This paper investigates transient natural convection flow of a chemically reactive fluid in a vertical annulus, considering the combined influence of heat source/sink and Soret diffusion. The governing conservation equations are treated using a semi-analytical method, and the resulting solutions are expressed in modified Bessel functions. The analysis predicts that internal heat generation enhances fluid velocity and thermal gradients, while heat absorption promotes faster stabilization of the flow field. The Soret effect introduces strong coupling between temperature and concentration, significantly influencing mass transfer and buoyancy-driven circulation. These interactions modify the Nusselt and Sherwood numbers, as well as skin-friction characteristics across the annular region. The study provides practical relevance to systems such as catalytic reactors, geothermal exchangers, and energy devices where coupled heat–mass transport under reactive conditions plays a critical role.