CJC Prelim H1 Chem 2008 P2 (worked soln) (updated)

CJC H1 Prelim Exam 2008 Paper 2 Solutions CATHOLIC JUNIOR COLLEGE H1 CHEMISTRY PRELIM EXAM PAPER 2 SOLUTIONS SECTION A 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 order of reaction with respect to X and write down the rate equation for this reaction. By inspection, when initial concentration of X increases 2 times, from 1.67 to 3.34 mol dm-3, the initial rate of reaction increases 4 times from 0.41 to 1.64 mol dm-3 s-1. ∴ Order of reaction wrt X is 2nd order. Rate equation: rate = k [X]2 (ii) Calculate the rate constant for this reaction, stating the units clearly. Using data from 1 st expt: 0.41 = k (1.67)2 ∴k = 0.41 / (1.67)2 = 0.147 dm3 mol-1 s-1 [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. Energy Ea of forward rxn = 130 kJ mol -1 Ea of catalysed reverse rxn = 43 kJ mol-1 ∆H Ea of reverse rxn = 72 kJ mol-1 1(d) ∆Hr = 130 – 72 = +58 kJ mol-1 [3]

CJC H1 Prelim Exam 2008 Paper 2 Solutions (c) What will be the effects of increasing pressure on the decomposition of X? By Le Chatelier’s Principle, increasing pressure will favour will not shift the position of equilibrium in either direction as both forward and reverse reactions produce the same number of gaseous molecules. Hence the decomposition of X will not be favoured. However , the rate of decomposition will still increase since pressure is increased. [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. ∆Ho 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 ∆Hr = ∑∆Hf (products) - ∑∆Hf (reactants) = (-92.3 -12.5) – (49 -109) = -44.8 kJ mol-1 [1] (ii) Write an equation to represent the standard enthalpy change of combustion, ∆Ho c of ethylbenzene. C6H5CH2CH3 (l) + 2 21O2 (g) → 8CO2 (g) + 5H2O (l) (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 ∆Hc (ethylbenzene) = ∑∆Hf (pdts) - ∑∆Hf(reactants) = 8(-393) + 5(-286) – (-12.5) = -4561.5 = - 4560 kJ mol-1 (3sf) [3] (b) (i) State the reagents and conditions used in Step II. Reagent: Cl2 in CCl4 (or) Cl2 (g) (or) Cl2 Conditions: FeCl 3 / Fe / AlCl 3 catalyst, room temperature, in the dark / absence of light.

CJC H1 Prelim Exam 2008 Paper 2 Solutions (ii) Draw the structure of another possible product that can be formed in Step II and briefly explain the formation of this product. CH2CH3 Cl [ 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 4-chloroethylbenzene can also be formed as the ethyl substituent on the benzene ring is 2, 4-directing. Hence the incoming Cl can substitute the H atom on the 2 nd or 4th position of the benzene ring. 1-chloro-2-phenylethane 2-chloroethylbenzene Chemical test: 1) Add excess NaOH(aq) to each compound separately and boil / heat. 2) Add HNO 3(aq), followed by addition of AgNO3(aq) Observation: 1-chloro-2-phenylethane reacts to form a white ppt of AgCl. But no ppt. observed to form for 2-chloroethylbenzene. Equation: CH2CH2Cl + OH- (aq) → CH2CH2OH + Cl- (aq) Ag + (aq) + Cl- (aq) → AgCl (s) [3] 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

CJC H1 Prelim Exam 2008 Paper 2 Solutions 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 Oppositely charged ions Oppositely charged ions Atoms Molecules Type(s) of bonds Ionic bonds Ionic bonds with covalent character Covalent bonds Covalent bonds and intermolecular Van der Waals’ forces [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] 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. X2O3 is Al2O3. Al2O3 + 6H+ Æ 2Al3+ + 3H2O Al2O3 + 2OH- + 3H2O Æ 2Al(OH)4 - Shape: V-shape or bent (NOT necessary to show bond angle in this case.) Accept diagram in terms of Z. WO is soluble in polar solvents like water. It is a basic oxide implies it is a metallic oxide (or ionic oxide), so it is expected to be able to hydrolyse water to form alkaline solution. (Or) It is an ionic oxide and is able to form favourable ion-dipole interactions with H2O molecules. The energy released is enough to compensate the energy to attract the ions from the ionic lattice.

CJC H1 Prelim Exam 2008 Paper 2 Solutions 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 lau