CJC Prelim H2 Chem 2008 P3

CATHOLIC JUNIOR COLLEGE PRELIMINARY EXAM 2008 CHEMISTRY 9746/03 CHEMISTRY 9746/03 Higher 2 Higher 2 Paper 3 Free Response 8 September 2008 Paper 3 Free Response 8 September 2008 2 hours 2 hours Additional Materials: Data Booklet Additional Materials: Data Booklet Answer Paper Answer Paper READ THESE INSTRUCTIONS FIRST READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. You may use a soft pencil for any diagrams, graphs or rough working. Do not use correction fluid. Do not use correction fluid. Answer any four questions. Answer any four questions. A Data Booklet is provided. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. 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 together. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 8This document consists of 8 printed pages and 0 blank page.

2 Answer any four questions. 1 Grignard reagents are organo-magnesium hali des, commonly used in synthesis to prepare a variety of organic compounds. The reaction scheme for the synthesis of 3-methylpentan-2-ol, B, using a Grignard reagent is shown below. The Grignard r eagent is prepared from the alkylhalide A and subsequently acts as a nucleophile before forming the product B. CJC 2008 9746/03/prelim/08 (a) (i) Describe the mechani sm for the reaction in step I. (ii) Hence explain why compound A may be found as an equimolar mixture of two isomers. [ 6 ] (b) An alternative method of preparing 3 -methylpentan-2-ol is through the use of 3-methylpentane as a starting material. (i) Suggest an alternative two-step met hod of preparing 3-methylpentan-2-ol from 3-methylpentane, stating clearly the reagents and conditions required for each step. (ii) Suggest a reason why the synth etic method stated in (b)(i) is not preferred to that of using Grignard reagents. [ 3 ] (c) 3-methylpentan-2-ol is easily converted to other useful organic compounds such as 2-methylbutanoic acid, which is widely used in perfumery. Outline how 3-methylpentan-2-ol may be converted to 2-methylbutanoic acid, giving the structure of any intermediate product(s) formed. [3] (d) There are certain practi cal issues with the use of Grignard reagents, notably slow formation of the Grignard reagent and susceptibility to water in air. (i) The slow starting rate may be offset by cutting the magnesium into smaller pieces or scraping the sides of the metal before use. Using a balanced equation, explain why the magnesium may not react as quickly as expected. (ii) The Grignard reagents have similar proper ties to that of magnesium chloride. Using balanced equations, show the reacti on of magnesium chloride in water and state the pH of the resultant solution. Hence, suggest why effort is required to ensure only anhydrous c hemicals are used during synthesis involving Grignard reagents. [ 5 ] (e) Compare the structure and bondi ng of the oxides of magnesi um in contrast to the oxides of silicon and sulphur. [3] [Total: 20] C 4H9Cl A Mg ether CH3CH2CHCH3 Grignard reagent H O CH3 1) 2) HCl (aq) C6H14O B CH3CH2CH=CH2 step I HCl MgCl

3 2 (a) Nitrogen and oxygen react in a hot car engi ne to form nitrogen monoxide which is a serious pollutant of the atmosphere. Nitrogen monoxide may be removed from the car exhaust by using a catalytic conver ter. In this proc ess, nitrogen monoxide reacts with carbon monoxide (a product of combustion of gasoline) in the presence of platinum catalyst as follows: NO (g) + CO (g) → 2 1 N2 (g) + CO2 (g) By using the following values of ent halpy change of formation, calculate the relevant enthalpy changes of reactions and explain why this re action is more efficient as a method of removing nitr ogen monoxide than decom posing it to its elements. [ΔHf / kJ mol–1 : NO = +90; CO = –111; CO2 = –394 ] [3] (b) In heavily polluted air, a brown haze is often seen due to the formation of nitrogen dioxide from nitrogen monoxide that passes into the atmosphere. 2NO (g) + O2 (g) → 2NO2 (g) ΔH < 0 In a series of experiments carried out to investigate this reaction, the following results were obtained. concentration / mol dm–3 Experiment NO (g) O2 (g) initial rate / mol dm–3 s–1 1 0.001 0.1 0.70 × 10–3 2 0.002 0.1 2.80 × 10–3 3 0.003 0.2 12.60 × 10–3 (i) Deduce the order of reaction with re spect to each reactant, and hence write the rate equation for the reaction. (ii) Calculate a value for the rate constant k, giving its units. (iii) Suggest, with explanation, how t he value of the rate constant k is affected (if at all) as the gases get cooler in the upper part of the atmosphere. [5] (c) The first step in a possible mech anism for the reaction in (b) is 2NO (g) → N 2O2 (g) fast (step 1) (i) Draw dot-and-cross diagrams to show the electronic structures of the two molecules, NO and N 2O2. What feature in the el ectronic structure of NO would suggest that this is a likely first step in the reaction? (ii) Suggest an equation for the slow rate-determining step (step 2). (iii) Sketch and clearly label the ener gy profile diagram for the proposed mechanism represented by steps 1 and 2. [ 6 ] CJC 2008 9746/03/prelim/08

4 (d) (i) When 2-bromocyclohexanol reacts wit h NaOH in hot ethanolic solution, two isomeric products are formed. Draw the structures of the two isomers. (ii) A different reaction occurs when 2-bromocyclohexanol reacts with aqueous NaOH. Draw the structural formula of the product formed. (iii) The kinetics of t he reaction in (d)(ii) is given by the rate equation rate = k [2-bromocyclohexanol] Outline a mechanism which is consist ent with the observ ed kinetics, using curly arrows to denote any electron transfer. [ 6 ] [Total : 20] CJC 2008 9746/03/prelim/08

5 3 Use of the Data Booklet is relevant to this question. (a) Explain the following observations with the aid of equations. (i) When sodium hydroxide was added to a solution of iron(II) ions, a green precipitate was observed. On standing, in the presence of ai r, the precipitate turned brown. (ii) When iodide ions were added to aqueous iron(III) chloride, a brown solution was obtained. However, no such observa tion was seen when the experiment was repeated in the presence of cyanide ions. [6] (b) One method for powering a heart pacem aker comprises zinc and platinum electrodes implanted within the body tiss ues. These electrodes in the oxygen- containing body fluid at a pH of 7.4 form a ‘biogalvanic’ cell in which zinc is oxidised and oxygen is reduced. (i) Write a balanced equation for the overall chemical reaction and calculate the E θ cell of this cell. (ii) Explain how the E θ cell value would change if the pH of the body fluid decreases. (iii) If a current of 4 x 10 -5 A was drawn from the cell, calculate how long a zinc electrode weighing 5 g will last before it needs to be replaced. Give your answer correct to the nearest day. [7] (c) Explain why a solution of Zn 2+ ions is colourless but a solution of Fe2+ is coloured. [3] (d) An aq