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bobby chopra Grade: 11

With the aid of an enthalpy diagram show how heats of combustion can be used to compare stabilities of geometric isomers of hydrocarbon alkenes. Is the conversion of a cis  to a trans-isomer exothermic or endothermic? Why ?

9 years ago

Answers : (1)

askIITIians Expert
21 Points

As shown in the figure below, the enthalpies of the products of alkene combustion, CO2 and H2O, are always less than those of the reactants,alkene and O2. 1 - Butene and cis- and trans -2 - butene are arbitrarily chosen. All combustions are exothermic and have - ?Hc's. Only ismoreic hydrocarbons can be compared because only then will the equivalent amounts of reactants and products be the same. The most stable isomer has the smallest enthalpy, and the least negative ?Hc. We can safely use ?Hc instead of Gc  because ?Sc is positive and constant for combustion of each isomer. The isomerisation is exothermic because cis has a larger enthalpy than trans.


9 years ago
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