It is said that activation energy does not change with temperature.

But I am unable to figure out why. If we increase the temperature, the kinetic energy of the molecules will increase and they will need less extra energy and hence lesser activation energy to overcome the threshold energy barrier.

When I surfed the net, at most of the places it has been explained that increasing the temperature increases the no. of molecules having more energy and hence reaction rate is more but activation energy is same. This makes some sense but I still don't get why the activation energy doesn't decrease as the KE of an individual molecule should also increase on increasing the temperature.

  • $\begingroup$ Activation energy isn't "extra" energy, but total. $\endgroup$ – Mithoron Apr 30 '15 at 12:59
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    $\begingroup$ Try thinking of the term "activation energy" as meaning the height of the barrier between the reactants and the products, rather than the amount of kinetic energy of the molecules in the system. Individual molecules need to have a kinetic energy comparable to the height of the barrier in order to react. When you increase the temperature, it increases the average kinetic energy of the molecules (and hence the number of molecules with sufficient energy to react), but does not change the height of the barrier. So the reactants cross over the barrier more frequently -- hence a faster reaction. $\endgroup$ – iad22agp Apr 30 '15 at 13:52

The two distribution plots shown here are for a lower temperature $T_{1}$ and a higher temperature $T_{2}$. The area under each curve represents the total number of molecules whose energies fall within particular range. The shaded regions indicate the number of molecules which are sufficiently energetic to meet the requirements dictated by the two values of $E_{a}$ that are shown.

It is clear from these plots that the fraction of molecules whose kinetic energy exceeds the activation energy increases quite rapidly as the temperature is raised. This the reason that virtually all chemical reactions (and all elementary reactions) are more rapid at higher temperatures.

Activation energy seems to be more of a material or reaction pertinent . Say a reaction has $10J$ as $E_{a}$ to proceed but has $20 J$ kinetic energy - I would say the the additional energy is transferred/carried on by the products after reaction is complete. So having more K.E shouldn't affect the character of a reaction but only its kinetics .


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