SAJC H2 CHEM P2 Ans Prelim

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 Preliminary Exam (TUTOR’S COPY) Chemistry Higher 2 9647/02 31 August 2016 Paper 2 2 hours Candidates answer in the spaces provided on the question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use: Question Marks 1 2 3 4 5 6 7 Total (72 marks) This document consists of 27 printed pages (including this page) and 1 blank page.

2 [Turn Over 1 A student found a bottle of solid benzoic acid in the school laboratory. She was tasked to determine the standard enthalpy change of neutralisation of benzoic acid. The standard enthalpy change of neutralisation is when one mole of water is formed between an acid and a base. H+ (aq) + OH− (aq)  H2O (l) You may assume you are provided with the following:  FA 1, aqueous sodium hydroxide, NaOH  Solid benzoic acid  Polystyrene (styrofoam) cups  Apparatus normally found in a school laboratory Before carrying out the experiment, an aqueous solution of 2.00 mol dm -3 benzoic acid (FA 2) is first prepared. Subsequently, in separate experiments, different volumes of FA 2 and FA 1 are mixed while keeping the total volume of the reaction mixture constant. In each experiment, the temperature rise, ∆T, is to be determin ed. A graph of ∆T against volume of FA 1 used is then plotted. Data from the graph can then be used to determine:  the ∆Tmax,  the concentration of FA 1,  the enthalpy change of neutralisation between FA 1 and FA 2. It is given that 4.18 J is required to raise the temperature of 1 cm 3 of any solution by 1 oC. (a) Write an equation to represent the standard enthalpy change of neutralisation between sodium hydroxide and benzoic acid. [1] C6H5COOH(aq) + NaOH(aq) → C6H5COO-Na+(aq) + H2O(l) (b) (i) Write a detailed plan to prepare 2.00 mol dm-3 solution of FA 2. [3] To prepare 250.0 cm3 of 2.00 mol dm–3 benzoic acid (FA 2) Molar mass of C7H6O2= 122 g mol–1

3 [Turn Over 1) Amount present in 250.0 cm3 of 2.00 mol dm–3 solution = 61.0 g 2) Use a weighing balance to measure accurately 61.0 g (minimum 61.0 g or maximum 62.0 g) of solid C 7H6O2 in a clean and dry 3) weighing bottle. Record the mass of the solid and the weighing bottle. 4) Dissolve the solid 5) in a 100 cm3 / 250 cm3 beaker with 30 cm3 of water. 6) Transfer the solution and washings 7) into a 250 cm 3 graduated flask/ volumetric flask and 8) make up to the mark dropwise with deionized / distilled water 9) Shake the solution to obtain a homogeneous solution and label it FA 2. 10) Reweigh the emptied weighing bottle. / Alternative: wash the weighing bottle in the procedure if student calculate 61g but use an alternative value ignore (ii) It is predicted that the maximum temperature change for the neutralisation would occur when the volume of FA 1 mixed is between 25 cm 3 and 30 cm3. The total volume of any mixture should be kept constant at 50 cm3. Write a detailed plan on how you could determine the temperature changes for the series of reactions between FA 1 and FA 2. Your plan should include details of:  all essential experimental details,  appropriate volumes of solutions to be used,  a tabulation of the experimental data to be collected (including volumes used),  how these measurements can be used to obtain the temperature change, ∆T Procedure 1) Place a dry polystyrene cup into a 250 cm3 beaker. 2) Use a 50 cm 3 / 100 cm 3 measuring cylinder/burette 3) to transfer 20 cm 3 of FA 1 into the styrofoam cup labelled FA 1 4) Use a 50 cm 3 / 100 cm 3 measuring cylinder/burette 5) to transfer 30 cm3 of FA 2 into another styrofoam cup labelled FA 2 6) Use a thermometer 7) to measure the temperature of the FA 1 and FA 2 solution separately. Record as TFA1 and TFA2 respectively.

4 [Turn Over 8) Transfer the contents of FA 2 cup to the FA 1 cup. 9) Stir the mixture gently using the thermometer provided. 10) Record the maximum/highest temperature reached. 11) Record as Tmax. Wash and dry both the FA 1 and FA 2 styrofoam cups. 12) Repeat steps 1 to 8 for Mixtures 2 to 5 Tabulation: Vol of FA 1 / cm3 Vol of FA 2 / cm3 Initial temperature of FA 1 / oC Initial temperature of FA 2 / oC Average temperature / oC Highest temp / oC ∆T, / oC 20.00 30.00 25.00 25.00 30.00 20.00 35.00 15.00 40.00 10.00

5 [Turn Over (b) (iii) The following plots were plotted on a grid after another similar experiment was conducted. Draw suitable graphs through the plotted points. [1] (c) By using the graph in (b)(iii), calculate  the concentration of FA 1, given that the total volume of the mixture is 50 cm3.  the enthalpy change of neutralisation for the reaction between FA 1 and FA 2. [2] Volume of FA 1 (NaOH) used when temperature change is maximum = 26.50 to 26.80 cm3 No of moles of benzoic acid used = (50-26.75)/1000 x 2.0 = 0.0465 mole NaOH ≡ benzoic acid No of moles of NaOH (FA 1) used = 0.0465 mole

6 [Turn Over Concentration of sodium hydroxide in FA 1 = 0.0465/ 0.02675 = 1.738 mol dm–3 = 1.74 mol dm–3 From graph, maximum ∆T = 12.15 oC q when ∆T is highest = 50  4.18  12.15 = 2539 J No of moles of water formed = 0.0470 Enthalpy change of neutralisation between NaOH and benzoic acid = − 2539 /0.0465 = − 54.6 kJ mol–1 [Total: 12]

7 [Turn Over 2 (a) Chromium carbonyl, also known as chromium hexacarbonyl, is a chemical compound with the formula Cr(CO) 6. Cr(CO) 6 is zerocovalent, meaning that Cr has an oxidation state of 0. Draw the structure of the complex and state the shape and bond angle around the Cr atom in the complex. Shape: Octahedral Bond Angle: 90o [2] (b) Chromium hexacarbonyl undergoes the following ligand replacement reaction. Cr(CO)6 + PR3 Cr(CO)5PR3 + CO

8 [Turn Over Two separate experiments were carried out to study the rate of this reaction. In the first experiment, the ligand PR3 was in a large excess and [Cr(CO)6] was measured against time. The results are shown on the graph below. In the second experiment, Cr(CO) 6 was in a large excess, and [PR 3] was measured against time. The following results were obtained. time / s [PR3] / mol dm–3 0 0.0100 120 0.0076 200 0.0060 360 0.0028 (i) On the graph, plot the data given in the table.

9 [Turn Over [1] (ii) Use the graphs to determine the order of reaction with respect to Cr(CO)6 and PR3. In each case explain how you arrived at your answer. Cr