ACJC H2 Chem 2013 Prelim P3 QP

2 © ACJC 2013 9647/03/Prelim/13 [Turn over Answer any four questions. 1 (a) Liquid bromine is added to propenyl ethanoate CH 2=CHCH2OCOCH3 in presence of concentrated ethanoic acid which acts as a solvent, slowly enough for the reaction to be followed by usual laboratory techniques. The following results were obtained at 25 oC. Experiment Number [propenyl ethanoate]/mol dm-3 [Br2]/mol dm-3 Rate/ mol dm -3 s-1 1 4.0 x 10-2 2.0 x 10 -2 1.29 x 10 -3 2 6.0 x 10-2 2.0 x 10 -2 1.94 x 10 -3 3 4.0 x 10-2 4.0 x 10 -2 5.16 x 10 -3 [10] (i) Write the equation for the addition of bromine to propenyl ethanoate (ii) Deduce the order of reaction with respect to propenyl ethanoate and bromine and hence write the rate equation for the reaction. (iii) Estimate how long it would take for 1% of ester to react in Experiment Number 3. (iv) Experiment Number 1 was repeated with 2.0 x 10 -2 mol dm-3 bromine under the same experimental condition, but using 4.0 x 10-2 mol dm-3 CH2=CHCHClOCOCH3 instead. Would you expect any change to the rate? Suggest a reason for this change if any. (v) What would be the product if water were to be used as a solvent instead of concentrated ethanoic acid? (vi) Would you expect the rate of reaction to be slower or faster if aqueous bromine were to react with propenyl ethanoate? Suggest a reason. 286

3 © ACJC 2013 9647/03/Prelim/13 [Turn over (b) In an experiment, 50.0 cm 3 of aqueous magnesium chloride were titrated with 1.00 mol dm-3 sodium hydroxide. The pH of the solution changed as shown in the diagram below. [10] 287

4 © ACJC 2013 9647/03/Prelim/13 [Turn over (i) Calculate the initial concentration of magnesium chloride. (ii) When 10.0 cm3 of NaOH have been added, calculate the (a) hydroxide ion concentration (b) hydrated magnesium ion concentration Hence, calculate the solubility product of magnesium hydroxide. (iii) Suggest how, if at all, the pH of a saturated solution of Mg(OH) 2 would change if the temperature were to be increased given that Ksp of Mg(OH)2 decreases with increasing temperature. (iv) By means of an equation, explain why initial pH of graph starts at 6. [Total: 20 marks] 288

5 © ACJC 2013 9647/03/Prelim/13 [Turn over 2 (a) A cycloalkane, C 5H10, gives only two mono-brominated products (ignoring any stereoisomers) with Br2 under uv light. [3] (i) Give the structural formulae of the cycloalkane and the two mono- brominated products formed. (ii) Suggest the ratio in which the two mono-brominated products will be formed. (b) Cyclopentane is an isomer of the cycloalkane identified in (a)(i). It can be used to synthesise a cyanohydrin. W Br2 uv light step I step II step III CN Br OH O [7] (i) Step I is a first order reaction. Write a balanced equation for the reaction, stating suitable reagents and conditions required. Hence, state the rate equation of the reaction. (ii) State the reagents and conditions required in step III. Describe the mechanism involved. (c) Aluminium is obtained from electrolysis of aluminium oxide which is mainly from the mining the mineral bauxite. Bauxite contains only 30 to 54% aluminium oxide and the rest being other solid impurities. Hence, bauxite must be purified prior to electrolysis. The Bayer process is the principal industrial means of refining bauxite to produce pure aluminium oxide. [10] (i) In order to separate aluminium oxide from the other solid impurities in bauxite, it is digested by washing with a hot solution of sodium hydroxide under pressure followed by filtration in the Bayer process. Write a balanced equation with state symbols for the chemical reaction above. (ii) In the subsequent step of the Bayer process, the above solution is cooled and treated by bubbling carbon dioxide into it, through which a solid is precipitated. When heated to a high temperature, this solid decomposes to aluminium oxide, giving off water vapour in the process. Identify the solid and write a balanced equation with state symbols for its formation. (iii) The purified bauxite ore of aluminium oxide is continuously fed into the electrolysis process and cryolite is added to lower the melting point and dissolve the ore. Write half-equations for the reactions at the cathode and anode during electrolysis. 289

6 © ACJC 2013 9647/03/Prelim/13 [Turn over (iv) Aluminium cannot be produced by the electrolysis of an aqueous aluminium salt. Instead, a molten aluminium salt could be used. In the Hall–Héroult process, aluminium oxide is dissolved in molten cryolite, Na 3AlF6 and electrolysed. Explain  why it is not feasible to obtain aluminium by the electrolysis of the aqueous aluminium salt;  why cryolite is added in the electrolysis of aluminium oxide. (v) A proposed aluminium plant is expected to consume 1.00 x 10 8 kg of bauxite per year. Assuming that the bauxite contains 54 % of aluminium oxide, calculate the mass of aluminium produced and the total amount of charge (in C) that flows through the electrolytic cell per year. [Total: 20 marks] 290

7 © ACJC 2013 9647/03/Prelim/13 [Turn over 3 (a) For most aldehydes and ketones, the most stable structural isomer has a C=O bond and this is called the keto form. The enol form of the aldehyde or ketone is derived from the keto form by transfer of a hydrogen atom and it has a C=C bond with an OH substituent. This interconversion between the keto and enol forms is called keto-enol tautomerism. At normal room conditions the keto form is the predominant form at equilibrium suggesting that the keto form is the more stable form. Using bond energy data from the data booklet, calculate the energy change for the above interconversion and hence expl ain why the equilibrium lies heavily towards the keto form. [2] (b) The above conversions can be catalysed by an acid which involves protonation of oxygen atom of the C=O bond followed by loss of H + from the α-position as shown in the mechanism below Equilibrium 1 Equilibrium 2 The first equilibrium step in the above mechanism can be written in a simplified form as given below: …Equilibrium 1 [7] (i) Given that all the equilibriums concerned are in a homogeneous system, write the K a expressions for the acid dissociation of H3O+, and (CH3)2C=OH+ (ii) Hence, by combining the two expressions, obtain the K c expression for equilibrium 1. (iii) Given that the pKa of H 3O+ is -1.7 and the pKa of (CH3)2C=OH+ is -7.2, calculate the Kc for the above equilibrium. 291

8 © ACJC 2013 9647/03/Prelim/13 [Turn over (iv) Given that ∆Go = -RT ln K, calculate the Gibbs Free energy change for the above equilibrium. (v) Based on your answers to the values of ∆Go and Kc, comment on the relative stability of the keto form versus the enol form. (c) It has been observed that the enol form of 1,3-dicarbonyl compounds are unusually stable and hence the equilibrium lies more towards the enol form as in the case of pentane-2,4-dione. It is also observed that the percentage of the enol form increases as the solvent used is changed from a polar solvent to a non polar solvent. [5] (i) By comparing the structures of th