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Ch_322a_4.013 - CH 3 CH 2 CH 2 CH 3(g 6.5 O 2(g 4CO 2(g 5H...

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Measurements of Relative Stabilities Heats of Combustion The heat of combustion of a compound is the enthalpy change for complete combustion of one mole of the compound. For a hydrocarbon the products are CO 2(g) and H 2 O (l) . The amount of heat evolved is measured in a calorimeter. In giving the heat of combustion, it is important to note the physical state of the reactant: liquid, gas or solid. If the state is liquid or solid, the heat of vaporization must be taken into account. An Example CH 3 CH 2 CH 2 CH 2 CH 3(g) + 8O 2 5CO 2(g) + 6H 2 O (l) ! H comb = -845.2 kcal/mol but for liquid pentane, ! H comb = -838.8 kcal/mol The difference of 6.4 kcal/mol is the heat required to change one mole of liquid pentane to one mole of gaseous pentane, the heat of vaporization.
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Stability of Isomers The relative stability of isomeric hydrocarbons may be determined by measuring their heats of combustion under identical conditions. An Example: The Isomeric Butanes The heats of combustion of the isomeric butanes are:
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Unformatted text preview: CH 3 CH 2 CH 2 CH 3(g) + 6.5 O 2(g) 4CO 2(g) + 5H 2 O (l) ! H comb = -687.5 kcal/mol CH 3 CHCH 3(g) + 6.5 O 2(g) CH 3 4CO 2(g) + 5H 2 O (l) ! H comb = -685.5 kcal/mol Analysis The analysis of the relative stabilities of the isomeric butanes from the above data is easiest to see in the energy state diagram below. Since the combustion reactions are exothermic , the product states are lower in energy than the reactant states. Since the same product state is produced in each combustion reaction, the levels of the two reactant states automatically are set by the amount of heat released. The difference in energy levels reflects the difference in energies of the isomeric butanes since O 2 is common to both reactant states. Enthalpy 4CO 2(g) + 5H 2 O (l) CH 3 CH 2 CH 2 CH 3(g) + 6.5 O 2(g) CH 3 CHCH 3(g) + 6.5 O 2(g) CH 3 ! H comb = -685.5 kcal/mol ! H comb = -687.5 kcal/mol 2.0 kcal/mol Isobutane is more stable than butane by 2.0 kcal/mol....
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