SAJC H2 CHEM P2 QP Prelim

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 Preliminary Exam 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.

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3 [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 determined. 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]

4 [Turn Over 1 (b) (i) Write a detailed plan to prepare 2.00 mol dm-3 solution of FA 2. …………………………………………………………………………………………….……….… ………………………………………………………………………………………………….….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… ………………………………………………………………………………………………….….… …………………………………………………………………………………………….…...….… ……………………………………………………………………………………………….…….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… ……………………………………………………………………………………………….…….… [3] (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 cm 3. 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.

5 [Turn Over 1 (b) (ii) …………………………………………………………………………………………….……….… ………………………………………………………………………………………………….….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… ………………………………………………………………………………………………….….… …………………………………………………………………………………………….…...….… ……………………………………………………………………………………………….…….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… …………………………………………………………………………………………….……….… [5]

6 [Turn Over 1 (b) (iii) The following plots were plotted on a grid after another similar experiment was conducted. Draw suitable graphs through the plotted points. [1]

7 [Turn Over 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] [Total: 12]

8 [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. [2] (b) Chromium hexacarbonyl undergoes the following ligand replacement reaction. Cr(CO) 6 + PR3 Cr(CO)5PR3 + CO Two separate experiments were carried out to study the rate of this reaction. In the first experiment, the ligand PR 3 was in a large excess and [Cr(CO)6] was measured against time. The results are shown on the graph below.

9 [Turn Over 2 (b) 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 [PR 3] / 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. [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(CO)6 ………………………………………………………………………………………….…… ………………………………………………………………………………………….…… PR 3 ………………………………………………………………………………………….…… ………………………………………………………………………………………….…… [2] (iii) Write the rate equation for the reaction, and calculate a value for the rate constant, stating the units of the rate constant. [3]

10 [Turn Over 2 (b) (iv) Two possible mechanisms for this reaction are given below. State the mechanism which is consistent with the rate equation you have written in (b)(iii) and explain your answer. Mechanism A Cr(CO)6  Cr(CO)5 + CO slow Cr(CO)5 + PR3  Cr(CO)5PR3 fast Mechanism B Cr(CO)6 + PR3  [OC- - -Cr(CO)4- - -PR3]  Cr(CO)5PR3 + CO (transition state) ………………………………………………………………………………………….…… ………………………………………………………………………………………….…… ………………………………………………………………………………………….…… [2] [Total: 10]