The weighted residual method was used to calculate . the isothermal effectiveness factors of spherical for first- and second-order reactions. Dirac functions were chosen as the weight functions. Because of the simplicity of the form of the trial function
the calculation could be completed with a calculator. The results were very close to the analytic solution for first-order reactions and closer to the numerical solution than those from Satterfield. Kjaer and Bischoff-Aris-Petersen methods for second-order reactions.