CJC H2 Chem 2012 Prelim P3 Soln

2 9647/03/CJC JC2 Preliminary Exam 2012 Answer any four questions. 1 This question relates to the chemistry of Be, Mg, Al and their compounds. (a) Beryllium compounds are toxic air pollutants. Inhalation of high levels of beryllium can cause inflammation of the lungs in humans and long-term exposure may cause chronic beryllium disease (berylliosis), in which granulomatous lesions develop in the lung. (i) Given that charge density ∝ ionic charge ionic radius , calculate the relative charge densities of Be2+, Mg2+ and Al3+, using relevant data from the Data Booklet. Be2+: 64.5, Mg2+: 30.8, Al3+: 60.0 (ii) Hence, predict what is observed when aqueous sodium hydroxide is gradually added to aqueous beryllium sulfate until the sodium hydroxide is in an excess . Write equations for all reactions that have taken place. - white ppt, which dissolves in excess NaOH to give a colourless solution Be2+(aq) + 2OH-(aq) ® Be(OH)2(s) Be(OH)2(s) + 2OH-(aq) ® Be(OH)42-(aq) (iii) Suggest the pH of the solution formed when beryllium chloride is dissolved in water. Give your reasoning. pH 3 Be2+ ions have high charge density , which polarises neighbouring H 2O molecules; hence, weakening O— H and H+ lost (iv) Magnesium ions are essential for the action of some enzymes (e.g. alkaline phosphatase found in the liver) by receiving electron pairs from oxygen and nitrogen atoms in the protein. It is thought that beryllium compounds are poisonous because they displace magnesium ions from these enzymes. Suggest a reason why beryllium ions should behave in this way. Be2+ ions have higher charge density (or greater polarising power) than Mg2+; hence has greater tendency to receive electron pairs to form dative covalent bonds. (v) Beryllium chloride may be used as a catalyst in the chlorination of benzene. Suggest a reason why this is possible. O utline the mechanism to show how beryllium chloride is involved in this reaction. [10] In BeCl2, Be atom has only 4 outer electrons and so, is able to act as lone pair acceptor (to generate Cl+ electrophile) BeCl2 + Cl2 à BeCl3- + Cl+

3 9647/03/CJC JC2 Preliminary Exam 2012 Cl+ Cl H Cl H BeCl3 -+ Cl BeCl2 HCl+ + (b) A student carried out a kinetics experiment using a roll of magnesium ribbon that had been exposed to air for some time. He placed a piece of magnesium ribbon of mass 0.12 g into a flask containing 15.0 cm 3 of 1.0 mol dm–3 hydrochloric acid. The progress of the reaction was followed by measuring the pressure of the system at different times. The graph below shows the results of the experiment. (i) Determine, by calculation, the limiting reagent for the experiment. Mg + 2HC l ® MgCl2 + H2 amt of Mg = 3 . 24 12 . 0= 0.00494 mol amt of HCl = 1.0 x 1000 0 . 15= 0.015 mol since Mg º 2 HCl, hence, amt of HCl required for reaction = 2 x 0.00494 = 0.00988 mol < 0.015 mol (initial amount of H