Eall reactantsEthe thermal reactants (3.11) EY2 exp(-ET /kpT)dET =可B了Eexp-所1keDr When we recognize that,strictly speaking,the activation energy is really a thermal average many puzzles go away.The activation energy is not,for example,the minimal energy for reaction.It is closely related to it because the reactive reactants need to have at least the minimal energy,but they can have more than the minimal amount and indeed the reaction cross section typically rises steeply in the post threshold regime,figure 3.1.Nor does the activation energy have to be independent of temperature.Atypically the activation energy can even be negative if low energy collisions preferentially lead to reaction3. Show,problem E,that this dependence tells us about the entropic requirements of the reaction.We provide a microscopic view of such requirements in chapter 5. MRD Chapter 3 page 14 ©R D Levine(2003)ET all reactants ≡ ET the thermal reactants = ET ET 1/2 exp(−ET / kBT)dET ET 1/2 ∫ exp(−ET / kBT)dET ∫ (3.11) When we recognize that, strictly speaking, the activation energy is really a thermal average many puzzles go away. The activation energy is not, for example, the minimal energy for reaction. It is closely related to it because the reactive reactants need to have at least the minimal energy, but they can have more than the minimal amount and indeed the reaction cross section typically rises steeply in the post threshold regime, figure 3.1. Nor does the activation energy have to be independent of temperature* . Atypically the activation energy can even be negative if low energy collisions preferentially lead to reaction4 . * Show, problem E, that this dependence tells us about the entropic requirements of the reaction. We provide a microscopic view of such requirements in chapter 5. MRD Chapter 3 page 14 © R D Levine (2003)