[yodalr]

HEAT - Kinetic movement of atoms. Heat energy release means just that atoms start moving faster in the system. Is this correct?

ENERGY RELEASE WHEN BONDS FORM(this I don't get) - Let's imagine a system with 2 hydrogen atoms. One atom is standing still and the other is flying towards it with speed 5. It hit's the other atom and forms a bond with it. The kinetic energy of the moving atom is transferred to the new bond and the system now has a moving H2 atom with a speed of 2.5. Where is energy release?

Another example, 2 hydrogen atoms are moving towards each other at speed 5 and -5. They hit each other, form a bond and the overall speed of H2 goes to 0. Where is energy release?

Please no Gibbs Free Energy explanations, something like ELI5 is prefered.

[Borek]

What about vibrational states and energy emitted as electromagnetic radiation?

[mjc123]

What is EL15? (Google says it's a battery, but there are no batteries in H₂.)
The answer to your question is that the excess energy is in the vibration of the H-H bond. If you think of the potential energy curve, two hydrogen atoms approaching each other have sufficient energy for the H₂ molecule to fall apart within the first vibration unless some energy is lost. If within the first vibrational period the new molecule collides with another molecule and transfers some of its vibrational energy to that, it is now in a stable bound state, and the other molecule has increased kinetic energy.
(It's analogous to the case where you put a mass on the end of a vertical spring and let it go. It doesn't stop at the equilibrium displacement, but oscillates about this point. To get it to stop you have to absorb that extra energy (with your hand, or by air resistance etc.)

[Enthalpy]

Heat is not only kinetic energy. For instance the deformation of bonds stores heat too - much so. And if a molecule comprises several atoms, the molecule can rotate; you may compute a kinetic energy for each atom, or for the centre of mass of the entire molecule, but then you must add the rotation energy of the molecule. There are more exotic forms of heat.

The formation of a molecule from two atoms requires often one additional atom or molecule that absorbs the released energy.

[yodalr]

Just watched this about molecular vibrations, but it seems rather old. Is it still relevant?
https://www.youtube.com/watch?v=3RqEIr8NtMI

Is electromagnetic radiation just pure energy?

ELI5 - Explain Like I'm 5

[Borek]

Just watched this about molecular vibrations, but it seems rather old. Is it still relevant?

Our basic understanding of molecular vibrations didn't change by much for at least eighty years.

Is electromagnetic radiation just pure energy?

No such thing as a "pure energy". Energy is a property and always requires a carrier, electromagnetic radiation is one of many of these carriers.