The reaction rate for a product in a particular reaction is defined as the amount (in moles or mass units) per unit time per unit volume that is formed or removed.

Knowledge of these rates is essential in chemical engineering and environmental engineering.

Reaction rates are expressed by the formula:

In this equation, [A] is the molar concentration of the reactant (there may be several) A, n is the order of the reaction, and k is the rate constant. Order establishes the mathematical relation of concentration to rate. For instance, in a second-order reaction, if the concentration doubles, the rate quadruples.

When there is more than one reactant involved in a reaction, the rate is simply the product of all of the reactants and their orders:

Rate is often expressed in the units ^mol/_Ls.

There are several factors that affect the rate of reaction:

• Temperature: Conducting a reaction at a higher temperature puts more energy into the system and increases the reaction rate. The influence of temperature is described by the Arrhenius equation, whose result is factored into the equation by k. As a rule of the thumb, the reaction rate doubles for every 10 degrees Celsius increase in temperature.
• Concentration: As reactant concentration increases, the frequency of collision increases and so therefore does the frequency of collisions having sufficient energy to cause reaction.
• Pressure: The rate of gaseous reactions usually increases with an increase in pressure. Increase in pressure in fact is equivalent to an increase in concentration of the gas.
• Light: Light is a form of energy. It may affect the rate or even course of a reaction. For example when methane reacts with chlorine in dark, the reaction rate would be very low. It can be speeded up when the mixture is put under diffused light. In bright sunlight, the reaction is explosive.
• Order: Clearly the order of the reaction has a major effect on its rate. The order of a reaction is found experimentally, and, for most basic reactions, is an integer value.
• A catalyst: The presence of a catalyst increases the reaction rate in both the forward and reverse reactions by lowering the activation energy of the reaction.
• State of sub-division of reactants: The larger the surface area compared to the volume, the faster a reaction can take place, as more simultaneous reactions can occur.
• The nature of the reactants: If a reaction involves the breaking and reforming of bonds (complex) compared to just the forming of bonds (simple) then it generally takes longer. The reactants position in the reactivity series also affects reaction rate.