SAJC H2 CHEM P3 Prelim

1 ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry Higher 2 9647/03 Paper 3 12 September 2011 2 hours Candidates answer on separate paper. Additional Materials: Writing paper, Data Booklet READ THESE INSTRUCTIONS FIRST: Write your name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. 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. This document consists of 8 printed pages including this page.

2 C O CH NH2 CH3 OH Answer any four questions 1 Deoxyribonucleic acid, DNA, is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints, like a recipe or a code, sin ce it contains the instructions needed to construct other components of cells, such as proteins. Part of human DNA is formed using alanine, glutamine and lysine (which are listed in the following table). Amino Acid Structure Isoelectric point Alanine (Ala) 6.01 Glutamine (Gln) 5.65 Lysine (Lys) C O CH NH2 NH2 CH2 CH2 CH2 CH2 OH 9.74 (a) With reference to these three different types of amino acids only, (i) describe and exp lain with the aid of a diagram what is meant by the primary structure of proteins given that t he pH of the bloodstream is 7.3. (ii) describe how an excess amount of ethanol in th e bloodstream would affect the function of proteins. [5] (b) Ethanol, which is a commonly used organic solvent, has been used as the starting component in the synthesis of a variety of organic by -products. State the reagents and conditions needed to convert ethanol into: (i) 2-hydroxypropanoic acid (ii) methanoic acid [5] [Turn Over

3 1 (c) Drink driving is the act of driving a motor vehicle with blood levels of alcohol in excess of a specified limit. Hence, with drink driving becom ing an increasingly prevalent issue, the breathanalyser technology has become more precise and one of the cruder initial prototype was based on the oxidation process of ethanol using acidified potassium dichromate (VI). (i) Derive the ion electron half equation for the oxidation of ethanol to ethanoic acid. Hence, using relevant information from the Data Booklet, write a balanced equation for the oxidation of ethanol using acidified potassium dichromate (VI). (ii) When potassium dichromate (VI) is reacted with sodium hydroxide, a colour change from orange to yellow was observed . Suggest the identity of the yellow solution. (iii) Explain why compounds of chromium are usually coloured. The driver would be asked to exhale a deep breath of air at a volume of 1 dm 3, 36.9oC and 101325 Pa into the breathanalyser. If the colour of the dichromate (VI) ions changes, the driver would then be sent to the hospital for further testing, before being charged with drink driving. (iv) Given tha t the breathanalyser contains 15.0 cm 3 of 0.017 mol dm -3 acidified potassium dichromate (VI), calculate the minimum volume of vaporised ethanol needed to turn all the potassium dichromate (VI) green. [8] (d) Ethanol, upon prolonged exposure to the atmosphere, would eventuall y oxidise to form ethanoic acid according to the following equation. CH3CH2OH (l) + O2 (g)  CH3COOH (l) + H2O (l) Use the following data, to construct an energy cycle to calculate the standard enthalpy change for the oxidation of ethanol. ∆H / kJ mol-1 Enthalpy change of combustion of ethanol -1371 Enthalpy change of combustion of ethanoic acid -876 [2] [Total: 20] [Turn Over

4 2 (a) Explain the following observations. (i) The fifth ionisation energy of chlorine is a smaller numerical value than the fifth ionisation energy of sulfur. (ii) 1 mole of h ydrogen fluoride gas occupies a smaller volume than 1 mole of hydrogen chloride gas at low temperature and pressure. (iii) The different relative rate of formation of precipitate upon reacting chloromethane and bromomethane with aqueous sodium hydroxide , followed by acidified silver nitrate. [7] (b) During the catalytic cracking of long chain and branched alkane m olecules, fragments of chained or branched molecules could be formed. An example of these fragments is 2-methylpropene. Outline the mechanism for the reaction of 2 -methylpropene and liquid iodine monochloride, ICl. [2] (c) 2-chloro-2-methylpropan-1-ol, can also be formed when aqueous chlorine is used in place of liquid iodine monochloride . It is then reacted with acidified potassium manganate (VII) solution to form compound HA. (i) Write out the displayed formula of compound HA. FA1 was made by dissolving 1.0 x 10 -3 mol of H A into 1 dm 3 of deionised water. At 40 oC, FA1 was found to have a 14.1% degree of dissociation. (Kw = 2.92 x 10-14 mol2 dm-6) (ii) Calculate the concentration of H A after dissociation. Hence, c alculate the acid dissociation constant of HA at 40 oC. (iii) 10.0 cm 3 of FA1 was titrated against a solution of 5 x 10 -4 mol dm -3 NaOH. Calculate the pH of the resulting solution, named FA2, after 10.00 cm3 of NaOH solution was added. (iv) When 10.00 cm 3 of 2.5 x 10 -4 mol dm -3 Ca(OH)2 was added to 20.0 cm3 of FA2, a salt solution was formed. (1) Calculate the concentration of the salt solution formed. (2) Write a balanced equation to show why the salt solution is alkaline. (3) Hence, c alculate the pH of the resulting solution taking into account the dissociation of water. [11] [Total: 20] [Turn Over

5 3 (a) Compound B has the molecular formula C 11H14O2. It is sparingly soluble in water. It does not decolourise aqueous bromine. Upon heating with acidified potassium manganate (VII) solution, it does not decolourise the purple solution, but forms two organic products, compounds C and D. Compound C forms a violet colourat ion when neutral iron (III) chloride solution is added. Upon reaction of C with aqueous bromine (in a stoichiometric ratio of 1:3), it forms a white precipitate and copious white fumes which turn moist blue litmus paper red. Compound D is able to rotate plane -polarised light. Upon combustion with excess oxygen, 0.1 mol of compound D produces 34.7 dm 3 of gas at 150oC and 101325 Pa. The volume is halved when the gaseous products was passed through anhydrous calcium chloride. Compound D also reacts with sodium carbonate powder, in a ratio of 2:1, to give effervescence. It is then reacted with lithium aluminium hydride in dry ether to form compound E. Compound E is then reacted with excess concentrated sulfuric acid, and subsequently decolourises acidified potassium manganate (VII) to form compound F with the evolution of a gas. Compound F has also been found to give orange crystals with 2, 4 dinitrophenylhydrazine. Deduce the structures of compounds B, C, D, E, and F. Explain the chemistry of the reactions described, writing balanced equations where appropriate. [14] (b) (i) Sketch a labelled diagram to show how the standard cell potential of the electrode systems between iodide and acidified potassium manganate ( VII) is measured . Hence, calculate the standard cell potential using relevant information from the Data Booklet. (ii)