The combination of non-equilibrium thermodynamics with Cauchy- Lagrange optimization procedures has resulted in a new, general principle for process design - the Principle of Equipartition of Forces. The principle states that the best trade-offs between entropy production and transfer area in transport processes are obtained when the thermodynamic driving forces are uniformly distributed over the transfer area. It has been applied so far to conduction of mass, heat and charge. In this paper the application to rate-controlled reactions will be shown, concluding that Delta rG/T for a rate-controlled reaction should be uniform through space and time in a reactor. Certain optimization results reported in the literature can thus be explained and generalized. The new principle is easy to apply and has a predictive power for improving chemical reactors as well as interacting transport processes.