Why Transition States always have Positive Enthalpy

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Albert Chong_1L
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Joined: Fri Sep 25, 2015 3:00 am

Why Transition States always have Positive Enthalpy

Postby Albert Chong_1L » Wed Feb 24, 2016 7:45 pm

Hi I was wondering why transition states always have positive enthalpy changes despite TS that involve solely bond formation.

One example would be the second step of electrophilic addition of HBr to Propene in which the second step involves a Br anion attaching to a carbocation.

Since the formation of bonds releases energy, why would this TS still have a positive enthalpy change?

Thanks.

304621080
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

Re: Why Transition States always have Positive Enthalpy

Postby 304621080 » Wed Feb 24, 2016 8:37 pm

The transition state has a positive enthalpy value because it requires energy to break the bonds resulting in an endergonic reaction.

Albert Chong_1L
Posts: 77
Joined: Fri Sep 25, 2015 3:00 am

Re: Why Transition States always have Positive Enthalpy

Postby Albert Chong_1L » Wed Feb 24, 2016 10:16 pm

Yes. That's right. But what about the example I gave?


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