MI H2 Chem 2013 Prelim P3 QP

Class Adm No Candidate Name: This document consists of 11 printed pages and 1 blank page. [Turn over 2013 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/03 Paper 3 Free Response Friday, 20 Sep 2013 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet Graph Paper READ THESE INSTRUCTIONS FIRST Write your name, class and index number 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. 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. Begin each question on a fresh sheet of paper. 837

2 Answer any four questions. 1. (a) State and explain in terms of electronic structure, two characteristic chemical properties of transition metals or their compounds. [4] (b) Transition element complexes such as those of iron are common reagents in colourful demonstrations of redox reactions. In one particular demonstration, the following observations were made. When aqueous iron( III) sulfate was added to aqueous sodium iodide, the reaction mixture turned brown. The brown colour is discharged upon addition of aqueous sodium hexacyanoferrate( II), Na4Fe(CN)6. On the other hand, when aqueous sodium hexacyanoferrate( III), Na3Fe(CN)6, was added to a fresh sample of the aqueous sodium iodide, the colour of the reaction mixture remained unchanged. (i) Explain the above observations using relevant data from the Data Booklet. (ii) Comment on the relative stability of the different oxidation states of iron in the presence of different ligands. [5] (c) Haemoglobin (Hb) is composed of 4 polypeptide chains: two alpha chains and two beta chains of polypeptides. Each chain contains one haem group, each of which contains one iron ion. The iron is the site of oxygen binding; each iron can bind one O 2 molecule thus each haemoglobin molecule is capable of binding a total of four O2 molecules. The iron–oxygen interaction is very weak; the two can easily be separated without damaging the haem unit or the oxygen molecule. The binding of an oxygen molecule to the iron in a haem unit is therefore completely reversible. (i) Explain how O 2 molecule binds to the Fe of the haem group. 838

[Turn over 3 1. (c) (ii) Each Hb molecule has a complex quaternary structure . Briefly describe what this means. (iii) A haemoglobin molecule in which the iron has separated from the oxygen molecule is called deoxyhaemoglobin. Blood containing red-blood cells filled with O 2 is brighter red. Suggest an explanation for the observation of this colour. (iv) Haemoglobin is susceptible to denaturation. Explain the term denaturation as applied to proteins and state two types of R group interactions that can be disrupted when heat is applied to proteins. [7] (d) Iron(II) ethanedioate, FeC 2O4, reacts with manganate(VII ), MnO 4 –, forming Fe 3+ and carbon dioxide. In an experiment, 0.144g of FeC 2O4 reacted with 20.0 cm3 of 0.150 mol dm-3 of MnO4 –. (i) Write an ion-electron equation for the oxidation of FeC 2O4 to Fe 3+ and carbon dioxide. (ii) Determine the oxidation number of manganese in the product formed after the reduction of manganate(VII). [4] [Total: 20] 839

2. ( a a) Malic a some o below: (i) (ii) (iii) (iv) (v) acid is an or other fruits D-M State a cha With the a water. Malic acid is an isome H CO H co State the ty Compound manganite( The structu Suggest ho in two step well as the rganic dicar . Malic aci alic acid aracteristic d aid of a di a can be de h er of compo CC H C O H C O ompound K ype of isom d L in ( (VII). Draw ure of 3-oxo ow you ca n ps, showing structure o 4 rboxylic acid d has two difference b agram, exp l hydrated to ound K. OH K erism exhib (a)(iii) de the structu opropanoic a C CH O H 3-oxoprop n make ma l g the reag e of the interm d found in w stereoisom L-M between D- lain why m o give com p bited. colourises re of the org acid is show H2C O OH panoic acid lic acid fro m ents and c o mediate. wines, sour meric form s Malic acid and L-Mali malic acid i s pound K. C C C O O H CO O compou acidified ganic produ wn below. m 3-oxopro onditions ne apples and s as show n c acid. s soluble i n ompound L C CH2 OH nd L potassium uct. panoic acid ecessary as [8 d n n L m d s ] 840

[Turn over 5 2. (b) A student carried out an experiment to investigate the rate of reaction between sodium and malic acid. A freshly cut piece of sodium weighing 0.0690 g was added to a large excess of malic acid. The volume of gas evolved was m easured at room temperature and pressure over a 30-min time interval. The following results were obtained. Time /min 0 5 10 15 20 25 30 Volume of gas /cm3 0.0 8.5 15.0 20.5 25.0 27.5 31.0 (i) Write a balanced equation for the reaction between sodium and malic acid. (ii) Calculate the maximum volume of gas evolved at room temperature and pressure if the reaction goes to completion. (iii) What do you understand by the term order of reaction? (iv) Using the graphical method, show that the overall kinetics of the reaction is first order. (v) In this experiment, the kinetics appears to be zero order with respect to malic acid initially. Suggest a reason for this. [7] 841

6 2. (c) 4-aminobutanoic acid, also known as GABA, can also be synthesised from 3-oxopropanoic acid in a few steps. CH2CH2 C O OHCH2N H2 GABA The two pK a values associated with 4-aminobutanoic acid are 4.23 and 10.43. A sample of GABA requires 20 cm 3 of 1.0 mol dm –3 NaOH for complete neutralisation. The following diagram is a titration curve, showing how the pH of GABA changes with volume of NaOH added. (i) Draw the structures of the predominant species of GABA at X, Y and Z. (ii) State the pH and the volume of NaOH added when Y and Z are equimolar. [5] [Total: 20] pH 10 20 Volume of NaOH added /cm3 X Y Z 842

[Turn over 7 3. (a) Calcium is a mineral that is found naturally in foods. Calcium is necessary for many normal functions of the body, especially bone formation and maintenance. Calcium sulphate, CaSO 4, is a salt of calcium. It is used as a coagulant in products like tofu. (i) Draw the dot-and-cross diagram to show the bonding in sulfate ion. State the shape and the bond angle. (ii) Using the following data, construct an energy level diagram to calculate the enthalpy change of solution of CaSO4. Include state symbols in your diagram. enthalpy change of hydration of calcium ion – 1562 kJ mol -1 enthalpy change of hydration of sulfate ion – 1166 kJ mol -1 lattice energy of calcium sulfate – 2710 kJ mol-1 (iii) Hence predict whether CaSO 4 becomes more or less soluble when temperature increases. Explain your answer. [9] (b) Calcium sulfate is a component of the grout used in between the tiles in swimming pools. Calcium chloride is added into swimming pools to prevent calcium sulfate in the grout from dissolving. The solubility product of CaSO4 at 25°C is 2.4 × 10-5 mol2 dm-6. (i) Write an expression for the solubility product, Ksp, of CaSO4. (ii) If 5.555 kg of calcium chlo