NYJC H2 Chem 2013 Prelim P3 QP

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 19 Sep 2013 2 hours Candidates answer Section A on the Question Paper Additional Materials: Answer Paper Data Booklet 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. 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. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Section B Answer all questions on separate answer paper. For Examiner’s Use Section A 1 2 3 4 5 Total This document consists of 11 printed pages 40

2 Answer any four questions. 1 The elements of Group VII of the Periodic Table are called halogens, which mean "salt formers". They lack only one electron to fo rm a complete shell, and are extremely active chemically. This question examines the chemistry of two reactions of halogens. (a) In the first reaction, a 1:2 mixture of H 2 and I 2 gases was mixed at a constant temperature of 430 oC and a constant pressure of 2 atm. The following equilibrium was set up. H2 (g) + I2 (g) 2HI (g) ∆H = −11 kJ mol1 (i) Given that degree of dissociation of H 2 is 86 %, calculate the equilibrium constant, Kp for the above reaction under these conditions. (ii) Suggest, with reasoning, a way to increase degree of dissociation of H 2 for the above reaction. (iii) Deduce how K p for the following system would be compared to your answer in (i). H2 (g) + Br2 (g) 2HBr (g) [7] In another reaction, halogens can react with unreactive alkanes to form mono- substituted product. An example is the reaction of chlorine with 2-methylpentane shown below. 2-methylpentane (b) (i) Explain why alkanes are generally unreactive. (ii) This reaction is seldom used for synthesis as there are many associated problems. Firstly, several isomeric pr oducts are formed. The relative ratio of the isomeric products may be more a ccurately determined if relative rates of abstraction of H atoms are taken into account. The relative rates of abstraction of H atoms are Type of H atoms Relative rate of abstraction Primary 1 Secondary 4 Tertiary 6 By examining the difference in stab ility of the intermediates formed when different types of H atom are abstracted, explain the trend in relative rate of abstraction. 41

3 H2 Chemistry 9647/03 NYJC J2/2013 Prelim [Turn over (iii) Predict the ratio of the following two products A and B, taking into account the relative rates of abstraction given in (ii). Explain your reasoning. Cl and A B Cl (iv) Describe suitable chemical test(s) to distinguish compounds A and B in (iii). [7] (c) Another problem of the reaction is multi-substitution. (i) Suggest the conditions that will give rise to formation of multi-substituted products. (ii) A di-substituted product C with molecular formula C 6H12Cl2 was obtained in the reaction of chlorine with 2-methylpentane. C does not exhibit optical activity. On warming with alcoholic KOH, a major product D is formed. When D is treated with hot acidified potassium manganate(VII) solution, effervescence was observed and the only organic product remaining gives a yellow precipitate with alkaline aqueous iodine. Use this information to deduce the structure of C and D , explaining your reasoning. [6] [Total: 20] 42

4 2 Precipitation plays an import ant function in growth of bones and teeth in our bodies. Teeth and bones are largely compos ed of calcium phosphate salt Ca 3(PO4)2. In order for this precipitation to occur, the concentrations of the ions in blood must exceed the solubility product in the immediate region of deposition. (a) Write an ionic equation with st ate symbols to show the formation of teeth and bones at the deposition region and hence an expression for the solubility product of the Ca3(PO4)2 deposits. [2] (b) (i) At a temperature of 37 oC, the solubility product for calcium phosphate Ca3(PO4)2 has a numerical value of 4 × 10 -27. In a sample of blood at a pH of 7.4, the concentrations of calcium and phosphate ions are found to be 1.2 × 10 -3 mol dm-3 and 1.6 × 10-8 mol dm-3 respectively. State and explain if the above composition in blood will resu lt in the growth or degeneration of teeth and bones. (ii) Hence, state and explain if t he pH environment of 7.4 in (i) is that for a growing child, a fully grown man or an old person. [3] (c) The pH at the area of growth may change due to metabolic processes in the body. The hydrogen ions that are present combine phosphate ions as hydrogen phosphate ions. This prevents growth of strong teeth and bones. PO 4 3- (aq) + H+ (aq) HPO4 2- (aq) Assuming that the [HPO4 2-] is maintained at 1.7 × 10-4 mol dm-3 by cell functions, (i) calculate the pH that will result in nei ther growth nor degeneration at the teeth and bones deposition region when the concentration of calcium ions in blood is maintained at 1.2 × 10-3 mol dm-3. [Ka of HPO4 2- = 4.4 × 10-13 mol dm-3] (ii) Suggest another acid–conjugate base e quation that accounts for the constant supply of HPO4 2- ions from cell functions. [3] 43

5 H2 Chemistry 9647/03 NYJC J2/2013 Prelim [Turn over (d) (i) Construct an energy level diagr am for the formation of Ca 3(PO4)2 from its elements using relevant data from the table below and other appropriate values in the Data Booklet. Hence calculate the lattice energy of Ca3(PO4)2. Enthalpy ∆H / kJ mol-1 Standard enthalpy change of formation of Ca3(PO4)2 −4132 Standard enthalpy change of atomisation of Ca +178 Standard enthalpy change of atomisation of P +315 P(g) + 4O(g) + 3e → PO4 3-(g) −2810 (ii) Using your answers to (i) and other enthalpy change value if any, explain why Ca 3(PO4)2 is insoluble. [6] (e) Use of the Data Booklet is relevant to this question. Group VII elements exist as ions and play a vital role in bodily functions. The following observations are made about Gro up VII elements and their reactions. Explain the observations made. (i) F 2 cannot be prepared by electrolytic oxidation of aqueous F- solutions. (ii) HI can be prepared by treating NaI with phosphoric acid, H3PO4 but not with concentrated sulfuric acid. (iii) Interhalogen compounds such as IF 3, IF 5 and IF 7, BrF 3, BrF 5, ClF 3, ICl 3 exists. On the other hand, ClF 5 is not easily formed and BrCl 3 does not exists. [6] [Total: 20] 44

6 3 (a) Period 3 elements react with oxygen to form the corresponding oxides of varying acid-base nature. Using magnesium, aluminium and phosphorus as examples, describe the acid-base nature of the ox ides across Period 3 with suitable equations.