An equation used extensively to describe chemical reactions. For a rate constant κ‎, it is given by

where k is the Boltzmann constant, T is the thermodynamic temperature, h is the Planck constant, Δ‎G^‡ is the free energy of activation, and K is a constant called the transmission coefficient, which is the probability that a chemical reaction takes place once the system has reached the activated state. A similar equation (without the K) has been used to describe transport processes, such as diffusion, thermal conductivity, and viscosity in dense gases and liquids. In these cases it is assumed that the main kinetic process is the motion of a molecule to a vacant site near it. The equation is derived by assuming that the reactants are in equilibrium with the excited state. This assumption of equilibrium is not necessarily correct for small activation energies. The Eyring equation is named after Henry Eyring, who derived it in 1935 and applied it widely in the theory of chemical reactions and transport processes. It was also derived independently that same year by Meredith Gwynne Evans (1904–52) and Michael Polanyi (1891–1976).