<Li> Chemical relaxation methods such as temperature jump and pressure jump, in which a pre-mixed system initially at equilibrium is perturbed by rapid heating or depressurization so that it is no longer at equilibrium, and the relaxation back to equilibrium is observed . For example, this method has been used to study the neutralization H O + OH with a half - life of 1 μs or less under ordinary conditions . </Li> <Li> Flash photolysis, in which a laser pulse produces highly excited species such as free radicals, whose reactions are then studied . </Li> <P> While chemical kinetics is concerned with the rate of a chemical reaction, thermodynamics determines the extent to which reactions occur . In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal (the principle of detailed balance) and the concentrations of the reactants and products no longer change . This is demonstrated by, for example, the Haber--Bosch process for combining nitrogen and hydrogen to produce ammonia . Chemical clock reactions such as the Belousov--Zhabotinsky reaction demonstrate that component concentrations can oscillate for a long time before finally attaining the equilibrium . </P> <P> In general terms, the free energy change (ΔG) of a reaction determines whether a chemical change will take place, but kinetics describes how fast the reaction is . A reaction can be very exothermic and have a very positive entropy change but will not happen in practice if the reaction is too slow . If a reactant can produce two different products, the thermodynamically most stable one will in general form, except in special circumstances when the reaction is said to be under kinetic reaction control . The Curtin--Hammett principle applies when determining the product ratio for two reactants interconverting rapidly, each going to a different product . It is possible to make predictions about reaction rate constants for a reaction from free - energy relationships . </P>

Two factors that affect the rate of reaction