1. (a) Answer the following short questions Nowadays, products manufactured from industrial process emphasize on a green approach. An organic compound is manufactured by two different approaches as shown below. Approach I: + 2CrO 3(s) + 6H + (aq) + 2Cr 3+ (aq) + 6H 2O(l) Approach II: + O 2(g) + 2H 2O(l) Suggest TWO aspects to show approach II is considered to be a greener method than using approach I. High atom economy No toxic heavy metal ion such as Cr 3+ is produced () () (ii) An experiment was carried out to study the progress of the reaction between iodine and propanone at 298 K as shown below. I 2(aq) + CH 3COCH 3(aq) CH 3COCH 2I(aq) + HI(aq) Describe with the use of titrimetric analysis and suitable graph plots to confirm the order of reaction with respect to I 2 is 1. (You need to state the expected findings from the graph.) Pipette a fixed volume of reaction mixture to the conical flask at regular time interval. Quench the solution and titrate against standard solution Na 2S 2O 3 with the use of starch as indicator Plot the graph of conc I 2 against time, determine the initial rate of I 2. Repeat the experiment by varying the concentration of I 2, while other parameters remain unchanged Plot the graph of initial rate against different [I 2], a plot of straight line is resulted, that indicates I 2 is the first order. (iii) Write a chemical equation for the production of syngas for manufacturing methanol and state the necessary conditions for the production. CH 4(g) + H 2O(g) 3H 2(g) + CO(g) () Conditions: Ni/NiO and 700-1000 o C and 10-20 atm ()
(b) Chlorine gas, hydrogen gas and sodium hydroxide solution can be produced from chloroalkali industry. Describe the working principle of a flowing mercury cell to produce chlorine gas, hydrogen gas and sodium hydroxide solution. (You are NOT required to write chemical equations.) The chloride ion will be preferentially discharged at anode of which undergoes oxidation to form chlorine gas The sodium ion is reduced and reacts with mercury to form sodium amalgam. The sodium amalgam reacts with water to form sodium hydroxide solution and hydrogen gas (ii) State ONE disadvantage of using flowing mercury cell over the membrane cell in chloroalkali industry. Toxic mercury is used. () (c) The single-step of the reaction shown below with an activation energy of + 27.1 kj mol 1. Cr 2O 7 2 - (aq) + 14H + (aq) + 3Sn 2+ (aq) 2Cr 3+ (aq) + 7H 2O(l) + 3Sn 4+ (aq) ΔH < 0 It is found that the order of reaction with respect to Cr 2O 7 2 - (aq) and Sn 2+ (aq) is 1 respectively, while the order of reaction with respect to H + (aq) is 0. At 64 o C, the rate constant of the reaction is 2.5 10 5 dm 3 mol 1 s 1. Write a rate equation for the above reaction based on the given information. Rate = k [Cr 2O 7 2 - ] 1 [Sn 2+ ] 1 () (ii) With the help of Arrhenius equation, calculate the rate constant at 30 o C. Given that gas constant R is 8.31 JK 1. ln (k 2 /2.5 10 5 ) = - 27.1 x 1000 / 8.31 ((1/243) (1/209)) k 2 = 0.282 x 10 5 dm 3 mol 1 s 1 () () (iii) In the presence of Fe 3+ (aq), the reaction can be catalysed with the following proposed steps: 2Fe 3+ (aq) + Sn 2+ (aq) 2Fe 2+ (aq) + Sn 4+ (aq) Cr 2O 2-7 (aq) + 14H + (aq) + 6Fe 2+ (aq) 2Cr 3+ (aq) + 7H 2O(l) + 6Fe 3+ (aq) (fast reaction) (slow reaction) (I) Suggested a reason to support Fe 2+ (aq) for the above conversion is a catalyst. Fe 2+ can be regenerated after the reaction that supports Fe 2+ can be repeated use. ()
(II) On the same diagram, sketch labeled energy profiles for the reaction in the absence and in the presence of Fe 3+ (aq). (You are not required to draw the energy profile to scale.) (4 marks) (iv) With reference to part of the electrochemical series given below, explain briefly whether the reaction can be catalysed by SO 4 2 - (aq) with the use of chemical equations. Sn 4+ (aq) + 2e - Sn 2+ (aq) 2 SO - 4 (aq) + 2H + (aq) + 2e - 2 SO - 3 + H 2O(l) Cr 2O 2-7 (aq) + 14H + (aq) + 6e - 2Cr 3+ (aq) + 7H 2O(l) SO 4 2 - (aq) + Sn 2+ (aq) + 2H + (aq) SO 3 2 - (aq) (aq) + Sn 4+ (aq) + H 2O(l) The strength of oxidizing agent of SO 4 2 - > Sn 4+ in electrochemical series, the reaction is feasible. Cr 2O 2-7 (aq) + 8H + 2 (aq) + 3SO - 3 (aq) 2Cr 3+ (aq) + 4H 2O(l) 2 + 3SO - 4 (aq) The strength of oxidizing agent of Cr 2O 2-7 2 > SO - 4 in electrochemical series, the reaction is feasible. So, the reaction can be catalysed by SO 4 2 -.
