MI H2 CHEM P3 MS Prelim

Class Adm No Candidate Name: This question paper consists of 12 printed pages. [Turn over 2011 Preliminary Examination II Pre-university 3 H2 CHEMISTRY (Marking Scheme) 9647/03 Paper 3 Free Response 14 September 2011 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet 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. You may use a soft pencil for any diagrams, graphs or rough workings. 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, arrange your answers in numerical order and fasten all your work securely together.

- 2 - 1(a) Ethanol is a renewable energy source becau se the energy is generated by using a resource, sunlight, which cannot be depleted. Creation of ethanol starts with photosynthesis causing a feedstock, such as sugar cane or corn, to grow. These feedstocks are processed into ethanol. Ethanol can be used in fuel cells, in t he presence of aqueous potassium hydroxide electrolyte to produce electricity. During the reaction, ethanol is oxidised to ethanal. (i) Write ionic equations to illustrate the reactions that are happening at the cathode and anode respectively. [2] Anode: CH 3CH2OH + 2OH-  CH3CHO + 2 H2O + 2e ; Cathode: O 2+ 2H2O + 4e  4OH- ; (ii) Write an equation to represent the overall equation [1] 2CH 3CH2OH + O2  2 CH3CHO + 2 H2O ; (iii) An ethanol fuel cell is said to provide an e.m.f of 1.43V. Determine the reduction potential of the ethanol half cell. [1] [O]: CH 3CH2OH + 2OH-  CH3CHO + 2 H2O + 2e E =a [R]: O2+ 2H2O + 4e  4OH- E = 0.40V a+ 0.40 = 1.43V a = 1.03 V Reduction potential = -1.03V ; (Reject if the answer is +1.03V) (iv) Assume that the current that is drawn by the ethanol fuel cell is 0.2 amperes per 10 hours, what is the mass of ethanol consumed by the ethanol fuel cell in a day? [2] Q = It = 0.2 x 10 x 60 x 60 = 7200 C Amt of electrons consumed in 10 hours = 96500 7200 = 0.07461 mol. Amt of ethanol consumed in 10 hours = 0.07461 x 0.5 = 0.03731 mol. ; Mass of ethanol consumed in 10 hours = 0.03731 x (2 x 12.0 + 6 x 1.0 + 16.0) = 1.72g Mass of ethanol consumed in a day = 1.72 x 10 24 = 4.12g (3sf) ; (v) Suggest reagents and conditions to obtain ethanal from ethanol. [1] Acidified K 2Cr2O7, distill. (reject KMnO4/ H+) 1(b) An organic compound P with molecular mass 180.0 contains the elements carbon, hydrogen and oxygen only. It was found to contain 66.7% carbon and 6.67% hydrogen by mass. P reacts with neutral FeC l 3 to give a violet colouration. When P was boiled under refluxed with aqueous NaOH, 2 compounds Q, C 8H9O2 -Na+ and CH 3CO2 -Na+ were formed. Q can rotate plane polarized light. Q reacts with acidified potassium dichromate to give R, C 8H8O2. R gives S and a yellow precipitate with aqueous alkaline iodine. R decolourises aqueous bromine to give a white precipitate of T, C 8H5O2Br3. Suggest the structures of P, Q, R, S and T. Explain the chemistry of the reactions [10]

involved. P: 66.7% carbon and 6.67% hydrogen by mass. M.F of P = C 10H12O3 P reacts with neutral FeC l3 to give a violet colouration. Phenol is present in P. P was boiled under refluxed with aqueous NaOH, 2 compounds Q, C8H9O2 -Na+ and CH 3CO2 -Na+ were formed. P is an ester. Or Hydrolysis in an alkaline medium Q can rotate plane polarized light. Chiral carbon is present in Q. Q reacts with acidified potassium dichromate to give R, C8H8O2. Oxidation reaction. No change in number of oxygen. Q is a secondary alcohol. R is a ketone. R gives S and yellow precipitate with aqueous alkaline iodine. R has the CH3 R O structure. R decolourises aqueous bromine to give a white precipitate of T, C8H5O2Br3. The side-chain is not at the 2,4,6 position as phenol is tri-substituted. Deductions: Max 5. P: C CH3 OC O CH3 OH H Q: C CH3 OH O - Na + H R: C CH3 O OH S: C O - Na + O O - Na + T: C CH3 O OH BrBr Br Structures: 1 mark per correct structure. 1(c) Explain the relative ease of hydrolysis of ethanoyl chloride, chlorobenzene and chloroethane. [3] Most readily hydrolysed: Ethanoyl chloride, chloroethane, chlorobenzene. ; Ethanoyl chloride hydrolyses the most readily in water because one of the carbon atoms is bonded to 2 highly electronegative elements, O and C l. Hence, the carbon atom is highly electron deficient, making it highly susceptible to nucleophilic attack.;

- 4 - The p-orbital of C l in chlorobenzene is able to overlap with the p-orbitals of benzene ring, hence, forming a partial pi bond which makes the C-C l bond resistant to hydrolysis.; [Total: 20 marks] 2(a) (i) An environmental chemist investigated the bromide content of river water. He decided to convert the bromide ions into bromine using acidified KMnO4. Suggest, with the aid of equations, whether using HC l or H2SO4 would be more suitable for his investigation. [3] The use of H 2SO4 will be more suitable. ; (no marks for merely quoting the correct answer) 2Cl-  Cl2 + 2e E = -1.36V MnO4 - + 8H+ + 5e  Mn2+ + 4H2O E=+1.52V Overall E = +1.52 – 1.36 = 0.16V > 0V ; If HCl was used to acidify the solution, KMnO 4 will be used to oxidise both C l- and Br- ; hence rendering the volume of KMnO 4 required to be more than actual. ; (bonus) Max 3 2(b) Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution or liquefied. In electrowinning, a current is passed from an inert anode through a liquid leach solution containing the metal so that the metal is extracted as it is deposited in an electroplating process onto the cathode. The most common electrowon metals are lead , copper, gold, silver, zinc, aluminium, chromium, cobalt, manganese, and the rare-earth and alkali metals. For aluminium, this is the only production process employed. Several industrially important active metals like sodium and potassium are produced commercially by electrolysis of their pyrochemical molten salts. (i) Suggest 1 similarity and 1 difference between electrowinning and electropurification of copper. [2] Similarity: Metals are deposited at the cathode.; Difference: For electroplating, the metal to be plated is placed at the anode (metal participates in the reaction.) In electrowinning, the anode is inert.; (ii) Draw a setup to illustrate how sodi um metal can be extracted from its molten salts, stating:  the name of the salt used,  the electrode used,  the equations at the respective electrodes. [3] Electrode: Graphite, Molten salt: Molten sodium choride. ; Setup; Equations at the respective electrodes.; (iii) Suggest why the reduction potential of reactive metals like sodium and potassium cannot be measured directly when they are connected to the standard hydrogen electrode. [1] Sodium and potassium electrodes will react vigorously with the electrolyte to form a hydroxide solution.;

2(c) (i) State two assumptions of the kinetic theory of ideal gas. [2]  There is insignificant intermolecular force of attraction between the particles. ;  The volume of the gas particles is insignificant when compared to the volume of the container.;  There is elastic collision between the particles.; (Any 2) (ii) Would you expect carbo