CJC H2 CHEM P3 QP Prelim

CHEM Paper 3 F Additiona READ T Write your Write in d NOT ALLO You may u Do not use Answer an You are r e The num b At the end CATH JC2 PR Higher MISTRY Free Respo al Material THESE INS r name and c ark blue or OWED] use a soft pe e staples, pa ny four ques eminded of th ber of marks d of the exam Thi HOLIC JU RELIMINAR r 2 Y onse ls: Data Bo Answer STRUCTIO class on all th black pen o n encil for any d aper clips, hig stions. Write he need for g is given in br mination, fast s document 9 UNIOR C RY EXAMIN ooklet r Paper ONS FIRST he work you n both side s diagrams, gr ghlighters, gl your answer good English rackets [ ] at en all your w t consists o 9647/03/CJC COLLEGE NATIONS T hand in. s of the pap e raphs or roug lue or correc rs on the ans h and clear p t the end of e work securely of 12 printed JC2 Prelimina E W er. [PILOT F R gh working. ction fluid. swer paper p presentation i each questio y together. d pages and ary Exam 2014 Wednesda RIXION ERAS rovided. in your answ on or part que d 0 blank pa 4 9 ay 27 Augu SABLE PENS wers. estion. age. [Tu 9647/03 ust 2014 2 hours S ARE urn over

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. (ii) Explain why the above complex is green in colour. (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 and calculate the amount of precipitate formed. With reference to the structure of the above complex, explain the above observations fully. (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. [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. (ii) One way to obtain nitriles is to synthesize them from chloroalkanes. State the reagents and conditions required to carry out this synthesis.

3 [Turn over 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. [11] [Total: 20]

4 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. (ii) Hence, deduce the overall reaction for the electrolysis of concentrated aqueous sodium chloride solution. (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. (iv) The anode and cathode compartments are s eparated 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]

5 [Turn over 9647/03/CJC JC2 Preliminary Exam 2014 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. (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. (iii) By selecting an appropriate E o value from the Data Booklet , explain why the reaction between hydrochloric acid and manganese( IV) oxide would not be expected to occur. (iv) Suggest a possible reason why the reaction does in fact occur. [6] (c) The composition of a mixture of two solid sodium halides was investigated in two separate experiments. Experiment 1 When a large excess of chlorine gas was bubbled through a concentrated solution of the mixture, reddish brown fumes and a black precipitate were produced. Experiment 2 0.545 g of the solid mixture was dissolved in water and a large excess of silver nitrate solution was added. The mass of the mixture of silver halide precipitates formed was 0.902 g. After washing the mixture of precipitates with an excess of concentrated ammonia, the mass of the final precipitate was reduced to 0.564 g. (i) With reference to Experiment 1 , identify the two halides present in the mixture and explain why the reactions occur. (ii) Use of the Data Booklet is required for this question. The atomisation of the reddish brown substance produced in Experiment 1 involves the vaporisation of the liquid, followed by the breaking of covalent bonds. By constructing an energy cycle, calculate the standard enthalpy change of vaporisation of the reddish brown liquid. [Given: ∆H atm o (reddish brown liquid) = +112 kJ mol-1]

6 9647/03/CJC JC2 Preliminary Exam 2014 (iii) Using data given in Experiment 2, calculate the mass of each halide ion present in the mixture. [7] [Total: 20]

7 [Turn over 9647/03/CJC JC2 Preliminary Exam 2014 3 (a) Use of the Data Booklet is relevant to this question. Lead( II) carbonate and zinc carbonate decompose on heating in the same way as calcium carbonate. (i) Write an ionic equation for the thermal decomposition of the carbonate anions. (ii) By listing the ionic radii of Zn 2+, Ca 2+ and Pb 2+, suggest and explain the order of decomposition temperature of these three carbonates. (iii) The graph below shows the decomposition of CaCO 3. Copy the graph and on the same axes, sketch the graph when the same number of moles of ZnCO 3 was heated under the same conditions. td is the time taken for the complete decomposition of CaCO3. [6] (b) Methanoic acid occurs naturally in ants. Ants secrete methanoic acid for attack and defense purposes. To treat ant stings, bicarbonate of soda (sodium hydrogencarbonate) is often used. (i) Write the equation for the reaction between sodium hydrogencarbonate and methanoic acid. mass of solid / g 0.5 0.25 time CaCO3 td

8 9647/03/CJC JC2 Preliminary Exam 2014 When an ant bites, it injects a solution containing 50% by volume of methanoic acid. A typ