A catalyst is any agent that functions to speed up a reaction or process without being used up or changed itself.
In chemical reactions, molecules are changed by moving or rearranging atoms or clusters of atoms. For each reaction, to achieve these chemical transitions from one molecule to an altered molecule, a certain amount of energy is normally required to prepare the molecule to undergo change. This is referred to as the activation energy. Activation energy can be thought of as a barrier that prevents molecules from changing from one form to another.
In a chemical reaction, catalysts function to hold a molecule in a certain position or influence the strength of the individual chemical bonds that undergo change during the reaction. Catalysts speed up reactions by lowering the activation energy necessary for the reaction to take place. In living systems, most chemical reactions are catalyzed by proteins called enzymes.
Catalysts can be homogeneous or heterogeneous. A homogeneous catalyst is one that exists in the same phase (gas, liquid, or solid) as the reacting chemical. In biology, for example, enzymes are distributed in the liquid environment inside of cells, and the reacting chemicals are dissolved in the liquid state there as well. In contrast, heterogeneous catalysts exist in a different physical state than the reacting chemicals. For example, in automobiles, the catalytic converter is a solid phase platinum-based catalyst found in the exhaust system, but the reacting chemicals are found in the exhaust gases that pass through after combustion of the gasoline.
Catalysts can be slowed when various inhibitors or poisons are present. Inhibitors are agents that physically interact with the surface of a catalyst to slow or interfere with a chemical reaction. Often, molecules that act as inhibitors for a certain catalyst have shapes and structures very close to the chemical that normally interacts with the catalyst. The inhibitors differ chemically from the reacting chemical, however, so that they are unable to be chemically altered by the normal action of the catalyst. In the case of enzymes, specific inhibitors may often be used in drugs, such as the popular statin drugs used to lower cholesterol. In the example of the catalytic converter, heavy metals such as lead function as poisons by irreversibly combining with the catalytic surface of the platinum, destroying its catalytic properties.
Among the many catalysts used in forensic testing, scientists use inorganic catalysts in the analysis of paint samples and biological catalysts when analyzing DNA .
see also Chemical equations; Endothermic reaction; Exothermic reactions.
cat·a·lyst / ˈkatl-ist/ • n. a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. ∎ fig. a person or thing that precipitates an event.