# What is the energy package released to the outside world?

Consider a chemical reaction where two different particles form another one

$$\ce{O + O_2 -> O_3}$$

I find it confusing how this can be an exothermic process. How to picture the release of $+\Delta H$ to the surroundings.

And am I right in viewing this energy value only as the net energy coming from an integration of the particle potentials as they get closer along the reaction coordinate?

• Can you provide a source for the thermodynamic data? I'd have thought it was an endothermic reaction. – CHM Aug 8 '12 at 15:55
• @CHM: I thought this is a exothermic one, but that doesn't really matter - the question goes for any exothermic reaction. (One could argue that the way I wrote it down doesn't even claim that this particular reaction is exothermic, but it's of course somewhat implied.) – Nikolaj-K Aug 8 '12 at 16:16

Essentially yes - if you consider the two particles colliding into a potential well, the kinetic energy associated with each will (depending on the angular specifics) be apportioned into rotational, vibrational, and even occasionally electronic excitations. So, the two particles collide, and are then vibrating and/or rotating about each other with all the translational energy converted in the collision.

Non-reaction collisions with other ambient particles allow relaxation into lower-energy states, transmitting energy to those particles. After some collisions the new species formed is in equilibrium, transferring between low-energy rotational and vibrational states as it collides further, and the change in potential energy has been dissipated as rotational, vibrational, and translational energy in other particles.

(I guess I could really use some sort of animation for this.)

• Mhm, my understanding of the energy package given to the outside, the so called system, hasn't really improved. – Nikolaj-K Aug 8 '12 at 18:26

The "packet" of energy exchanged fall into one of three forms: heat, work, or electromagnetic radiation. The first two are kinetic energy phenomena.

$$\Delta H ^0_{rxn}=\sum \Delta H ^0_{f}(products)-\sum \Delta H ^0_{f}(reactants)$$ $$\Delta H ^0_{rxn}= \Delta H ^0_{f}(\text{O}_3 )- \Delta H ^0_{f}(\text{O})$$ $$\Delta H ^0_{rxn}= 142.67~~\text{kJ/mol}-249.18~~\text{kJ/mol}=-106.51~~\text{kJ/mol}$$