External Pressure and Solvent Effects on Reaction Rates

11 External Pressure and Solvent Effects on Reaction Rates 

Not only the internal pressure of a solvent can affect chemical reactions (see Section 5.4.2 [231, 232]), but also the application of external pressure can exert large effects on reaction rates and equilibrium constants [239, 429-433, 747-750]. According to Le Chatelier s principle of least restraint, the rate of a reaction should be increased by an increase in external pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules, whereas the rate of reaction should be decreased by an increase in external pressure if the reverse is true. The fundamental equation for the effect of external pressure on a reaction rate constant k was deduced by Evans and Polanyi on the basis of transition-state theory [434]  

AF is related to the well-known volume contraction normally observed on dissolution of electrolytes and called electrostriction. The contraction of the solvent surrounding an ion is best expressed by the Drude-Nemst equation (5-172)  

The solvent is assumed to be a continuum of relative permittivity fir, and the ion to be a hard sphere of radius r with charge z e Na is the Avogadro number. Eq. (5-172) represents the electrostatic contraction change in volume for a mole of ions. According to the Drude-Nernst equation, electrostriction should be proportional to the square of the charge on the ions, inversely proportional to the ionic radii, and should increase in proportion to the value of l/s )/ 8si/8p), which depends on the nature of the solvent. It follows that A Ve will vary strongly in reactions where charges are created or neutralized, which, in turn, should show up as a solvent dependence of AE.  

AEA represents the change in volume due to changes in bond lengths and angles. It is this contribution to AE that is connected to the reaction mechanism in terms of the relative positions of the atoms in reactants and the activated complex. The absolute size of A has been concluded to be approximately +10 cm mol for bond cleavage and approximately —10 cm moE for bond formation in reactions of organic molecules [430]. 

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