The effects of substitution on reaction rates are at the heart of both physical chemistry and organic chemistry. In this paper, we present a computational laboratory module to explore the effects of group substitution and isotopic substitution on reaction rates by using a readily accessible graphical interface, a computational engine, and spreadsheet software. The module focuses on simple unimolecular isomerization reactions, often used as a classic example for kinetics in first-year, organic, and physical chemistry courses. In the context of a physical or physical organic chemistry course, the module is used to illustrate substituent and/or isotope effects on reaction rates, as well as chemical reaction rate calculations.