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