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I admit that my knowledge of collision theory may be lacking, but, as I understand it, when particles collide, a reaction will not occur without overcoming the activation energy.

That being said, as the temperature of the environment in which the collision takes place is decreased, I believe it is logical that the kinetic energy of these particles will also decrease. Hence, to me, it would make sense if none of these particles had the energy required to react.

So, my question is: Why do reactions still occur when the environment's temperature around a collision decreases? That is, shouldn't there be a point (such as the temperature in a freezer, perhaps?) in which the activation energy cannot be overcome at all?

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    $\begingroup$ The activation energy varies a lot for various reactions. As long as T > 0 K, there is still disordered kinetic energy in the molecules. But the reason gasoline doesn't spontaneously combust, is that the temperature isn't high enough to overcome the activation energy - a spark is needed. So you are onto something, just remember that the activation energy varies with each reaction. $\endgroup$ – Brian Dec 10 '13 at 7:34
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    $\begingroup$ Collision theory is not perfect and cannot describe well some types of reaction, making them appear to have a negative activation energy. Tunnelling is also a factor in the kinetics of some reactions, so reaction rates might be higher than expected at very low temperatures. If you wait a long, long time, a reaction can proceed to equilibrium even arbitrarily close to 0 K simply by tunnelling effects. $\endgroup$ – Nicolau Saker Neto Dec 10 '13 at 11:03
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    $\begingroup$ Besides tunnel effect, the kinetic energy of molecules at equilibrium is a distribution of various values (e.g. Boltzman). At low but not zero temperature, there are fewer but still some molecules with high velocity, which could pass the barrier. $\endgroup$ – user26143 Dec 10 '13 at 12:22
  • $\begingroup$ Very interesting comments from all of the above - thank you. @Brian perhaps you could post something like that as your answer $\endgroup$ – nmagerko Dec 10 '13 at 13:30
  • $\begingroup$ On the contrary of what you are saying exothermic reactions on decreasing temperature are more forward favorable. $\endgroup$ – Ayush Pateria Dec 15 '13 at 14:42
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The activation energy varies a lot for various reactions. As long as T > 0 K, there is still disordered kinetic energy in the molecules. But the reason gasoline doesn't spontaneously combust, is that the temperature isn't high enough to overcome the activation energy - a spark is needed. So you are onto something, just remember that the activation energy varies with each reaction.

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