Collisions continuously transfer energy between the different degrees of freedom and the average energy in each degree of freedom is the same. If work is done on the molecules which increases their vibrational energy, the amplitude of the vibrations increases, and eventually the chemical bonds break. Most free atoms quickly form new bonds. If the new bonds are stronger, i.e. if the new springs are stiffer, then they do more work pulling the atom towards their new equilibrium positions than was needed to break the old bonds and the atoms will have more kinetic energy as they pass through the equilibrium positions. This kinetic energy is quickly shared with the other degrees of freedom, the energy of all degrees of freedom increases, i.e. the thermal energy increases.
Thermal energy is released by a chemical reaction .
The temperature increases. To burn fuel, work must first be done to break the chemical bonds in the fuel. This work provides the activation energy , the energy needed to start the chemical reaction. The free atoms and molecules then bond with oxygen. The new bonds with the oxygen atoms are much stronger than the broken bonds.