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"This reaction requires XYZ activation energy, therefore it cannot happen at room temperature." is a sentence I find often in organic chemistry textbooks. Activation energies are steadily given in textbooks too. Even though it is a nice piece of info, which gives a feeling of how (not) sluggish a reaction can be, it would be nice to know how much energy can such a system actually supply? Like, I don't know - 30 kJ/mol or 35 kJ/mol. How much energy do particles in a system at normal conditions (25°c, 1 atm) have? I am note sure if there even is an answer, taking into consideration Maxwell-Boltzmann distribution, and an endless array of reactions... but maybe there is?

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    $\begingroup$ R = 8.314 J/K/mol is a good place to start... $\endgroup$ – Jon Custer Sep 21 '15 at 15:13
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Echoing Jon Custer's comment:

R = 8.314 J/K/mol

Multiply by 298 K, and you have a rough-and-ready rule-of-thumb number:

The "thermal energy" at room temperature is about 2.5 kJ / mol.

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    $\begingroup$ Or about 23 meV if you prefer those units, particularly for semiconductor physics... $\endgroup$ – Jon Custer Sep 21 '15 at 15:42

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