CJC H2 Chem 2012 Prelim P3 QP

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Friday 24 August 2012 2 hours Candidates answer on separate paper. Additional Materials: Answer 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. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] 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. 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 of the question. At the end of the examination, fasten all your work securely together. This document consists of 14 printed pages and 0 blank page. [Turn over

2 9647/03/CJC JC2 Preliminary Exam 2012 Answer any four questions. 1 This question relates to the chemistry of Be, Mg, Al and their compounds. (a) Beryllium compounds are toxic air pollutants. Inhalation of high levels of beryllium can cause inflammation of the lungs in humans and long-term exposure may cause chronic beryllium disease (berylliosis), in which granulomatous lesions develop in the lung. (i) Given that charge density ∝ ionic charge ionic radius , calculate the relative charge densities of Be2+, Mg2+ and Al3+, using relevant data from the Data Booklet. (ii) Hence, predict what is observed when aqueous sodium hydroxide is gradually added to aqueous beryllium sulfate until the sodium hydroxide is in an excess . Write equations for all reactions that have taken place. (iii) Suggest the pH of the solution formed when beryllium chloride is dissolved in water. Give your reasoning. (iv) Magnesium ions are essential for the action of some enzymes (e.g. alkaline phosphatase found in the liver) by receiving electron pairs from oxygen and nitrogen atoms in the protein. It is thought that beryllium compounds are poisonous because they displace magnesium ions from these enzymes. Suggest a reason why beryllium ions should behave in this way. (v) Beryllium chloride may be used as a catalyst in the chlorination of benzene. Suggest a reason why this is possible. O utline the mechanism to show how beryllium chloride is involved in this reaction. [10]

3 9647/03/CJC JC2 Preliminary Exam 2012 [Turn over (b) A student carried out a kinetics experiment using a roll of magnesium ribbon that had been exposed to air for some time. He placed a piece of magnesium ribbon of mass 0.12 g into a flask containing 15.0 cm 3 of 1.0 mol dm–3 hydrochloric acid. The progress of the reaction was followed by measuring the pressure of the system at different times. The graph below shows the results of the experiment. (i) Determine, by calculation, the limiting reagent for the experiment. (ii) Account for the change in pressure of the system as shown in the graph at points A, B, and from C onwards. [4] (c) An alloy of aluminium and magnesium is used in boat-building. A 1.75 g sample of the alloy was dissolved in the minimum volume of 4 mol dm –3 hydrochloric acid and the solution was then made alkaline by the addition of aqueous sodium hydroxide until no further reaction occurred. The resultant mixture was filtered and the residue, X, rinsed with distilled water, all washings being added to the filtrate, Y. After air drying, 0.18 g of X was obtained. Carbon dioxide was passed into Y and a white solid, Z, which contained aluminium, was collected. Heating Z to constant mass gave a residue of mass 3.16 g. Suggest the identities of X, Y and Z, and determine the percentage composition by mass of the alloy. [6] [Total: 20] C D A 0 B time pressure of the system

4 9647/03/CJC JC2 Preliminary Exam 2012 2 2-chlorobutane undergoes hydrolysis with NaOH(aq) via two different reaction pathways, in the same reaction, to form a mixture of two enantiomeric products. CH3CHClCH2CH3 + NaOH → CH3CH(OH)CH2CH3 + NaCl In one of the hydrolysis reaction pathways, only one product is formed and inversion of configuration occurs in the product. In the other reaction pathway, a racemic mixture is formed. (a) In an experiment, one optical isomer of 2-chlorobutane undergoes hydrolysis and two enantiomeric products in a ratio of 95%:5% are formed. (i) Draw the structures of the two enantiomeric products. (ii) One enantiomer is formed in a much higher percentage compared to the other. Explain clearly how this disparity arises by examining the mechanisms of both reaction pathways. You should name both mechanisms involved but an outline of the mechanism is not required. (iii) Write a rate equation for the reaction pathway that results in the inversion of the configuration and draw its energy profile diagram, given that the enthalpy change of the hydrolysis is exothermic. (iv) Suggest the percentage of 2-chlorobutane that undergoes hydrolysis via the reaction pathway in (a)(iii). (v) Hence deduce how much faster the rate of this reaction pathway in (b)(ii) compares to that of the other reaction pathway. [9] (b) 2-chlorobutane is commonly produced from but-1-ene via reaction with hydrogen chloride. CH 2=CHCH2CH3(g) + HCl(g) à CH3CHClCH2CH3(l) (i) Name the other possible product in the above reaction. (ii) Predict the sign of ΔS for this reaction, showing your reasoning. (iii) Using relevant bond energy values from the Data Booklet, calculate the approximate value of ΔH for the reaction in (b). (iv) Bond energies quoted from the Data Booklet are average values. Other than this, explain why the method in (b )(iii) is not the most accurate for determining Δ H of the reaction. (v) Deduce how the rate of reaction of but-1-ene with hydrogen halides will vary from H-F to H -I, and give your reasoning.

5 9647/03/CJC JC2 Preliminary Exam 2012 [Turn over (vi) While HCl react readily with alkenes under room conditions, HCN does not. Based on concepts of chemical bonding, suggest possible reasons for this. [10] (c) HCl can be prepared by adding concentrated sulfuric acid to solid sodium chloride. However when concentrated sul furic acid is added to sodium iodide, the yield of H I is very low. Explain. [1] [Total: 20]

6 9647/03/CJC JC2 Preliminary Exam 2012 3 This question explores the chemistry of zinc in biochemistry, organic chemistry and electrochemistry. (a) Angiotensin I, a simple protein, undergoes hydrolysis with the aid of an enzyme, known as angiotensin-converting enzyme (ACE) to form angiotensin II. Angiotensin II is an important hormone that causes blood vessels to constrict, resulting in a rise in blood pressure. (i) State how proteins can be hydrolysed to form a mixture of their constituent amino acids. Some of the amino acids found in angiotensin II are shown below. CH2N H CO2H CH CH3 CH3 CH2N H CO2H CH2 OH CH2N H CO2H CH2CH2CH2 N H C NH H2N valine tyrosine arginine (val) (tyr) (arg) The side chains (R -groups) of angiotensin II could bind to targeted proteins through suitable R-group interactions. The R -group interactions are also used to maintain two specific protein structures. (ii) Briefly describe one protein structure that involves R-group interactions. (iii) Suggest three different types of R-group interactions in which the side chains of angiotensin II could bind to targeted proteins. Your answer should clearly indicate the side chains that might be involved. (iv) Another enzyme that