The pi bond is weaker than the sigma bond so is broken more readily. The C-C bond has an enthalpy of 347kJmol-1 whilst the C=C bond has an enthalpy of 612 kJmol-1Alkenes undergo addition reactions. These include the addition of hydrogen, hydrogen halides, halogens and steam. Hydrogen is mixed with an alkene and passed over a hot nickel catalyst at 423K to form an alkane. Hydrogen is added across the double bond. This is known as hydrogenation.Halogenation occurs at room temperature. Alkenes react with halogens or hydrogen halides to form halogenoalkanes. Alkenes react with steam to form alcohols. The mechanism of the reactions is electrophilic addition.