text: For a first-order reaction (including a unimolecular one-step process), there is a direct relationship between the unimolecular rate constant and the half-life of the reaction: . Transition state theory gives a relationship between the rate constant  and the Gibbs free energy of activation  a quantity that can be regarded as the free energy change needed to reach the transition state.  In particular, this energy barrier incorporates both enthalpic  and entropic  changes that need to be achieved for the reaction to take place: The result found from transition state theory is  where h is the Planck constant and R the molar gas constant.  As useful rules of thumb, a first-order reaction with a rate constant of 10 s will have a half-life (t) of approximately 2 hours.  For a one-step process taking place at room temperature, the corresponding Gibbs free energy of activation (ΔG) is approximately 23 kcal/mol.