TJC Prelim P4 Qn Ans

1 9729/04/TJC Prelims/2017 [Turn over s CANDIDATE NAME CIVICS GROUP / CENTRE NO. / INDEX NO. / CHEMISTRY 9729/04 Paper 4 Practical 28 August 2017 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your Civics Group and name 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 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 on pages 14 and 15. 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 This document consists of 15 printed pages. For Examiner’s Use 1 / 15 2 / 24 3 / 16 Total / 55 2017 JC2 PRELIMINARY EXAMINATIONS HIGHER 2

2 9729/04/TJC Prelims/2017 [Turn over Answer all the questions in the spaces provided. 1 Determination of change in oxidation number of a transition metal ion, M2+ You are to determine, by titration, the change in oxidation number of a transition metal ion, M2+, when reacted with acidified potassium manganate(VII). FA 1 FA 2 FA 3 is 0.0200 mol dm3 potassium manganate(VII), KMnO4 is 0.0470 mol dm3 transition metal salt, MSO4 is 1.00 mol dm3 sulfuric acid, H2SO4 (a) Method (i) 1. Fill the burette with FA 1. 2. Using a pipette, transfer 25.0 cm3 of FA 2 into the conical flask. 3. Using a measuring cylinder, transfer 25.0 cm3 of FA 3 to the same conical flask. 4. Carry out as many accurate titration s as necessary to obtain consistent results. Add FA 1 until the contents of the conical flask turns a permanent pale pink colour. 5. Record below, in a table form, all of your burette readings and the volume of FA 1 added in each accurate titration. Results Final burette reading /cm3 23.50 23.50 Initial burette reading /cm3 0.00 0.00 Volume of FA 1 /cm3 23.50 23.50   ●Appropriate headings and units for accurate data where headings should match readings. ●All accurate burette readings recorded to 0.05 cm3 ●Has two uncorrected, accurate titres value within ±0.10cm3 ●●●3m – accuracy [6] (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. ●Average titre = 𝟐𝟑.𝟓𝟎+𝟐𝟑.𝟓𝟎 𝟐 = 23.50 cm3 25.0 cm3 of FA 2 required _23.50______________ cm3 of FA 1 [1] (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the amount of potassium manganate(VII) present in the volume of FA 1 calculated in (a)(ii). ●Amount of MnO4 = 𝟐𝟑.𝟓𝟎 𝟏𝟎𝟎𝟎 × 𝟎. 𝟎𝟐 = 4.70 x 104 mol

3 9729/04/TJC Prelims/2017 [Turn over amount of KMnO4 = 4.70 x 104 mol [1] (ii) Determine the amount of MSO4 in 25.0 cm3 of FA 2. ●Amount of MSO4 in 25.0 cm3 of FA 2 = 0.0470 x 𝟐𝟓 𝟏𝟎𝟎𝟎 = 1.175 x 103 mol = 1.18 x 103 mol (3sf) amount of MSO4 in 25.0 cm3 = 1.18 x 103 mol [1] (iii) Use your answer to (b)(i) and (b)(ii) to calculate the number of moles of MSO4 that react with 1 mole of KMnO4 ●Number of moles of MSO4 reacting with 1 mole of KMnO4 = 𝟏.𝟏𝟕𝟓×𝟏𝟎−𝟑 𝟒.𝟕×𝟏𝟎−𝟒 = 𝟐. 𝟓𝟎 mol amount of MSO4 = 2.50 mol [1] (iv) Two possible equations for the reaction of acidified KMnO4 with MSO4 are shown below: Equation 1: 2KMnO4 + 10MSO4 + 8H2SO4  K2SO4 + 2MnSO4 + 5M2(SO4)3 + 8H2O Equation 2: 2KMnO4 + 5MSO4 + 8H2SO4  K2SO4 + 2MnSO4 + 5M(SO4)2 + 8H2O State and explain which of the above two equations is consistent with your answer in (b)(iii). ●Equation 2 as ratio of KMnO4: MSO4 is 2:5 OR 1:2.5 Hence it is 2.5 mol of MSO4 reacting with 1 mol of KMnO4 [1] (v) Use your answer in (b)(iv) to state the oxidation number of the transition metal M in the product of the reaction. ●Oxidation state of M in product is +4 Workings: M(SO4)2  x+2(-2) = 0  x= +4 [1] (c) (i) A student suggested that using a burette to measure 25.0 cm3 of FA 2 would give a more accurate result than using a pipette. The percentage error of a 25.0 cm3 pipette is 0.24%.

4 9729/04/TJC Prelims/2017 [Turn over Calculate the percentage error of using a burette and deduce if the above claim by the student is correct. Using the burette to measure 25.0 cm3 of FA 2 involve 2 readings, thus the max error would be 2x (±0.05) = ±0.10 ●%error would be 𝟎.𝟏𝟎 𝟐𝟓 × 𝟏𝟎𝟎% = 𝟎. 𝟒𝟎𝟎% ●Since %error using burette > %error using pipette (0.24%). The claim is incorrect as it is less accurate to use a burette to measure the 25.0 cm3 of FA2. [2] (ii) Another student decided to use a 25.0 cm3 pipette instead of a measuring cylinder to measure the volume of FA 3 in Step 3. State and explain whether this alteration will improve the accuracy of the experimental results. ●No this alteration will not improve the accuracy of the experiment as the acid was used in excess. [1] [Total: 15]

5 9729/04/TJC Prelims/2017 [Turn over 2 Determination of the enthalpy change of neutralisation, ΔHn You are to determine the enthalpy change for the neutralisation reaction given below. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) FA 4 FA 5 is 1.80 mol dm–3 HA is aqueous sodium hydroxide, NaOH (a) Method Read through the instructions carefully and prepare a table below for your r esults before starting any practical work. 1. Support the styrofoam cup in the 250 cm3 beaker. 2. Rinse and fill the burette with FA 4. 3. Use the measuring cylinder to transfer 25 cm3 of FA 5 into the styrofoam cup. 4. Place the thermometer in the styrofoam cup and record the temperature of the solution. Tilt the cup if necessary to ensure the thermometer bulb is fully immersed. 5. Run 5.00 cm 3 of FA 4 into the cup. Stir, a nd record the new temperature of the solution and the volume of FA 4 added. 6. Run a second 5.00 cm3 of FA 4 into the cup. Stir and record the new temperature and the total volume of FA 4 added. 7. Continue adding FA 4 in 5.00 cm 3 portions. Stir and record each new temperature and total volume of FA 4 until a total of 45.00 cm3 has been added. Results Total Vol. FA 4 /cm3 Temp./0C 0.00 27.5 5.00 31.0 10.00 34.0 15.00 36.5 20.00 38.0 25.00 39.0 30.00 38.0 35.00 37.0 40.00 36.5 45.00 36.0 ●Table with initial temp of solution and 9 sets of results with appropriate headers ●Record every volume to 2dp and temperature recorded to 1dp. [2] (b) (i) Plot a graph of temperature (y-axis) against total volume of FA 4 added (x-axis) on the grid below. The temperature axis should allow you to include a point at least 2 °C greater than the maximum temperature recorded. Draw two best-fit lines connecting all the plotted points with • an increasing trend • a decreasing trend.

6 9729/04/TJC Prelims/2017 [Turn over Answer: ●Appropriate labels of the axis and appropriate scale of the graph ● All points accurately plotted (within ½ small square) ● Two lines of best fit drawn [3] (ii) Extrapolate the two lines until they cross. The intersection gives the maximum temperature, Tmax, at equivalence point. Use your graph to determine the following and show clearly on your graph how you obtained these answers.  Maximum temperature a