In a chemical reaction, the reaction rate is faster when there is a higher concentration of reactants because there are more molecules present that can collide with one another. Collision theory explains this phenomenon, which is especially true for chemical systems that contain either gases or liquids as reactants.
Conversely, a system that has a lower concentration of reactants has a slower reaction rate. Another factor that can affect reaction rates is particle size. Smaller particles lead to more collisions within a system and a faster reaction rate.
Increasing the temperature or pressure of a system also speeds up the reaction rate of a chemical reaction. However, increasing the pressure to speed up a reaction rate is more effective for a system that contains gases. Another way to increase the reaction rate is to use a substance called a catalyst to facilitate a chemical reaction.