MI H2 Chem 2013 Prelim P3 Soln

201 1. 13 MI PU3 (a) Sta che    Any 1m exp (b) Tra rea dem Wh rea add On Na 3 the (i) H2 CHEM te and ex p emical prope Transition number of stability. Transition accessible from ligan d complexes Transition compounds and avail a Transition catalyst be y two for eac h planation nsition el e gents in c o monstration en aqueous ction mixt u dition of aqu the othe 3Fe(CN)6, w colour of th Explain Booklet Fe3+ + [Fe(CN ½ I2 + When F feasible  Eθ cel MISTRY PR plain in te r erties of tra metal ion s 4s and 3d e metal ion s vacant orb ds to form . metals hav s are good ability of d metals a n cause of th characte r ement com olourful dem , the followi s iron(III) s ure turned ueous sodiu r hand, w was added t he reaction n the above t. + e- Fe 2+ N)6]3- + e- e- I- Fe3+ is adde e. l = +0.23 V RELIM II PA rms of el e nsition meta s possess electrons c s form co m bitals to ac c dative bo n ve catalytic heterogen d orbitals nd their c heir ability to ristic with mplexes su c monstrations ing observa sulfate was brown. Th e um hexacya when aqu e to a fresh s mixture rem e observatio + [Fe(CN) ed to I-, the (Eθ cell > 0; r APER 3 S ectronic str u als or their variable o an be remo mplexes; th e commodate nd which r e properties; eous catal y for tempo r compounds o exist in va 1m for e ch as tho s s of redox r ations were added to aq e brown c o anoferrate(II eous sodi u ample of th mained unch ons using re Eθ = 6]4- Eθ = Eθ = e oxidation reaction is fe OLUTIONS ucture, two compounds oxidation s oved to form e ions ha v the lone p esults in t h transition m yst because rary bond are goo d riable oxida each com p se of iron reactions. In made. queous sod olour is di s I), Na4Fe(C um hexac y he aqueous hanged. elevant data = +0.77 V = +0.36 V = +0.54 V of I- to brow easible) S o characte r s. states; vari m ions of sim ve energeti c pair of elec t he formati o metals and e of the v a formation d homoge n ation states plete mat c n are com n one parti c dium iodide scharged u CN)6. yanoferrate sodium iod a from the D wn I2 by Fe 1 ristic able milar cally rons n of their cant OR nous . ching [4] mon cular , the upon (III), dide, Data e3+ is 849

2 When [Fe(CN)6]4- is added to I2, brown I2 is reduced to I-.  Eθ cell = +0.18 V (Eθ cell > 0; reaction is feasible) The oxidation of I- to brown I2 by [Fe(CN)6]3- is not feasible.  Eθ cell = -0.18 V (Eθ cell < 0; reaction is not feasible) 1m for quoting all 3 correct Eθ from Data Booklet 1m for each calculation of Eθ cell to explain observation (ii) Comment on the relative stability of the different oxidation states of iron in the presence of different ligands. Since the E θ (Fe 3+/Fe2+) is more positive compared to the E θ ([Fe(CN)6]3-/[Fe(CN)6]2-), in the presence of the CN - ligand, Fe 3+ is stabilised with respect to Fe2+. 1m for complete answer [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 O2 molecule binds to the Fe of the haem group. Oxygen contains lone pairs which can coordinate to the centre Fe ion through coordinate (or dative) bonding. 1m for explanation (ii) Each Hb molecule has a complex quaternary structure . Briefly describe what this means.  The quaternary structure refers to the three dimensional structure of proteins consisting of more than one polypeptide chain coming together to form the complete protein.  The structure is stabilized by R group interactions such as hydrogen bonding, disulphide bridges, van der Waals’ forces and ionic linkages. 1m for each point 850

3 (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. When O2 binds to Fe ion centre, the light-absorbing property of the heme group is changed due to the change in structure. Oxyhaemoglobin absorbs light in the blue-green range, and reflect red light. 1m for correct answer (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. Denaturation is a process where the original conformations of secondary, tertiary and quaternary structures are destroyed through the breaking of non-covalent interactions and disulfide linkages. ; Van der Waals’ or hydrophobic interactions ; hydrogen bonds ; [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. FeC2O4  Fe3+ + 2CO2 + 3e ; (ii) Determine the oxidation number of manganese in the product formed after the reduction of manganate(VII). amount of FeC2O4 = 0.144/(55.8 + 24.0 + 64.0) = 0.001001 mol amount of e = 3 × 0.001001 = 0.003003 mol ; amount of MnO 4 - = 0.020 × 0.150 = 0.00300 mol 1 mol of MnO4 - takes in 1 mol of e ; oxidation no. of Mn in product = +7 – 1 = +6 ; [4] [Total: 20] 2. (a) Malic acid is an organic dicarboxylic acid found in wines, sour apples and some other fruits. Malic acid has two stereoisomeric forms as shown 851

belo (i) (ii) (iii) (iv) (v) ow: D State a The en D. Do not With t h water. 1m fo r betwee shown. 1m for s Malic a is an is State th Structu Compo mangan CO(CO The str Sugges in two well as D-Malic acid characteris antiomers r accept ‘The he aid of a r diagram s en malic ac i stating hydr cid can be d omer of com C H C O O H comp he type of is ral isomeris ound L in nite(VII). D OOH)2 ; ucture of 3- st how you steps, show the structu d stic differen rotate plane ey are mirro diagram, e showing a t id and wat e rogen bond dehydrated mpound K. C H C O OH pound K somerism e sm ; n (a)(iii) raw the stru -oxopropan C O H 3-oxop can make wing the r e re of the int ce between e-polarized l or image of explain wh y t least on e er, with pa r d d to give com H xhibited. decolouris ucture of the oic acid is s CH2C O O propanoic a malic acid eagents and termediate. L-Malic aci n D- and L-M light in oppo each other. y malic ac i e hydroge n rtial charges mpound K. C C O O H CO O compou ses acidifi e organic pr shown below OH cid from 3-oxo d conditions id Malic acid. osite directi .’ id is solub l n bond fo r s and lone Compound C CH2 OH nd L ied potas s roduct. w. opropanoic s necessar 4 on? le in med pair L sium acid y as 852

5 C CH2C O H O OH HCN, trace NaCN or NaOH 10 - 20oC C CH2CH O OH OH C N C CH2CH O OH OH C OH O dil H2SO4 heat 1m for each set of reagents and conditions 1m for intermediate [8] (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