Thermodynamically stable vs Kinetically stable


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Hyein Cha 2I
Posts: 103
Joined: Fri Sep 29, 2017 7:05 am
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Thermodynamically stable vs Kinetically stable

Postby Hyein Cha 2I » Sat Mar 17, 2018 9:58 am

Can someone please explain the difference between a reaction that is considered to be under kinetic control vs reaction considered to be under thermodynamic control? Like what control dominates at low temperature vs high temperatures? Which pathway has larger activation energy? etc..

Aijun Zhang 1D
Posts: 53
Joined: Tue Oct 10, 2017 7:13 am

Re: Thermodynamically stable vs Kinetically stable

Postby Aijun Zhang 1D » Sat Mar 17, 2018 10:25 am

Image
You draw a graph like this. P1 refers to product 1, P2 refers to product 2. A is the reactant. The two high peaks are two transition states.
You can see that P1 is thermodynamically more stable than P2, so P1 is more favorable in thermodynamics. However, the activation energy to form P1 is higher than P2, so it is unfavorable in kinetics.
If the temperature is low, the reaction is under kinetic control since it can only overcome the reaction with the lower energy barrier. If temperature is high, kinetics now does not control anymore since the reaction can cross both activation energies. Therefore, the reaction favors the side which forms a stable product, which means the enthalpy of the product should be the lowest. In this way, thermodynamics control the reaction.


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