<Li> Activation energy, which is defined as the amount of energy required to make the reaction start and carry on spontaneously . Higher activation energy implies that the reactants need more energy to start than a reaction with a lower activation energy . </Li> <Li> Temperature, which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time, </Li> <Li> The presence or absence of a catalyst . Catalysts are substances which change the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take place . A catalyst is not destroyed or changed during a reaction, so it can be used again . </Li> <Li> For some reactions, the presence of electromagnetic radiation, most notably ultraviolet light, is needed to promote the breaking of bonds to start the reaction . This is particularly true for reactions involving radicals . </Li>

It refers to a substance that speeds up chemical reactions