Collision theory and transition state theory explain how chemical reactions occur and why reaction rates differ. Collision theory, proposed independently by Max Trautz and William Lewis in 1916-1918, qualitatively explains chemical reactions. Transition state theory, developed by Henry Eyring in 1935, describes an activated complex or transition state that forms between reactants and products. For a reaction to occur, the transition state must have sufficient concentration and break apart to form products rather than reforming reactants.