APRIL. 1952
A NOTE ON THE KINETIC SALT EFFECTS JOHN H. WOLFENDEN Dartmouth College,Hanover, New Hampshire
TEEinterpretation of the influence of salts on rates on another positive ion in its vicinity is reduced by
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of reaction in terms of the variation of activity coefficients with ionic strength has reduced to a rational pattern what had previously been an unrelated multitude of observations. For some students a t any rate the primary and secondary salt effects of Brfinsted open up the first crack in a very solid, if private, conviction that activity coefficients are "faking factors" for which the symbol f seems very appropriate. The thermodynamic basis of the primary salt effect, however, has been known to raise difficulties in the minds of students. It does not seem to be generally realized that the sense of the primary salt effect can be accounted for in a qualitative way in terms of electrostatic influence on molecular eucount,ers. This "collisional" interpretation was pointed out to the writer some 15 or more years ago by Professor C. N. Hinshelwood in conversation; but, although it is implicit in the point of view underlying pages 369-70 of "The Structure of Physical Chemistry,"' I have never seen it explicity stated. It seems worth while putting in black and white a way of looking a t the primary salt effect that has proved helpful in teaching. The ionic, atmosphere around a given ion is of opposite charge to the ion itself2 and accordingly operates to reduce the intensity of the field of force due to the ion. Thus the repulsion exerted by a positive ion ~
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HINSHELWOOD. C. N.. "The Structure of Phvsical Chemistry," Clarendon press, oxford, 1951. a This is reflected in the derivation of the Dehye-Hiickel limiting equation for the free energy of an electrolyte where the expression for the electrical potential oantainns a t e r n due to the "centrd'I' ion and one of oppositesign due to its ionic atmosphere.
its ion atmosphere. This mitigation of ordinary electrostatic repulsions and attractions that is caused by the ion atmospheres will clearly grow more marked as the ionic strength of the solution increases. It therefore becomes obvious in a qualitative way that encounters between mutually repulsive ions (of like charge) will be facilitated by a high ionic strength while encounters between mutually attractive ions (of unlike charge) will, on the other hand, be hindered by an increase of ionic strength. This consideration provides a clear if over-simplified physical picture of the primary salt effect and a means of predicting its sense. To the student whose sense of neatness is offendedby the reference of two kinds of salt effect and who finds their nomenclabure a little arbitrary it is also perhaps worth pointing out that the secondary salt effect can be regarded as a consequence of the primary salt effect. Thus, for example, the primary salt effect should slow down the rate of recombination of the ions of a weak acid when the ionic strength of the solution is increased. while the rate of ionization should be little affected; it is accordingly evident that added salts must displace the ionization equilibrium in the sense leading to a typical "secondary" salt effect. This point of view is open to the possible criticism that the kinetic picture of the ionization of a weak electrolyte is not unambiguous; the intervention of the solvent mav raise difficulties in the identification of the "slow step:" It is, however, perhaps permissible to ignore this somewhat sophisticated difficulty in a first presentation of the kinetic salt effects.
