It is shown that the activation energy of a chemical reaction depends mainly on the difference between the hardness of the initial state of a reaction and the hardness of the transition state, that these two quantities may be estimated from the softnesses of the reacting molecules and the softnesses of the molecular fragments that characterize the transition state, by making use of the additive properties of the softness, and that the proportionality constant that characterizes the softness of the transition state may provide information about the structure and the looseness of the latter. In addition, it is shown, through the sign of the reaction energy, that reactions tend to go in the direction that produces the hardest possible species. Finally, it is demonstrated that the gap between the eigenvalues of the highest occupied and the lowest unoccupied molecular orbitals takes its minimum value at the transition state.