How does the law of conservation of energy apply to chemical changes?

The law of applies to all chemical reactions.

We know that chemical is just another form of potential energy, so when we react chemicals together we can either release that energy (exothermic reactions, like burning a piece of wood) or take energy from the surroundings to store more energy (endothermic reactions, like photosynthesis in plants.

Basically we can say that in any closed reaction, physical, chemical or otherwise, the total amount of energy is constant and just gets changed from one form to another,