3200
F. NAKSIIALL UEKINGEK AND E.MELVINGINDLEK
1'01. 7'7
[CONTRIBC.IIC)N DRUM 1 H E DEPARTMEN1 O F CFrEMISlRY O F THE POLYTECHKIC INbTII UTE O F B R O U K L Y Y ]
Ion Pairs a s Intermediates in Irreversible Reactions of Electrolytes'" B Y F.
h\IARSIIALL
BERINCER.4ND E.
AIELVIN
GIWDLER~~'
RECEIVED OCTOBER 20, 1951 Kinetic expressions are developed for a system in which free ions are iti mobile equilibrium with ioti pairs, which iiiav collapse irreversibly t o products. If the concentration of such ion pairs is substantial, even at constant ionic strength the cxperimental second-order rate constant depends on reagent concentrations. As the concentration of ion pairs decreases, t h e rate constant becomes independent of reagent concentrations, and its behavior may approach t h a t of the Debye-IIiickcl limiting law. Relevant published equations and data are discussed.
Introduction Results obtained in this Laboratory indicate that in water-dioxane mixtures ion pairs are intermediates in the irreversible reaction of diphenyliodonium and phenoxide ions to give diphenyl ether and iodobenzene. A preliminary report of these results already has been made2; a fuller exposition directly follows this paper.3 The results have prompted a formulation of the kinetics of irreversible reactions of ions of opposite charge on the basis of the following assumptions. 1. Free ions in solution are in equilibrium with ion pairs. This equilibrium is described by a mass action law formulated in terms of activities; the requisite activity coefficients are approximated by equations in the literature. 2. The ion pairs decompose irreversibly to products by way of an activated complex or transition state, the concentration of which determines the rate of the reaction. Ion Pair Equilibria : Historical.-The incomplete dissociation of the electrolytes has long been proposed by B j e r r u ~ n Onsagert5 ,~ Nernst,6 Fuoss and Kraus' and other^.^^^ hiore recently Sl'icke and Eigen'O have developed a new distribution formula for the ionic atmosphere in the theory of Debye and Huckel, have applied it to the incomplete dissociatioii of electrolytes in water and have obtained substantiating data from sound absorption measurements. Likewise Narshall and Grunwaldl' have used ion association in an interpretation of the activity coefficients of hydrogen chloride in methanol, ethanol and dioxane-water mixtures. Other workers have found it convenient to consider ion associa( I ) (a) This paper is taken from a dissertation submitted by E Melvin Gindler in partial fulfillment of t h e degree of Doctor of Philosophy, June. 1956. i b ) D u Pont Postgraduate Fellow, 1953-10d.5. (2) American Chemical Society Meeting, S e w York City, S e p tember 13-18, 10.54; see Abstracts of Papers. y, 1 I R . (3) F. M , Reringer and E:. .If Gindler, 'THIS J O I ~ R N A I . , 77, 3208 (1065). (4) . ' h Bjerrum, I i ~ 1 Doiiahc . I7i,Ic,zskab .>cl51;06, , l l ~ ~ t l i - . f ~Ilc,!tl., s. 7. 0 (1928). ( 5 ) I,. Onsagcr, Pliysik. Z . , 28, 2 i 7 1 1 0 2 7 ) . (0) W, Nernst and W, O r t h m a n n . %. p h y s t k . ( hein , 135, 1!!9 (1!128) ( 7 ) R.h l . Fuoss a n d C A . Kraiis, T H I S J O U R N A I . , 6 5 , '$76, 2387, 3614
(1933). (8) F o r a detailed expoTition see H S. Harned and B. B Owen. "The Physical Chemistry o f Electrolytic Solutions," Second Ed , Reinhold Puhl Corp., Xew York, N . V , I Y i O (9) F o r a recent s u r v e ) of I O U asswidtiuu see C A h r a u s , .I P h y r Chein., 58, 673 ( i g 5 4 ) 110) E . Wicke and A1 I
