TJC Prelim P2

TEMASEK JUNIOR COLLEGE Preliminary Examinations Higher 2 CANDIDAT E NAME CIVICS GROUP / CHEMISTRY 9746/02 Paper 2 Structured Questions 28 September 2009 1 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet READ T HESE INSTRUCTIONS FIRST Write your Civics Group and candidate name on all the work you hand in. Write in dark blue or black pen. 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 all questions. You are reminded of the need for good English and clear presentation in your answers. You may lose marks if you do not use appropriate units. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 3 4 5 Total This paper consists of 16 printed pages.

Answer all questions in the spaces provided. 1 This question is about chlorine and its compounds. (a) A teacher instructed a student to r eact some chlorine gas with 250 cm 3 of 0.100 mol dm -3 aqueous sodium hydroxide, an d left the lab. After he returned, he realized that he had not s pecified the reaction temperature. The teacher found that the pH of the solution had become 12.0. After acidifying the solution and adding excess silver ni trate, 2.69 g of silver chloride was obtained. He then concluded that the student had heated the sodium hydroxide. For this question, you may assume that all of the chloride ions are precipitated by the addition of silver ions. For Examiner’s Use (i) Determine how the teacher arrived at his conclusion. (ii) If the student had not heated the aqueous sodium hydroxide and the same amount of chlorine gas was us ed, what would be the resulting mass of silver chloride obtained when excess silver nitrate is added after acidifying the solution? [5] [Turn over] 2

(b) The melting point of POCl 3 is 1.25 oC whereas the melting point of PC l5 is 166.8 oC. (i) Name and draw diagrams to illustrate the shapes of POCl 3 and PCl5. (ii) Explain, in terms of st ructure and bonding, why PC l5 has a higher melting point. For Examiner’s Use [4] [Turn over] 3

(c) Phosphorous pentachloride is commonly used in organic reactio ns such as the following: CH3OH(l) + PCl5(s) CH 3Cl(g) + POCl3(l) + HCl(g) For Examiner’s Use (i) With reference to the Data Booklet and the following bond enthalpies, calculate the enthalpy change of the above reaction. Bond Bond Energy / kJ mol -1 P=O 460 P-Cl 331 (ii) Deduce whether the entropy change is positive or negative and hence explain why the above reaction occurs at room temperature. [Turn over] 4

(iii) Given that the actual standard enthal py change of reaction is 104.3 kJ mol1, suggest two reasons for the discrepancy. [7] For Examiner’s Use (d) A student mixed up three test tubes containing chlorobenzene, trichloromethane and ethanoyl chloride separately. (i) Describe a chemical test to first identify ethanoyl chloride from among the three test tubes. [Turn over] 5

(ii) Describe a second chemical test to then identify tr ichloromethane from among the remai ning two test tubes. [4] [Total: 20 marks] For Examiner’s Use [Turn over] 6

2 The use of the Data Booklet is required for this question. (a) Bromine is the only liquid non-metallic element at room temperature . It reacts vigorously with metals, especially in the presenc e of water, as well as most organic compounds, especially upon illumination. Bromine is also used in the zinc -bromine flow battery which is a rechargeable battery. The two elec trode chambers are separated by a micro-porous membrane which prevents bromine from reaching the zinc electrode where it would react with zinc, causing the battery to self-discharge. During the charging of the battery, bromide is converted to bromine. For Examiner’s Use (i) In the table below, indicate the polarity (+/ ) of the electrodes and write the half-equations for the electrode processes that occur when the battery discharges. Electrode Polarity Half-equation Zinc Carbon carbon zinc mixture of ZnBr2 (aq) and Br2 (aq) ZnBr2 (aq) micro-porous membrane [Turn over] 7

(ii) 4.2 g of bromine is formed in the cell when a current of 2.0 A is passed through it during charging. Determine the l ength of time required to form 4.2 g of bromine. For Examiner’s Use (iii) In a research laboratory, a resear ch worker first charged a zinc-bromine battery fully before adding 10 cm 3 of 0.2 mol dm 3 sodium bromide solution to each of the two electrode chambers. What is the effect on the electrical output of the battery when it is discharged? [8] [Turn over] 8

(b) Ethanol fuel cells are more practical t han hydrogen fuel cells since ethanol is easier to store and transport than hydrogen. Ethanol is oxidised into carbon dioxide and water when the fuel cell is operated. For Examiner’s Use (i) Write the half-equations taking place at each of the electrode: Anode: ____________________________________________ Cathode: ____________________________________________ ] (ii) Use appropriate data from the Data Booklet to explain why an acidic electrolyte is often preferred to an alkaline or neutral electrolyte. [4] H2SO4 (aq) ethanol air wire connecting a pair of graphite electrodes [Turn over] 9

(c) Ethanol can be oxidised to form ethanoi c acid, which is used to prepare pyruvic acid, CH3COCO2H in a 3-step synthesis. For Examiner’s Use (i) State the reagents and conditions for stages I and III. Stage I: ____________________________________________ Stage III: ____________________________________________ (ii) Identify the intermediate compound, L. [3] [Total: 15 marks] CH3CO2H I CH3COCl L CH3COCO2H pyruvic acid KCN III [Turn over] 10