Units and typical measurement conventions

Summary

Activation energy is typically measured as an amount of energy per mole of reactant, most commonly in kilojoules per mole (kJ/mol) or, less commonly, kilocalories per mole (kcal/mol). It is represented by Eₐ and describes the minimum energy barrier that reacting molecules must overcome. In the Arrhenius equation, temperature is expressed as an absolute temperature, usually in kelvins (K), while R denotes the universal gas constant and k denotes the reaction rate coefficient. Related transition-state quantities use symbols such as ΔG‡, ΔH‡, and ΔS‡ for the Gibbs energy, enthalpy, and entropy of activation. A superscript ° may be used to indicate that a quantity is evaluated under standard-state conditions, although it is sometimes omitted for simplicity.

Key Points

• 1

  Activation energy is conventionally reported in kJ/mol or kcal/mol.

• 2

  Temperature in the Arrhenius equation is normally measured in kelvins (K).

• 3

  Activation energy is represented by Eₐ; activation-related thermodynamic quantities include ΔG‡, ΔH‡, and ΔS‡.

• 4

  A superscript ° indicates evaluation under standard-state conditions, though it may be omitted in simplified notation.