Catalysis is a process in which the rate and/or the outcome of a chemical reaction is influenced by the presence of a substance known as a catalyst. Catalysts are substances that increase the reaction rate of a chemical reaction without being consumed in the process/14%3A_Chemical_Kinetics/14.07%3A_Catalysis). They are not consumed by the reaction and remain unchanged after it. Catalysts make chemical reactions more efficient by lowering the activation energy, which is the energy barrier that must be surmounted for a chemical reaction to occur. This makes it easier for atoms to break and form chemical bonds to produce new combinations and new substances. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the non-catalyzed mechanism. Catalysts can be classified as either homogeneous or heterogeneous. Homogeneous catalysis involves catalysts that are in the same phase as the reactants, while heterogeneous catalysis involves catalysts that are in a different phase from the reactants/14%3A_Chemical_Kinetics/14.07%3A_Catalysis). Enzymes and other biocatalysts are often considered as a third category. Catalysts are ubiquitous in the chemical industry, and estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture. Catalysts also have a key property called selectivity, by which they can direct a reaction to increase the amount of desired product and reduce the amount of unwanted byproducts.