Heat energy is either absorbed or released during a reaction, but it is not wasted. Therefore, the products and reactants may have different amounts of energy depending on the kind of reaction they are going through: endothermic or exothermic.
How can products and reactants have different amounts of energy without violating the law of conservation of energy?
Answer: The way that products and reactants have different amounts of energy without violating the law of conservation of energy is by having the energy change from one form into another.
