# 42.2: Introduction


Chemical reactions may occur seemingly instantly, or may take some time to progress.  There are several factors that determine the rate at which a chemical reaction occurs.  In general, the more energetic and numerous collisions are between reactants, the faster reactions occur.  Sometimes reaction rates are intentionally slowed or inhibited.  Factors that may affect reaction rates include nature of reactants (complexity), temperature, concentration, surface area, and use of a catalyst.