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Heat of hydrogenation of an alkene is the standard enthalpy of catalytic hydrogenation of an alkene.  Catalytic hydrogenation of an alkene is always exothermic.  Therefore, heat of hydrogenation of alkenes is always negative.eg:

Standard enthalpy of this reaction is -30.3 kcalmol-1. Thus, heat of hydrogenation of 1-butene is -30.3 kcalmol-1.Heat of hydrogenation of alkenes is a measure of the stability of carbon-carbon double bonds.  All else being the same, the smaller the numerical value of heat of hydrogenation of an alkene, the more stable the double bond therein.  Based on heats of hydrogenation of alkenes, the trend in the stability of carbon-carbon double bonds is tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted.Heat of hydrogenation of alkenes is additive, provided that the double bonds are not conjugated.
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it  can be used to measure the relative stability of isomeric alkenes. Heats of hydrogenation correlate with structure just like heats of combustion do. The greater the heat of hydrogenation, the less stable the alkene.
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