2019 TMJC H2 Chem Prelim P4 QP

Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry CANDIDATE NAME CIVICS GROUP _______________________________________________________________________________ H2 CHEMISTRY 9729/04 Paper 4 Practical 4 September 2019 2 hours 30 minutes Candidates answer on the Question Paper. Additional materials: As listed in the Confidential Instructions _______________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name and Civics Group in the spaces at the top of the page. 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 17 and 18. 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 18 printed pages. TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Shift Laboratory For Examiner's Use 1 2 3 Total

2 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry Answer all the questions in the spaces provided. 1 Determination of the concentration of a base and the enthalpy change of the neutralisation reaction FA 1 is 2.0 mol dm–3 sulfuric acid, H2SO4. FA 2 is aqueous sodium hydroxide, NaOH. The reaction of sulfuric acid and sodium hydroxide is exothermic. In separate experiments, you will add increasing volumes of FA 2 to a fixed volume of FA 1. In each experiment you will measure the maximum temperature rise, ∆T. As the volume of FA 2 is increased, this maximum temperature rise, ∆T, will increase and then decrease. By measuring the maximum temperature rise for different mixtures of the two reagents , you are to determine the following:  the concentration of sodium hydroxide, NaOH, in FA 2  the enthalpy change when 1 mol of H2SO4 is neutralised by NaOH (a) Method  Fill the labelled burette with FA 1.  Support a styrofoam cup in the 250 cm3 beaker.  Run 10.00 cm3 of FA 1 from the burette into the styrofoam cup.  Measure 20.0 cm3 of FA 2 using a measuring cylinder.  Place the thermometer in the FA 2 in the measuring cylinder and record the steady temperature of the solution.  Pour the FA 2 into the styrofoam cup, stir and record the maximum temperature obtained in the reaction.  Empty and rinse the styrofoam cup; shake dry the styrofoam cup. Rinse the thermometer.  Carry out the experiment three more times. Each time use 10.00 cm3 of FA 1.  Use 30.0 cm3, 40.0 cm3 and 50.0 cm3 of FA 2 in these different experiments. Carry out two further experiments. Choose volumes of FA 2 which will allow you to investigate more precisely the volume of FA 2 that produces the highest temperature rise when added to 10.00 cm3 of FA 1. Results Record your results below in an appropriate form showing, for each experiment, the volumes of solutions used, temperature measurements and the temperature rise, T. [4]

3 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry (b) Plot a graph of temperature rise, ∆ T, on the y-axis against the volume of FA 2 added on the x-axis. Draw a line of best fit through the points where the temperature rise is increasing and another line through the points where the temperature rise is decreasing. The intersection of these lines represents the temperature rise for the volume of FA 2 that exactly neutralises the sulfuric acid present in 10.00 cm3 of FA 1. [3] (c) Read from the graph the volume of FA 2 that gives the maximum temperature rise. Volume of FA 2 giving the maximum temperature rise = cm3 [1]

4 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry (d) (i) Calculate the amount of NaOH required to neutralise the amount of H2SO4 at the maximum temperature rise. Amount of NaOH required = mol [1] (ii) Hence, calculate the concentration of NaOH in FA 2. Concentration of NaOH in FA 2 = mol dm3 [1] (e) Read the maximum temperature rise from the graph and use this to calculate the enthalpy change when 1 mol of H2SO4 is neutralised by NaOH. Give your answer in kJ mol–1. [4.18 J are absorbed or released when the temperature of 1 cm3 of solution changes by 1 °C.] Enthalpy change = kJ mol1 [2]

5 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry (f) The enthalpy change of neutralisation, Hneut, between a strong acid and a strong base is –57 kJ mol1. Explain why the enthalpy change calculated in (e) is significantly more exothermic than Hneut. [1] (g) A student suggested that the experiments carried out in (a) would be more accurate if volumes of 20.00 cm3 of 1.0 mol dm–3 H2SO4 were used instead. State and explain whether you agree or disagree with the student’s suggestion. evaluation of student’s suggestion explanation [1] [Total: 14]

6 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry 2 Determination of the percentage by mass of sodium carbonate in a mixture of sodium hydroxide and sodium carbonate Sodium carbonate is neutralised by hydrochloric acid in two steps: Na2CO3 + HCl  NaCl + NaHCO3 NaHCO3 + HCl  NaCl + H2O + CO2 This step-wise neutralisation can be observed when the acid is added slowly to the sodium carbonate. The percentage by mass of sodium carbonate in a mixture of sodium hydroxide and sodium carbonate can be determined by carrying out titrations using two different indicators. Since both sodium hydroxide and sodium carbonate react with acids, through careful selection of the indicators used for the titration, the volume of acid required to react with only the sodium carbonate can be found. FA 3 is 0.125 mol dm3 hydrochloric acid, HCl. FA 4 is an aqueous solution containing sodium hydroxide, NaOH, and sodium carbonate, Na2CO3. You are also provided with bromophenol blue indicator. In this question, you will carry out titrations to determine the percentage by mass of sodium carbonate in the mixture of sodium hydroxide and sodium carbonate in solution FA 4.

7 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry (a) (i) Titration of FA 4 against FA 3 using bromophenol blue 1. Fill a burette with FA 3. 2. Pipette 25.0 cm3 of FA 4 into a conical flask. 3. Add four to five drops of bromophenol blue indicator. 4. Titrate the mixture in the flask with FA 3 until the blue-violet colour of the solution changes to yellow. 5. Record your titration results , to an appropriate level of precision, in the space provided. Repeat steps 2 to 5 to obtain consistent results. Titration results [6] (ii) From your titration results, obtain a suitable volume of FA 3, VFA 3, to be used in your calculations. Show clearly how you obtained this volume. VFA 3 = [1]

8 Tampines Meridian Junior College 2019 JC2 Prelim H2 Chemistry (b) When the titrations in (a) were repeated using phenolphthalein as the indicator, 25.0 cm3 of FA 4 required 15.00 cm3 of FA 3. The following explains why different results are obtained using two different indicators. • When phenolphthalein is used as the indicator, the fo