RJC Chemistry H2 P3 with Ans Prelims

RAFFLES JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 © RJC 2008 [Turn over 1 HIGHER 2 CHEMISTRY 9746/03 Paper 3 Free Response 12 September 2008 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, civics tutorial group and index number in the spaces provided on the cover page on page 10 and the writing papers. Write in dark blue or black pen on both sides of 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. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good Eng lish and clear present ation 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 secu rely together, with the cover page on top. ________________________________________________________________________ This question booklet consists of 9 printed pages and one cover page.

Answer any four questions. Begin each question on a fresh sheet of paper. 1 This question involves Period 3 elements and their compounds. (a) An aqueous mixture of sodium hydrogencarbonate and sodium carbonate can act as a buffer solution. (i) Define the term buffer solution. (ii) With the aid of two equations, explain how a solution of sodium hydrogencarbonate and sodium carbonate can act as a buffer. (iii) Calculate the pH range in which the buffer is most effective. Take the K a for HCO3 − to be 5.61 x 10−11 mol dm−3 at 25 °C. (iv) Calculate the change in pH of the solution if the ratio 2 3 3 [CO ] [HCO ] − − was increased from 0.30 to 0.96. [7] (b) Explain, with the aid of equations, why (i) a solution of aluminium sulphate turns blue litmus red. (ii) aluminium oxide dissolves on additi on of excess aqueous sodium hydroxide. [ 4 ] (c) Sketch the graph of the pH of the resulting mixture when the oxides of Na to S are separately added to water. [1] © RJC 2008 [Turn over 2

(d) Deuterium (D or 2H) is a heavy isotope of hydrogen. Chlorine can react with deuterium to form deuterium chloride, DC l. When DCl is dissolved in methylbenzene, no reaction occurs. However, if an anhydrous catalyst W is added to the solution, a reaction takes place, and a compound X is one of the products. If more DC l is present, a compound Y is formed. CH3 D CH3 DD D D D X Y (i) Suggest a suitable identity for the catalyst W. Explain why W must be anhydrous and why it can act as the catalyst in the reaction. (ii) With the aid of equations, state and describe the mechanism involved in the formation of compound X from methylbenzene. (iii) In the presence of excess DC l, would compound Z be formed? Explain. [8] CD3 DD D D D Z [Total: 20] © RJC 2008 [Turn over 3

© RJC 2008 [Turn over 4 2 The halogens are elements in Group VII of the Periodic Table and display similar chemical properties. They are good oxidisi ng agents and can exhibit variable oxidation states. (a) Using relevant data from the Data Booklet, describe and explain the reactions of the (i) elements, X2 (where X = Cl, Br and I), with aqueous sodium thiosulphate. (ii) halide ions, X –, with concentrated sulphuric acid. Give an equation for any reaction that occurs. [7] Chlorine, which is a Group VII element, has been extensively used in the manufacture of many compounds. One such compound is hous ehold bleach, which contains sodium chlorate(I), NaClO. The chlorate(I) ion acts as a powerful oxidising agent in acid solution: 2ClO – + 4H+ + 2e– ⇌ Cl2 + 2H2O, E{ = +1.64 V (b) (i) What do you understand by the term standard electrode potential? (ii) Draw a labelled diagram to show how you could measure the standard electrode potential of the C lO–(aq)/Cl2(g) electrode system. You are to indicate clearly the anode and cathode and to show the direction of the flow of electrons. (iii) Predict whether a reaction will occur when chlorate(I) in acid solution is mixed with H 2O2(aq). Calculate E { cell and write an equation for any reaction that occurs. [7] Bromine, which is another Group VII element, is used to manufacture silver bromide. Silver bromide is often found in photographic film. (c) Silver bromide is insoluble in water. Explain why silver bromide dissolves when concentrated aqueous ammonia is added. [2] [2]

Another bromine–containing compound is titanium bromide. An electrolysis experiment was conducted using the apparatus shown below to determine the correct formula for titanium bromide, TiBrx. © RJC 2008 [Turn over 5 heat crucible molten titanium bromide carbon electrodes ammeter I II A Crystals of titanium bromide were placed in the crucible and heated. The molten bromide was then electrolysed using carbon electrodes. Results for the experiment were: Ammeter reading 0.50 A Duration of current flow 1 hour and 30 minutes Mass of titanium produced 0.45 g (d) (i) Identify the electrode ( I or II) at which titanium is produced. (ii) Determine the value of x. [4] [Total: 20]

3 Carbon is one of the most abundant elements on Earth. While it is found mainly in organic substances, carbon does exist as inorganic compounds such as carbon dioxide, carbonates and hydrogencarbonates. (a) In order to determine the titration curve of the reaction between HCO 3 −(aq) and HCl(aq), a student took a 25.0 cm 3 of solution containing 0.100 mol dm −3 HCO3 − ions and titrated it with standard 0.100 mol dm−3 dilute HCl. ( Kb for HCO3 − = 2.34 x 10−8 mol dm−3 at 25 °C) (i) Calculate the pH of the solution before the addition of dilute HC l. (ii) Calculate the pH of the solution at the equivalence point. (iii) Calculate the pH of the solution after 37.5 cm 3 of dilute HCl was added. (iv) Using your answers from (i) to (iii), sketch the pH titration curve. [9] (b) Industrially, inter–conversions between organic compounds are common. Devise a two–step synthesis for the following conversion. CH3 CH3 Na+-O2CCH2CH2CH2CH2CO2 -Na+ Indicate the reagents and conditions for each step, and draw the structure of the intermediate. [2] (c) Organic compounds can be distinguished using simple chemical tests. Suggest how you can distinguish between the following pairs of isomers. State clearly the reagents and conditions used as well as the observations for each compound. (The –OCH 3 group is inert and can be disregarded in this question.) (i) C H O OCH3 and C CH2OH O (ii) CH2 CH2 O C O and CH2 CH2 C OO [5] © RJC 2008 [Turn over 6 (d) Observations from kinetics studies often allow scientists to deduce the mechanisms of organic reactions.

© RJC 2008 [Turn over 7 The hydrolysis of (CH 3)3CBr with aqueous sodium hydroxide is found to be first order with respect to (CH 3)3CBr and zero order with respect to OH −. The half− life for the reaction is 5 minutes. (i) Write the rate equation. Calculate the value of the rate constant, giving its units. (ii) If the initial concentration of (CH 3)3CBr used is 1.6 mol dm–3, sketch a graph of concentration of (CH3)3CBr against time for the bromoalkane to decrease to 6.25 % of its original concentration. [4] [Total: 20]

4 A neutral compound P has mo