2021 MI Prelim H2 Chemistry P4 (9729 04) Answers

Class Adm No Candidate Name: This question paper consists of 19 printed pages and 1 blank page. 2021 Preliminary Examinations Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 1 Sept 2021 2 hours 30 min Candidates answer on the Question paper. READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number 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 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 at the back of the Question Paper. 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. Question 1 2 3 4 Total Marks 13 20 10 12 55 Shift Laboratory

2 1 Determine the formula of the ion IOx– by titration In this experiment you will determine the formula of the ion, IOx–. Iodine, I2, is formed when IOx– ions reacts with an excess of iodide ions. The equation for this reaction is: IOx– + y I– + z H+ → (y+12) I2 + z2 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 0.500 mol dm–3 potassium iodide, KI. FA 4 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Method • Pipette 25.0 cm3 of FA 1 into a conical flask. • Using a measuring cylinder, add 20.0 cm3 of FA 2 to the conical flask. • Using a measuring cylinder, add 10.0 cm3 of FA 3 to the conical flask. The solution will turn brown as iodine is produced. • Fill the burette with FA 4. • Add FA 4 from the burette until the solution in the conical flask turns yellow. • Add 1 cm3 of starch indicator to the conical flask. The solution will turn blue-black. • Continue to add more FA 4 dropwise from the burette until the blue-black colour just disappears. This is the end-point of the titration. • Carry out as many accurate titrations as you think necessary to obtain consistent results. • Record the burette readings and the volume of FA 4 added in each accurate titration in the space below. Results Initial burette reading / cm3 0.00 0.00 Final burette reading / cm3 22.50 22.50 Volume of FA 4 added / cm3 22.50 22.50 ✓ ✓ M1 table w headers and units M2 readings to 0.05 cm3 + correct calc M3 2 consistent values within 0.10 M4 accuracy M5 accuracy

3 [Turn over (b) From your titration results, obtain a value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. Volume of FA 4 added = !!.#$%!!.#$! = 22.50 cm3 25.0 cm3 of FA 1 required 22.50 cm3 of FA 4. M6 correct calc w working (c) Calculations (i) Use your answer to (b) to calculate the amount of iodine that was formed. Amount of S2O32– = 0.100 x !!.#$&$$$ = 0.00225 mol Amount of I2 = ½ x 0.00225 = 1.13 x 10-3 mol (3sf) Amount of I2 = 1.13 x 10-3 mol M7 (ii) Calculate the amount of IOx– ions in 25.0 cm3 of FA 1. Amount of IOx– = 0.0150 x !#.$&$$$ = 3.75 x 10-4 mol amount of IOx– ions= 3.75 x 10-4 mol M8

4 (iii) Using your answers in (c)(i) and (c)(ii), calculate the value of y. IOx– + y I– + z H+ → (y+12) I2 + z2 H2O Amount of I!Amount of IO'(=𝑦+121 1.125x10−3 3.75 x 10−4=𝑦+12 y = 5 (must round off to nearest integer) y = 5 M9 M10 (c) ans to 3sf / integer + correct units (iv) Hence, determine the value of x in IOx– ion. IOx– + 5I– + z H+ → 3I2 + z2 H2O Total charge on LHS = Total charge on RHS -1 – 5 + z = 0 + 0 z = 6 IOx– + 5I– + 6H+ → 3I2 + 3H2O ∴ x = 3 x = 3 M11 (d) (i) The maximum error in the volume measured using the pipette is ±0.05 cm3. Calculate the maximum percentage error in the volume of FA 1 used. Max % error = $.$#!#.$ x 100% = 0.200 % Maximum percentage error: 0.200 % M12

5 [Turn over (ii) A student suggested that a more accurate value of x could be obtained if a burette is used to measure FA 3 instead of a measuring cylinder. Do you agree with the student? Explain your answer. Disagree. FA 3 is added in excess, hence there is no need to use a precise instrument. M13 [Total: 13]

6 2 Determination of the enthalpy change of the decomposition of NaHCO3 You are required to determine the enthalpy change of the decomposition of NaHCO3. NaHCO3 → ½Na2CO3 + ½H2O + ½CO2 FA 5 is anhydrous sodium hydrogencarbonate. FA 6 is anhydrous sodium carbonate. FA 7 is 1.0 mol dm-3 hydrochloric acid. You will determine the enthalpy change of reaction for each of FA 5 and FA 6 with the excess addition of FA 7. You will then use these values to calculate the enthalpy change of the decomposition of sodium hydrogencarbonate. (a) Determining the enthalpy change of reaction between FA 5 and FA 7 NaHCO3 + HCl → NaCl + H2O + CO2 Method • Weigh an empty weighing boat. • Weigh about 4.0 g of FA 5 in a weighing boat. • Using a 50 cm3 measuring cylinder, transfer 50.0 cm3 of FA 7 into a 250 cm3 beaker. • Stir the solution and record the initial temperature. • Start timing and do not stop the stopwatch until the whole experiment has been carried out for 7 minutes. • Record the temperature of the solution mixture every minute for 2 minutes. • At exactly 3 minutes, carefully add FA 5 into the beaker and stir the mixture. You do not need to measure the temperature of the solution at the 3rd minute. Break up any clumps while stirring. • Record the temperature at t = 3.5 min. • Repeat the measurement every 0.5 min until t = 7min. • Reweigh the weighing boat containing residual sodium hydrogencarbonate. • In an appropriate format in the space provided below, prepare tables in which to record results for your experiment in (a): o all weighings to an appropriate level of precision, o all values of temperature, T, to an appropriate level of precision, o all values of time, t, recorded to the nearest 0.5 min. Results Mass of empty weighing boat / g 3.11 Mass of weighing boat with FA 5 / g 7.11 Mass of weighing boat with residual FA 5 / g 3.13 Mass of FA 5 transferred / g 3.98 t / min 0.0 1.0 2.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 T / °C 30.0 30.0 30.0 27.2 25.6 25.4 25.4 25.5 25.6 25.8 25.8 M14 3 mass readings + time, temp M15 units

7 [Turn over (b) Plot a graph of temperature, T, on the y-axis, against time, t, on the x-axis on the grid in Fig. 2.1. Draw a best-fit straight line taking into account all of the points before t = 3.0 min. Draw another best-fit straight line taking into account all of the points after the temperature of the mixture has started to rise steadily. Extrapolate both lines to t = 3.0 min and determine the theoretical fall in temperature. Fig. 2.1 M16 axes labelled with units M17 scale M18 all points plotted correctly theoretical fall in temperature = ………………………………… M19 2 best fit lines, extrapolated to t = 3.0, 2nd line must only use increasing / same value pts

8 (c) Calculations (i) Using the answer from (b), calculate the heat energy taken in during the reaction of FA 5 with FA 7. [4.2 J is required to raise the temperature of 1 cm3 of solution by 1 oC] Q = mcΔT = (50.0)(4.2)(5.0) = 1050 J Heat energy take