MI Prelim P4 Ans

Class Adm No Candidate Name: This question paper consists of 18 printed pages and 0 blank pages. 2018 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 10th Sept 2018 2 hour 30 mins 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 26 13 6 10 55 Shift Laboratory

2 1 Investigation of acid-base titrations involving sodium hydrogen carbonate According to the Arrhenius theory of acids and bases, an acid produces H +(aq) ions and a base produces OH–(aq) ions. The reaction of these two ions to form water molecules is known as acid- base neutralisation. The equation for this neutralisation reaction is given below, and strong acid–strong base reactions are known to have an enthalpy change of neutralisation (∆Hneu) of approximately –57.1 kJ mol-1. H+(aq) + OH–(aq) → H2O(l) ∆Hneu = –57.1 kJ mol-1 However, the ∆Hneu for weak acid–strong base reactions are known to differ. Sodium hydrogen carbonate, NaHCO3, is an example of a weak acid. FA 1 is 1.8 mol dm-3 sodium hydrogen carbonate, NaHCO3. FA 2 is sodium hydroxide, NaOH, of concentration between 0.9 – 1.2 mol dm-3. NaHCO3 + NaOH → Na2CO3 + H2O ∆Hneu’ As the precise concentration of FA 2 is unknown, determination of ∆Hneu’ can be done using a thermometric titration to simultaneously determine both the concentration of FA 2 as well as ∆Hneu’. Thermometric titration is a technique whereby equivalence points of a reaction can be located by observing temperature changes, hence eliminating the need for an indicator. In 1(a), you will perform a weak acid–strong base thermometric titration. The data from this titration will be used to determine: • the titration value at equivalence point, Veq, • the precise concentration of FA 2, [NaOH], • the maximum temperature change, ∆Tmax, • the enthalpy change of neutralisation, ∆Hneu’. For Examiners’ Use

3 [Turn over (a) Determination of Veq and ∆Hneu’ using thermometric titration For this experiment, you will need to measure the maximum temperature of the reaction mixture when specified volumes of FA 1 have been added. In an appropriate format in the space provided on the next page, prepare a table to record your results. Record all values of temperature, T, to 0.1°C, and each total volume of FA 1 added. Note: You should aim to perform each subsequent addition of FA 1 quickly. 1. Fill a burette with FA 1. 2. Using a pipette, transfer 25.0 cm 3 of FA 2 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm3 glass beaker. 3. Stir and measure the temperature of this FA 2. Record this temperature. 4. Add 2.00 cm 3 of FA 1 from the burette to the FA 2 in the Styrofoam cup. 5. Using the thermometer, stir the mixture thoroughly and record the maximum temperature reached and the volume of FA 1 added. 6. Repeat steps 4 and 5 until a total volume of 30.00 cm3 of FA 1 has been added. For Examiners’ Use

4 Results Vol of FA 1 added / cm3 T / °C 0.00 29.4 2.00 30.4 4.00 31.5 6.00 32.2 8.00 32.9 10.00 33.4 12.00 34.0 14.00 33.9 16.00 33.6 18.00 33.3 20.00 33.0 22.00 32.8 24.00 32.6 26.00 32.4 28.00 32.3 30.00 32.2 [2] M1 all 16 readings of vol and T tabulated with correct headers and units M2 T to ±0.1 °C

5 [Turn over (i) Plot a graph of temperature, T, on the y-axis, against volume of FA 1 added, on the x-axis on the grid in Fig. 1.1. The temperature axis should allow you to include a point at least 1.5 °C greater than the maximum temperature recorded. Fig. 1.1 M3 axes labelled with units M4 all points accurately plotted to ½ square and appropriate choice of scale M5 2 best-fit curves drawn that cross (and not just touch) Draw two most appropriate best-fit lines in Fig. 1.1 , taking into account all of your plotted points. Extrapolate (extend) these two best-fit lines until they cross each other. [3]

6 (ii) From your graph in Fig. 1.1, determine: • the titre at equivalence point, Veq, • the maximum temperature reached, Tmax, • the maximum temperature change, ∆Tmax. On your graph, show clearly how you obtained these values. ∆T max = 34.125 – 29.4 = 4.7 °C Veq = 12.70 cm3 Tmax = 34.1 °C ∆Tmax = 4.7 °C [3] For Examiners’ Use M6 both construction lines shown on graph M7 both readings correct to ½ square M8 accuracy of ∆Tmax, student value ≤0.6 from supervisor’s value (iii) Determine the concentration of NaOH, [NaOH], in FA 2. Amount of NaHCO3 reacted = 1.8 x 12.70 x 10-3 = 0.02286 mol [NaOH] = ଴.ଶଶ଼଺ ଶହ.଴×ଵ଴షయ = 0.914 mol dm-3 [NaOH] in FA 2 = 0.914 mol dm-3 [1] M9 (iv) Determine the enthalpy change of neutralisation, ∆Hneu’. NaHCO3 + NaOH → Na2CO3 + H2O ∆Hneu’ Assume that the reaction mixture has a density of 1.00 g cm -3 and a specific heat capacity, c, of 4.18 J g-1 K-1. m = 25.0 + 12.70 = 37.7 g q = mc∆T = (37.7)(4.18)(4.7) = 740.6 J ∆H = − ௤ ௡ಽೃ = − ଻ସ଴.଺ ଴.଴ଶଶ଼଺ = –32.4 kJ mol-1 (3 sf) ∆Hneu’ = –32.4 kJ mol-1 [3] M10 correct mass used M11 correct calculations, do not mark for sign M12 all calc in (a) to 3 or 4 sf and have correct units and correct sign for ∆H (v) Comment on your value of ∆Hneu’ obtained compared to ∆Hneu = –57.1 kJ mol-1. The value of ∆Hneu’ obtained is less exothermic, as some of the heat energy released was used to completely dissociate the weak acid NaHCO3. [1] M13 comparison unambiguous (do not accept “value s more/less”)

7 [Turn over (vi) From your graph in Fig. 1.1, explain the shape of your best-fit line before equivalence point. The curve is increasing with a decreasing gradient as the exothermic reaction releases the same amount of heat energy for each addition of FA 1, which causes a smaller temperature rise as the mass of the mixture increases. [1] M14

8 (b) Determination of titration value at equivalence point, Veq’, using ‘regular’ titration FA 3 is hydrochloric acid, HCl, of unknown concentration. Sodium hydrogen carbonate, NaHCO3, is also able to act as a weak base. In 1(a), NaHCO3 acts as the acid while in 1(b), NaHCO3 is acts as the base. For this experiment, you will titrate FA 3 against FA 1 to determine the titration value at equivalence point, Veq’, using methyl orange as the indicator. For Examiners’ Use (i) The use of thermometric titration in 1(a) eliminated the need for an indicator as the equivalence point was located by observing temperature changes. ‘Regular’ acid-base titrations however, require the use of an indicator. Explain why an indicator is required for ‘regular’ acid-base titrations (such as in 1(b)). To give a visible observation that corresponds to completion of reaction. [1] M15 Titration of FA 3 against FA 1 1. Fill the burette with FA 1