<P> When side products or reaction intermediates are formed, the IUPAC recommends the use of the terms rate of appearance and rate of disappearance for products and reactants, properly . </P> <P> Reaction rates may also be defined on a basis that is not the volume of the reactor . When a catalyst is used the reaction rate may be stated on a catalyst weight (mol g s) or surface area (mol m s) basis . If the basis is a specific catalyst site that may be rigorously counted by a specified method, the rate is given in units of s and is called a turnover frequency </P> <Ul> <Li> The nature of the reaction: Some reactions are naturally faster than others . The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can greatly influence the rate of a reaction . </Li> <Li> Concentration: Reaction rate increases with concentration, as described by the rate law and explained by collision theory . As reactant concentration increases, the frequency of collision increases . </Li> <Li> Pressure: The rate of gaseous reactions increases with pressure, which is, in fact, equivalent to an increase in concentration of the gas. The reaction rate increases in the direction where there are fewer moles of gas and decreases in the reverse direction . For condensed - phase reactions, the pressure dependence is weak . </Li> <Li> Order: The order of the reaction controls how the reactant concentration (or pressure) affects reaction rate . </Li> <Li> Temperature: Usually conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory . However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions (when bonds are formed between reactants). The influence of temperature is described by the Arrhenius equation . </Li> </Ul> <Li> The nature of the reaction: Some reactions are naturally faster than others . The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can greatly influence the rate of a reaction . </Li>

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