SRJC H2 Chem 2013 Prelim P3 QP

1   Turn Over] SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CLASS CHEMISTRY 9647/03 Preliminary Examination 27 Aug 2013 Paper 3 Free Response Questions 2 hours Additional Materials: Data Booklet, Cover Page Writing Papers 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 work. 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 The number of marks is given in the brackets [ ] at the end of each question or part question. Start each question on a fresh piece of paper. At the end of the examination, fasten all your work securely together with the cover page provided and ensure your script is arranged in an orderly manner. This document consists of 11 printed pages 1281

2   Turn Over] 1 Phosphorus is the thirteenth element in the world to be discovered and due to this is sometimes referred to as the Devil’s element. The reference is also made due to its versatility in forming chemical species with explosive and toxic nature. (a) One example of a highly reactive phosphorus-containing ion is phosphonium, PH 4 +. (i) Write a balanced chemical equation for the reaction between phosphonium iodide and potassium hydroxide to form phosphine, PH3. (ii) Hence, suggest a suitable method of measuring the rate of reaction. (iii) Draw a ‘dot-and-cross’ diagram showing the electrons (outer shells only) in the phosphonium ion, and use the VSEPR (valence shell electron pair repulsion) theory to predict its shape. You may find both a written description and a 3-dimensional sketch useful in your answer. (iv) The bond angle in ammonia is 107.8 o. Predict the H – P – H bond angle in phosphine and justify your answer. [7] (b) Although phosphorus is present in numerous harmful chemicals, it is a useful feature in organophosphorus compounds. Ylides are a class of organophosphorus compounds that are well-known for their role in the Wittig reaction. The synthesis of methylpropene via the Wittig reaction is shown below. P                     PC H 3                      PC H 2 (i) Name the type of reaction that occurs in step I and outline the mechanism for the reaction. (ii) Suggest why 2-bromo-2-methylpropane is not preferred in step I. (iii) State the role of butyl lithium, C4H9Li, in step II. [5] C4H9Li II CH3Br I O III 1282

3   Turn Over] (c) Two resonance structures exist to stabilise the ylide used in step III above. PC H 2                       PC H 2 In this step, tetrahydrofuran, THF, is used as an aprotic solvent – one that lacks an acidic hydrogen. (i) Explain why, unlike phosphorus, nitrogen cannot form organic compounds similar to ylides. (ii) Explain why THF is a suitable solvent to employ in step III whereas water is not. [4] (d) -carotene is a food colouring that can be extracted from the pigmentation found in red-orange plants and fruits such as carrots. It can be synthesised using excess of an aldehyde and a diylide, a compound with two equivalents of phosphorus in its structure. -carotene Suggest the structures of the aldehyde and a symmetrical diylide with 6 stereoisomers that can be used to produce -carotene. [2] (e) Compounds X and Y are isomers that can undergo the Wittig reaction with the following ylide. PC H ( C H3) The products formed have the same molecular formula, C7H14, but only the product formed from X displays stereoisomerism. X and Y both do not react with ammonical silver nitrate. Draw the structures of X and Y. [2] [Total : 20] 1283

4   Turn Over] 2 (a) Deuterium (symbol D or Hଵ ଶ ) was discovered in 1931. Deuterium accounts for approximately 0.0156% of all the natur ally occurring hydrogen in the oceans, while the most common isotope (hydrogen-1) accounts for more than 99.98%. Chemically, deuterium behaves similarly to ordinary hydrogen. (i) On the same diagram, sketch how a beam of singly positively-charged deuterium ions and a beam of hydrogen ions will behave in an electric field. In your diagram, indicate clearly the angle of deflection for each beam. (ii) Suggest the difference in the melting point and thermal stability of DC l, DBr and D I. (iii) Chloride and iodide ions are known to react differently with concentrated deuterium sulfate, D 2SO4. For the reaction involving iodide, it was observed that the yield of D I was significantly lower as compared to the yield of DCl for the reaction using chloride. It was also noted that a foul rotten egg smell was detected for the reaction involving the iodide. With reference to the Data Booklet and with the aid of equations , explain the above mentioned observations. [10] (b) Deuterium can replace the normal hydrogen in water molecules to form heavy water, D 2O. This difference increases the strength of water’s hydrogen-oxygen bonds which makes it more difficult to undergo electrolysis. Some data between light water and heavy water are given below. Property D2O (Heavy water) H2O (Light water) Freezing point (oC) 3.82 0.00 Boiling point (oC) 101.4 100.0 Density at standard condition (g/mL) 1.1056 0.9982 Heavy water and light water can be tested using their freezing points and boiling points. Suggest with reasoning, how a scientist can deploy another physical method to differentiate the two types of water without the use of a temperature measuring device. [2] 1284

5   Turn Over] (c) During World War II, German scientists were known to conduct experiments using heavy water. (i) With reference to the data given below, determine why it is more difficult for heavy water, D 2O, to undergo electrolysis. Bond type Bond length (m) Cl–Cl 1.76 x 10–10 D–Cl 1.27 x 10–10 H–Cl 1.29 x 10–10 (ii) Heavy water is processed via electrolysis using a concentrated potassium hydroxide-light water mixture. During the process, oxygen gas is produced at the anode and hydrogen gas is produced at the cathode. Write ion-electron half equations at the respective electrodes and suggest how heavy water can be obtained from the mixture. (iii) Suggest why distillation is not effective as the initial stage in separating heavy water from the potassium hydroxide-light water mixture. [8] [Total : 20]   1285

6   Turn Over] 3 Chlorine is often used as a disinfectant in swimming pools to remove organic molecules and pathogens. The disinfecting action is due to the presence of hypochlorous acid, HC lO, and chlorate(I) ion, OCl - formed by the reaction of chlorine with water. Cl2(aq) + H2O(l) HC lO(aq) + H+(aq) + Cl– (aq) --- (1) Hypochlorous acid ionises as a weak acid with dissociation constant of 3 x 10-8 moldm-3. HClO(aq) H +(aq) + OCl– (aq) --- (2) The hypochlorous acid and chlorate( I) ions make up ‘free available chlorine’ (FAC). When chlorine is added, FAC combines with the contaminants in the pool, called combined available chlorine (CAC). The total available chlorine (TAC) is the sum of free available chlorine (FAC) and combined available chlorine (CAC). It is undesirable when CAC exceeds 0.2 ppm as chloroamine