CJC H2 CHEM P3 QP Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Thursday 27 August 2015 2 hours Additional Materials: Data Booklet Answer Paper 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. Write your answers on the answer paper 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 14 printed pages and 0 blank page. [Turn over 9647/03/CJC JC2 Preliminary Exam 2015

2 9647/03/CJC JC2 Preliminary Exam 2015 Answer any four questions. 1 Butanone is an industrial organic solvent used in the manufacture of plastics, lacquers and varnishes. (a) Butanone reacts with iodine in the presence of an acid according to the equation: CH3CH2COCH3 + I2  CH3CH2COCH2I + HI In a series of experiments, the reaction was carried out with different concentrations of reagents, and the following initial rates were obtained: Expt. [CH3CH2COCH3] / mol dm–3 [I2] / mol dm–3 [H+] / mol dm–3 Initial rate / mol dm–3 min–1 1 0.010 0.010 0.010 8.31 x 10–8 2 0.010 0.010 0.013 10.8 x 10–8 3 0.013 0.010 0.013 14.0 x 10–8 4 0.018 0.013 0.018 26.9 x 10–8 (i) Using the data given, deduce the order of reaction with respect to each of the three reagents, clearly showing how you arrive at your answers . Hence, write a rate equation for the reaction. (ii) Explain how the rate of reaction would change if the solution was diluted with an equal volume of inert solvent, using r to represent the original rate before dilution. (iii) The reaction can also be carried out in the presence of a strong base instead of an acid. State what would be observed when a strong base is used and w rite an equation for the reaction that occurs. [7] (b) Elemental iodine, I2, has limited solubility in water. Its solubility is greatly increased in a solution containing iodide ion, I–, due to the formation of triiodide ion, I3 –, as shown: I2(aq) + I–(aq) ⇌ I3 –(aq) (i) Write an expression for the equilibrium constant, Kc, of this reaction, stating its units. (ii) Experiments have shown that when [ I– ] = 1.4 x 10–3 mol dm–3, the concentrations of I2 and I3 – are equal. Use this information to calculate a value for Kc. (iii) Using your answer from (b)(ii), calculate the concentration of iodide ions necessary for 99 % of the iodine to be dissolved. [4]

3 [Turn over 9647/03/CJC JC2 Preliminary Exam 2015 (c) Explain, using Maxwell -Boltzmann distribution curve s, how the rate of a reaction would change if the temperature was increased. [3] (d) Grignard reagents are organo -magnesium halides, commonly used in synthesis to prepare a variety of organic compounds. The carbon-magnesium bonds in Grignard reagents are highly polar and this makes it extremely useful in organic synthesis as it is able t o react with other polar organic molecules to form carbon-carbon bonds. An example of the use of a Grignard reagent is the two -step reaction of CH 3CH2MgBr with butanone, CH3CH2COCH3, to form 3-methylpentan-3-ol. (i) Suggest the type of reaction that has taken place in step 1. (ii) Suggest the structural formula of the organic compound to be used with CH3CH2MgBr to form propan -1-ol, if the reaction undergoes a similar two -step process. (iii) Suggest a suitable Grignard reagent and another organic compound to be used if propan-2-ol is to be prepared using a similar two-step process. [4] (e) There are certain practical issues with the use of Grignard reagent, notably slow formation of the Grignard reagent and its susceptibility to reaction with water in air. The slow formation of the Grignard reagent may be offset by cutting the precursor magnesium into smaller pieces or scraping the sides of the metal before use. Using a balanced equation, explain why magnesium, which has been stored in dry ambient conditions, may not react as quickly as expected. [2] [Total: 20]

4 9647/03/CJC JC2 Preliminary Exam 2015 2 The direct oxidation of alcohols in a fuel cell represents potentially the most efficient method of obtaining useful energy from renewable fuel. The diagram below illustrates the functional parts of a direct -ethanol fuel cell (DEFC). The anode and cathode compartments are separated by the proton exchange membrane . The protons are transported across the proton exchange membrane to the cathode where they react with oxygen to produce water. Diagram of a DEFC (a) (i) Explain, in terms of the change in average oxidation number of the carbon atoms , whether electrode 1 or 2 acts as the anode. (ii) Write the half-equations for the reactions which take place at the electrodes of the DEFC, and hence an overall equation for the cell reaction. (iii) The cell is capable of producing an e.m.f. of 1.47 V. By using suitable data from the Data Booklet , calculate a value for the E  of the CO2/CH3CH2OH electrode reaction. (iv) By quoting relevant E  values from the Data Booklet , explain why an acidic electrolyte is preferred to an alkaline electrolyte in the DEFC. (v) Describe and explain the effect on the cell potential when the concentration of ethanol is increased. [9] H2SO4(aq) External circuit CO2 CH3CH2OH H2O O2 Electrode 2 Electrode 1

5 [Turn over 9647/03/CJC JC2 Preliminary Exam 2015 (b) Chromium plating is an important industrial application of electrolysis, where a steel object is plated with a thin layer of chromium to make it resistant to rusting and scratches. In an electroplating experiment, a researcher connected the DEFC to an e lectrolytic cell as shown below. The aqueous solution containing chromium( III) chloride was electrolysed using a piece of pure chromium and the steel object to be electroplated as the electrodes. (i) The total time taken for 10 g of chromium to be plated was 4 hours. Calculate the current produced by the DEFC. (ii) Calculate the volume of ethanol used when 10 g of chromium metal was plated onto the steel object under room conditions, given that the density of ethanol is 0.789 g cm–3. [3] (c) Chromium is a transition element. (i) Explain what is meant by the term transition element. (ii) Chromium(III) chloride, CrC l3, reacts with water to form a violet hydrate [Cr(H2O)6]Cl3. This violet hydrate is isomeric to the dark green [CrCl2(H2O)4]Cl.2H2O and the pale green [CrCl(H2O)5]Cl2.H2O Based on the above information, give one property of transition elements or their compounds that is not shown by s-block elements. (iii) When 0.03 mol of aqueous silver nitrate was added to 0.01 mol of one of the chromium-containing compounds in (c)(ii), a white precipitate weighing 1.44 g was formed. Deduce the formula of the compound to which the silver nitrate was added. [3] CrCl3 (aq) pure chromium steel object DEFC

6 9647/03/CJC JC2 Preliminary Exam 2015 (d) The following reaction scheme shows the chemistry of some chromium -containing species in aqueous solution. (i) Identify the chromium -containing species present in A. Describe how this species accounts for the formation of carbon dioxide in Step III. Include any relevant equations in your answer. (ii) Give a bala