Several factors can influence the rate of a chemical reaction, including pressure, temperature, concentration and the presence of catalysts. These factors are important to professional chemists, many of whom make their living by improving the speed and efficiency of chemical reactions in industry, science and medicine.
TL;DR (Too Long; Didn't Read)
Pressure, temperature, concentration and the presence of catalysts can affect the rate of chemical reactions.
Pressure of Gases
For reactions involving gases, pressure strongly affects the reaction rate. With increasing pressure, the free space between molecules decreases. The chance for collisions between molecules increases, so the reaction rate increases. The reverse is true when you decrease the pressure.
Concentration of Solutions
In reactions involving solutions, the concentration of the substances in the solution directly affects the rate: Higher concentrations lead to faster reactions. The reason is much the same as for pressure and gases; molecules in a highly concentrated solution are packed more closely together, and the chance for them colliding and reacting with other molecules increases.
Heat and Cold
Temperature strongly influences the rate of almost all chemical reactions. When objects become hotter, the molecules vibrate more strongly and become more likely to collide with each other and react. At very cold temperatures, molecular vibrations are very weak, and reactions are infrequent. Temperature effects work over a limited range, however; when substances become too hot, undesired reactions can take place. Substances can melt, burn or undergo other unwanted changes.
Exposed Surface Area
A reaction between a liquid and a solid is limited by the ability of the molecules in the liquid to reach those of the solid. The solid’s outside surface is all the liquid “sees”; the outer layers prevent reactions with the liquid until they dissolve. For example, for a lump of metal dropped into a beaker of acid, the acid at first affects only the outer parts of the lump; the inner parts react only when the outer ones dissolve away. On the other hand, an equal amount of metal powder reacts more quickly to the acid, because the powder form exposes more of the metal. The same applies to reactions between gases and solids, and to a lesser extent between liquids. Reactions between gases, by contrast, aren’t limited by surface area since all the molecules are exposed and move freely.
Catalysts and Activation Energy
A catalyst is a chemical substance that doesn’t act as a product or reactant; instead it serves only to speed up the reaction. Many chemical reactions have an activation energy requirement; the molecules need an energy “kick” for the reaction to take place, such as the spark needed to ignite the gasoline in a car engine. The catalyst reduces the activation energy requirement, allowing more molecules to react under the same conditions.
Sensitivity to Light
Some chemical substances are light-sensitive; certain wavelengths of light add energy to reactions, greatly speeding them up. For example, polystyrene and other plastics are sensitive to the ultraviolet waves present in sunlight. The ultraviolet breaks down the bonds between the atoms in the plastic, causing it to deteriorate over time. Chlorophyll and other organic molecules are also sensitive to light, allowing plants to produce useful biomolecules from carbon dioxide in the air; the amount of light directly affects the plant’s health.