Rale constants solvent effects

The reaction rate constant k is not truly a constant it is merely independent of the concentrations of the species involved in the reaction. The quantity k is referred to as either the specific reaction rale or the rale constant. It is almost always strongly dependent on temperature. It depends on whether or not a catalyst is present, and in gas-phase reactions, it may be a function of total pressure, in liquid systems it can also be a function of other parameters, such as ionic strength and choice of solvent. These other variables normally exhibit much less effect on the specific reaction rate than temperature does with the exception of supercritical solvents, such as super critical water. 

The buzzword polarity , derived from the dielectrie approach, is certainly the most popular word dealing with solvent effects. It is the basis for the famous rale of thumb similia similibus solvuntur ( like dissolves like ) applied for diseussing solubility and miseibility. Unfortunately, this rale has many exceptions. For instanee, methanol and toluene, with dielectric constants of 32.6 and 2.4, respectively, are miscible, as are water (78.4) and isopropanol (18.3). The problem lies in exactly what is meant by a like solvent. Originally, the term polarity was meant to be an abbreviation of statie dipolarity and was thus associated with solely the dielectric properties of the solvent. Later on, with the advent... 

This rale follows immediately from Stokes s law for the motion of spherical bodies in viscous fluids when assuming constant radii. It is applicable in particular for the change in ionic mobility that occurs in a particular solvent when the temperature is varied. Between solvents it remains valid when the electrolytes have poorly solvated ions, such as N(C2H5)4l. For other electrolytes we find rather significant departures from this rale. These are due in particular to the different degrees of solvation found for the ions in different solvents, and hence their different effective radii. 

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