JJC H2 Chem 2013 Prelim P3 QP

[Turn over JURONG JUNIOR COLLEGE 2013 JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 17 September 2013 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and exam index number 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 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 question. At the end of the examination, fasten all your work securely together. This document consists of 11 printed pages and 1 blank page. 619

2 © Jurong Junior College 9647/03/PRELIM/2013 Answer any four questions. 1 Nitric acid, HNO 3, is a common laboratory reagent and an important industrial chemical for the manufacture of fertilisers and explosives. (a) Nitric acid can be used to prepare lead( II) nitrate, Pb(NO 3)2, which does not occur naturally. When lead is dissolved in aqueous nitric acid, a brown gas is evolved during the reaction. (i) Use the Data Booklet to construct an equation for the reaction of aqueous nitric acid with lead. A 40 cm 3 s a m p l e o f P b ( N O3)2(aq) was prepared and then mixed with 60 cm3 of 0.100 mol dm3 Al2(SO4)3(aq), which is in excess. Upon mixing, a precipitate is formed which is removed. The final solution X that remained is found to be weakly acidic. (ii) With the aid of an equation, explain why solution X is acidic. (iii) Given that the acid dissociation constant, K a, of solution X is 1.4  105 mol dm3, calculate [H+] in solution X. [5] (b) During the industrial manufacture of nitric acid, NO 2 is hydrolysed by passing the gas through water. 3NO2(g) + H2O(l)  2HNO3(aq) + NO(g) Hr The following enthalpy changes are given: H /kJ mol1 N2(g) + O2(g)  2NO(g) +183 2NO(g) + O2(g)  2NO2(g) 116 2N2(g) + 5O2(g) + 2H2O(l)  4HNO3(aq) 256 (i) Using relevant data from the table above, calculate the enthalpy change of formation of NO 2(g). (ii) Using your answer in (b)(i) and any relevant data in the table, calculate Hr, the enthalpy change of reaction for the hydrolysis reaction of NO2. [3] (c) When concentrated nitric acid is reacted with concentrated sulfuric acid, the nitronium ion, NO 2 +, is produced, according to the following equation. HNO3 + H2SO4  NO2 + + HSO4  + H2O (i) Draw a dot-and-cross diagram of the HNO3 molecule. (ii) Hence, suggest why HNO3 acts as a Bronsted base in this reaction. [2] 620

3 © Jurong Junior College 9647/03/PRELIM/2013 [Turn over 1 (d) Benzophenone can undergo the following two-step process. O benzophenone A B C13H11NO conc HNO3, conc H2SO4 step 1 step 2 Draw the structural formulae of A and B and write a balanced equation for step 1. Suggest the reagents and conditions for step 2. [4] (e) Unlike nitric acid which is a strong acid, HCN is a weak acid. (i) With the aid of an equation, explain what is meant by the term weak acid using HCN as an example. In organic reactions, HCN can react with aldehydes and ketones as shown in the mechanism below using ethanal as an example. Step 1: +C N O O CN Step 2: + HCN O CN OH CN +C N (ii) The reaction is carried out at a carefully controlled pH. Suggest why this reaction occurs more slowly at both high and low concentrations of H+ ions. When primary amines are used instead of HCN, aldehydes and ketones react differently to form imines which can be converted to other amines, as illustrated by ethanal below. (iii) State the type of reaction for reactions I and II. (iv) Using the similar series of reactions as above, suggest the structural formulae of a possible pair of starting compounds that can be used to produce compound C. [6] [Total: 20] 621

4 © Jurong Junior College 9647/03/PRELIM/2013 2 Iron(III) chloride, FeCl3, is an industrial scale commodity chemical compound. (a) Anhydrous FeC l3 can be prepared by passing chlorine gas over hot iron metal, kept heated at temperatures below 320 °C. However, if the metal is heated beyond 320 °C, the Fe2Cl6 vapour that is formed will slowly decompose to give FeCl2 and Cl2. Fe2O3 can also be prepared by passing oxygen gas over hot iron metal. Unlike the preparation of FeC l3, the metal can be heated at temperatures as high as 1000 °C. (i) Explain why Fe 2O3, unlike FeC l3, can be prepared by heating iron metal with oxygen gas at very high temperatures. (ii) FeCl3 and Fe2O3 have the following melting points. compound melting point /°C FeCl3 305 Fe2O3 1565 Explain, in terms of structure and bonding, why the melting points of FeC l3 and Fe2O3 are significantly different. [4] (b) One important industrial application of FeC l3 is the etching of copper, via a two–step redox reaction as shown below, in the production of printed circuit board. Step 1 FeCl3(aq) + Cu(s)  FeCl2(aq) + white solid D Step 2 FeCl3(aq) + white solid D  FeCl2(aq) + blue solution E (i) Suggest the formula of compound D and of the complex ion present in E. (ii) Explain fully why D is not coloured but E is coloured. [6] (c) In the laboratory, FeC l3 is commonly employed as a Lewis acid (an electron–acceptor) as illustrated in the following example: FeCl3 + Cl2  FeCl4  + Cl+ It is used for catalysing Friedel–Crafts reac tion of aromatic compounds as shown by benzene below. (R represents an alkyl group.) (i) State the type of reaction undergone by benzene during Friedel-Crafts reactions. 622

5 © Jurong Junior College 9647/03/PRELIM/2013 [Turn over 2 (c) Using the two Friedel-Crafts reactions described on page 4 as parts of a synthesis route, a student suggested the following route to form 1–tetralone, from benzene. (ii) Give appropriate reagents and conditions necessary for steps 3 and 4. (iii) Predict the structure of G. (iv) The student suggested that F has the following structure: By quoting appropriate data in the Data Booklet , explain why the student’s suggestion is not correct. (v) Suggest a chemical test to be performed on F to prove that the student’s suggestion is not correct. (vi) Both H and J can also undergo Friedel–Crafts reactions in the presence of FeCl3. Suggest two reasons why H would require a harsher condition for Friedel–Crafts reaction to occur, as compared to J. [10] [Total: 20] 623

6 © Jurong Junior College 9647/03/PRELIM/2013 3 A breathalyser is a portable device used by the traffic police to estimate the blood alcohol content of intoxicated suspects via breath samples. (a) One type of breathalyser uses the fuel cell technology and calculates the blood alcohol content based on the voltage measured. The fuel cell contains platinum electrodes immersed in an acid electrolyte. At one electrode, ethanol from a breath sample is converted to ethanoic acid whereas at the other electrode, oxygen is converted to water. (i) State the oxidation state of the carbon atom that is attached to the oxygen atom in one molecule of ethanol. (ii) Write an overall equation for the reaction that occurs in the breathalyser. (iii) Explain how the voltage of this breathalyser will change when the ethanol concentration increases. [3] (b) An earlier version of a breathalyser consists of a chamber of acidified potassium dichromate(VI), K 2Cr2O7, which converts ethanol to ethanoic acid. Ethanol in the breath sample will cause a colour chan