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