NOTES
2451
Table I : Dissociation Energy of Fen
Temp,
Run
Fe-ZrOz-Ta
O K
2073 2041 2004 2062 2049 2070 2106 2158 2176
I(Fez +)/ I(Fe+) x 108
0.925 0.676 0.510 0.800 0.715 0.871 1.17 1.83 2.01
Doo from the third-law method AHTO from baaed on the the aecondmean multiplicity law method
29.3 29.3 29.0 29.3 29.4 30.0 29.9 30.4 30.4
Fe-ThOz-Ta
1963 2050 2108 2188 2112 2085 2102 1988 1931 2028 2096 2168 2098 2045
0.487 0.703 1.87 2.75 2.10 1.12 1.14 0.55 0.386 0.691 1.09 2.77 1.73 0.748
29.8 29.3 31.8 31.4 32.2 30.3 29.9 30.0 30.2 29.7 29.8 32.0 31.7 29.7 __ 3 0 . 6 =!= 1 . 6
+
+
29.7 rt 0.5
the first period and the atomic valence state energies of the corresponding atoms. The appearance potential of Fez+was measured to be 2 f 0.2 eV less that of Fe+ [I.P.(Fe) = 7.90 f 0.01].4 The reaction Fe Fe+ = Fez+as a spurious source of dimer is improbable because the ratio I(Fez)/I(Fe) was found to be independent of repeller voltage, and it requires a three-body collision. The charge-exchange reaction Fe2+ AB = Fe+ A + B (AB is a neutral molecule), as discussed in a previous paper,” which could affect the dimer ion intensity, was eliminated by using ionizing electrons (20 eV) of energy less than the Fez+ appearance potential.
l8
*
+ +
(8) R. Hultgren, R. L. Orr, P. D. Anderson, and K. K. Kelley, “Selected Values of Thermodynamics Properties of Metals and Alloys,” John Wiley & Sons, Inc., New York, N. Y.,1963. (9) The maximum value of the heat function may be obtained by maximizing the quantity (AHT - AHo), = Zg,E,e-fldRT/(l Fgie-EdRT) with respect to E , assuming E = E”,g = Zgi and using g = 180 which is obtained from the sum of states, IKA, 1*A, . , , LA, 1 6 2 , 1 3 2 , , , , 12. (10) A. Kant and B. Straws, J. Chem. Phys., 41, 3806 (1964). (11) A. Kant, S.-S. Lin, and B. Straws, ibid., 49, 1983 (1968).
+
The Molecular Hydrogen Yield in Irradiated 2-Propanol Vapor1 18 f 3
by M. G. Bailey and R. S. Dixon2 Atomic Energy of Canada Limited, The dissociation energy can also be obtained from the Whiteshell Nuclear Research Establishment, plot of log [I(Fez+)/I(Fe+)1 vs. reciprocal temperature, Pinawa, Manitoba, Canada (Received November 16, 1968) known as the second-law method. A least-squares treatment of the data of Table I, using the heat of vaporization of iron from Hultgren, et uZ.,* results in The effect of propene on G(H2)3from irradiated 2AHo2~00= 18 f 3 kcal for the reaction Fez(g) = Fe (g) propanol vapor has indicated the presence of an unfor both the thoria and zirconia liners. Reduction of scavengable or molecdar yield of hydrogen, g(Hz) = this value to absolute zero using the tabulated heat 1.7 =!= 0.2.4 The effect of propene is to capture thermal function of iron and the previously mentioned vibraH atoms 3) tional frequency of Fez gives DOo(Fez) = (13 H C3He --t C3H7 (1) (AHT - AHo),kcal where the last quantity, the molecular electronic heat function, is unknown and must be thus preventing them from forming hydrogen by abapproximated. The minimum and estimated maxistraction mum contributions of the molecular electronic heat H CsH70H --t Hz C3HsOH (2) function are 0 and 12 kcal19 respectively. Using the mean value of 6 f 6 kcal for the heat function, 19 f 7 I n this study, the possible abstraction reaction from kcal is obtained for the second-law value of Doo(Fez). propene It should be noted that the third-law computations, H C3Ha -+ Hz C3H5 (3) based on 16A and $A molecular states, give values of Doo(Fez) of 23 f 5 and 25 f 5 kcal which are not in was considered to be negligible. Clearly this molecular much disagreement with the second-law result. Apparhydrogen yield which persists at very high propene ently both a high degeneracy and large number of lowlying energy levels are required to bring the second- and (1) Issued as AECL No. 3277. (2) T o whom correspondence should be addressed. third-law values into agreement. (3) Primary yields are written as g(X) and experimentally measured The relative low dissociation energy of Fez is consisyields as G(X). tent with a previous correlationlo between the dissocia(4) R. S. Dixon and M. G. Bailey, Advances in Chemistry Series, tion energies of transition element diatomic molecules of No. 82, American Chemical Society, Washington, D. C., 1968, p 247.
