Microwave-assisted chemical reactions have been widely used, but the mechanism is still unclear, which limits further applications. For example, when chemical reactions are exposed to electromagnetic waves, some electromagnetic energies dissipate and these processes are poorly understood. In this paper, we investigate the power loss density in unimolecular reactions under exposure of electromagnetic waves. By using the equivalent circuit approach, the energy problem can be transformed into the energy stored or dissipated in equivalent circuit elements. The power loss density of electromagnetic waves in unimolecular reactions can be clearly obtained from an equivalent time-varying resistance, which is related to the amount of substances in reactions. Specifically, when the applied electromagnetic wave is monochromatic and the reaction rate is slow, the derived expression of the power loss density can be reduced to the classical formula in the frequency domain.