At room temperature all atoms in a solid vibrate about their equilibrium position. If work is done which increases their kinetic 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. the new springs are stiffer, they do more work pulling the atom to their new equilibrium positions than was needed to break the old bonds. This work is converted into random kinetic energy, i.e. thermal energy.
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.