A simple experiment about thermal effects in chemical reactors is described, which can be used to illustrate chemical reactor models, the determination and validation of their parameters, and some simple principles of heat transfer and sensor dynamics. It is based in the exothermic reaction between aqueous solutions of sodium thiosulfate and hydrogen peroxide in a discontinuous reactor that exchanges heat with the environment. Two models: with and without considering the thermometer time lag, are compared. (Contains 4 figures and 1 table.)