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Author: Subject: kinetically inhibited reaction
aeacfm
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cool.gif posted on 9-10-2010 at 06:06
kinetically inhibited reaction


what is meant by kinetically inhibited reaction ?





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DJF90
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[*] posted on 9-10-2010 at 06:56


It means that, although a reaction might be thermodynamically favourable, it will not occur/will be very slow because of the kinetic stability of one of the species. Off the top of my head, perchlorate is a good example, because although its redox potential is so high (and hence is a very strong oxidiser), in solution it is not so good because of its kinetic stability, i.e. the reaction is very thermodynamically favourable, but kinetically inhibited. Peroxodisulfate is the same, and often a trace of Ag (I) salt is added to make the oxidation of other species occur at a much more reasonable rate, i.e. the silver species acts as a catalyst to overcome the kinetic stability of the (S2O8)2- ion.

[Edited on 9-10-2010 by DJF90]
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[*] posted on 9-10-2010 at 16:03


To generalize a bit and continue with DJF90's explanation:

Thermodynamics is very deterministic. It tells you whether something will happen or whether it will not happen.

However, thermodynamics does not say anything (unless you want to talk about transition state theory, which you probably don't) about the time it takes for the reaction to occur--that is described by the reaction kinetics.

A good example would be the transformation of diamond into graphite. If you've heard the saying, "diamonds are forever," it's not quite true--because thermodynamically, they'll convert to graphite. However, the process is *very* slow, so diamonds are essentially forever.





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[*] posted on 9-10-2010 at 17:54


Even a mixture of H2 and O2 exhibit this, they'll remain as H2 and O2 for a very long time until initiated by a spark or something hot enough. OTOH mixing solution of AfNO3 and NaCl gives instant reaction.

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[*] posted on 10-10-2010 at 02:47



Quote: Originally posted by not_important  
Even a mixture of H2 and O2 exhibit this, they'll remain as H2 and O2 for a very long time until initiated by a spark or something hot enough. OTOH mixing solution of AfNO3 and NaCl gives instant reaction.



aha , but this dependent on reaction type , conditions or there are general rules control it ?

[Edited on 10-10-2010 by aeacfm]






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[*] posted on 10-10-2010 at 22:56


There are no general rules. Thermodynamics only tells something about the amount of energy released (or consumed) in a certain reaction, it does not tell anything about the mechanistic pathway for the reaction.

An analogon may make this more clear.

Suppose you have a ball with mass M and it is at height h above sea level. The potential energy stored in the ball is M*g*h (where g is the gravitational acceleration). A ball, very high in the sky has more potential energy than a ball just above sea level. If both balls are released and allow to plunge into the sea, then the ball from the greater height will have a much more violent impact than the ball which only is a little above sea level.

Now imagine that both balls are on the top of a mountain. One ball is on a mountain, which has a hole on its top, the other ball is on a mountain without a hole, the mountain is just flat near the top and has a slope downwards around the top.

Now you can see that the ball which is on the mountain with the hole on the top never will roll down and plunge into the sea, regardless of the height of that mountain. The other ball easily rolls down, a little wind, a slight push, etc. can move it from the top and once it goes it will continue its path downwards. The ball in the hole may have considerable potential energy, but it is not made manifest if it just is allowed to lay in its hole. Someone needs to give it a push, such that it is pushed over the rim around the hole.

In this analogon, the shape of the mountain from which the ball must roll can be compared with the mechanism of the reaction, the initial height of the ball above sealevel can be compared with the thermodynamic properties of the compound (i.e. an energetic compound can be compared with a ball at very great height). But if the mountain is covered with all kinds of obstacles, then even though the ball has high potential energy, it will not roll off the mountain and release that energy. This is the same with an oxidizer like perchlorate. It contains a lot of energy, but one needs to push it quite strongly (e.g. by heating) before it can release that energy (look at it as a ball in a deep hole on a very high mountain).




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[*] posted on 10-10-2010 at 23:43


I guess physical ability to form intermediates and other reaction pre-reqs has nothing to do with this then?

I'm thinking like how tert-butyl has a habit of getting in the way.




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[*] posted on 11-10-2010 at 05:35


Quote: Originally posted by psychokinetic  
I guess physical ability to form intermediates and other reaction pre-reqs has nothing to do with this then?

I'm thinking like how tert-butyl has a habit of getting in the way.


Transition states have everything to do with kinetics. If a reactant molecule requires a huge change in potential energy to reach a certain transition state, then that reaction will proceed slowly. On the other hand, if only a slight change in potential energy is required to reach a transition state, that particular pathway will proceed readily.

Formally, this is explained through the Hammond postulate.

[Edited on 10-11-10 by DDTea]




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[*] posted on 11-10-2010 at 17:06


I see. I got a little confused at the introduction of thermodynamics. I'm still learning about applying TD to everything.



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