• Catalysts aid chemical reactions by modifying their mechanism in a way that lowers the activation energy barrier. This barrier is the amount of energy that must be supplied for the reaction to take place. Although it is possible to take the opposite approach and supply more energy (for example, as heat), this may not be desirable due to cost, the danger of heating the reactants and the fact that increasing the temperature may result in undesirable products. For all these reasons, catalysts are widely used in chemical industries, such as petroleum refining.
  
  

Types of Catalysts

• Catalysts are classified as homogeneous and heterogeneous. A homogeneous catalyst is one that is present in the same phase as the reactants--for example, gases--while a heterogeneous catalyst is present in a different phase. For example, the chlorofluorocarbons (CFCs) that catalyze the destruction of ozone do so as homogeneous catalysts because they are gases. In the Antarctic, solid ice microcrystals function as heterogeneous catalysts, a phenomenon not prevalent in the Arctic. This is the reason why the ozone hole is much bigger in the Southern Hemisphere. In the case of heterogeneous catalysts, the reactants must be transferred from one phase to another, or the reaction will not occur. This extra step adds complexity to the mechanism, and for this reason chemists know less about these reactions than about those involving homogeneous catalysts. What is known is that the reactions proceed in four steps: (1) the reactants bind to the catalyst, (2) they move along it until they are close to each other, (3) they react to form the product or products and (4) the product or products are released from the catalyst.
  
  

Biological Catalysts

• Living organisms rely on chemical reactions for virtually every process, from obtaining energy from food to using it to grow or reproduce. At room or body temperature, many of these reactions are too slow to be compatible with life and must be accelerated. This goal is achieved using biological catalysts, known as enzymes. Although there are many types of enzymes, and each one catalyzes a specific chemical reaction, most follow a general mechanism that has several steps. First, the enzyme binds to a reactant (or substrate), and this causes a change in its shape. This change affects the substrate, making it more susceptible to interacting with another reactant to form the product. As a rule, the product does not bind to the enzyme, and so it is released. When the enzyme is free, it resumes its original shape, and the process can recommence.