We present a mathematical procedure for the determination of the driving force distribution in a chemical reactor that has minimum lost work for a given production rate. It is shown how the path of minimum lost work is determined from knowlegde of reaction kinetics, using the reaction A --> B as an example. The normal chemical reaction has a nonlinear relation between the rate, r, and the driving force, -A/T, where A is the affinity and T is the absolute temperature. Minimum lost work is obtained when A/T + r d(A/T)dr is constant. This criterion is converted into a more practical criterion for the operating temperature along the reactor. The inverse operating temperature should be parallel to the inverse equilibrium temperature, when the enthalpy of reaction is constant.