Chemical reactions occur when chemical bonds between atoms are formed or broken. reactants come together and produce a reaction at the end known as the products. a reactant combined with another is written as a chemical formula. usually in a chemical formula an arrow is drawn between the reactants and products to indicate the direction of the chemical reaction. some chemical reactions run in one direction until the reactants are used up. these are known to be irreversible reactions. other reactions, however, are classified as reversible. reversible reactions can go in both the forward and backward.