Composition Concentration of Reactants

Chemical Composition Concentration of Reactants. The first important studies of reaction systems were begun with a study of the effect of the concentrations of the reacting species on the rate of a reaction. For gas reactions, concentrations are directly related through the equation of state to pressure, volume, and temperature. For liquid reactions the pressure assumes secondary importance as a variable, the volume of the system being very insensitive to either pressure or temperature changes. 

Since the chemical stoichiometry of the reaction fixes relations between 

Chemical Composition Concentration of Inert Substances. In a great many studies it has been found that an added substance which is not consumed in the chemical reaction and may thus be considered as inert has a pronounced effect on the rate of reaction. Thus in the case of the gas-phase thermal decomposition of certain hydrocarbons, ethers, and aldehydes it has been found that added quantities of II2, He, or N2 may under certain conditions increase the rate of the reaction. (II2 is probably chemically active in these systems see Chap. XIII.) 

In liquid-phase reactions such effects may be much more pronounced. It is frequently found that a change in solvent may change the rate of a liquid reaction, in some cases by many orders of magnitude. A most spectacular example of this is the reaction of ethyl iodide with trimethyl-amiiie to form the quaternary salt. The reaction rate is almost 1000 times greater in nitrobenzene than in cyclohexane under comparable conditions. 

Chemical Composition Catalysis. Careful study has shown that certain substances present in a reaction system in only small quantities may have a considerable effect on the rate of the reaction. In the cases in which these substances are not consumed chemically the phenomenon is referred to as catalyvsis. 

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