In a chemical reaction, molecules maybe have the following 3 changes: (1) molecules attaching to each other to formlarger molecules.(2) A Molecule breaking apart to form two or more smaller molecules.(3) Rearrangements of atoms within molecules.Chemical reactions often involve changes in energy due to thebreaking and formation of bonds. Reactions in which energy is released are exothermic reactions, while those that take in heat energy are endothermic.