CJC Prelim H1 Chem 2008 P2

Name Class: 2T CATHOLIC JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 CHEMISTRY 8872/02 Higher 1 Paper 2 26 August 2008 2 hours Candidates answer Section A on the Question Paper and Section B on separate paper. Additional Materials: Paper 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. You may use a soft pencil for any diagrams, graphs or rough working. Do not use paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer any two questions. 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 Examiner’s Use Section A B5 B6 B7 Total There are a total of 12 printed pages including this cover page. 8872/02/CJC Prelim 08 1

SECTION A Answer all the questions in this section in the spaces provided. 1 The equation for the decomposition of gaseous X at 373 K is shown below. 2X (g) Y (g) + Z (g) The values for the initial rates of decrease in concentration of X at various initial concentrations have been determined. These are shown in the table below. Initial concentration / mol dm -3 1.67 3.34 5.01 6.68 Initial rate / mol dm-3 s-1 0.41 1.64 3.69 6.56 (a) (i) Determine the order of reaction with respect to X and hence write the rate equation for this reaction. (ii) Calculate the rate constant for this reaction, stating the units clearly. [4] (b) (i) At 373 K, the activation energy for the forward reaction is 130 kJ mol -1 and that for the reverse reaction is 72 kJ mol-1. Sketch and label the energy profile diagram for this reaction. (ii) Hence, determine the value of the enthalpy change of the decomposition reaction. [3] 8872/02/CJC Prelim 08 2

(c) What will be the effects of increasing pressure on the decomposition of X? [2] (d) In the presence of a catalyst, the activation energy of the reverse reaction is 43 kJ mol -1. On the same axes in (b)(i), draw the energy profile of the catalysed reaction and label it clearly. [1] [Total: 10] 2 2-chloroethylbenzene can be formed stepwise from benzene via the following reaction scheme. CH2CH3 CH2CH3 Cl Step I Step II (a) Step I shows how ethylbenzene is synthesised from benzene via a reaction known as Friedel-craft alkylation. The equation below represents the chemical reaction that occurs in Step I. C 6H6 (l) + CH3CH2Cl (g) → C6H5CH2CH3 (l) + HCl (g) (i) Use the following data to calculate the enthalpy change of the above Friedel-craft alkylation reaction. ∆H o f (C6H6) = +49.0 kJ mol-1 ∆Ho f (CH3CH2Cl) = -109 kJ mol-1 ∆Ho f (C6H5CH2CH3) = -12.5 kJ mol-1 ∆Ho f (HCl) = -92.3 kJ mol-1 [1] (ii) Write an equation to represent the standard enthalpy change of combustion, ∆H o c of ethylbenzene. 8872/02/CJC Prelim 08 3

(iii) Calculate the standard enthalpy change of combustion, ∆Ho c of ethylbenzene given the following data: ∆Ho c (carbon) = -393 kJ mol-1 ∆Ho c (hydrogen) = -286 kJ mol-1 [ 3 ] (b) (i) State the reagents and conditions used in Step II. (ii) Draw the structure of another possible product that can be formed in Step II and briefly explain the formation of this product. [3] (c) Describe a simple chemical test that can be used to distinguish between 1-chloro-2- phenylethane and 2-chloroethylbenzene. State any observation made and write equation(s) where appropriate. CH2CH2Cl CH2CH3 Cl 1-chloro-2-phenylethane 2-chloroethylbenzene [3] [3] [Total: 10] 8872/02/CJC Prelim 08 4

3 Physical properties of the oxides of some Period 3 elements W, X, Y and Z are given below. Formula of oxide Melting point / oC Appearance at r.t.p. Conductivity in molten state Acidic/Basic nature of oxide WO 1132 White solid Good Basic X2O3 2054 Solid (variable colour) Good Amphoteric YO2 1650 White solid None Acidic ZO2 -72 Colourless gas None Acidic (a) State the type(s) of particles present in the solid lattices of the four oxides above. In addition, identify all type(s) of bonds present in each oxide. WO X2O3 YO2 ZO2 Type(s) of lattice particles Type(s) of bonds [4] (b) Based on considerations of the properties of the respective oxides above, (i) Draw and name the shape of the molecule of ZO2. (ii) Suggest, with reasons, whether WO is soluble in water. (iii) State the possible identity of X2O3 and hence write down balanced equations to show the amphoteric nature of X2O3. [6] [Total: 10] 8872/02/CJC Prelim 08 5

4 Every year, Singapore celebrates its independence on the 9 th of August with a large-scale celebratory event – the National Day Parade. During the Parade, the most highly anticipated item is the fireworks display, which has never failed to captivate the hearts of the audience. To create these spectacular visual effects, chemistry is actually involved. In fact, each firework that is launched into the sky comprises of chemicals and fuels that are precisely formulated so as to produce different special effects. The power needed to lift each firework into the air is provided by the highly exothermic combustion of the ‘black powder’, a slow-burning combination of 75 % potassium nitrate, 15 % charcoal (carbon), and 10 % sulphur. For a typical 1-kg firework, it contains approximately 500 g of black powder. This composition of the black powder was first used in China about 1000 years ago, and has undergone little change since then. When the black powder burns in open air, the heat and gases generated dissipate quickly. Hence, in order to successfully launch the firework high up into the atmosphere, the heat and gases generated during combustion need to be trapped at the bottom of the shell for enough pressure to build up prior to the launch. When the black powder combusts, potassium nitrate in the powder will decompose according to the following equation: 2 KNO 3(s) Æ K2O(s) + N2(g) + 2 5 O2(g) The other two reagents in the black powder, sulphur and charcoal (carbon), will react with oxygen (from both air and potassium nitrate) to produce sulphur dioxide and carbon dioxide respectively: S(s) + O 2(g) Æ SO2(g) ΔHc = -296.83 kJ mol-1 C(s) + O2(g) Æ CO2(g) ΔHc = x kJ mol-1 The above combustions not only produce gases but also release a lot of heat energy. As such, the gases which are produced are simultaneously heated up and therefore rapidly expand. These rapidly expanding gases will become the explosive force of the reaction. The colours of the fireworks are produced by heating the added metal salts, such as calcium chloride or sodium nitrate. Part of the heat energy which is produced from the combustions of sulphur and charcoal (carbon) will be absorbed by the atoms of the metal salts. As a result, the amou