RI H2 Chem 2013 Prelim P3 QP

-1- RAFFLES INSTITUTION 2013 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 24 September 2013 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided on the cover page. Write in dark blue or black pen on both sides of the paper. 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 any four questions. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. 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. At the end of the examination, fasten all your work securely together, with the cover page on top. This document consists of 14 printed pages. 212

-2- Answer any four questions. Begin each question on a fresh sheet of paper. 1 Esters are sweet -smelling organic compounds that have been used as flavourings in food and perfumes. For example, ethyl propanoate is an ester with pineapple-like smell. (a) Explain why the rate of t he following reaction gradually increases at first and then decreases. CH3CH2COOCH2CH3 + H2O  CH3CH2COOH + CH3CH2OH ethyl propanoate [2] (b) The kinetics of the hydrolysis of ethyl propanoate in the presence of aqueous sodium hydroxide solution at T K was studied. Two separate experiments were carried out with different concentrations of aqueous sodium hydroxide solution. For each experiment, the concentration of ethyl propanoate was determined at regular time intervals as the reaction progressed. The graph on page 13 shows the results of Experiment 1 with [NaOH] = 2.0 mol dm3. The results of Experiment 2 with [NaOH] = 1.0 mol dm3 are shown below. Time / min Experiment 2 with [NaOH] = 1.0 mol dm3 [CH3CH2COOCH2CH3] / mol dm3 0 0.0200 25 0.0152 50 0.0115 75 0.0088 100 0.0067 125 0.0051 1 (i) Using the same axes on page 13, plot the graph for Experiment 2. 3 (ii) Use the two graphs to determine the order of reacti on with respect to CH3CH2CO2CH2CH3 and to NaOH, showing your working clearly. 1 (iii) Hence write the rate equation for the above reaction. 2 (iv) Calculate the initial rate from E xperiment 1, and use it, together with your rate equation, to calculate the rate constant for the reaction. 1 (v) In a third experiment, 0.0100 mol dm 3 of ethyl propanoate was reacted with 2.0 mol dm3 of sodium hydroxide. On the same axes given on page 13, sketch the graph expected for this experiment, indicating clearly two half-lives. 213

-3- (vi) The hydrolysis of ethyl propanoate by sodium hydroxide is known to occur as follows:  The first step involves a nucleophilic attack of the hydroxide ion on the carboxyl carbon to form a tetrahedral intermediate.  The second step involves formation of a carbon–oxygen  bond and the leaving of an ethoxide ion.  The third step involves the protonation of the ethoxide ion by propanoic acid. Using curly arrows, describe the mechanism for the hydrolysis of ethyl propanoate by sodium hydroxide. [11] (c) The Claisen c ondensation of esters involves the formation of a new carbon -carbon bond between two esters. The products are a keto -ester and an alcohol. The reaction takes place in the presence of a strong base e.g. CH3O. O OR + + O OR O OR O ROH a keto-ester (i) The first step in Claisen condensation involves the strong base CH 3O removing an -hydrogen atom (i.e. the hydrogen atom bonded to the carbon atom next to the carbonyl carbon atom) in an acid-base reaction as shown below. H3C C O OR + CH3O H2C C O OR + CH3OH Suggest a reason why the -hydrogen atom of the ester is acidic. (ii) With reference to the acid -base reaction mentioned in (i), explain why each of the following reactions does not take place in the presence of CH3O–. (1) CH3CH3  –CH2CH3 (2) CH3COOH  –CH2COOH (iii) Draw the structures of all the keto -esters formed when a mixture of CH 3CO2CH2CH3 and CH3CH2CO2CH2CH3 undergo the Claisen condensation. (iv) Draw the structure of the reactant that can be used to prepare the fol lowing compound by the Claisen condensation. O OCH3 O [7] [Total: 20] [Turn over -hydrogen 214

-4- 2 Nickel is a typical transition element in the d-block of the Periodic Table. It is commonly used as a catalyst and as an alloying ingredient. (a) Explain why nickel is regarded as a transition element. [1] (b) In an experiment to study the reaction between nickel and the metal ion, M 2+(aq), an electrochemical cell was set up using the Ni2+/Ni and M2+/M half-cells at 298 K. During the experiment, the concentration of Ni 2+ ions in the Ni 2+/Ni half -cell was kept at 1.0 mol dm –3 but t he concentration of M 2+ ions in the M 2+/M half -cell was changed. The e.m.f. of the cell was measured with each change of solution in the M 2+/M half-cell, and the results obtained are tabulated below. [M2+] / mol dm–3 10–1 10–2 10–3 10–4 10–5 10–6 e.m.f. / V 0.091 0.061 0.032 0.002 –0.028 –0.057 lg [M2+] –1.0 –2.0 –3.0 –4.0 –5.0 –6.0 (i) Plot a graph of e.m.f. against lg [M2+]. (ii) With the aid of the graph plotted and relevant data from the Data Booklet, determine the standard electrode potential of the M2+/M half-cell. (iii) The above electrochemical cell is ‘dead’ when the overall cell reaction reaches equilibrium. With the aid of the graph plotted, calculate the equilibrium constant, Kc, for the cell reaction shown below at 298 K. Ni(s) + M2+(aq) ⇌ Ni2+(aq) + M(s) (iv) Using your answer from (ii) and relevant data from the Data Booklet , predict what might be observed when a rod of metal M is dipped into an aqueous copper( II) nitrate(V) solution. [8] (c) Nickel forms many hydrated salts such as NiC l2.6H2O and NiSO 4.7H2O. When some NiCl2.6H2O is dissolved in water, a green solutio n containing [Ni(H 2O)6]2+ ions forms. Addition of excess aqueous ammonia to this green solution results in a blue solution containing [Ni(NH3)6]2+ ions. (i) State and explain whether the H –O–H bond angle in H 2O in an isolated gaseous H 2O molecule is larger or smaller than that in the [Ni(H2O)6]2+ ion. (ii) Explain briefly why the two nickel(II) ion-containing solutions have different colours. [4] 215

-5- (d) Nickel is able to form neutral complexes. One such complex, tetracarbonylnickel(0), Ni(CO) 4, was first prepared in 1888 by passing carbon monoxide over finely divided nickel. This complex is a colourless liquid at room temperature. (i) Draw the displayed formula of tetracarbonylnickel(0), showing clearly all the dative covalent bonds. (ii) Define the term bond energy. (iii) With the aid of a labelled energy level d