In this letter, we study the nonequilibrium effects which appear in a thermally activated exothermic reaction A + A --> products using the molecular dynamics for reactive hard spheres. We have found that the rate constant is reduced with respect to its equilibrium value and the relative decrease of rate constant is similar to that observed for a thermoneutral reaction with the same activation energy. It is demonstrated that a simple phenomenology, which assumes a Maxwellian distribution of the energetic states for the reactant and for the system as a whole, gives a good estimation of the nonequilibrium rate constant.