SRJC H2 Chem 2012 Prelim P3 QP

1 SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CLASS CHEMISTRY 9647/03 Preliminary Examination 22 August 2012 Paper 3 Free Response Questions 2 hours Additional Materials: Data Booklet 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. At the end of the examination, fasten all your work securely together with the cover page provided. This document consists of 13 printed pages and 1 blank page 2012 JC2 PRELIM 9647 P3 Turn Over]

2 2012 JC2 PRELIM 9647 P3 Turn Over] Answer any four questions 1 (a) Aqueous sodium hydroxide is an important reagent in many organic reactions where the hydroxide ion can act as both a base and a nucleophile. In the Reimer-Tiemann reaction, phenol is heated with chloroform in the presence of aqueous sodium hydroxide to give 2-hydroxybenzaldehyde, compound B . The reaction involves the formation of a reactive intermediate, CCl2, through an acid- base reaction. CHCl3 + OH- ® CCl2 + H2O + Cl – chloroform (i) State the type of reaction that occurred in reaction I. (ii) Suggest a chemical test to differentiate compound s B and A under room conditions. (iii) Compound B can undergo further reaction to form the product below. Name and describe the mechanism involved, using curly arrows to show the movement of electrons and indicating all charges. (iv) Comment on the relative acidity of phenol and compound B. [9] NaOH (aq) heat II OH + CCl2 OH CHCl2 OH CHO B A I OH C OH CN H B OH CHO

3 2012 JC2 PRELIM 9647 P3 Turn Over] (b) Besides its exten sive use in organic chemistry, aqueous sodium hydroxide is also used in photographic development. The black-and-white photography process involves four steps: Step 1 Photo-taking: Interaction of light with silver halides (AgX) on film. Step 2 Developing: Redox reactions to develop the actual image. Step 3 Fixing: Changing the solubility of chemical compounds with different conditions to preserve the image on the film. Step 4 Printing: Shining light through the film onto photosensitive paper, producing the final photograph using the same chemistry as step 1 to 3. When light is shone on the film during the phototaking process, a very small number of X- ions in AgX are oxidised. (i) A small amount of silver is produced as a resu lt of the oxidation reaction of X-. The silver produced catalyses the reduction of the surrounding AgX to black silver metal in the presence of a reducing agent, D, during the developing process. 2AgX + D + 2OH- ® 2Ag + E + 2H2O + 2X- State and explain why the rate of reaction changes as the developing process proceeds. (ii) Under appropriate conditions, chloride can be oxidised to chlorine gas. With the aid of an equation, suggest how C l2 will react with hydroxide ions under room conditions. (iii) E, formed from the oxidation of D, has the molecular formula C6H4O2. E forms an orange precipitate when warmed with 2,4-dinitrophenylhydrazine. One mole of E decolourises two mole of bromine dissolved in tetrachloromethane. Heating E with acidified potassium manganate( VII) produces only one product. D has the molecular formula C 6H6O2. It is insoluble in water but can dissolve in aqueous sodium hydroxide. Addition of sodium carbonate to D does not result in effervescence but addition of sodium metal to D gives mild effervescence. Suggest the structures of D and E, explaining your reasoning. (You are not required to explain the oxidation process of E from D) [11] Total 20 marks

4 2012 JC2 PRELIM 9647 P3 Turn Over] 2 Halogens are highly reactive and they are the only group in the Periodic Table which contains elements in all three familiar states of matter at 25 oC and 1 atm. The halogens form hydrogen halides with hydrogen, all of which are strong acids with the exception of HF. (a) Discuss the thermal stability trend of the hydrogen halides , HCl, HBr and H I and variation in volatility of the halogens from fluorine to iodine. [5] (b) Concentrated sulfuric acid will react with solid sodium halides NaX. (X = Cl -, Br - or I-). Using relevant data from the Data Booklet, describe and explain why hydrogen chloride can be prepared by the reaction between sodium chloride and concentrated sulfuric acid while hydrogen iodide cannot be synthesised using a similar method with sodium iodide and concentrated sulfuric acid. You are to include relevant chemical equation(s) in your discussion. [5] (c) Halogens form many compounds with metals. These compounds range from ionic compounds such as lithium chloride, monomeric covalent compounds such as uranium hexafluoride, and polymeric covalent compounds like palladium chloride. The Corey-Posner, Whitesides-House reaction is an organic reaction that involves the reaction of a lithium dialkyl cuprate with an alkyl halide to form a new alkane, an organocopper compound and a lithium halide. The formation of propane from a halogeno compound using the Corey-Posner, Whitesides-House reaction is as shown. (CH 3)2CuLi + CH3CH2X ® CH3CH2CH3 + CH3Cu + LiX (X = Cl -, Br - or I-) (i) Draw the structural formulae of the three alkanes produced when (C3H7)2CuLi and C3H7Br undergo the Corey–Posner, Whitesides–House reaction. (ii) Suggest with reasoning which of the alkanes in (c)(i) has the lowest boiling point. [6]

5 2012 JC2 PRELIM 9647 P3 Turn Over] (d) Chlorine and fluorine react exothermically to form an interhalogen compound, ClF3. (i) With the help of a Lewis structure, state the shape of the ClF3 molecule. (ii) The interhalogen compounds are very strong oxidising agents. When gaseous C lF3 is added to water, a vigorous reaction occurs, giving three gases as the only products. The gaseous product mixture appears as white fumes which forms a weak acid. The gaseous mixture not only turns moist blue litmus paper red but also bleaches it. In add ition, it relights a glowing splint. State the three gases in the gaseous mixture. Hence, c onstruct a balanced equation, including state symbols, for the reaction. [4] Total 20 marks

6 2012 JC2 PRELIM 9647 P3 Turn Over] 3 (a) Nitrous acid , HNO 2, is an unstable, weakly acidic compound. It can easily be prepared by acidifying a solution of a nitrite. Barium nitrite, Ba(NO 2)2 is often used since the insoluble barium sulfate can be easily filtered off. Ba(NO2)2 (aq) + H2SO4 (aq) à 2HNO2 (aq) + BaSO4 (s) Nitrite ions can be oxidised by potassium manganate(VII) to form nitrate ions: 5NO2- (aq) + 2MnO4- (aq) + 6H+ (aq) → 5NO3- (aq) + 2Mn2+ (aq) + 3H2O (l) The following are some t