CJC H2 CHEM P2 ANSWER Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS 2T CHEMISTRY 9647/02 Paper 2 Structured Questions TUESDAY 13 SEPTEMBER 2011 2 Hours Candidates answer on the Question Paper. Additional Materials: 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. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] 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. 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 brackets [ ] at the end of each question or part of the question. At the end of the examination, fasten all work securely together. ANSWERS This document consists of 17 printed pages and 1 blank page. [Turn over]

2 9647/02/CJC JC2 Prelim Exam 2011 1 Planning (P) A mixture of sodium carbonate, Na2CO3 and sodium hydrogencarbonate, NaHCO3 is heated strongly with a bunsen burner. Only sodium hydrogencarbonate, NaHCO 3 decomposes on heating. 2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g) You are to design an experiment to determine the percentage by mass of sodium hydrogencarbonate, NaHCO3 in a 5.00 g sample of a mixture of sodium carbonate, Na 2CO3 and sodium hydrogencarbonate, NaHCO3, by heating alone. The only apparatus available consists of a boiling-tube and holder, a heat-proof mat, a chemical balance and a bunsen burner. (a) Outline step by step, the practical sequence for the method you would use to  make appropriate weighings,  decompose NaHCO 3 in the sample by heating,  ensure that decomposition was complete. Weigh accurately the empty dry boiling-tube, using a chemical/electronic balance. Add 3.00 g of the mixture of the sample and weigh the boiling-tube and contents (or any reasonable specified amount). Heat the sample in the boiling-tube strongly, for about 5 minutes, using a test-tube holder and bunsen burner. Allow the boiling-tube and the contents to cool on a heat-proof mat. After cooling, reweigh the boiling-tube and its contents. Repeat the heating, cooling and reweighing until there is no further loss in mass. This is to ensure that all the NaHCO3 has decomposed completely. [4] (b) Show how you would tabulate your results. Include in your table and other masses you would calculate from the experimental results to enable you to determine the percentage by mass of NaHCO3 in the sample of the mixture. Insert in your table, the letters A, B, C etc. to represent each measurement of mass. Mass of boiling-tube and sample/g B Mass of empty boiling-tube/g A Mass of sample/g B - A Mass of boiling-tube and contents/g after 1st heating/g after 2nd heating/g C C(showing constant mass) Loss in mass/g B - C [3]

3 9647/02/CJC JC2 Prelim Exam 2011 (c) Use the letters you have entered in (b) to show how you would process the results to find (i) the mass of NaHCO 3 in the sample of the mixture 2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g) m g (say) (B – C) g Amount of NaHCO 3 = 84 m mol Amount of CO 2 = amount of H2O = 2 1  84 m = 168 m mol  168 m (44.0) + 168 m (18.0) = (B – C) g 168 62m = (B – C) m = 2.71(B – C) g (ii) the % by mass of NaHCO 3 in the sample of the mixture. % by mass of NaHCO3 = AB CB   )(71.2  100 % [4] (d) Suggest an alternative experimental method, not involving weighing, which could be used to determine the % by mass of NaHCO3 in the sample of the mixture. By heating the sample and collect (and measure) the volume of CO2(g) liberated using a gas syringe at r.t.p, OR By adding excess H 2SO4(aq)/HCl(aq) to the sample and collect (and measure) the volume of CO2(g) liberated using a gas syringe at r.t.p. OR By titrating an aqueous solution of the sample with standard H2SO4(aq)/HCl(aq) by a double indicator method (phenolphthalein and methyl orange). [1] [Total: 12]

4 9647/02/CJC JC2 Prelim Exam 2011 2 Most automobiles today are powered by internal combustion engine, where a certain fuel is used and burned with a mixture of air to produce useful energy. In Singapore, methane (marketed as compressed natural gas, CNG), has been actively promoted and used as an alternative fuel to petrol (contains mainly iso-octane). Biofuels such as methanol and ethanol have been considered viable alternative fuels as well. Some properties of the fuels are listed in the table below. Types of Fuel Hc o Standard enthalpy change of combustion (kJ mol-1) Boiling point (oC) Solubility in water (g/cm3) CH4 −891 -161 35 iso-octane (C8H18) −5461 99 insoluble CH3OH −726 60 very soluble CH3CH2OH −1300 74 very soluble (a) Iso-octane has the IUPAC name 2,2,4-trimethy lpentane. Draw the structural formula of iso-octane. (You may wish to use skeletal formula.) [1] (b) Boiling point is an important practical consideration when selecting fuel. (i) Account for the difference in boiling point between iso-octane and methane. ………………….…………………………………………………………………………… ……………..………………………………………………………………………………… ………..……………………………………………………………………………………… …..…………………………………………………………………………………………… (ii) Suggest a possible benefit of using petrol (iso-octane) compared to methane as fuel due to its boiling point. ………………………………………………………………………………………….…… [3] Iso-octane is a larger molecule with a larger electron cloud/number of electrons,which results in more extensive van der waal’s forces of attraction as compared to methane. Hence more energy is required to break the intermolecular forces of iso-octane during the boiling process. Ease of storage or transport as octane is liquid at r.t.p, while methane is a gas.

5 9647/02/CJC JC2 Prelim Exam 2011 (c) Another important factor for suitable alternative fuel is the amount of heat released which can be related to the standard enthalpy change of combustion, Hc of the compound. (i) Define the term standard enthalpy change of combustion. ………………………………………………………..……………………………………… …………………………………………………………..…………………………………… ……………………………………………………………..………………………………… (ii) From the data given in the table, H c of methanol is −726 kJ mol-1. However, when the value is calculated from bond energy data, the Hc of methanol is −886 kJ mol-1. Account for the large difference in the values. …………………………………………………..…………………………………………… …………………………………………………….………………………………………… (iii) With reference to the table, iso-octane releases the most energy. But for practical reasons, the most ideal fuel is best quantified by the highest energy released per unit mass. By considering this, state which fuel is most ideal and energy released per gram. Fuel: …………………………... Energy evolved: …………….…kJ g -1 (iv) A common fear of installing CNG tank is the possible leakage and explosion of the methane gas. However, methane will only burn at 1200 K without any spark or ignition. Suggest the reason for the need of a high temperature. ……………………………………………………………………………………………..… ………………………………………………………………………………………………. [5] The standard enthalpy change of combustion is defined as the enthalpy change when one mole of a compound is completely burnt in excess oxygen under standard conditions of 298K and 1 at