Portmess Homework Solutions 44

# Portmess Homework Solutions 44 - 38 ALKANES(b 15O2(c 10H2O...

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2.34 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.
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