VJC Prelim P1 P2 ANS

© VJC 2009 8872/02/PRELIM/09 [Turn Over 1 VJC 2009 H1 Chemistry 8872/2 Suggested Answers Section A Answer all questions in the space provided. 1 (a) Methylacetylene is an alkyne with the chemical formula CH3C≡CH. It is a component of MAPP gas along with its isomer propadiene, which is commonly used in gas welding. Methylacetylene exists in e quilibrium with its isomer, the mixture of methylac etylene and propadi ene being called MAPD: C H3 CC H(g) H⇋ H C2 CC 2(g) 1 2 3 Methylacetylene propadiene, (i) State the type of hybrid isation, shape and number of sigma and pi bonds about I. C1 and C2 of methylacetylene, and II. C3 of propadiene in the table below. Carbon Hybridisation Shape No. of sigma bonds No. of pi bonds C1 sp3 Tetrahedral 4 0 C2 sp Linear 2 2 C3 sp2 Trigonal planar 3 1 (ii) When methylacetylene reacts with Br 2 in CC l4, a compound having the molecular formula of C 3H4Br2 is formed. Name the type of isomerism shown by the compound and draw all its possible isomers. [5] Cis-trans isomerism or geometric isomerism CC Br CH3 Br H CC Br CH3 Br H cis trans

© VJC 2009 8872/02/PRELIM/09 [Turn Over 2 (b) An experiment was carried out as follows to determine the standard enthalpy change of combus tion of methylacetylene contained in the gas cylinder of a gas stove. A large beaker of water was plac ed on the stove and heated. The temperature rise was recorded. The cylinder was weighed before and after the experiment to determine the ma ss of methylacetylene used. This process was known to be only 70% e fficient. The following results were obtained. Mass of methylacet ylene before heating = 4.66 g Mass of methylacetylene after heating = 2.16 g Mass of water heated = 750 g Temperature rise = 25.8 oC Assume the heat capacity of water = 4.2 J K 1 cm3 Based on the given data, calculate (i) the heat absorbed by water Heat absorbed by water = Vc∆T = 750(4.2)(25.8) = 81270 J = 81.3 kJ (ii) the heat given out by methylacetylene in the experiment Assuming 100% heat transfer, Heat given by heating methylacetylene = 81270 x 100/70 = 116 100 J = 116 kJ (iii) enthalpy change of combustion of methylacetylene enthalpy change of combustion of methylacetylene = -116 100 / [(4.66- 2.16)/40] = -1.86 x 10 6 J mol-1 = -1860 kJ mol-1 (iv) Hence, use the answer in (b)(iii) to calculate the enthalpy change of reaction for the conversion of methylacetylene to propadiene, given that the enthalpy change of combustion of propadiene is -1830 kJ mol-1. [5] Hrxn =  Hc(reactants) –  Hc(products) = –1860 – (–1830) = – 30 kJmol-1 [Total: 10]

© VJC 2009 8872/02/PRELIM/09 [Turn Over 3 2 In nature, chlorine is abundantly found as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans. The pH of the solutions of chlorides for the elements s odium to phosphorus is given below. compound Sodium chloride Magnesium chloride Alumin ium chloride Silicon tetrachloride Phosphorus pentachloride pH of solution 7 6.5 3 2 2 (a) Suggest the type of st ructure and bonding present in sodium chloride and silicon tetrachloride. NaCl : giant ionic stru cture; ionic bonding SiCl4 : Simple molecular structure; covalent bonding [2] (b) When solid aluminium chloride is heated above 180 oC, a vapour is formed which has Mr = 267. When this vapour is heated above 800oC, the vapour has Mr = 133.5. Draw the displayed formula of the vapour at 180oC to show its bonding. [1] Al Al Cl Cl Cl Cl Cl Cl (c) Write equations to account for the pH of aqueous solutions of (i) Alumin ium chloride : [Al(H2O)6]3+ + H2O [Al(H2O)5(OH)]2+ + H3O+ (ii) Phosphorus pentachloride : PCl 5 + 4 H2O  H3PO4 + 5 HCl [2]

© VJC 2009 8872/02/PRELIM/09 [Turn Over 4 Chlorine forms various oxides and oxoanions as shown in the following. Chlorine oxides Cl2O C lO2 C l2O6 C l2O7 Chlorine oxoacids HClO HC lO2 HClO3 HClO4 (d) Cl2O6 exists as singly charged ions in the solid state. The oxidation states of chlorine are +5 and +7 in the cation and anion respectively. Suggest the formulae of the ions and draw the dot-and-cross diagram for the cation. Formulae of the ions: [ClO2]+ and [ClO4]- [2] (e) When chlorine gas reacts with water, hypochlorous acid, HC lO is formed according to the equation below. Cl2(aq) + H2O (l) HClO (aq) + H+ (aq) + Cl-(aq) At 25oC, 0.1 mole of the chlorine gas is bubbled into 2 dm3 of distilled water. 55% chlorine remains in the equilibrium solution. (i) Write an expression for K c, stating its units. Kc = [HC lO][H+][Cl-] mol2dm-6 [Cl2] (ii) Calculate the value of Kc for this reaction. Cl2(aq) HClO(aq) H+ (aq) Cl-(aq) Initial no. of moles 0.1 - - - Change -0.045 +0.045 +0.045 +0.045 Equilibrium 0.055 0.045 0.045 0.045 Concentration, mol dm -3 0.055/2 0.045/2 0. 045/2 0.045/2 Show working

© VJC 2009 8872/02/PRELIM/09 [Turn Over 5 Kc = [HClO][H+][Cl-] / [Cl2] = 0275 . 0 0225 . 03 = 4.14 x 10-4 mol2dm-6 [ 3 ] [Total: 10] Hydrogen cyanide was first isolated from a blue dye (Prussian blue) which had been known from 1704 and is now known to be a c oordination polymer, Fe4[Fe*(CN)6]3. In the year 2000, 732,552 tonnes of HCN were produced in the US. The most important process is the Andrussov oxi dation invented by Leonid Andrussow at IG Farben in which methane and ammonia r eact in the presence of oxygen and platinum at about 1200 °C: 2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O The energy needed for the reaction is pr ovided by the parti al oxidation of methane and ammonia. (1 tonne = 1 000 kg) HCN is also obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almonds, fr om which almond oil and flavoring are made. Many of these pits c ontain small amounts of cyanohydrins such as mandelonitrile and amygdalin , whi