Rates of reactions and their temperature dependence

All chemical reactions, whether of the hydrolysis, acid-base, or combustion type, take place at a definite rate and depend on the conditions of the system. The most important of these conditions are the concentration of the reactants, the temperature, radiation effects, and the presence of a catalyst or inhibitor. The rate of the reaction may be expressed in terms of the concentration of any of the reacting substances or of any reaction product that is, the rate may be expressed as the rate of decrease of the concentration of a reactant or the rate of increase of a reaction product. 

A stoichiometric relation describing a one-step chemical reaction of arbitrary complexity can be represented by the equation [2.2, 2.3] 

The reason for following this complex notation will become apparent shortly. The law of mass action, which is confirmed experimentally, states that the rate of disappearance of a chemical species i, defined as RRit is proportional to the product of the concentrations of the reacting chemical species, where each concentration is raised to a power equal to the corresponding stoichiometric coefficient that is, 

In many systems M7 can be formed not only from a single-step reaction such as that represented by Eq. (2.3), but also from many different such steps, leading to a rather complex formulation of the overall rate. However, for a single-step reaction such as Eq. (2.3), not only represents the overall order of the reaction, but also the molecularity, which is defined as the number of molecules that interact in the reaction step. Generally the molecularity of most reactions of interest will be 2 or 3. For a complex reaction scheme, the concept of molecularity is not appropriate and the overall order can take various values including fractional ones. 

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