A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the repulsion between their electrons. This energy is called activation energy. Factors influencing the rate of reaction either lower the activation energy or increase the likelihood of a successful collision.

Concentration and Pressure:

The more particles present within a given space, the more likely those particles are to bump into one another. This means that chemists can speed up chemical reactions not only by increasing the concentration of particles—the number of particles in the space—but also by decreasing the volume of the space, which would correspondingly increase the pressure.

Temperature:

The kinetic energy of subatomic particles increases in response to increases in thermal energy. The higher the temperature, the faster the particles move and the more likely they are to come in contact and react. This means that chemical reactions occur at a faster rate at higher temperatures.

Properties of the Reactants:

If chemical reactions are to occur quickly, the atoms in the reactants must have easy access to one another. In general, the greater the surface area of the reactants, the more readily they will interact. Generally, gases tend to react faster than liquids or solids. Energy is required to separate particles of a substance, and gases already have space between their particles. Similarly, the larger the molecule, the greater the number of total bonds, so reactions involving smaller molecules with fewer total bonds would be expected to proceed faster.

A chemical reaction between two or more substances requires intimate contact between the reactants. When reactants are in different physical states or phases (solid, liquid, gaseous, dissolved), the reaction takes place only at the interface between the phases. Consider a heterogeneous reaction between a solid phase and either a liquid or gaseous phase. Compared with the reaction rate for large solid particles, the rate for smaller particles will be greater because the surface area in contact with the other reactant phase is greater. This means that for heterogeneous reactions, the rate of reaction depends on the surface area of the reactant in a more condensed phase.

Enzymes and Catalysts:

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change. A catalyst does this by lowering the activation energy needed to start a chemical reaction. The most important catalysts in the human body are enzymes. Enzymes are critical to the body's healthy functioning. They assist, for example, with the breakdown of food and its conversion to energy. Most of the chemical reactions in the body are facilitated by enzymes. This means that their deficiency can cause serious health problems. For example, in Tay-Sachs disease, a hexosaminidase enzyme deficiency leads to the accumulation of gangliosides in the brain and spinal cord.

This text is partially adapted from Openstax, Anatomy, and Physiology 2e, Section 2.3: Chemical Reactions and Openstax, Chemistry 2e, Section 12.2 Factors Affecting Reaction Rates