1 x = 3 oxidation number of I in IOx– ion = ……………… x = ……………… [2] (v) A student suggested that a more accurate value of x could be obtained if a 10.0 cm3 pipette was used to measure FA 3 rather than the measuring cylinder. State whether you agree with the student. Explain your answer. [1] I do not agree as KI is in excess so precision of the apparatus does not matter.

5 [Turn Over (vi) Explain how the titre volume of Na2S2O3 will change when the value of x in IOx– is greater. [1] When x is greater, higher amount of iodine will be produced, which require a higher titre volume of S2O32–. (d) Chlorine is also able to form more than one oxyanion, ClOx–. Similar to IOx–, oxyanions of chlorine are also oxidising agents. The table below shows the standard electrode potential of different chlorine oxyanions. Electrode Reaction Eo / V ClO– + H2O + 2e– ⇌ Cl– + 2OH– +0.89 ClO2– + 2H2O + 4e– ⇌ Cl– + 4OH– +0.78 ClO3– + 3H2O + 6e– ⇌ Cl– + 6OH– +0.63 ClO4– + 4H2O + 8e– ⇌ Cl– + 8OH– +0.56 Cl2 + 2e– ⇌ 2Cl– +1.36 However, unlike IOx–, the value of x in the oxyanion of chlorine, ClOx– cannot be determined by reacting ClOx– with its corresponding halide, Cl–. Use the data given in the above table, explain why it is not possible to determine the value of x in the oxyanion of chlorine, ClOx–, with the reaction with Cl–, under standard conditions. [1] Eocell is negative for all reactions between ClOx– and Cl–, hence reaction is not spontaneous / feasible. [Total: 13]

6 [Turn Over 2. Investigation of the effect of [S2O82–] on the rate of reaction between I– and S2O82– Sulfur forms the peroxodisulfate anion, S2O82–. This ion can oxidise iodide ions, I–, to iodine, I2, as shown in the equation. 2I– (aq) + S2O82– (aq) → I2 (aq) + 2SO42– (aq) You will carry out a series of experiments to investigate how the rate of this reaction is affected by changing the concentration of the solutions. The rate can be measured by adding thiosulfate ions, S2O32–, and starch indicator. As the reaction between S2O82– and I– occurs, iodine is produced. The I2 produced reacts immediately with the thiosulfate. I2 (aq) + 2S2O32– (aq) → 2I– (aq) + S4O62– (aq) When all the thiosulfate has reacted, the iodine will remain in the mixture and cause the starch indicator to turn blue-black. The rate of reaction may be determined by measuring the time taken for the reaction mixture to turn blue-black. FA 5 is 0.0200 mol dm–3 potassium peroxodisulfate, K2S2O8. FA 6 is 1.00 mol dm–3 potassium iodide, KI. FA 7 is 0.00500 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) (i) Procedure Experiment 1 1. Use the marker to label one of the 100 cm3 beakers ‘A’ and the other 100 cm3 beaker ‘B’. 2. Use the marker to label one of the measuring cylinders ‘A’ and the other measuring cylinder ‘B’. 3. Use the measuring cylinder A to transfer 20.0 cm3 of FA 5 into beaker A. 4. Use the measuring cylinder B to add 20.0 cm3 of FA 6 into beaker B. 5. Use the measuring cylinder B to add 10.0 cm3 of FA 7 to beaker B. 6. Add 10 drops of starch indicator to beaker B. 7. Add the contents of beaker A to beaker B. Start the stopwatch during this addition.

7 [Turn Over 8. Stir the mixture once and place the beaker on a white tile. 9. Stop the stopwatch when the solution first turns blue-black. 10. Record this reaction time to the nearest second. 11. Wash out both