I will present a general mathematical framework for constructing deterministic models of chemical reactions. In such a model, an underlying dynamical system drives a process in which a particle undergoes a reaction (changes color) when it enters a certain subset (the catalytic site) of the phase space and (possibly) some other conditions are satisfied. This framework allows us to define the entropy of reaction precisely and does not rely on a stochastic mechanism to generate additional entropy. Rates of reaction are also handled in this framework, but are independent of the entropy of reaction. This is joint work with L. Bunimovich.