COMMUNICATIONS TO THE EDITOR
2808
The Volume Change on Neutralization of Strong Acids and Bases
Sir: We recently reported' a limiting apparent molal volume of -5.25 ml. mole-' for aqueous sodium hydroxide at 25". From this value and those for hydrochloric acid and sodium chloride given by Harned and Owen2 we obtained a volume change a t zero ionic concentration of 21.86 (sign error in ref. 1) ml. mole-' for the reaction H+(aq)
+ OH-(aq)
=
HzO
Dr. Redlich has kindly pointed out3that the limiting value for the apparent molar volume for hydrochloric acid listed in ref. 2 is based on an incorrect extrapolation to zero concentration. Accordingly, we have redetermined apparent molal volumes of hydrochloric acid a t five concentrations from 0.002 to 0.024 M by the dilatometric method.' The results confirm the theoretical limiting slope4 and extrapolate to +v" (HC1) = 17.82 0.02 ml. mole-'. Our results are in excellent agreement with the earlier work of Redlich and Bigele i ~ e n but , ~ are not in agreement with the results of Geffcken and Wirth or the +v0 values (18.20 and 18.07) quoted by Harned and Owen2 that are based on extrapolations with limiting slopes considerably smaller than the theoretical slope. Combination of +v0 (HCI) = 17.82 ml. mole-', +v" (NaOH) = -5.25 ml. mole-' from our previous work,l +v0 (NaC1) = 16.61 ml. mole-' from the work of KrUis6 (obtained by extrapolation based on the correct limiting slope), and the molar volume of water leads to AVO = 22.11 ml. mole-' for the neutralization reaction represented above. The change from our previous value arises entirely from the revision in dv0 (HCI) from the values reported by Harned and Owen.2
*
(1) L. G. Hepler, J. M. Stokes, and R. H. Stokes, Trans. Faraday SOC.,61,20 (1965).
(2) H. S. Harned and B. B. Owen. "The Phvsical Chemistrv of Eiectrolytic Solutions, 3rd Ed., Reinhold Pudlishing Corp., New York, N. Y.,1958,p. 361. (3) 0. Redlich, personal communication to L. G. H., 1965. (4) 0.Redlich and D. M. Meyer, Chem. Rev., 64, 221 (1964). (5) 0.Redlich and J. Bigeleisen, J . Am. C h m . SOC.,64, 758 (1942). (6) A. Kruis, 2. physik. Chem., 34B, 1 (1936).
PHYSICAL CHEMISTRYDEPARTMENT UNIVERSITIOF NEW ENGLAND ARMIDALE, N.S.W., AUSTRALIA DEPARTMENT OF CHEMISTRY CARNEGIE INSTITUTE OF TECHNOLOGY
PITTSBURGH, PENNSYLVANIA RECEIVEDJUNE 14, 1965
The Journal of Physieal Chemistry
L. A. DUNN R. H. STOKES L. G. HEPLER
A Suggested Mechanism for the
Hydrogen-Fluorine Reaction. 11. The Oxygen-Inhibited Reaction Sir: Recently, Levy and Copeland have studied the rate of reaction between hydrogen and fluorine, first in a flow system diluted with nitrogen' and secondly in a static system, in the presence of oxygen2in the temperature range 122-162". A previous communicationa suggested a mechanism for reaction in the absence of oxygen; the present communication indicates how that mechanism can be expanded to describe the oxygen-inhibited reaction. Levy and Copeland2 found that small amounts of oxygen greatly decrease the reaction rate but that soon the rate reaches a limiting value and is unaffected by further oxygen addition. The limiting rate was found to be approximately proportional to the fluorine concentration and the square root of the hydrogen concentration. This result can be explained by the reactions
(F- 2HF + (wall?)
'/2
0 2
The numbering of the last three reactions, indicated by primes, conforms to the numbering frequently used for these hydrogen-oxygen chain reaction steps. When small amounts of oxygen are added to hydrogen-fluorine mixtures, reaction 6 competes with the step
H
+
ks F2
+HF
+F
(2)
At large oxygen concentrations reaction 2 is over(1) J. B. Levy and B. K. W. Copeland, J . Phys. C h a . , 67, 2156 (1963). (2) J. B. Levy and B. K. W. Copeland, ibid., 69,408 (1965). (3) R.9. Brokaw, %%id., 69,2488 (1965). (4) B.Lewis and G. Von Elbe, "Combustion, Flames, and Explosions of Gases,'' Academic Press Inc., New York, N. Y.,1951.
