A breakdown of the rates of reactions in which ions are formed from neutral molecules into heats and entropies of activation reveals the following regularities with change in solvent: the heats of activation do not change greatly and are more frequently larger in polar solvents than in nonpolar ones; the entropies of activation are always negative and become more negative as the polarity of the solvent decreases. An explanation is given in terms of solvent molecules ``frozen'' around the incipient ions of the activated complex. Nonpolar solvents suffer a greater loss in entropy in such ``freezing'' than polar solvents. An electrostatic calculation is given which is satisfactory for all solvents whose molecules contain a permanent dipole moment, but which must break down for nonpolar solvents.
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