YJC H2 CHEM P3

9647 / YJC / 2014 / Preliminary Examination / Paper 3 2 Answer any four questions 1 This question involves Period 3 elements and their compounds. (a) Describe the reactions, if any, of the oxides Na2O and SO3 with H2O. Include the approximate pH value of any resulting solutions and w rite equations for any reaction that occur. [3] (b) An aqueous mixture of NaHCO3 and Na2CO3 can act as a buffer solution. (i) Define the term ‘buffer solution’. (ii) With the aid of two equations, explain how a solution of NaHCO 3 and Na2CO3 can act as a buffer solution. (iii) Calculate the pH range in which the buffer is most effective, given the following information. HCO3 − + H2O ⇌ CO3 2− + H3O+ Ka = 5.61 × 10−11 mol dm−3 at 25 °C (iv) Calculate the pH change in the solution when t he ratio of [CO 3 2−] to [HCO 3 −] was increased from 0.50 to 0.85. [7] (c) When dry chlorine gas is passed over heated aluminium in a hard glass tube, a vapour is produced. The vapour condenses to a yellow-white solid at the cooler part of the tube. At low temperatures, the vapour has the empirical formula A lCl3 and a relative molecular mass of 267. When a few drops of water are added to the yellow -white solid, steamy white fumes are evolved and a white solid remains. The solid is insoluble in water. However, when a large amount of water is added to the solid, a weakly acidic solution is formed. (i) Suggest the molecular formula of the vapour with empirical A lCl3 and draw a ‘dot -and- cross’ diagram to describe the bonding. (ii) Identify the white solid formed when a small amount of water is added. (iii) With an appropriate equation, account for the acidic solution formed. Suggest a pH for this solution. [5]

9647 / YJC / 2014 / Preliminary Examination / Paper 3 3 (d) When chlorine gas and anhydrous A lCl3 is added to met hylbenzene, the following reaction occurs: CH3 2 + 2Cl2 CH3 Cl + CH3 Cl + 2HCl AlCl3 (i) State the type of reaction which occurred. (ii) With the aid of equations, outline the mechanism of the above reaction. Use curly arrows to show the movement of electrons. (iii) Suggest the structure of the organic product which may form when excess chlorine gas is added to methylbenzene in the presence of sunlight. [5] [Total: 20 marks]

9647 / YJC / 2014 / Preliminary Examination / Paper 3 4 2 Use of the Data Booklet is relevant in this question. Sulfuric acid is one of the most important commodity and industrial chemicals, and more of it is produced in the world each year than any other manufactured chemical. It has widely varied uses and is involved in the production of almost all manufactured goods. (a) Draw the ‘dot -and-cross’ diagram of H 2SO4 and suggest the shape and bond angle with respect to S. [2] (b) Compound L shown below is able to react with concentrated H 2SO4 under varied conditions to yield different products. For each of the conditions listed below, draw the structure of the organic product. OH Compound L (i) Concentrated H2SO4, 170 °C (ii) CH3COOH with concentrated H2SO4 (iii) NaBr with concentrated H2SO4, heat (iv) Concentrated H2SO4, followed by H2O/heat [4] (c) The oldest type of rechargeable batteries invented is lead-sulfuric acid batteries, patented by French physicist Gaston Plant é in 1859. When the battery discharges, both the positive and negative plates forms lead(II) sulfate, as shown by the equations below: Pb(s) + H2SO4(aq)  PbSO4(s) + 2H+(aq) + 2e− PbO2(s) + HSO4 −(aq) + 3H+(aq) + 2e−  PbSO4(s) + 2H2O(l) (i) Using values from the Data Booklet, calculate the Eᶱcell of the lead -acid battery during the discharge phase. (ii) Write an equation for the overall reaction of the recharge process.

9647 / YJC / 2014 / Preliminary Examination / Paper 3 5 The graphs below measure how the e.m.f. of the lead -sulfuric acid cell and the e.m.f. of another cell X change with time. (iii) Explain why the e.m.f. of the lead-acid cell decreases over time. (iv) Suggest an identity for cell X and explain why the e.m.f. of cell X remains constant. [4] (d) One application of sulfuric acid is in the making of paints. One such paint is chrome yellow, a natural yellow pigment made of lead( II) chromate, which is insoluble in water. The solubility product of PbCrO4 at 15 °C is 1.69 × 10−14 mol2 dm−6. (i) Write an expression for the solubility product, Ksp of lead( II) chromate and hence calculate the solubility of lead(II) chromate in mol dm−3. (ii) Concentrated lead(II) nitrate is added dropwise to 0.010 mol dm −3 K2CrO4, potassium chromate(VI). Assuming the concentration of K 2CrO4 does not alter with the addition of Pb(NO3)2, calculate the maximum concentration of Pb 2+ ions in the solution befor e the first trace of precipitation. (iii) In the presence of acid, CrO 4 2− forms Cr2O7 2− readily. In the presence of alkali, CrO 4 2− is preferred. Explain this observation. [5] (e) White lead, Pb(OH) 2.PbCO3 used to be the primary pigment u sed in paints. However, it has been replaced largely by BaSO4 and TiO2, due to the toxicity of lead-based paint. When white lead is heated to decomposition, steam and a colourless gas are evolved. The colourless gas forms a white precipitate when it is passed through limewater. (i) Write an equation for the decomposition of white lead. (ii) Using your answer in (e)(i), calculate the percentage loss in mass upon heating a sample of white lead until no further change occurs. (iii) Assuming Ca( II) compounds decomposes to form similar compounds as Pb( II) compounds, explain whether you would expect the temperature required for the decomposition of Ca(OH)2.CaCO3 to be higher or lower than that of white lead. [5] [Total: 20 marks]

9647 / YJC / 2014 / Preliminary Examination / Paper 3 6 3 By observing the kinetics of organic reactions, scientists are able to deduce their mechanisms. (a) In an experiment, t he hydrolysis of 2 -bromo-2-methylpropane, (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 of the reaction is found to be 5 minutes. (i) Calculate the value of the rate constant, giving its unit. (ii) The initial concentration of (CH 3)3CBr is 1.6 mol dm −3. Sketch a graph of concen tration of [(CH 3)3CBr] against time, clearly indicating the time taken for 2 -bromo-2- methylpropane to decrease to 6.25 % of its original concentration. [3] (b) In another experiment, the hydrolysis of 2 -bromopropane, ( CH3)2CH(Br) is being investigated. Samples of 2 -bromopropane are dissolved in dilute aqueous ethanol (80% ethanol and 20% water by volume) and reacted with sodium hydroxide solution under heat. The rate equation is found to be as follows: Rate = (0.24 × 10−5) [(CH3)2CH(Br)] + (4.7 × 10−5) [(CH3)2CH(Br)][OH−] (i) The rate equation obtained shows the hydrolysis of 2 -bromopropane exhibiting a mixture of first and second order kinetics, following mechanisms of S N1 and S N2 respectively. Suggest why this may be so. (ii) Describe the reaction mechanism of the SN2 part of the reaction of 2-bromopropane with OH− ions. Use curly arrows to indicate the movement of electrons. (iii) The percentage at which the respective mechanisms contribute to the overall rate is known as the percentage rate. For the second order kinetics in this reaction, the percentage rate given is as follows: % rate = 0.24 ]4.7[OH ]4.7[OH   × 100 Using the above information, calculate the percentage rate for the S N2 mechan