* +
+
+
+
+
+
Volume 73, Number 7 July 1969
2452
NOTES
I
1.0
I
-O.O
10
0.0 I 0
I
50
-1
20
I
I
150
100
I
200
I
250
1
[c3H 70H [C2 b]
acc
0 I0
005
ELECTRON
FRACTIOV
ADDITIVE
Figure 1. Effect of additives on the hydrogen yield from irradiated 2-propanol vapor a t 125': 0, ethylene; 0, nitric nitric oxide 1 mol yo ~ F B . oxide;
,.
+
I
I
1
I
I
10
20
30
40
SO
concentrations does not arise from thermal H atoms,
jC3H7OH
It should, therefore, persist in the presence of additives
[NO1
which scavenge thermal H atoms and/or their precursors in irradiated 2-propanol, unless the thermal H atom and molecular Hz yields have a common precursor. We have extended our study of the unscavengable yield of hydrogen in irradiated 2-propanol vapor4 by examining the effect of other additives on G(H2) at 125". For this study we have chosen to look at the effect of nitric oxide and ethylene, both of which react rapidly with H atoms by addition.636 H atoms abstract to a negligible extent from ethylenee8 The experimental details have been described previ~usly.~Irradiations were carried out at -700 Torr pressure and 125". The dose rate was 1.9 X lo1*eV g-l min-I (estimated by the ethylene dosimeter' using G(H2) = 1.31), and total doses were in the range 1019-1020eV g-l. G values were based on the energy absorbed by 2propanol only. Corrections for hydrogen produced from the ethylene were made using G(Hz)cl~,= 1.31,' assuming this to be unchanged in the presence of 2propanol. The effects of ethylene and nitric oxide on G(Hz) from 2-propanol vapor are shown in Figure 1. The rapid decrease in G(H2) on addition of as little as electron fraction NO, followed by a slower fall-off as the NO concentration increases, is indicative of ?SO reacting with more than one precursor of hydrogen. The effect is similar to that of CC14 in 2-propanol vapor4 and, as in the case of CCI4, addition of sufficient SFs to capture all electrons gives values of G(H2) which fall on the same curve as those in the absence of SFa (Figure 1). This shows that in the absence of SFa, NO is completely suppressing the formation of H atoms from electron-positive ion combination by reacting efficiently The Journal of Physical Chemistry
1
Figure 2. Reciprocal plots for hydrogen yields from irradiated 2-propanol vapor containing various additives a t 125': X, ethylene; 0, nitric oxide; 0, nitric oxide 1 mol % ' SFB. (Inset shows larger scale plot for the lower [CaH7OH]/ [C~HI] ratios.)
+
with either the electron or positive ion to give a product which does not yield H atoms. Since G(H2) = 5.4 in the presence of SFs a t zero NO concentration* and assuming any further reduction in G(H2) to be caused by the capture of residual H atoms by NO
H
+ NO -+HNO
(4)
in competition with reaction 2, then the competition may be expressed by the equation
where g(H) is the yield of H atoms which do not arise from electron-positive ion combination. The plot of l(5.4 - G(Hz)) vs. [C,H,OH]/[NO] shownin Figure 2 is a good straight line, and from it g(H) = 3.6 f 0.2 and h / k Z = 21 are obtained. Thus, by subtraction, the molecular hydrogen yield g(Hz) = 1.8 f. 0.4 in good agreement with the value found for p r ~ p e n e . ~ This agreement justifies the neglect of reaction 3 in the propene study. The effect of ethylene on G(H2) is less drastic than that of NO (Figure 1) and appears more like the effect (5) E. W. R-Steacie, "Atomic and Free Radical Reactions," Vol. 1, Reinhold Publishing Corp., New York, N. Y., 1964. (6) R. J. Cvetanovib, Advan. Photochem,, 1, 115 (1963). (7) G.G.Meisela, J . Chem. Phys., 41,51 (1964).
