What is meant by excess energy in exothermic reaction?
The energy release is because of the total energy of valence electrons in the product bonds is lower  than in reagents.

In fact, the change of the mean total  potential energy of valence electrons is twice as much as the change of their total energy, as the half of that change is compensated by their increased mean total kinetic energy.

As a consequence, the energy released by forming chemical bonds of products is bigger than energy spent on breaking bonds of reagents.