I was reading this article on exothermic reactions online: Exothermic reactions

It states that "In exothermic reactions, the products have less enthalpy than the reactants, and as a result, an exothermic reaction is said to have a negative enthalpy of reaction. This means that the energy required to break the bonds in the reactants is less than the energy released when new bonds form in the products. Excess energy from the reaction is released as heat and light."

I'm just curious, is the excess energy from the bonds being formed, or is it because of the fact that products have more potential energy than the reactants, and because of law of convervation of energy, is released as heat energy?

  • 5
    $\begingroup$ There is no "or". Both alternatives are correct, except that you have the second one upside down. Products have lower potential energy than the reactants, hence the extra energy. $\endgroup$ Apr 26, 2019 at 7:15

1 Answer 1


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.

  • $\begingroup$ But I thought the energy is released from bonds being formed? Is that incorrect? $\endgroup$ Apr 26, 2019 at 7:00
  • $\begingroup$ Yes, That is right, it is the consequence of "low level quantum mechanics". See the answer update. $\endgroup$
    – Poutnik
    Apr 26, 2019 at 7:32

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