NJC_Prelim_CHEM_P3

NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Mon 14 Sept 2015 2 hour READ THESE INSTRUCTIONS FIRST Answer any four questions. A Data Booklet is provided. The use of an approved scientific calculated is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely behind the cover page. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 10 printed pages.

NJC Preliminary Exam 9647/03/15 [Turn over 2 Answer any four questions. 1 A student analysed a can of beer to verify its alcohol content. The alcohol content is the percentage by volume of pure ethanol in the beer at room temperature. The alcohol content printed on the can is 4.2 %. The analysis is carried out by titrating 10.0 cm 3 of the beer with 0.20 mol dm −3 acidified potassium manganate( VII). It is found that 17.80 cm 3 of potassium manganate( VII) is required to reach the end-point. (a) (i) Write an overall equation for the reaction between potassium manganate( VII) and ethanol. (ii) Using the titration result, calculate the number of moles of potassium manganate(VII) reacted and hence calculate the mass of ethanol in 10.0 cm 3 of the beer sample. (iii) Given that the density of ethanol is 0.789 g cm−3, determine the alcohol content of this beer sample. (iv) Give a reason why the alcohol content of the beer calculated is different from that printed on the can. (v) Explain how you would determine the end-point of this titration. (vi) Ethanol is highly soluble in water. Explain why the solubility of alcohols in water decreases with increasing carbon chain lengths. [9] (b) In the brewing of beer, yeast is added to break down the starch from malt or barley into glucose, C 6H12O6, which is further converted to ethanol and carbon dioxide. The structure of glucose is as shown: (i) Write a balanced equation for the conversion of glucose to ethanol. (ii) Describe a simple chemical test to distinguish between glucose and ethanol. Give an equation for the reaction. [4]

NJC Preliminary Exam 9647/03/15 [Turn over 3 (c) Ethanol can be used to synthesise ether. Due to the low reactivity of ether, it is a suitable solvent for many chemicals reactions. One method to synthesise ether, RCH 2OCH2R’, is through the Williamson ether synthesis as shown in the equation below. RCH2O– + R’CH2Cl → RCH2OCH2R’ + Cl – (i) Describe the mechanism for the above reaction, including curly arrow