CJC H2 CHEM P3 Prelim

CATHOLIC JUNIOR COLLEGE Preliminary Examination 2009 Higher 2 CHEMISTRY 9746/03 Paper 3 Free Response 15 September 2009 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS CAREFULLY Write your name and class in the spaces on the writing paper. Write in dark blue or black pen on both sides of the writing paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use 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 the answer scripts securely together. This document consists of 8 printed pages and no blank pages. CJC Chemistry Department © CJC 2009

 CJC 2009 9746/03/prelim/09 2 Answer any four questions 1 Fireworks are an integral part of the Olympics opening ceremony. One of the most spectacular displays in recent history took place at the 2006 T orino Winter Olympics, where the largest ever Olympic rings were lit up in fireworks. (a) The colours of fireworks are due to various colour -producing chemicals present, of which compounds of the Group II metals are particularly important. Another component of fireworks is the oxidiser, w hich decomposes upon heating to provide oxygen for burning of the fireworks fuel. One such oxidiser in common use is potassium nitrate, KNO 3, which gives potassium nitrite, KNO 2, and oxygen gas when heated. (i) Write a balanced equation for the decompositi on of KNO 3. (ii) From your answer in (a)(i), explain, includi ng any relevant equations, why potassium nitrate is a better choice as an oxidiser as compared to magnesium nitrate. [3] (b) When a 2.75 g mixture of potassium nitrate and a G roup II carbonate from a red firework was heated thoroughly, enough oxygen was produced t o oxidise 0.0141 mol of aluminium, which is used as fuel in fireworks. In addition, wh en the gases produced were cooled to room temperature and pressure, it was found that 10 0 cm 3 of carbon dioxide was produced. Given that complete decomposition of the nitrate and carbonate took place, find the identity of the Group II metal used. [4] (c) Phosphorus and sulfur have various applications in pyrotechnic materials, including fireworks. (i) Predict, with reasons, how the radius of P 3– compares with that of S 2–. (ii) Both phosphorus and sulfur exhibit two diff erent oxidation states in their oxides and chlorides. Explain why phosphorus can form chlorides of two different oxidation states, while nitrogen can only form one chloride, NC l 3. [4] (d) Another fuel also used in fireworks is gallic acid . A closely related compound which is used as a base material in the production of Tamiflu, an antiviral influenza drug, is shikimic acid. State one chemical test, other than reaction with oxidising agents, by which gallic acid and shikimic acid can be distinguished from each other, and describe what will be observed. [2] C O 2 H O H O H O H C O 2 H O H O H O H gallic acid shikimic acid

 CJC 2009 9746/03/prelim/09 3 (e) (i) Outline how the following transformation can b e achieved, providing details of any necessary reagents and conditions, as well as the s tructures of any intermediates formed. (chloromethyl)benzene (ii) When warmed with aqueous sodium hydroxide fo llowed by an excess of dilute nitric acid and aqueous silver nitrate, (chloromethyl)benz ene gives a white precipitate. However, neither 2-chloromethylbenzene nor (fluorom ethyl)benzene gives a precipitate. Suggest explanations for the above observations. [7] [Total: 20] 2 (a) Two hydrocarbons of molecular formula C 4H10 are n-butane and i-butane. n-butane is a straight chained butane molecule whereas i-butane is a branched chain molecule. (i) Draw the full structural formula of both forms of butane and explain which form is expected to have the higher boiling point. (ii) Give the IUPAC name of i-butane. (iii) i-butane is used in refrigeration systems in place of CFCs and HCFCs that cause ozone depletion. Give a reason why i-butane does not cause ozone depletion. (iv) n-butane is sold as bottled fuel for cooking and cam ping. Given that the enthalpy change of the combustion of n-butane is -2877 kJ mol -1, calculate the mass of n-butane needed to bring 1 dm 3 of water at room temperature to boiling point. Assume the burner is only 85% efficient. (Given: M r of n-butane = 58.0; specific heat capacity of water = 4.2 J cm -3 oC-1) (v) Excess n-butane reacts with C l2 in the presence of uv light to give a mixture of t hree monochlorinated products. Draw clearly the three i someric products formed and name the mechanism of the reaction. [14] (b) Sodium chlorate(V), NaC lO3, is an ingredient in weed-killer. In the industry, sodium chlorate(V) is synthesized from the electrolysis of hot NaC l solution in a mixed electrode tank: NaC l + 3H 2O → NaC lO3 + 3H 2 (i) Suggest another method of synthesising sodium chlorate(V) and write a balanced equation for the reaction. (ii) NaC lO3 obtained in (b)(i) is purified by a process called crystallisation. NaC lO3 can undergo a disproportionation reaction to form NaC l and NaC lO4. The half equation showing C lO3 - being reduced to C l- is shown below: ClO3 - + 6H + + 6e - → C l- + 3H 2O Write the half equation to show C lO3 - being oxidised to C lO4 -, and hence write a balanced equation to show the disproportionation of C lO3 -. (iii) Suggest, with a reason, whether the dispropo rtionation of C lO3 - is pH dependent. [6] [Total: 20] C H 2 C l C H 2 C O 2 H

 CJC 2009 9746/03/prelim/09 4 3 H2O2 reacts with I- ions to give I2 and water. H2O2(aq) + 2 I-(aq) + 2 H +(aq) → I2(aq) + 2 H 2O( l) The kinetics of this reaction vary with pH. (a) To study the rate of this reaction, fixed amounts of sodium thiosulfate, Na 2S2O3, and starch solutions were added to different concentrations of the reagents. Suggest why these solutions were needed. Include any relevant equation in your answer. [2] (b) A series of experiments was carried out at 25 oC and the following results were obtained. In each case, [H +] was kept constant by using a buffer solution. Expt number pH of buffer [H 2O2]/mol dm -3 [I-]/mol dm -3 Initial rate/ mol dm -3 s -1 1 5.0 0.10 0.10 1.2 × 10 -4 2 5.0 0.20 0.10 2.5 × 10 -4 3 5.0 0.20 0.20 4.7 × 10 -4 4 4.0 0.20 0.20 4.8 × 10 -4 5 0.53 0.20 0.20 2.1 × 10 -3 6 0.40 0.20 0.20 2.8 × 10 -3 (i) Using data from experiments 1-4, find the order of reaction with respect to [H +], [H 2O2] and [ I-] respectively. Hence, write a rate equation for th e reaction when the pH is between 4 and 5. (ii) Evaluate the rate constant for the reaction ov er this pH range. (iii) Using the data from experiments 5 and 6 , write the rate equation for the reaction when the pH is low. (Assume that the orders of reaction with respect to both [H 2O2] and [ I-] do not change with pH). (iv) What conclusion can you draw from the rate equations obtained above? (v) Suggest a 3-step mechanism for the reaction at pH 5 given the following: • The first step is the rate-determining step. • One of the intermediate products is O I - which can act as a base. [10]

 CJC 2009 9746/03/prelim/09 5 (c) The stability constant for a complex ion, K stab , gives a measure of the stability of a complex. Consider the equation for the formation of the complex ion, [Fe(SCN)(H 2O) 5]2+ : [Fe(H 2O) 6]3+ (aq) + SCN -(aq) [Fe(SCN)(H 2O) 5]2+ (aq) + H 2O(l) Th