J . Phys. Chem. 1990, 94, 541 2-541 3
5412
new results for n-GaAs and n-GaP in both acidic and basic aqueous solutions can be understood on the basis of our theoretical approach. It follows that a detailed study of the rest potential as a function of the hole injection rate can be seen as a method (experimentally
the simplest one) to study the mechanism of electroless etching using "simple" hole injecting oxidizing agents. However, it is expected that also for the study of more complex etching solutions the relationship between the rest potential and the hole injection rate will be relevant.
COMMENTS TABLE 1: Bond Dissociation Enthalpies Given in kcal/mol
Comment on the Calculation of Bond Dissociation Enthalpies from Redox Potentials Sir: In a recent letter' it was claimed that the gaseous bond dissociation enthalpies, AHDBE, can be determined for reaction 1 from purely aqueous redox data. Here, XOH and XO' denote XO-H(g)
-.
XO'(g)
+ H'(g)
(li
hydroxylic species with the same charge, which may be 0 or - - I . However, what is really being calculated in ref I is
ACz
+ 7.8 kcal/mol
XOH(aq)
-
= AG2 + T U ,
XO(aq)
+ H'(g)
(2)
To obtain AH,, the additional steps (3) and (4) are required: XOH(aq) XO'(aq)
-
XOH(g)
(3)
XO'(g)
(4)
substance
calcd in
XO-H
this work I I9
HO-H 0-H -0-H HOO-H -00-H 00-H
PHBDE calcd in ref 1 I19
ex~tl~
99
106
119 102
87 66 51
1 I6 90 80 59
88 59 47
I n conclusion, contrary to the claim in ref 1, revision of the AHBDE values hitherto accepted cannot be justified.
Departments of Physical and Nuclear Chemistry Royal Institute of Technology S - 10044 Stockholm, Sweden
Gibor Merenyi* Johan Lind
Received: January 16, 1990
Then AH! == AG2
+ 7.8 - 4G3 + AG4
In the equations presented the standard states are (aq) 1 M solution except for H 2 0 where it denotes the pure liquid; (g) denotes the gaseous state at NTP. For neutral species differing in the number of hydrogen bonds the difference, AG3 - AG4, is usually not negligible and it can become quite considerable for ionic species. Utilizing the same redox data as ref 1 and with the best literature values of AG(hydration) we recalculate AH,,, (Table I) for the species treated in ref 1. For comparison the values calculated in ref 1 as well as the experimental values cited therein are given. It is readily seen that our recalculated values are close to the experimental estimates. In particular, the large deviation found in ref 1 in the last two entries has been reduced. Now, the aqueous redox data combined with the T U , value is accurate to within 2 kcal/mol. The same error limit should apply to AC3 - AG4 for the neutral species while it is probably as high as 4 kcal/mol for the anionic ones. Allowing for these possible errors no significant difference can be established between the accepted literature values and the calculated ones in this work.
Bond Dissociation Enthalpies for Gases Calculated from Aqueous Redox Potentials Sir: In a recent letter Sawyer' claims that "accurate values" for for H-OH, H e , gas-phase "bond dissociation energies (AHDBE)n H-0-, H-OOH, H-OO-, and H-OO' may be obtained from the formula AHDBE
=
D.T. J . Phys. Chem. 1989, 93, 1911. (2) Pearson, R. G . J . Am. Chem. SOC.1986, 108, 6109. (3') Schwarz, H. A., Dodson, R. W . J . Phys. Chem. 1984, 88, 3643. (4) Kinetically estimated values cited for comparison i n ref 1 0022-3654/90/2094-5412$02.50/0
+ TASDBE
(1)
where AGBF (bond formation) for the reaction of interest is calculated from aqueous solution redox potentials. No justification is given for this formula, but inherent in it is the notion that -- AGBF(aq)= AGoDBE(g). This would be the case only if the Gibbs energy of transfer of the parent species from solution to the gas phase is balanced without significant error by the Gibbs energy of transfer of the dissociated products from gas phase to solution. In addition, for all the cases listed above Sawyer uses a constant ASDBEof 26 cal/K, which is ASo for the reaction H-OH@)
( 1 ) Sawyer,
-AGgF
thereby yielding a constant
-
H'(g)
TASDBE
+ 'OH(g) = 7.8 kcal/mol. Neither of
( I ) Sawyer, D. 'I. J Phys. Chem. 1989, 93, 7911.
0 I990 American Chemical Society
