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.