DHS H2 Chem 2012 Prelim P3 QP

Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/03 Paper 3 Free Response 18 September 2012 2 hours Additional Materials: Data Booklet Graph Paper INSTRUCTIONS TO CANDIDATES Write your name, register number and class on this question paper and on the Cover Sheet provided. 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. Start each question on a fresh sheet of paper. A Data Booklet is provided. 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 work securely together with the Cover Sheet on top. · Hand in the question paper separately. The total marks for this paper is 80 marks. This question paper consists of 8 printed pages and 0 blank page. @DHS 2012 [Turn over

2 © DHS 2012 9647/03 [Turn over Answer any four questions. 1 The study of organic compounds includes the collection of kinetic data and thermodynamic data of the compounds. (a) The kinetics of the hydrolysis of the ester, CH 3CH2CO2CH3, may be investigated by the following method. CH3CH2CO2CH3 + H2O ® CH3CH2CO2H + CH3OH In a 1 dm3 mixture, 0.350 mol of the ester was hydrolysed by heating with water and using hydrochloric acid as catalyst. The following results were obtained. Time/s Concentration of CH3CH2CO2H/mol dm–3 0 0 340 0.105 680 0.185 1080 0.243 1440 0.278 (i) Suggest how the progress of this reaction may be followed in order to obtain the results as stated. (ii) By drawing a suitable graph using the data given above, show that the reaction is first order with respect to the ester. It has been found that the hydrolysis reaction is first order with respect to the hydrochloric acid. (iii) Deduce the units of the rate constant. (iv) State and explain the effect of a catalyst on the rate constant. (v) Using suitable bond energy values from the Data Booklet, calculate the ∆H for the hydrolysis of the ester. (vi) Given that the standard enthalpy change of reaction for the hydrolysis is +7.6 kJ mol –1, suggest a reason for the difference between this given value and the value that you have calculated in (a)(v). [11] (b) Hydrolysis of ester can be achieved in the biological system by enzymes known as esterase. Enzymes are proteins that catalysed a specific chemical transfo rmation in the biological system. Such enzymes are generally quaternary proteins. (i) Sketch and explain the graph showing how the rate of hydrolysis changes with increasing concentration of the ester. (ii) Explain the meaning of quaternary structure of proteins. (iii) List 2 other major functions of proteins in the body. (iv) Suggest and explain a chemical method for distinguishing the following pair of esters. You should state the expected observations. HCO2CH2CH3 and CH3CH2CO2CH3 [9] [Total: 20]

3 © DHS 2012 9647/03 [Turn over 2 This question is about the chemistry of the transition metal, nickel and its compound. (a) Explain why the colour of [Ni(NH3)6]2+(aq) is blue. [3] (b) Ni(CO)4 is a compound formed by the reaction between nickel and carbon monoxide. The Mond process was developed by Ludwig Mond to extract and purify nickel from its ores. One of the stages of this process involves the decomposition of Ni(CO) 4 at 227 °C to give nickel as shown in this equation below: Ni(CO)4(g) Ni(s) + 4CO(g) The equilibrium constant, K p, for the equilibrium at 227 °C is 1.01 atm 3. A sample of gaseous Ni(CO)4 was placed in a 2 dm3 evacuated container at 227 °C. At equilibrium, the partial pressure of CO was 2.00 atm. [1 atm = 1.01 x 105 Pa] (i) Sketch the shapes of the hybrid orbitals around the C atom in carbon monoxide. (ii) Write an expression for Kp. (iii) Calculate the total pressure of the system at equilibrium. (iv) Calculate the mass of Ni(CO)4 placed in the container initially. [8] (c) Ni is commonly used in catalytic hydrogenation r eactions. One such example is given below. Compound A, C 10H12NOCl, has a chiral centre and dissolves in dilute sulfuric acid. It reacts with 2,4– dinitrophenylhydrazine to form an orange precipitate, but does not react with Tollens’ reagent. A reacts with H 2 in the presence of Ni catalyst followed by addition of aqueous bromine to form B, C 10H11NOClBr3. When 1 mol of compound B is heated under reflux with aqueous iodine and excess alkali, followed by careful acidification, compound C, C8H4NO4Br3 is formed together with 2 mol of yellow solid CHI3. Deduce the structures of compounds A, B and C, giving reasons for your answer. [9] [Total: 20]

4 © DHS 2012 9647/03 [Turn over 3 This question is about the varied chemical uses of HCl. (a) (i) In the electrolysis of a solution of hydrochloric acid with inert platinum electrodes, an ammeter showed a reading of 0.450 A in the external circuit of the cell. 110 cm 3 of gas was evolved at the cathode after 30 minutes under room temperature and pressure. Determine the actual current used in the electrolysis process. Hence, determine the percentage error (if any) of the ammeter reading. (ii) A student carried out a reaction involving acidified potassium dichromate (VI), together with an unknown half– cell which consists of transition metal ions. The Gibbs Free Energy of the reaction was known to be –324 kJ mol –1 and the procedure involved transfer ring 6 mol of electrons from the unknown s olution to potassium dichromate(VI). Using the expression of DGq = –nFEqcell, where n is the number of moles of electrons transferred, F is the Faraday constant and E qcell is the overall cell potential, determine the Eqcell value and hence deduce the identity of the unknown half cell. [4] (b) A sample of sodium hypochlorite, N aOCl, was dissolved in 100 cm 3 of 0.123 mol dm –3 HOCl (pKa = 7.50) solution forming a buffer of pH 6.20. The buffer is then used to absorb HCl gas. Calculate the concentration of gaseous HCl (in mol dm–3) that is required to be added to the buffer solution until it reaches pH 6. [5] (c) The decomposition temperature of hydrogen chloride is close to 3000 K. Predic t, with reasoning, if the decomposition temperature of hydrogen astatide would be higher or lower than 3000 K. [3] (d) Concentrated hydrochloric acid is commonly used in the reduction of trinitrobenzene (2,4,6–trinitrobenzene). (i) Suggest the complete set of reagents, together with concentrated HC l, to be used in this reduction process. (ii) Suggest the structural formula of the product formed. (iii) Explain the relative basicity between the product formed in (d)(ii) and the melamine molecule as shown below in terms of their structures. The melamine molecule contains a triazine ring with 3 nitrogen atoms present in the ring. N N N NH2 N H2 NH2 Melamine [5]

5 © DHS 2012 9647/03 [Turn over (e) 2,4,6–trinitrobenzene can also undergo Friedel–Crafts alkylation with CH 3Cl to form 2,4,6–trinitromethylbezene. The alkyl side chain can then be further reacted to form useful halogen derivatives. However, some side products like hydrogen chloride gas and compound D can also be formed in the process. NO2 CH2CH2O2