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In solution phase, the activation energy of a reaction is usually determined through the study of kinetics and the Arrhenius equation. However, I am confused by the notation, and have trouble interpreting the Ea term, almost universally called "activation energy". Is it free energy? Or is it enthalpy? This is important, because I want to compare some experimental and theoretical values and in the output of quantum chemistry software the differences between energy/enthalpy/free energy are quite clear, and I want to be sure that I compare the right ones.

So my question is: The Ea calculated via the Arrhenius eq. is the enthalpy change going from the reactants to the transition state, or is it the free energy change?

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