Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Spontaneous (exothermic) chemical reactions often require a push from the addition of externally supplied energy. This energy is often called activation energy. What does activation energy actually do? What is the energy hill that the activation energy surmounts?

share|improve this question

1 Answer 1

up vote 2 down vote accepted

It essentially is the energy required to start breaking up the molecules involved. This happens through collisions. So, the activation energy is a measure of the amount of heat (via temperature, which is a measure of translational energy of molecules) required for the collisions to start breaking things up when they hit.

For instance, CH4 and O2 at 200K won't really react. This is because the translational energy is too low; when an O2 hits a CH4, energy is exchanged but not enough to break the bond in CH4 or O2.

The form of energy transfer is also important. It can enter different modes: translational-translational, translational-rotational, translational-vibrational, etc.. It's the T-V collisions that matter. Once enough energy is transferred from translational energy to vibrational energy, the bonds snap and reactions occur.

share|improve this answer
Thanks. I guess that answers it. –  RussAbbott Feb 13 '12 at 8:01
Is there something that needs clarified or more explaining? –  tpg2114 Feb 14 '12 at 4:04
No. I checked the check mark. –  RussAbbott Feb 14 '12 at 7:51

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.