2. (a) Answer the following short questions : Cellulose and chitin are natural polymers and their structures are shown below : Cellulose Chitin With reference to the structures of cellulose and chitin, account for their difference in water absorbance. There are more hydroxyl groups for interacting with water molecules in cellulose than in chitin, [] Cellulose has a higher water absorbance than chitin./cellulose can absorb water but chitin cannot. [] (ii) Alloys can be classified into two types. The diagram below shows one of the types of alloys. (1) Name this type of alloys. Substitutional alloy [] (2) Account for the difference in electrical conductivity between the above alloy and its constituent metals. The alloy has a lower electrical conductivity than its constituent metals. [] Because the irregular structure of alloy hinders the movement of delocalized electrons. [] (iii) Compare the molecules in nematic phase and those in cholesteric phase of liquid crystals. Molecules in cholesteric phase are chiral but molecules in nematic phase may not be chiral. [] Also accept: Molecules in cholesteric phase are chiral but molecules in nematic phase are not chiral.
(b) The diagram below shows a plastic drinking bottle made from a polycarbonate (PC) : It is known that PC can be produced from the polymerisation between bisphenol-a and phosgene. Hydrogen chloride is the unique by-product of the polymerisation. The structures of bisphenol-a and phosgene are shown below : Bisphenol-A Phosgene Write a chemical equation for the above polymerization. [] (ii) Name the type of polymerisation involved in forming PC. Condensation polymer [] (iii) Polycarbonates are a group of thermoplastics. State the meaning of the term thermoplastic. A thermoplastic is a plastic which can be repeatedly softened by heating and hardened again on cooling. [] (iv) Poly(methyl methacrylate) (PMMA) is an alternative to PC. The structure of poly(methyl methacrylate) is shown below: (1) Give the systematic name of the monomer of poly(methyl methacrylate). Methyl 2-methylpropenoate [] (2) With reference to the polymerisation, give TWO reasons why it is greener to produce PMMA than PC. The atom economy of producing PMMA is higher than that of PC. [] No hazardous chemical produced from manufacture of PMMA while HCl is produced from that of PC. []
(b) (v) Aeroplane windows are made of PMMA instead of glass. Suggest TWO properties of PMMA rendering it suitable for replacing glass for making aeroplane windows. Any 2 out of 3 [1+1] Transparent Stiff / Rigid / Tough / Strong / High impact strength / Hard Lower density than glass (c) Magnesium, calcium and barium are Group II elements but they have different crystal structures in solid state. Calcium exists in face-centred cubic structure while barium exists in body-centred cubic structure. Suggest the type of crystal structure in which solid magnesium exists. Hexagonal close-packed structure [] (ii) Draw a unit cell of magnesium. [] (iii) Deduce the number of magnesium atoms in the unit cell. 12 x 1/6 + 2 x 1/2 + 3 [] = 6 [] (iv) What is the coordination number of the magnesium atom? 12 [] (v) Magnalium is an aluminium-magnesium alloy. A student said that magnalium is a metal compound. Suggest why magnalium should NOT be considered as a compound. Elements in magnalium are not held together in a definite / fixed ratio. [] END OF SECTION B
3. (a ) Compound A contains 10.9 % hydrogen, 65.6 % carbon, 10.9 % nitrogen and 12.6 % oxygen. Upon hydrolysis of compound A with dilute hydrochloric acid, only compound B (C 2H 5NH 3 + Cl - ) and an organic acid C (HOOCCH 2(CH 2) 6CH 2COOH) are formed. State all functional group(s) presence in compound A. The structure of compound A should contains amide linkage. (ii) Calculate the mole ratio of carbon atom to nitrogen atom in compound A. (The relative atomic masses: C = 12.0 and N = 14.0) The mole ration between carbon to nitrogen = (65.6 / 12) : (10.9 / 14) = 7 : 1 () (iii) Draw the structure of compound A. C 2H 5NHOCCH 2(CH 2) 6CH 2CONH C 2H 5 (iv) Outline an experimental procedure, based on a solvent