2016 HCI H1 Chemistry Prelims P2 Answers

HWA CHONG INSTITUTION Preliminary Examination Higher 1 CANDIDATE NAME CT GROUP 15S CHEMISTRY Paper 2 Candidates answer Section A on the Question Paper. Additional Materials: Data Booklet Writing paper; Graph paper 8872/02 30 August 2016 2 h READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue, correction fluid or tapes. Section A Answer all questions. Section B Answer two questions on separate answer paper. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. FOR EXAMINERS’ USE ONLY Paper 1 Paper 2 TOTAL Multiple Choice Section A (Structured) Section B (Free Response) Q1 /10 Q4 / 20 Q2 /14 Q5 / 20 Q3 /16 Q6 / 20 / 30 Subtotal / 40 Subtotal / 40 This question booklet consists of 13 printed pages. 110

2 Section A Answer all the questions in this section in the spaces provided. 1 (a) An equilibrium exists between aqueous chromate (VI) ions and dichromate (VI) ions as shown by the expression below: 2CrO42-(aq) + 2H+(aq)  Cr2O72-(aq) + H2O(l) Yellow orange (i) State the meaning of the term dynamic equilibrium. ………………………………………………………………………………………. ………………………………………………………………………………………. ……………………………………………………………………………………[1] (ii) Write an expression for the equilibrium constant, Kc, for this reaction. Kc = [Cr2O72-] / [CrO42-]2[H+]2 [1] (iii) The initial concentration of CrO42- ions and H+ ions ae 0.85 mol dm−3 and 1.20 mol dm−3 respectively. After equilibrium is reached, the concentration of Cr2O72- ions is 0.200 mol dm−3. Calculate the value of Kc for this equilibrium and state its units. 2CrO42-(aq) + 2H+(aq)  Cr2O72-(aq) + H2O(l) I 0.85 1.20 0 0 C -0.400 -0.400 +0.200 E 0.45 0.80 0.200 Kc = 0.200/(0.45)2(0.8)2 = 1.54 mol-3 dm9 Kc = ……………………. units = ……………………. [3] (b) (i) State Le Chatelier’s Principle. ………………………………………………………………………………………. ………………………………………………………………………………… …………………………………………………………………………………[1] Volume of dilute HNO3 added / cm3 Dynamic equilibrium is achieved in a reversible reaction when rate of forward reaction equals the rate of backward reaction. LCP states that when a system in equilibrium is subjected to change in conditions which disturbs the equilibrium the position of equilibrium will shift in a way to reduce that change.

3 (ii) Describe and explain what happens to the colour in the beaker when aqueous sodium hydroxide is added. ………………………………………………………………………………………. ………………………………………………………………………………………. ……………………………………………………………………………………[2] (iii) When the beaker containing aqueous chromate (VI) and dichromate (VI) ions in equilibrium is heated, the solution becomes more yellow. Explain whether the equilibrium is exothermic or endothermic. ………………………………………………………………………………………. ………………………………………………………………………………………. ………………………………………………………………………………………. ………………………………………………………………………………………. ……………………………………………………………………………………[2] [Total: 10] The alkali will neutralize the H+ reducing the [H+]. Hence, by Le Chatelier’s Principle, the position of equilibrium will shift left and the colour will more yellow. The forward reaction is exothermic while backward reaction is endothermic. When temperature increase the backward reaction, which reduce temperature is favoured.

4 2 (a) A, B, C, D, E and F are all structural isomers with the molecular formula C4H8O A, B and C all give an orange precipitate when treated with 2,4- DNPH but only A and B give a brick-red precipitate when warmed with Fehling’s solution. (i) Draw the structural formulae of A, B and C A B C (ii) Name the type of structural isomerism shown by A and B Chain isomerism …………………………………………………………………………………………….. (iii) State what you would see when a sample of A is warmed with Tollens’ reagent. …………silver mirror………………………………………………….. …………………………………………………………………………………………….. ……………………………………………………………………………………………. (iv) State the reagents used that give a positive test for C but not A and B. ……………iodine/ NaOH(aq) ……………………… [6]

5 (b) (c) D, E and F all decolourise bromine and effervescence slowly with sodium metal. E shows geometrical isomerism. Only D has branched chain. None of these isomers contains C=O (i) Give the structures of D, E and F. Show the two stereoisomers of E and label the stereoisomerism shown. D E …………… E ……………….. F (ii) Write balanced equation showing how one of these isomers react with sodium metal. ROH + Na  RO- + ½ H2(g) [6] Another compound, G, C3H6O, gives a silver mirror when Tollen’s reagent was added to it. Give equations for the reactions of G with acidified potassium dichromate(VI) and sodium tetrahydridoborate, NaBH4, using [O] or [H] as appropriate. (i) reaction with acidified potassium dichromate(VI) C3H6O + [O]  CH3CH2CO2H…… (ii) reaction with sodium tetrahydridoborate, NaBH4 C3H6O + 2[H]  CH3CH2CH2OH…… [2] [Total: 14]

6 3 ‘Water gas’ is an equimolar mixture of hydrogen and carbon monoxide. It is used as a gaseous fuel in the industry. It is produced when steam is blown through white-hot coke in the following reaction. H2O(g) + C(s)  H2(g) + CO(g) Another widely-used industrial fuel is natural gas, which consists mainly of methane. Hoc values are given in the table below. Substance Standard enthalpy change of combustion, Hoc/ kJ mol–1 CH4 -890 H2 -242 CO -283 C -394 (a) (i) Define the term standard enthalpy change of combustion,Hoc …………………………………………………………………………………….. …………………………………………………………………………………….. ……………………………………………… ……………………………………………………………………………………..[2] (ii) Using the data given, calculate the volume of methane required to produce 1 MJ of heat energy when burned. [2] nCH4 required = 1 x 106 / 890 x 103 = 1.1236 mol volume of methane = 1.1236 x 22400 = 25200 cm3 (iii) Calculate the volume of “water gas” required to produce the same amount of heat energy as methane. (1 mole of gas occupies 22400 cm3 at stp) [2] a x 242 + a x 283 = 1000 kJ a = 1.905 moles of each gas total = 1.905 x 2 = 3.81 moles of gas Vol of gas = 3.81 x 22400 = 85344 ( 85300) cm3 at stp The heat evolved when 1 mole compound is completely burn in excess oxygen gas under standard conditions.

7 (b) In recent years, there has been worldwide interest in the extraction of ‘shale gas’ as an important energy source. One of the problems associated with us