Kinetic and Thermodynamic stability???

ive always had problems with this, so plz help

what exactly is
1. kinetic stability
2. thermodynamic stability

i know kinetic is like the activation energy but ony enuf to get 1 mark.

thanks!

Reply 1

Eau

Yes, kinetic stability is related to activation energy.

Thermodynamic stability is related to enthalpy - the lower a system is on an enthalpy diagram, the more stable.

Reply 2

rohitkhannak

Hmm, I look at it this way:

If you draw an energy level diagram, exothermic rxns have the pdts BELOW the reactants. So, technically, they are MORE stable then the reactants.
So, in this case, the products are Thermodynamically (i.e. with relation to heat/energy) more stable than the reactants.

Kinetically stable means that the reaction proceeds too slowly for any reaction to be seen. So, if a rxn is kinetically unstable, the rxn is fast and observations can be made.
This leads to the conclusion that if Ea (Act. Energy) is high, the Reaction is Kinetically stable and if Ea is low, Rxn is kinetically unstable.

Now, rxns can be both kinetically stable/unstable and thermodynamically stable/unstable. Picture an exo- rxn with a low Ea. Here, the pdts of the rxn are thermodynamically more stable than the reactants, so as a whole, the rxn is thermodynamically stable. Since there is a low Ea, the reaction is kinetically unstable.
Picture an endo- rxn with a low Ea. The rxn is thermodynamically unstable and kinetically unstable. But, the thing is, that this stability concept is theoretically possible, but most endo- rxns, or atleast this is what I have seen, dont really have a low Ea. So, this may be theoretically plausible but whether this is pragmatic or not is open to interpretation I guess.

Hope that helped :smile:

Reply 3

Excalibur

Thermodynamic stability - depends on the relative enthalpies. The products in an exothermic reaction is thermodynamically more stable relative to the reactants because the products have a lower enthalpy (ie. lower down on a reaction profile diagram). Thus, exothermic reactions are thermodynamically feasible and may happen.

Kinetic stability - whether the activation energy is too high. Reactions may not happen even if it is thermodynamically feasible because the activation energy is too high. If the activation energy is too high for a reaction to proceed then the reactants are said to be kinetically stable.

This explanation does not take into account entropy (the level of disorder in the system) though, but you don't need that at A level.

Reply 4

rohitkhannak

hmm Excalibur, I've heard that entropy is A2? (For Edexcel atleast).

Reply 5

Excalibur

Heh, I wouldn't know, maybe it is introduced at A2.

Reply 6

cujo

Simple way to remeber it

Thermodynamically stable is when the entalpy level of the products is much higher then the reactants

in an exothermic reaction the substances on the left are said to be thermodynamically unstable to the substances on the right

in an endothermic reactoin the substances on the left are thermodynamically stable to those on the right

kinetic stability is when the reacion mixture has such a high activation energy that none of the molecules possess the sufficient activation energy to react example is a mixture of air and petrol it must be heated before it reacts/

Reply 7

Wangers

Entropy isnt even on the OCR A2 - its usually done at uni - to do with cheleate complexes and such, very interesting though.

Reply 8

Apprentice

Surely it's the phase with the lowest gibbs free energy, which is not necessarily the one with the lowest enthalpy, that is thermodynamically favourable.