Chemical reactions are classed as endothermic or exothermic.

Defn: exothermic

(of a reaction or process) accompanied by the release of heat.

The heat is released when the chemical bonds in the product are formed.

Defn: heat

heat is energy in transfer to or from a thermodynamic system, ... The mechanisms include conduction, ...; or radiation between separated bodies; or ....

So, I have three questions:

1 - if the heat is released via conduction, is this equivalent to an increase in the kinetic energy of the product(s) compared to the kinetic energy of the reactant(s)? Also, how does the formation of a chemical bond produce kinetic energy?

2 - if the heat is released via radiation, how is radiation created in the formation of a chemical bond?

3 - are conduction and radiation the only forms of heat transfer when a bond is created?

Edit - to clarify the question - the issue is addresed in the OPENSTAX AP chemistry text - which covers one example of 2 H atoms coming together to form H2 in detail as follows:

When the atoms are far apart ..... by convention the sum of their energies is 0. As the atoms move together their orbitals begin to overlap. Each electron begins to feel the attraction of the nucleus in the other atom. In addition, the electrons begin to repel each other, as do the nuclei. While the atoms are still separated, the attractions are slightly stronger than the repulsions, and the energy of the system decreases. (A bond begins to form.) ... At some specific distance between the atoms, which varies with the atoms involved, the energy reaches its lowest (most stable) value. This optimum distance between the two bonded nuclei is the bond distance between the two atoms.

The text doesn't specify what energy is decreasing, and why. But I can answer these questions I think. The energy that is decreasing is the potential energies of the attractive fields of the atoms, and that energy is being transformed to kinetic energy as the atoms accelerate toward each other.

So we have the nuclei accelerating towards each other. What happens next?

According to the text ... continuing where we left off

The bond is stable because at this point the attractive and repulsive forces combine to create the lowest possible energy configuration.

So there is no mention of the release of energy, and the atoms just arrive at the stable combined state.

Here is what I would expect, given that the nuclei are accelerating towards each other - at some point the repulsive forces become stronger than the attractive forces, and the atoms begin to decelerate. If there is no damping force on the motion the atoms will oscillate about the point where the attractive and repulsive forces are equal. So, there must be some damping force on the atoms, that causes them to come rest at the bond distance of H2.

Now comes the mystery, which is, how does the formation of a chemical bond release energy?

What happens to the kinetic energy of the two nuclei accelerating towards each other? If the two hydrogen atoms are coming together head on there will be a collision that would appear to cancel out the motion, i.e. kinetic energy, of both atoms. If there is a damping mechanism that generates the heat, what is it?

Since the energy is released as heat, I think it must be in the form of kinetic or electromagnetic energy, but what is the specific mechanism?

Or, am I trying to get too much out of the kinetic model?

  • 2
    $\begingroup$ Further, the subject is covered in my high school kid's chemistry text, OPENSTAX Chemistry, in the chapter 'Chemical Bonding and Molecular Geometry' as follows: 'plus the energy released when all bonds are formed', and 'the excess energy is released as heat', and that's it. $\endgroup$
    – DrWill
    Apr 2, 2019 at 15:26
  • 1
    $\begingroup$ I think the issue is that you ask a couple different subquestions and any one of these has the potential to turn into a long answer depending on how much you know and how much detail you want. I think the overall question is interesting and it got a good, so I'll vote to reopen, but I think a full answer to the question could cover excited states, various types of radiative and non radiative transitions, how collisions exchange energy between a system and its environment, and so on. $\endgroup$
    – Tyberius
    Apr 4, 2019 at 20:19
  • 1
    $\begingroup$ @Tyberius - I'm not looking for a full treatment, just one simple example would do, suitable for a high school class. To that end I've edited the question with an example from a high school text that describes the formation of a bond, but leaves out the part about the release of energy. $\endgroup$
    – DrWill
    Apr 5, 2019 at 18:37
  • 1
    $\begingroup$ Also, the college board has an article, 'Ending Misconceptions About the Energy of Chemical Bonds', link: apcentral.collegeboard.org/courses/ap-chemistry/… which does not answer the question. $\endgroup$
    – DrWill
    Apr 6, 2019 at 12:29
  • 3
    $\begingroup$ Rather than deleting the question (which we will not do because it is answered) and asking it anew, it is preferable to edit the question for clarity. Now this is a bit of a problem here, as the question is already answered. In such cases it would be best to ask a new question as a follow-up to this question. When in doubt, please consult Chemistry Meta to ask for advice. $\endgroup$ Apr 8, 2019 at 12:00

1 Answer 1


It is easier first to think of what happens when a bond is broken in an exothermic reaction. The energy is initially released into vibrations, rotations and translational motion of the two fragments. This energy is then lost as these fragments collide with other gas molecules or with solvent molecules and so eventually ends up as heat.

In exothermic bond formation the extra energy vibrationally and rotationally excites the new molecule and this is then lost via collisions with surrounding gas or solvent molecules and again ends up as heat.

(If the new molecule were isolated in space (so no collisions) it would remain 'hot' until the energy is radiated away from vibrational levels and until it (radiatively) comes into equilibrium with the surroundings. This is a slow process, by many orders of magnitude, compared to collisions in solution or in a gas at normal atmospheric pressure )

In a very few types of reaction an electronically excited state is produced in the product and then photons radiate (as visible light) some of the energy away. This is called chemiluminescence.

  • 3
    $\begingroup$ Energy is not released when a bond is broken. It requires energy to break a bond. $\endgroup$
    – DrWill
    Apr 2, 2019 at 15:12
  • 2
    $\begingroup$ The question is about an exothermic reaction in which case energy is released $\endgroup$
    – porphyrin
    Apr 3, 2019 at 7:13
  • 2
    $\begingroup$ Still, energy is not released when a bond is broken, even in an exothermic reaction. You need to separate two atoms which have an attractive force between them. That has to require energy. In an exothermic reaction, generally, more bonds form than break, so there is an energy surplus. $\endgroup$
    – Deltab
    Apr 5, 2019 at 18:53
  • 1
    $\begingroup$ You are not quite correct. You are effectively talking about the activation energy, or energy barrier, between reactant and product, when this is surmounted more energy is released in an exothermic reaction than needed to overcome the barrier. $\endgroup$
    – porphyrin
    Apr 5, 2019 at 21:23
  • 2
    $\begingroup$ Energy is never released when a bond is broken in isolation. An exothermic reaction involves stronger bonds forming than are broken, it is the formation that releases the energy. Activation energy doesn’t affect this as it only involves the energy that must be overcome during the reaction pathway, not the potential energies of the bonding states before and after. $\endgroup$
    – Withnail
    Apr 5, 2019 at 21:36

Not the answer you're looking for? Browse other questions tagged or ask your own question.