2019 MI Prelim H2 Chem P4 ANS

Class Adm No Candidate Name: This question paper consists of 19 printed pages. 2019 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 4th Sept 2019 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 18 22 9 6 55 Shift Laboratory

2 1 Determination of titration value at equivalence point The reaction between acid and carbonates is well known. In the presence of excess acid, the following reaction occurs: CO32–(aq) + H+(aq) ⇌ HCO3–(aq) pKa(HCO3–) = 10.32 equation 1 HCO3–(aq) + H+(aq) ⇌ H2CO3(aq) pKa(H2CO3) = 6.37 equation 2 H2CO3(aq) ⇌ CO2(aq) + H2O(l) equation 3 CO2(aq) is then released from the solution as CO2(g), which is observed as effervescence. The entire reaction is known to release heat. FA 1 is 1.8 mol dm-3 of aqueous potassium carbonate, K2CO3. FA 2 is nitric acid, HNO3, of concentration between 1.9–2.1 mol dm-3. Assuming that the first equivalent of H + fully reacts with CO32– before reacting with the HCO3– produced, the reaction between FA 1 and the first equivalent of FA 2 can be simplified as: CO32–(aq) + H+(aq) → HCO3–(aq) ∆Hrxn And the reaction between FA 1 and the second equivalent of FA 2 can be simplified as: HCO3–(aq) + H+(aq) → H2CO3(aq) As the precise concentration of FA 2 is unknown, determination of ∆Hrxn can be done using a thermometric titration to simultaneously determine both the concentration of FA 2 as well as ∆Hrxn. 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 an acid -carbonate thermometric titration. The data from this titration will be used to determine:  the titration value at the first equivalence point, Veq1,  the precise concentration of FA 2, [HNO3],  the maximum temperature change, ∆Tmax,  the enthalpy change of reaction, ∆Hrxn. For Examiners’ Use (a) (i) 25.0 cm3 of FA 1 is reacted with FA 2. Calculate the theoretical volume of FA 2 needed for the first equivalence point, Veq1’, and the second equivalence point, Veq2’, of the reaction between FA 1 and FA 2 . Assume [HNO3] to be 2.0 mol dm-3. Amount of K2CO3 reacted = 1.8 x 25.0 x 10-3 = 0.045 mol Amount of H+ for first equivalence point = 0.045 mol Veq1’ = 0.045 2.0 = 0.0225 dm3 = 22.5 cm3 Veq2’ = 22.5 x 2 = 45.0 cm3 Veq1’ = …………… cm3 Veq2’ = …………… cm3 M1 M2

3 [Turn over [2] Determination of Veq1 and ∆Hrxn using thermometric titration For this experiment, you will need to measure the maximum temperature of the reaction mixture when specified volumes of FA 2 have been added to FA 1. In an appropriate format in the space provided below, prepare a table to record your results. Record all values of temperature, T, to 0.1°C, and each total volume of FA 2 added. Note: You should aim to perform each subsequent addition of FA 2 quickly. 1. Fill a burette with FA 2. 2. Using a pipette, transfer 25.0 cm3 of FA 1 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 1. Record this temperature. 4. Add 4.00 cm3 of FA 2 from the burette to the FA 1 in the Styrofoam cup. 5. Using the thermometer, stir the mixture thoroughly and record the maximum temperature reached and the volume of FA 2 added. 6. Repeat steps 4 and 5 until a total volume of 48.00 cm3 of FA 2 has been added. For Examiners’ Use Results Vol of FA 2 added / cm3 Maximum T / °C 0.00 32.7 4.00 33.7 8.00 34.5 12.00 35.0 16.00 35.4 20.00 35.6 24.00 35.6 28.00 35.4 32.00 35.2 36.00 35.0 40.00 34.7 44.00 34.5 48.00 34.2 [2]

4 (ii) Plot a graph of temperature, T, on the y-axis, against volume of FA 2 added, on the x-axis on the grid in Fig. 1.1. The temperature axis should allow you to include a point at least 1.0 °C greater than the maximum temperature recorded. Fig. 1.1 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] For Examiners’ Use

5 [Turn over (iii) 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. ∆Tmax = 35.7 – 32.7 = 3.0 °C Veq1 = 22.40 cm3 Tmax = 35.7 °C ∆Tmax = 3.0 °C [3] For Examiners’ Use (iv) Determine the concentration of HNO3, [HNO3], in FA 2. Amount of K2CO3 reacted = 1.8 x 25.0 x 10-3 = 0.045 mol Amount of HNO3 reacted at Veq1 = 0.045 mol [NaOH] = 0.045 22.40×10−3 = 2.01 mol dm-3 (3sf) [HNO3] in FA 2 = …………… [1] (v) Determine the enthalpy change of reaction, ∆Hrxn. CO32–(aq) + H+(aq) → HCO3–(aq) ∆Hrxn 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 + 22.40 = 47.4 g q = mc∆T = (47.4)(4.18)(3) = 594.4 J ∆Hrxn = − 𝑞 𝑛𝐿𝑅 × CLR = − 594.4 0.045 × 1 = –13.2 kJ mol-1 (3 sf) ∆Hrxn = …………… [3]

6 (b) (i) With reference to your graph in Fig. 1.1, explain why the titre at second equivalence point, Veq2, cannot be determined from this experiment. The temperature does not increase / reaction is not exothermic for equation 2. OR There is only one intersection point OWTTE [1] For Examiners’ Use (ii) Suggest, using chemistry concepts, a possible explanation for the observation in (b)(i). Equation 2 is not exothermic / the temperature does not increase as equation 3 could be endothermic. or pKa(H2CO3) is small compared to pKa(HCO3–) and the energy released from the reaction is used to complete the reaction. [1] (iii) The value of Veq1 could also have been determined via a regular acid-base titration with a suitable indicator. Suggest which titration method is likely to give a more accurate value of Veq1 and give two reasons for why this is so. Method: Accept any reasonable answer for both Regular titration: heat loss to surroundings does not affect value of Veq1 unlike thermometric method; The first equivalent of H + fully reacts with CO 32- before reacting with HCO3- may not be true. Thermometric titration: does not require use of indicator which may affect pH of solution and end point may not match exactly with equivalence point [2] [Total: 18]

7 [Turn over 2 Investigation of the kinetics of an acid-carbonate reaction via gravimetric analysis FA 1 is 1.8 mol dm-3 of K2CO3. FA 3 is dilute nitric acid, HNO3, of concentration 1.0 mol dm-3. K2CO3 + 2HNO3 → 2KNO3 + CO2 + H2O The rate equation of the reaction between FA 1 and FA 3 is express