Kinetics is the branch of physical chemistry dealing with reaction rates and mechanisms. In a chemical system, several reactions may be possible according to thermodynamics, but in practice the fastest reaction predominates, not necessarily the most energetically favorable. The reaction rate – the rate at which the concentration of one reactant decreases – is normally proportional to a certain power of the concentration of the reactants, the sum of the exponents being called the reaction order. Thus for the reaction A + B → C + D it may be found that the rate is given by

 

   d[A] / dt = k[A] ² [B]

 

Such a reaction is third order overall (second order in A, first order in B). The rate constant, k, depends exponentially on the absolute temperature (so that at room temperature most reactions double in rate for a 10K rise in temperature) and on the activation energy. Catalysis speeds up a reaction by providing an alternative mechanism with lower activation energy. Reaction rates are studied by measuring concentration as a function of time, regular or continuous chemical analysis being used.