CJC H2 CHEM P3 SOL Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Wednesday 27 August 2014 2 hours Additional Materials: Data Booklet Answer 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. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] 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. Write your answers on the answer paper provided. You are reminded of the need for good English and clear presentation in your an swers. 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. [Turn over Answers

2 9647/03/CJC JC2 Preliminary Exam 2014 Answer any four questions. 1 (a) When an aqueous solution of Co 2+ is reacted with hydrochloric acid and ethane -1,2- diamine (en) under oxidising conditions, a green mixed -ligand cobalt( III) complex, [Co(en)2Cl2]+Cl-, with the following structure is formed: , where L = en (i) State the coordination number of the above complex. 6 (ii) Explain why the above complex is green in colour. Co3+: [Ar] 3d6 In the presence of ligands, the incompletely filled (or partially filled) 3d - orbitals become non -degenerate or split into 2 different energy levels. Hence d-d* electronic transition takes place as radiation in the red region of the visible spectrum is absorbed. The light energy not absorbed is thus seen as the colour of the complex, which is green. (iii) When 0.03 mol of aqueous silver nitrate is added to 0.01 mol of the above aqueous complex, a white precipitate weighing 1.44 g is formed. Identify an d calculate the amount of precipitate formed. With reference to the structure of the above complex, explain the above observations fully. The white precipitate is AgCl. No. of mol of AgCl = = 0.0100 mol AgCl ≡ [Co(en)2Cl2]+Cl- Although [Co(en)2Cl2]+Cl- has three chloride ions, only one of them is free to be precipitated out by Ag+, as the other two are covalently bonded to Co3+ . (iv) The formula [Co(en)2Cl2]+Cl- may refer to a structure that is different from what is shown above. Draw this other structure of [Co(en)2Cl2]+Cl- and state its relationship with the above structure.

3 9647/03/CJC JC2 Preliminary Exam 2014 , where L = en [9] (b) Cobalt is also found in nitrile hydratase, a bacterial enzyme which catalyses the conversion of nitriles to their corresponding amides: Nitrile hydratase has a quaternary structure with four subunits. Each pair of subunits binds a cobalt atom, which serves as a catalytic centre for the above reaction. (i) Identify the type of reaction performed by nitrile hydratase, and propose a three - step synthesis pathway to perform the same conversion, stating clearly all intermediates, reagents and conditions. Hydration/Addition of water/nucleophilic addition (ii) One way to obtain nitriles is to synthesize them from chloroalkanes. State the reagents and conditions required to carry out this synthesis. KCN in ethanol, heat under reflux

4 9647/03/CJC JC2 Preliminary Exam 2014 (iii) Haemoglobin, like nitrile hydratase, has a quaternary structure. With reference to the haemoglobin (Hb) molecule, describe and explain what is meant by the terms secondary, tertiary and quaternary structures of proteins. You should state the type of bonding or interaction involved in each case. Secondary structure: -helix In the -helix, the polypeptide chain coils in a helical manner , with hydrogen-bonding between the hydrogen of N-H of one peptide bond with the oxygen of C=O of another peptide bond holding the helix together. Tertiary structure: The polypeptide chain bends and folds to give its overall 3-dimensional shape due to the R -group interactions between the R groups of the different amino acids in the polypeptide. These R -group interactions are: hydrogen bonding, ionic bonds, van der Waals forces, disulfide linkages . Quaternary structure: There are four subunits of haemoglobin, each with its own haem group. Two are  subunits and the other two are β subunits . The subunits are brought together by van der Waals forces or similar R group interactions as tertiary structure of the subunits. [11] [Total: 20]

5 9647/03/CJC JC2 Preliminary Exam 2014 2 Most commercial chlorine is produced by the electrolysis of a concentrated aqueous sodium chloride solution in a chloralkali cell. An example of a chloralkali cell is the membrane cell as shown below. Diagram of a membrane cell (a) (i) Write the ion-electron half-equations, with state symbols, for the reactions taking place at the anode and cathode. Anode (oxidation): 2 Cl-(aq)  Cl2(g) + 2 e- Cathode (reduction): 2 H2O(l) + 2 e-  H2(g) + 2 OH-(aq) (ii) Hence, deduce the overall reaction for the electrolysis of concentrated aqueous sodium chloride solution. 2 NaCl + 2 H2O  Cl2 + H2 + 2 NaOH OR 2 Cl- + 2 H2O  Cl2 + H2 + 2 OH- (iii) In the membrane cell, current was passed through the concentrated aqueous sodium chloride for 20 minutes. At room temperature and pressure, 450 dm 3 of chlorine gas was liberated at the anode. Calculate the current used in the electrolysis. Anode (oxidation): 2 Cl-(aq)  Cl2(g) + 2 e-

6 9647/03/CJC JC2 Preliminary Exam 2014 No. of moles of electrons involved = 24 450 x 2 = 37.5 mol Quantity of charge = 37.5 x 96500 = 3.62 x 106 C Current used = t Q = 6020 1062.3 6 x x = 3.02 x 103 A (iv) The anode and cathode compartments are separated by a polymeric ion-exchange membrane. The membrane can exchange cations, and only permits Na + ions to migrate from the anode to the cathode compartment. With the aid of a balanced equation , state the reaction that occurs when the membrane is removed and the products are allowed to mix at the high temperature of the cell. [7] When the membrane is removed, t he chlorine produced can undergo disproportionation with the OH- ions. Equation: 3 Cl2 + 6 NaOH  5 NaCl + NaClO3 + 3 H2O OR 3 Cl2 + 6 OH-  5 Cl- + ClO3 - + 3 H2O In the laboratory, chlorine can also be prepared by reacting concentrated hydrochloric acid with manganese(IV) oxide. The half-equation involving manganese(IV) oxide is shown below: MnO2(s) + 4 H+(aq) + 2 e- Mn2+(aq) + 2 H2O(l) Eo = +1.23 V (b) (i) Define the term standard electrode potential, Eo. Standard electrode potential , E o , is defined as the potential difference between a standard hydrogen electrode and a metal (the electrode) which is immersed in a solution containing metal ions at 1 mol dm–3 concentration at 25 °C and 1 atmospheric pressure. (ii) Describe with the aid of a labelled diagram, the set -up used to measure the standard electrode potential of manganese (IV) oxide and the reactions occurring at each electrode.

7 9647/03/CJC JC2 Preliminary Exam 2014 Reactions Anode (oxidation): H2(g) 2 H+(aq) + 2 e- Cathode (reduction): MnO2(s) + 4 H+(aq) + 2 e- Mn2+(aq) + 2 H2O(l) (iii) By selecting an appropriate E θ value from the Data Booklet , explain why the reaction between hydrochloric acid and manganese( IV) oxide would not be expected to occur. Oxidation half-equation: 2 Cl-(aq) Cl2(g) + 2 e- Eθ = -1.36