Catalysis (/kəˈtælÉ™sɪs/) is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (/ˈkætÉ™lɪst/), which is not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only very small amounts of catalyst are required to alter the reaction rate in most cases. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst usually reacts to form a temporary intermediate, which then regenerates the original catalyst in a cyclic process. A substance that provides a mechanism with higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance that does reduce the reaction rate is not considered a catalyst but a reaction inhibitor (see below). Catalysts may be classified as either homogeneous or heterogeneous