Are all electrophilic additions exothermic?

[Glenda Marshall DIS 3M]

Homework problem 4.29 says

"draw and label the reaction profile with potential energy as the y-axis for the electrophilic addition reaction of hydrogen bromide, HBr, to propene, CH3CHCH2, producing 2-bromopropane, CH3CHBrCH3"

the graph they drew was an exothermic reaction and I am wondering how they determined that from the given data. Thanks!

[Justin Le 2I]

I think we can assume that the reactions will be exothermic because they will usually be spontaneous. If the reaction were endothermic, it would not be spontaneous.

[Chem_Mod]

The majority of electrophilic addition reactions across a double bond are exothermic because the original carbon-carbon pi bond is generally weak compared to the newly formed single bonds.

Be careful when making statements about spontaneity. Remember that delta G (exergonic or endergonic) is the thermodynamic quantity that tells whether or not a reaction will occur spontaneously. Knowing delta H (exothermic or endothermic) alone does not necessarily tell us if a reaction is spontaneous.