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 ?

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 - ?H_c'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 ?H_c. We can safely use ?Hc instead of G_c  because ?S_c is positive and constant for combustion of each isomer. The isomerisation is exothermic because cis has a larger enthalpy than trans.