NJC Prelim P2 Question Paper

NJC SH2 Preliminary Examination 9729/02/17 [Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on Question Paper. Additional Materials: Data Booklet 9729/02 Thursday 24 August 2017 2 hours READ THESE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid/tape. Answers all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. Appropriate significant figures and units are expected for final numerical answers. For Examiner’s Use 1 /14 2 /16 3 /11 4 /16 5 /18 Paper 2 Total /75 This document consists of 19 printed pages and 1 blank page.

2 NJC SH2 Preliminary Examination 9729/02/17 1 Nitrous oxide, N2O and nitrogen dioxide, NO2, are atmospheric pollutants. Nitrous oxide has an atmospheric residence time as long as 20 to 30 years but nitrogen dioxide has a residence time of only approximately 4 days. (a) Suggest how NO 2 is formed in a car engine and how it may be removed from car exhaust gases. [2] (b) (i) Draw the dot -and-cross diagrams of N 2O and NO 2, given that nitrogen is the central atom in both species. [2] (ii) Hence, explain for the difference in residence times between N2O and NO2. [1]

3 NJC SH2 Preliminary Examination 9729/02/17 [Turn over (c) In the gaseous state, NO2 can dimerise as follows. 2NO2(g) N2O4(g) The following data are for NO2(g) and N2O4(g) at 298 K. ΔHfo / kJ mol‒1 ΔSfo / J mol‒1K‒1 NO2(g) +33.2 +240 N2O4(g) +9.2 +304 (i) Calculate ΔHo and ΔSo for the above reaction. [2] (ii) Explain the signs of your calculated ΔHo and ΔSo. [2] (iii) The two gases in the above reaction can co-exist in an equilibrium. Calculate the temperature at which a dynamic equilibrium is established. [2]

4 NJC SH2 Preliminary Examination 9729/02/17 (iv) At 294 K, N2O4 liquefies. Given that the molar entropy change of vaporisation of N2O4 is +88 J mol ‒1K‒1, calculate the molar enthalpy change of vaporisation of N2O4. [1] (v) With reference to your answers in (c)(i) and (iv), calculate ΔH and ΔS for th e reaction below. 2NO2(g) → N2O4(l) [2] [Total: 14]

5 NJC SH2 Preliminary Examination 9729/02/17 [Turn over 2 This question is about the reactions of iodine and its compounds. (a) In the 18 th and 19th centuries, iodine was industrially produced from kelp, a large seaweed. Combustion of kelp converts the organic substances to ash, and sodium halides (mainly sodium iodide) are obtained. In the laboratory, a similar process can be done according to the following procedure. 1. Fill a large crucible on a tripod with the seaweed. Heat with a strong Bunsen flame until all the seaweed has been turned to ash. 2. Boil the ash with about 20 cm3 of purified water in a beaker, and filter while hot. Collect the filtrate in a second beaker and allow to cool. 3. Add about 2 cm3 of dilute sulfuric acid to the filtrate, followed by hydrogen peroxide solution. 4. Transfer the mixture to a separating funnel and add 10− 20 cm 3 of a suitable organic solvent . Stopper the separating funnel and shake vigorously for about 30 s. With the separating funnel inverted, release any pressure that has built by opening the tap briefly. 5. Clamp the funnel and allow the layers to separate. 6. Run off the aqueous layer into a 250 cm3 conical flask. 7. Run the organic layer into an evaporating basin, and set aside to evaporate in the fume cupboard to obtain the iodine crystals. (i) State and explain, with the aid of a relevant equation, what you would observe during procedure 3. [2] (ii) With reference to the Data Booklet, explain why hydrochloric acid is not used in the acidification process in procedure 3. [2]

6 NJC SH2 Preliminary Examination 9729/02/17 (iii) Suggest a suitable organic solvent for the extraction of iodine in procedure 4 and state what you would observe. [2] (iv) Suggest a reason why in procedure 7, evaporation is employed instead of direct heating over a strong Bunsen flame. [1] (v) In order to check if the kelp contained significant amount of chloride anions, a student transferred 1 cm 3 of the filtrate obtained in procedure 2 into a test tube and added acidified silver nitrate solution. A cream precipitate was obtained. State and explain what reagent should be added to verify if there was a significant amount of chloride mixed with iodide. [2]

7 NJC SH2 Preliminary Examination 9729/02/17 [Turn over (b) Sodium iodate, NaIO4 (Mr = 214.0), is a powerful oxidising agent that can be used to oxidise thioethers into sulfoxides. Thioanisole (Mr = 124.1), C6H5SCH3, is a thioether. In a reaction between thioanisole and sodium iodate, it was found that 0.9 cm3 of thioanisole (density = 1 .06 g cm −3) reacts completely with 15.40 cm3 of 107 g dm⁻3 solution of sodium iodate to give methyl phenyl sulfoxide, C6H5SOCH3. (i) Deduce the stoichiometry ratio for the reaction between thioanisole and sodium iodate. [2] (ii) Hence, write the balanced chemical equation for the reaction. [1] (iii) Thiophenol, C6H5SH, has similar reaction s as phenol. Suggest how thioaniso le can be synthesized from thiophenol in two steps. [2]

8 NJC SH2 Preliminary Examination 9729/02/17 (c) Hydrogen peroxide, H2O2, is another common redox reagent which can act as either an oxidising agent or reducing agent. H 2O2 is a weak acid that dissociates according to the following equilibrium. H2O2 HO2 + H+ With reference to the Data Booklet, deduce whether H2O2 is a better reducing agent in acidic or alkaline condition. [2] [Total: 16]

9 NJC SH2 Preliminary Examination 9729/02/17 [Turn over 3 The pinacol rearrangement is a 1,2–rearrangement procedure which converts 1,2–diol to a carbonyl compound. This rearrangement process occurs under acidic condition (e.g. H2SO4). The diagram below shows the key steps in the mechanism, with movement of electrons pairs represented by curly arrows, needed to generate the carbonyl from 1,2–diol. In a particular synthetic route, an alkene U, of molecular formula of C8H14, was first converted into a diol W. Compound W then reacted with H2SO4 to form two carbonyls X and Y as major products, in a reaction similar to pinacol rearrangement. Compound X produces a yellow precipitate when heated with alkaline I2 solution but not for compound Y. Compound Y gives a silver mirror when warmed with ammonical solution of silver nitrate. (a) Suggest the structure of alkene U. [1] (b) State the rea