VJC Prelim P2

2 Section A Answer all the questions in this section in the spaces 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 equilibrium with it s isomer, the mixture of methylacetylene and propadiene being called MAPD: C H3 CC H(g) H⇋ H C2 CC 2(g) 1 2 3 methylacetylene propadiene (i) State the type of hybridisation, shape a nd number of sigma and pi bonds about I C 1 and C2 of methylacetylene, and II C 3 of propadiene in the table below. Carbon Hybridisation Shape No. of sigma bonds No. of pi bonds C1 C2 C3 (ii) When methylacetylene reacts with Br 2 in CCl 4, 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. Type of isomerism: ……………………………………………………. Isomers: [5] © VJC 2009 8872/02/PRELIM/09 [Turn Over

3 (b) An experiment was carried out as follows to determine the standard ent halpy change of combustion 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; (ii) the heat given out by methylacetylene during the experiment; (iii) the enthalpy change of combustion of methylacetylene. © VJC 2009 8872/02/PRELIM/09 [Turn Over

4 (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.