RI H2 CHEM P3 Prelim

RAFFLES INSTITUTION 2011 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 13 September 2011 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 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 c lear 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 11 printed pages and 1 blank page.

2 Answer any four questions. Begin each question on a fresh sheet of paper . 1 This question is about the chemistry of halogens and their compounds. (a) Many compounds of chlorine are manufactured from br ine, concentrated NaC l(aq). The electrolysis of brine produces C l2(g) and NaOH(aq). In some industrial electrolytic cells, these two substances are allowed to react further. The products formed in this second reaction depend on the operating conditions used. (i) Write balanced equations for the reaction between Cl2(g) and • cold aqueous NaOH, • hot aqueous NaOH. (ii) Suggest why F 2(g) cannot be manufactured in a similar way using concentrated NaF(aq). [3] (b) The standard electrode potentials, E/ring2, and standard Gibbs free energy changes, ∆ G/ring2, of different chlorine-containing species are tabulated below. Half-equation E /ring2/ring2 /ring2/ring2 / V ∆∆ ∆∆ G/ring2/ring2 /ring2/ring2 / kJ mol −− −− 1 1 C lO4 − + 2H + + 2e − ⇌ ClO3 − + H 2O +1.19 − 230 2 2C lO3 − + 12H + + 10e − ⇌ Cl2 + 6H 2O +1.47 ? 3 Cl2 + 2e − ⇌ 2C l− +1.36 − 262 These electrode potentials can be summarised using the Latimer diagram shown below. In a Latimer diagram, the most highly oxidis ed form of the element is on the left, with successively lower oxidation states to the right. The electrode potentials are shown on the arrows. ∆ G /ring2 and E/ring2 are related by the following equation. ∆ G/ring2 = − zFE /ring2 where ∆ G/ring2 is the standard Gibbs free energy change in joules per mole , z is the number of moles of electrons transferred during the redox reaction and F is the Faraday constant. (i ) Calculate ∆ G/ring2 for half-reaction 2. 2C lO3 − + 12H + + 10e − ⇌ Cl2 + 6H 2O ClO4 − ClO3 − Cl2 Cl− +1.47 V +1.36 V +1.45 V +1.19 V

3 (ii) The standard electrode potential of converting C lO3 − to C l− is not the summation of +1.47 V and +1.36 V. This is because the number of electrons transferred in each step must be taken into account. /onesans/onesans /onesans/onesans Write a half-equation for the conversion of C lO3 − to C l− . /twosans/twosans /twosans/twosans Using your knowledge of Hess’ Law for ∆ G/ring2 and your answers to (b)(i), show with the aid of an energy cycle that the E/ring2 for the conversion of C lO3 − to C l− is +1.45 V. (i ii) With the help of the Latimer diagram provided, calculate E/ring2 cell of the reaction below and hence determine its ∆ G/ring2. 4C lO 3 − (aq) → 3ClO4 − (aq) + C l− (aq) [7] (c ) Halogens and their compounds are commonly used in o rganic reactions to synthesise new compounds. 6.7 g of compound A, C 9H10 O, reacts with 1.2 dm3 of chlorine gas (at r.t.p.) to give compound B. On addition of PC l5, A gives white fumes. Treatment of A with NaBr and concentrated H 2SO 4 under reflux gives compounds C and D. Both C and D have the same molecular formula C 9H10 Br 2 and each contains only one chiral centre. (i) Use the information to deduce the structural formul ae of A – D. Explain the reactions that occur. In particular, state clearly the type of reaction(s ) undergone by A with NaBr and concentrated H 2SO 4 described above. (ii) State and explain which compound, C or D, is produced in greater quantity. (i ii) Describe the mechanism of the reaction between A and Cl2, including curly arrows showing the movement of electrons, and all charges. [10] [Total: 20]

4 2 (a) Histidine, an essential amino acid, is present in m any proteins and enzymes and plays a vital role in the structure and oxygen-binding function of haemoglobin. N HN OH O NH2 histidine There are three p Ka values associated with histidine: α -carboxyl = 1.82, R group = 6.00 and α -amino = 9.17. (i) Sketch the titration curve when 25 cm 3 of protonated form of histidine is being titrated with 75 cm 3 of NaOH(aq) of the same concentration. Your sketch should show clearly where the three p Ka values occur. (i i) Indicate clearly the isoelectric point of histidine on your sketch with an “X”. (iii ) Histidine is considered one of the best amino acid buffers under physiological condition. Write an equation to illustrate how histidine can maintain the pH of a solution at pH 6 when a small amount of OH − (aq) is added. Show the structure of histidine clearly in your equation. (iv) Calculate the final pH of a 100 cm 3 solution of 0.1 mol dm − 3 histidine at pH 6 if 1 dm 3 of 0.001 mol dm − 3 HC l was added. You may represent the acid as HA and the conjugate base as A − in your working. [9]

5 (b) Decarboxylation is an important reaction of amino a cids in many biological processes. Histamine, which causes runny noses and itchy eyes, is synthesised in the body by decarboxylation of histidine with the a id of an enzyme catalyst, L-histidine decarboxylase. N HN OH O NH2 CO2 N HN NH2 histaminehistidine (i) State the hybridisation of the nitrogen, labelled a, in histamine. Hence, explain why it is less basic than the nitrogen, labelled b. (i i) Sketch a graph to illustrate the relationship betwe en histidine concentration and the rate of reaction using a fixed amount of L-histidine decarboxylase. [4] (c) The first known synthesis of an amino acid occurred in 1850 in the laboratory of Adolf Strecker. (i) Name the types of reactions in step I and step II . (ii) Suggest suitable reagents and conditions for step IV . (iii) In step III , the reaction proceeds via 2 parts: /onesans Acid-base reaction between N in imine and HCN /twosans Followed by a nucleophilic attack on C by CN − Propose a mechanism for step III , showing clearly the movement of electrons using curly arrows. (iv) Suggest, with a reason, if there is any difference in the optical activity of a sample of amino acid synthesised by Strecker’s meth od and that of a naturally occurring amino acid. [7] a b

6 [Total: 20]

7 3 (a) The Nernst equation was developed by the German che mist, Walter Hermann Nernst, to calculate the cell potential under non-standard conditions. Two identical concentration half-cells, each consis ting of a copper electrode immersed in 90 cm 3 of 0.0100 mol dm − 3 of copper( II ) nitrate, are prepared. 10 cm 3 of 0.500 mol dm − 3 NH 3(aq) is