extraction process, to isolate pure compound C from the reaction mixture in good yield. Add ether and dilute NaHCO 3(aq) solution to the reaction mixture. Shake in a separating funnel and dilute HCl(aq) should be added to the aqueous layer to regenerate HOOCCH 2(CH 2) 6CH 2COOH Extract HOOCCH 2(CH 2) 6CH 2COOH with ether and shaking the mixture in a separating funnel, Collect the ether layer, add anhydrous sodium sulphate to the ether layer until the solution becomes clear, filter the mixture, and distill ether in the filtrate to obtain HOOCCH 2(CH 2) 6CH 2COOH (b) Chlorine bleach contains hypochlorite ion (OCl - ) as an active ingredient. The amount of hypochlorite ions in a sample was determined by titrimetric analysis. Step 1: Step 2: Excess dilute sulphuric acid was added to the diluted chlorine bleach solution. The resulting solution was then mixed with excess postassium iodide solution.
Step 3: Step 4: The reaction mixture was then pipetted into a conical flask and titrated again standard sodium thiosulphate solution. Starch was used as an indicator in the titration for the end point detection. Write the chemical equation for the reaction involved in Step 1. OCl - + Cl - + 2H + Cl 2 + H 2O (ii) By using a suitable instrument, briefly describe how to ensure excess dilute sulphuric acid and potassium iodide is added to the diluted sample in Step 2. Using colorimeter the absorbance data corresponding to the colour intensity of I 2 remained constant that indicate the acid and iodide solution are in excess. (iii) Explain why the above titrimetric analysis has to be performed rapidly. To minimize the air oxidation of iodide ions. 3. (c ) Consider the following silica gel thin layer chromatography (TLC) plate showing the compound A, B and C of three different colours are separated from the mixture D. The solvent used is hexane. Describe the principle how the three compounds A, B and C can be separated from the mixture D using TLC. In TLC, silica gel used as the stationary phase can be considered to be polar while the organic solvent used as the mobilephase is non-polar.
Since the components of the mixture have different in polarity, the components in a mixture have different tendencies to adsorb onto the silica gel or dissolve in the organic solvent. (ii) Which of the above compounds is most non-polar? Explain your answer. Compound A is the most non-polar as compund A travel the furthest from mixture D, it has high solubility in non-polar solvent hexane. / It has the highest R f value. (iii) It is discovered the separation between compound A and compound B are closed to each other. With the use of hexane and ethanol, briefly describe how to obtain compound A from the mixture of D. Using another solvent with different ratio of hexane and ethanol, until a larger difference of the separation of two extract is obtained. Using the techniques of column chromatography, since the components of the mixture have different in polarity, the components in a mixture have different tendencies to adsorb onto the silica gel or dissolve in the organic solvent. The less polar component has a weakest interaction with the silica gel and is therefore not readily adsorbedon the silica gel strongly. Therefore, it will be separated out first due to gravity. (iv) Suggest ONE advantage of paper chromatography over TLC. Low cost () (d ) One of the organic components inside Glycyrrhiza radix called neoflavonoid. The infrared spectrum of neoflavonoid is shown below.
By referring to the Characteristics Infra-red Absorption Wavenumber Ranges (Stretching modes) given in the table below, suggest which of the following structure (W, X or Y) may be the structure of neoflavonoid. Explain your answer. W: X: Y: There is no region fall on the board absorption peak at the wavelength number ranged from 3230 to 3670 cm -1, no alcohol functional group is present. A peak of wavelength number of 1680-1800 cm -1 indicates the presence of ketone or ester group. However, the is a sharp peak of wavelength number 1610-1680cm -1 indicates the presence of C=C So W is the answer. END OF SECTION C