Catalysts offer an alternative route for a reaction with lower activation energy, thereby making the reaction proceed at a higher rate. Catalysts do not change the equilibrium of a reaction; they can only increase the reaction rate. Without a catalyst, the reaction would therefore still proceed in the same direction, just slower.

Figure 1. A reaction from reactant to product is a transition from one energy state to another. A transition state exists between the substrate and the product. This state has a higher energy level than both the substrate and the product. A catalyst will lower this energy level, so the transition energy is reached more easily. This results in a faster reaction rate.

Types of catalysts

Catalysts can be classified as homogeneous or heterogeneous, depending on their specific application. A homogenous catalyst has the same phase as the reactant mixture where it's applied, while a heterogeneous catalyst has a different phase as the reactant mixture.

For example:

  • Homogenous catalysts: Chlorine from gaseous chlorofluorocarbon (CFC) in the atmospheric breakdown gaseous ozone.

  • Heterogeneous catalysts: Solid nickel in the hydrogenation of liquid ethylene.

Catalysts can also be found naturally in biochemical reactions. These biological catalysts are called enzymes. Enzymes are large molecules, usually proteins, that bind with specific substrates and reorient them so that they can react easily. As a result, the presence of an enzyme will speed up biological reactions and processes.