NYJC H2 Chem Prelim 08 P3

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME CHEMISTRY 9746/03 Paper 3 Free Response 10 September 2008 2 hours Candidates answer Section A on the Question Paper Additional Materials: Answer Paper Data Booklet Graph Paper READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 2 3 4 5 Section B Answer all questions on separate answer paper. Total This document consists of 10 printed pages and 2 blank pages. [Turn over

2 BLANK PAGE H2 Chemistry 9746/03 NYJC J2/08 PX [Turn over

3 Answer any four questions. 1 Some organic and inorganic compounds are classified under a category called ‘Non-Existent Compounds’. They are called non-existent because so far chemists had been unable to synthesise them. Some of the reasons why these compounds are unstable are: • unfavourable bond energy terms • a redox incompatibility of the ions making up the compound • inability of the element concerned to pr ovided orbitals of suitable energy for effective bond-forming overlap with neighbouring atoms • reaction occurring between the ions For the following cases, suggest an explanation for each observation. You may use data from the Data Booklet to assist you in your answe rs. Give relevant equations to support your answers. (a) Both phosphorus and nitrogen are elements in Group V of the Periodic Table. PCl5 can be formed but not NCl5. [ 3 ] (b) It is possible to find MnCl2 in the laboratory but impossible to get MnCl3. [3] (c) When aqueous sodium carbonate is added to aqueous aluminium chloride, a colourless gas is liberated and the precipit ate formed is not aluminium carbonate, Al2(CO3)3. [ 5 ] (d) Caprolactum, a monomer of t he polymer nylon 6, exists as an alicyclic ring structure and not as 6-aminohexanoic acid, a straight chain aliphatic st ructure as most monomers do. O H C N H 2NCH2CH2CH2CH2CH2CO2H Caprolactum 6-aminohexanoic acid [4] (e) Hydration of ethyne, H−C C−H, should produce ethenol, H−C=C−H H OH but, instead, it produces ethanal, CH 3CHO. [5] [Total: 20] H2 Chemistry 9746/03 NYJC J2/08 PX [Turn over

4 2 A current research in alternative source s of renewable energy is in the area of Microbial Fuel Cell (MFC). MFCs convert chem ical energy directly into electricity using bacteria as catalyst. H2 Chemistry 9746/03 NYJC J2/08 PX [Turn over An MFC consist of an anode compartment and a cathode compartment separated by a cation exchange membrane which allows only cations to pass through. The bacteria live in the anode compartmen t and feed on organic substrates, such as ethanoate ion, CH 3CO2 − that is continuously introduced. The bacteria will convert the substrate into hydrogen carbonate ions, HCO 3 −, protons and electrons. The HCO3 − ions and protons will pass into the anode electrolyte while the electrons will be transferred to the anode. The flow of electrons through the external circuit can be harnessed to do electrical work. At the cathode, oxygen in the cathode electrolyte is reduced to form water. (a) (i) Write ionic equations for the reactions occurring at the anode and cathode. (ii) Suggest, with reasoning, the function of the cation exchange membrane. [ 3 ] (b) Under standard conditions, the reduction potential for the HCO 3 −/CH3CO2 − system is +0.19 V when measured against the standard hydrogen electrode. (i) Draw a labeled diagram to show how the standard electrode potential of the HCO3 −/CH3CO2 − system could be measured. (ii) Calculate the standard cell potential that can be obtained in the MFC. [4] (c) A current of 0.50 A is pr oduced when the MFC is in oper ation. Calculate the mass of CH 3CO2 − processed by the MFC in one hour. [2] CH 3CO2 − Anode Compartment Cathode Compartment Bacterium Cation Exchange Membrane Pt anode e− flow Pt cathode O2 + H2O

5 (d) In actual MFCs, the anode electrolyte c ontains a mixture of sodium dihydrogen phosphate, NaH2PO4 and sodium hy drogenphosphate, Na2HPO4 to help maintain the pH at 7. This solution can be obtai ned by dissolving phosphorus (V) oxide, P4O10 in water followed by careful addition of aqueous sodium hydroxide. In an experimental setup, solid P 4O10 was dissolved in 50 cm 3 of distilled water and added to 50 cm 3 of 0.224 mol dm −3 aqueous sodium hydroxide. The resulting solution was used as the anode electrolyte. (i) Write equations for the above reactions. (ii) Explain with the help of equations the roles of NaH 2PO4 and Na2HPO4. (iii) The acid dissociation constant, K a of H 2PO4 − is 6.2 x 10 −8 mol dm −3. Determine the ratio of the amount of HPO 4 2− to H2PO4 − in 100 cm 3 of anode electrolyte. (iv) Using your answer in (d)(iii) , calculate the mass of P 4O10 dissolved in the 50 cm3 of distilled water. [ 7 ] (e) Chemical mediators are used in MFCs to help in the transfer of electrons. One such mediator is the compound neutral red. N N N CH3 H3C NH2 CH3 Neutral Red In order to understand the relative basicity of the various nitrogen groups on neutral red, a comparison is done with similar nitrogeneous bases. The pK b values of these relevant bases are given below. Methylamine Trimethylamine Pyridine Phenylamine 3.34 4.19 8.75 9.37 NH2 CH3 N CH3 CH3 N NCH3 H H (i) Explain the relative basicity of • methylamine and trimethylamine, • pyridine and phenylamine (ii) Rank the four nitrogen groups on neutral red in order of increasing basicity. [ 4 ] [Total: 20] H2 Chemistry 9746/03 NYJC J2/08 PX [Turn over

6 3 Arenediazonium ions have the general formula ArN 2 +, where Ar represents an aryl group. The benzenediazonium ion is an example of an arenediazonium ion. + NN benzenediazonium ion Aqueous solutions of arenediazonium sa lts are stable at low temperatures. (a) Warming an aqueous solution of a benzenediazonium salt produces phenol and nitrogen gas. C6H5N2 +(aq) + H2O(l) → C6H5OH(aq) + N2(g) + H+(aq) The reaction is first order with respect to benzenediazonium ion. In an experiment to determine the rate constant for the reaction at 48°C, 100 cm 3 of a 0.00800 mol dm−3 solution of benzenediazonium chloride was warmed to 48 °C. The volume of nitrogen gas evolved was measured at 48 °C and 101 kPa over a 40 min time interval. The following results were obtained. Time / min Volume of N2 / cm3 0 0.0 3 3.5 6 6.3 10 9.5 15 12.5 22 15.4 30 17.6 40 19.2 (i) Calculate the maximum volume of N 2 (at 48 °C and 101 kPa) that can be obtained if the reaction goes to completion. (ii) What do you understand by the terms order of reaction and rate constant? (iii) Confirm that the reaction is first order with respect to benz