A chemical equation can also show heat changes that occur. Endothermic reactions cause the reaction vessel to feel cooler because the reaction absorbs energy. The energy is used in the reaction, so energy is a reactant. Exothermic reactions release energy. Consequently, we consider heat or energy to be a product of an exothermic reaction. In our example, the burning of hydrogen is highly exothermic. The reaction can be rewritten as
According to the Law of Conservation of Mass (Matter), mass (matter) cannot be created nor destroyed. During a chemical reaction, since atoms are a form of matter, atoms cannot be created nor destroyed. We say that atoms are conserved. This means the number of each kind of atom in the reactants must equal the number of that kind of atom in the product .
CHEMICAL REACTIONS The Law of Conservation of Mass & Balancing Equations