COMMUNICATIONS TO THE EDITOR
3107
Thermodynamics of Ionization of
Ten determinations of the heat of solution of liquid aniline (in amounts corresponding to a range of 0.019 to 0.028 mole 1.-') lead to AH," = 0.45 f 0.03 kcal mole-' for
Aqueous Anilinium Ion
Sir: Numerous studies have illustrated the value of AH" and ASc' values for acid ionization reactions. Since most interest centers on comparison of A G O , AH", and A S values for ionization of a parent acid with corresponding values for substituted acids, accurate data are required. Data for ionization of aqueous anilinium ion, relevant to several studies of substituent effects, are in poor agreement and we undertook this investigation t o try t o resolve the resulting uncertainties in AH" and AS" values. Equilibrium measurements at several temperatures by Pedersen mere used by Everett and Wynne-Jones' in calculating aHozg8= 7.105 kcal mole-' for ionization of aqueous anilinium. Subsequent calorimetric measurements by Zawidzki, PapBe, Canady, and Laidler2 led these workers to report AH" = 6.74 f 0.09 kcal mole-', but Chen and Laidler, later indicated that this value and those reported in the same paper for substituted anilinium ions may be in error. Biggs4 has determined the ionization constant at several temperatures and calculated AH" = 6.52 kcal mole-'. Most recently, equilibrium measurements at three temperatures have led Aboul-Seoud and Doheim6 to report AH" = 5.78 I t 0.02 kcal mole-' for the ionization of aqueous anilinium. Measurements were made in three calorimeters, all similar to one described by O'Hara, Wu, and Hepler.6 The calorimetric dewar used a t the Naval Academy held SO0 ml, and lead storage batteries in the heater circuit were replaced by a Iiepco power supply. I n the calorimeters used at Carnegie Institute, the transposed bridge circuit was replaced by a nickel resistance thermometer and Leeds and Sorthrup Jlueller G-2 bridge and H. s. galvanometer. ,411 measurements were made at IG.0 0.15". Eastman White Label aniline was distilled in a small (all glass) apparatus .Ir.ith Vigreux column. Center cuts of distillations carried out immediately before each calorimetric run were used for all measurements. We have made 14 determinations of the heat of sohtion of liquid aniline in 0.095 lV! HC1, using amounts of aniline corresponding to a range of 0.015 to 0.034 mole 1.-'. On the basis of these heats, which are expected and observed to be nearly independent of concentration, we obtain AH1" = -6.79 f 0.09 kcal mole-' for
*
+ H+(aq) = C6HjNH3+(aq)
CsH5SHz(1)
(1)
C6H5NHz(l)= CsH6NHz(aq)
(2)
We calculate the standard heat of ionization of anilinium ion represented by C6HjNHs+(aq)
=
H+(aq)
+ C6HSNH2(aq) (3)
as AH," = AH," - AH1" = 7.24 f 0.1 kcal mole-'. This value is in satisfactory agreement with the AH" calculated by Everett and Wynne-Jones' from the equilibrium data of Pedersen, but is in poor agreement with the other values cited in the second paragraph of this note. Results of several investigators are in good agreement concerning Kzg8for the ionization and we adopt1v4 AG," = 6.27 f 0.02 kcal mole-'. We then calculate AS," = 3.3 f 0.3 cal deg-l mole-'. Our preliminary calorimetric measurements indicate AH" = 4.5 kcal mole-' for the ionization of the mnitroanilinium ion, which is also the value calculated from B i g g ~results. '~ Earlier measurements by Haldna' apparently lead to AH" = 5.6 kcal mole-' for this ionization.
Acknowledgment. Part of this research has been supported by the National Science Foundation. (1) D . H . Everett and W. F. K. U'ynne-Jones, Trans. Faraday Soc., 35, 1380 (1939).
(2) T. W. Zawidzki, H. M. Papbe, W. J. Canady, and K. J. Laidler, i b a . . 55. 1738 (1959). (3) D. T.Y. Chen and K. J. Laidler, %bid.,58, 480 (1962). (4) A. I. Biggs, J. Chem. SOC.,2572 (1961). (5) A. Aboul-Seoud and 31.Doheim, Can. J . Chem., 44, 521 (1966). (6) W. F. O'Hara, C. H. Wu, and L. G. Hepler, J . Chem. Educ., 38, 512 (1961). (7) u. Haldna, Izv. Vysshikh Uchebn. ZaLedemii Rhim. E;hzm. Tekhnol., 6, 233 (1933). (8) Work done at U. S.Naval Academy. (9) Work done at Carnegie Institute of Technolow.
W. F. O'HARA* H. C. KoQ
SCIENCE DEPARTMENT
u. s. KAVAL ACADEMY
ANNAPOLIS, MARYLAND DEPARTMENT OF CHEMISTRY 31. N. ACKERMANN~ CARNEGIE INSTITUTE OF TECHNOLOGY L. G. HEPLER' PITTSBURGH, PENNSYLVANIA 15213
RECEIVED MAY29, 1967
Volume 7 1 Number 9 Alcgust 1967 I