NOTES of p r ~ p e n e . ~Assuming, therefore, that the reduction in G(Hz) by ethylene is due to a competition between reactions 2 and 5
the competition may be expressed by the equation
2453 Thermodynamics of Ionization of Acetic and Chloroacetic Acids in Water-Ethanol Mixtures1&
by Frank J. Millero,lb Ching-hsien W U , ’ ~ and Loren G. Hepler’d Contribution Number 104.6 from the Institute of Marine Sciences, University of Miami, Miami, Florida $8149 and Department of Chemistry, University of Lethbridge, Lethbridge, Alberta, Canada (Received December 19, 1968)
where g(H)t is the total number of H atoms from all precursors. The plot of 1/(8.9 - G(H2)) US. [C~HVOH]/[CzH4] is shown in Figure 2. Although the plot is linear at the lower values of [C3H70H]/[CzH4Ilthe slope of it changes at higher values. This may be due to ethylene reacting with some species (possibly a precursor of H atoms) in addition to H atoms, but, if so, the identity of this species is not immediately apparent. Since simple olefins have negative electron affinities in the gas phase,s the reaction of ethylene with electrons may be eliminated. The ionization potential of ethylene (10.51 eV)8 and the proton affinity of 2-propanol (2193 + 9 kcal/mol)4 are such that neither charge nor proton transfer to ethylene should occur. However, reactions between olefins and alkane positive ions have been shown to occur in the mass spectrometerlo and in gas-phase radiolysis,” the predominant reaction being the transfer of H or Hz from the alkane ion to the olefin. Thus olefins could react with intermediates other than H atoms, and although we have no positive evidence in our system, we cannot rule out the possibility of ethylene reacting with some positive ion intermediate and altering the neutralization process to one which does not yield H atoms. Reaction of ethylene with other possible precursors such as excited molecules would be purely speculative. Thus the reason for the deviation a t high [C3H70H]/[CzH4]values is not clear, but extrapolation of the linear portion of the plot (shown in the insert of Figure 2 ) shows the total yield of hydrogen which is removed by ethylene to have a value G = 7.2 0.2. Since G(H2) = 8.9 f 0.2 in the absence of ethylene, the unscavengable or molecular yield is 1.7 f 0.4, in excellent agreement with the values found with propene4 and nitric oxide. Although the three additives studied behave in somewhat different ways in 2-propanol, they all give the same limiting yield of hydrogen at “infinite” additive concentration. The average value of this unscavengable or molecular yield is g(Hz) = 1.75 + 0.4.
*
Hoyland and L. Goodman, J . Chem. Phys., 36, 21 (1962). (9) V. I. Vedeneyev, L. V. Gurvich, V. N. Kondrat’yev, V. A. Medvedev, and Ye. L. Frankevich, “Bond Energies, Ionization Potentials and Electron Mnities,” Edward Arnold Ltd., London, 1966. (10) F. P. Abramson and J. H. Futrell, J . Phys. Chem., 71, 1233 (1967). (11) P.Ausloos and S. G . Lias, J . Chem. Phys., 43, 127 (1965). (8) J. R.
Earlier calorimetric investigations of ionization of aqueous acids have yielded useful thermodynamic data that have sometimes (for example, ref 2) contributed to our understanding of solute-solvent interactions in relation to acid strengths. Attempts to understand these and other data have in turn led to a number of theoretical investigations3-5 of linear free energy relations and substituent effects in relation to solute-solvent interactions. Substituent effects on acidities are often considered in terms of the general proton transfer reaction represented by HA(S)
+ R-(S)
=
A-(S)
+ HR(S)
(1)
in which HR represents a reference acid (such as acetic acid) and HA represents a substituted acid (such as chloroacetic acid), all in some solvent S. Earlier investigation^^-^ have shawn that it is useful to consider the thermodynamics of reactions of type 1in terms to be of AH” and AS” values, each of which is taken4&*6 a sum of external (environmental) and internal contributions as follows
AH” =
AHint
AS” =
ASint
+ AH,,$ + ASext
(2) (3)
T h e ~ e ~ aand - ~ other investigations (for example, see ref 7 ) provide evidence that compensation of AHext by TASextcauses AG” to be approximately equal to (or at least proportional to) AHint for reactions of type 1. Although this approach has led to improved under(1) (a) The experimental part of this research was conducted a t Carnegie Institute of Technology, Pittsburgh, Pa. (b) University of Miami. (c) University of Pennsylvania. (d) University of Lethbridge. (2) L. P. Fernandez and L. G. Hepler, J . Amer. Chem. Soc., 81, 1783 (1959). (3) C. D. Ritchie and W. F. Sager, Progr. Phys. Org. Chem., 2, 323 (1964). (4) (a) L. G. Hepler, J . Amer. Chem. SOC.,85, 3089 (1963); (b) 33, 3961 (1968).
J. W. Larson and L. G. Hepler, J . Org. Chem.,
Note that eq 17 in this paper should be printed as a fraction with (1 - p / T ) in the denominator. ( 5 ) J. W. Larson and L. G. Hepler in “Solute-Solvent Interactions,” by J. F. Coetzee and C. D. Ritchie, Ed., Marcel Dekker, Inc., in press. (6) E. J. King, “Acid-Base Equilibria,” Pergamon Press, Oxford, 1965. (7) D. J. G. Ives and P. D. Marsden, J . Chem. Soc., 649 (1965). Volume 73, Number 7‘ July 1969
