Chemical reactions that take place in the presence of a catalyst are called catalytic reactions. In a chemical reaction, some of the original chemical bonds of the reacting molecules must dissociate and form new ones, which requires a certain activation energy. In some systems where chemical reactions are difficult to occur, the addition of a third substance (catalyst) that helps rearrange the chemical bonds of the reacting molecules acts to reduce the activation energy of the reaction, thus accelerating the chemical reaction and controlling the selectivity and steric regularity of the products.