38 ALKANES +150 10CO2+10H2O Carbon Water dioxide CH3+150 10C0,+ 10H.O Methylcyclononane Oxygen (C10H2) (d) 10CO,+9H,O yclopentylcyclopentane Carbon Water (CodIS) 2.34 To determine the quantity of heat evolved per unit mass of material, divide the heat of combustion by the Methane Heat of combustion =890 kJ/mol(212.8 kcal/mol) Molecular weight 16.0 g/mol Heat evolved per gram=55.6 k/g(13.3 kcal/g) Heat of combustion= 2876 kJ/mol(687. 4 kcal/mol) Molecular weight= 58.0 g/mol evolved per gram =49.6 k/g(11. 8 kcal/g) When equal masses of methane and butane are compared, methane evolves more heat when it is Equal volumes of gases contain an equal number of moles, so that when equal volumes of methane and butane are compared, the one with the greater heat of combustion in kilojoules(or kilocalories) per mole gives off more heat. Butane evolves more heat when it is burned than does an equal volume of methane 2.35 When comparing heats of combustion of alkanes, two factors are of importance 1. The heats of combustion of alkanes increase as the number of carbon atoms increases 2. An unbranched alkane has a greater heat of combustion than a branched isomer. (a) In the group hexane, heptane, and octane, three unbranched alkanes are being compared. Oc tane( C&His) has the most carbons and has the greatest heat of combustion. Hexane(C6H14) has the fewest carbons and the lowest heat of combustion. The measured values in this group are as follows Hexa Heat of combustion 4163 kJ/mol(995.0 kcal/mol) Heptane Heat of combustion 4817 kJ/mol (1151.3 kcal/mol) Octane Heat of combustion 5471 kJ/mol (1307. 5 kcal/mol) b) Isobutane has fewer carbons than either pentane or isopentane and so is the member of the group with the lowest heat of combustion. Isopentane is a 2-methyl-branched isomer of pen tane and so has a lower heat of combustion Pentane has the highest heat of combustion among (CH3)3CH Heat of combustion 2868 kJ/mol(685.4 kcal/mol) Isopentane (CH3)2CHCH,CH Heat of combustion 3529 kJ/mol (843. 4 kcal/mol) Pentane CH CH.. CH Heat of combustion 3527 kJ/mol (845.3 kcal/mol) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website2.34 To determine the quantity of heat evolved per unit mass of material, divide the heat of combustion by the molecular weight. Methane Heat of combustion  890 kJ/mol (212.8 kcal/mol) Molecular weight  16.0 g/mol Heat evolved per gram  55.6 kJ/g (13.3 kcal/g) Butane Heat of combustion  2876 kJ/mol (687.4 kcal/mol) Molecular weight  58.0 g/mol Heat evolved per gram  49.6 kJ/g (11.8 kcal/g) When equal masses of methane and butane are compared, methane evolves more heat when it is burned. Equal volumes of gases contain an equal number of moles, so that when equal volumes of methane and butane are compared, the one with the greater heat of combustion in kilojoules (or kilocalories) per mole gives off more heat. Butane evolves more heat when it is burned than does an equal volume of methane. 2.35 When comparing heats of combustion of alkanes, two factors are of importance: 1. The heats of combustion of alkanes increase as the number of carbon atoms increases. 2. An unbranched alkane has a greater heat of combustion than a branched isomer. (a) In the group hexane, heptane, and octane, three unbranched alkanes are being compared. Oc￾tane (C8H18) has the most carbons and has the greatest heat of combustion. Hexane (C6H14) has the fewest carbons and the lowest heat of combustion. The measured values in this group are as follows: Hexane Heat of combustion 4163 kJ/mol (995.0 kcal/mol) Heptane Heat of combustion 4817 kJ/mol (1151.3 kcal/mol) Octane Heat of combustion 5471 kJ/mol (1307.5 kcal/mol) (b) Isobutane has fewer carbons than either pentane or isopentane and so is the member of the group with the lowest heat of combustion. Isopentane is a 2-methyl-branched isomer of pen￾tane and so has a lower heat of combustion. Pentane has the highest heat of combustion among these compounds. Isobutane (CH3)3CH Heat of combustion 2868 kJ/mol (685.4 kcal/mol) Isopentane (CH3)2CHCH2CH3 Heat of combustion 3529 kJ/mol (843.4 kcal/mol) Pentane CH3CH2CH2CH2CH3 Heat of combustion 3527 kJ/mol (845.3 kcal/mol) Cyclopentylcyclopentane (C10H18) Oxygen 29 2 O2 Carbon dioxide Water (d) 10CO2 9H2O CH3 15O2 (c) 10CO2 10H2O Methylcyclononane (C10H20) Oxygen Carbon dioxide Water 15O2 (b) 10CO2 10H2O Cyclodecane (C10H20) Oxygen Carbon dioxide Water 38 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website