NOTES
Kov., 19G1 metallic rhodium which previously has not been studied quantitatively. The recent survey of Honig' cites only an estimat'ed vapor pressure and heat of sublimat,ion. Experimental Apparatus and Techniques The apparatus used was a microbalance inside a vacuum system and has been described previously along with the %) calculation methods.2 The high purity wire (99.99 was suspended from a single crystal hIgO hook and no reaction was observed.
+
Discussion of Results The result,s of the sublimat,ion studies on R h are summarized in Table I. Monatomic Rh(g) was assumed to be t.he only important gaseous species. Combination of the vapor pressures with free energy funct'ions from Stull and Sinke3allows computation of a third law heat of vaporization a t 298°K. Aplot of Pvs. l/Tyields the heat of sublimation a t the high temperature from the slope and correction t'o 298OK. can be accomplished by using heat content data from Stull and Sinkee3 From Table I, the heat of sublimation of Rh at 298OK. is 134.2 f 0.8 kcd./mole and the extrapolated normal boiling point is 3900 f 100'K. The second law treatment of log P us. 1/T data yields AHZg8O= 135 f 2 kcal./mole, in good agreement.
2107
data for rhodium which indicate AHzgs = 132.5 It 2.0 kcal./ mole, while Panish and Reif6 have reported aHzss = 132.8 f 0.3 kcal./mole.
Acknowledgment.-The authors are pleased to acknowledge the financial support of this research by the Wisconsin Alumni Research Foundation and the Atomic Energy Commission. Samples of high purity Rh wire were generously provided by the International Nickel Company through the courtesy of E . 31.Wise. ( 5 ) M. B. Panish and L. Reif, J . Chem. Phys., 34. 1918 (1961).
THERMODYNAMICS O F IONIZATION OF AQUEOUS meta-CHLOROPHENOL BY W. F. O'HARAA N D L. G. HEPLER Department of Chemastry, Unazeraty of Varganza, Charlottesvzlie V a . Recezved June 1 1961
It has been found' that the differences in free energies of ionization of the three mono nitrophenols in aqueous solution are due to entropy rather than enthalpy effects. The entropies of ionization are different because the anions of onitrophenol and p-nitrophenol distribute their negative charge to the nitro group more than do anions of m-nitrophenol, in accord with conventional ideas about the small tendency of meta TABLE I substituents, as compared to ortho and para subVAPORPRESSURE DATAFOR RHODIUM METAL stituents, to take part in resonance with the The log P us. 1/T plot gives AH = 137.7 a t T g y g= 1925 phenolic function. The differences in charge diswhich when corrected to 298'K. using Stull and Sinke's data tribution cause differences in solute-solvent in= 135 f 2.0 kcal./mole. gives teractions, which show up most clearly in the Run no. T, Pmm AHmO (kcal./mole) entropies of ionization. 12 1942 1.17 x 10-5 138.5" Enthalpies and entropies of ionization of o3.50 x 10-5 134.2 13 1942 chlorophenol and p-chlorophenol also were investi14 1942 2.72 X 10-6 135.2 gated.' The heat of ionization of aqueous m-chlo15 19i5 4.16 X 135.8 rophenol has been investigated in a continuation 16 2068 5.32 x 10-4 131.6 of earlier work and the results are presented and 133.8 17 2068 3.09 x 10-4 discussed here. OK.
18 19 20 21 22 23 25 26 27 28 29
a
2068 2068 2068 1962 1975 20013 1744 1857 1857 1803 1803
3.60 x 3.33 x 3.01 x 1.33 x 7 . 19 x 1.19 x 4.73 x 7.29 X 5.80 x 2.22 x 2.00 x
10-4 10-4 10-4 10-4 10-5 10-4
10-7 10"
10" 10-6 10-8
133.2 133.5 133.9 130.4" 133.8 133.7 135.7 134.2 135.1 134.7 135.1
AH029s= 134.2 f 0 . 8 kcal./mole Large deviation. Given 0 %-eightin average.
XOTE ADDEDIN PROOF.-TWO recent reports verify the results reported here. Hampson and Walker4have report'ed (1) R. C. Honig, "Vapor Pressure Data for the More Common ElementR," R. C. A. Laboratories, David Sarnoff Research Center, Princeton. N. J , 1957. (2) L. H. Lheger and J. L. Margrave, J. Phys. Chem., 6 4 , 1323
(1960) (3) D. R. Stull and G. C. Sinlie, "Thermodynamic Properties of the
Elenients," Advances in Chemistry Series, No. 18, Amer. Chem. SOC. (l96G).
R. F. Hampson a n d R. F. Walker, J . Reaearck Natl. Bur. Standards. 618. 289 (1961); Abstracts. XVII Intern. Congr. Pure and Aki11'. Chern , ,\Iontrea!. August, 1961 p 101. (4) (a)
Experimental The solution calorimeter we used has been described.z All heats of solution and heats of reaction with KaOH(aq) were measured in a volume of 950 ml . a t 25 .O i0.1. Eastman white label m-chlorophenol was doubly distilled in an all-glass apparatus with Vigreux column a t atmospheric pressure. Only the fraction having 1 2 4 0 ~1.5560 and melting between 32.0 and 33.1" was used. The m-chlorophenol was used in the supercooled state in the calorimetric runs because it was conveniently handled in the liquid state and was found not to solidify in the sample bulbs during an experiment. Sodium hydroxide solutions were prepared and standnrdized by common procedures. O
Results and Discussion Heats of solution of liquid m-chlorophenol in 950 ml. of water were determined. The calorimetric reaction equation is m-CP(1iq) = m-CP(aq)
AH1
(1)
Results of these experiments, listed in Table I, were extrapolated to zero concentration t o give AH10= 674 f 15 cal./mole. Heats of neutralization of m-chlorophenol by (1) L. P. Fernandea and L. G. Hepler, J . A m . Chem. Soc., 81, 1783 (1959). (2) C. N. Muldrow and L. G. Hepler, ibdd., 79, 4045 (1957). (3) R. L. Graham and L. G Hepler, abad., 78,4846 (1958).
2108
Vol. 65
SOTES
and
TABLEI
HEATSOF SOLUTION OF m-CP( LIQ) h l d r s m-CP/930 nil Ir20
AH? = AHmt
3111 (ral / r u o l ~ )
663 678 664 658
0.01080
,01887 ,02825 ,03737
ASint(0-CP) = AS,at(m-CP) = AS,n,(p-CP)
aqueous KaOH also were determined. The calorimetric reaction equation is m-CP(liq)
+ OH-(aq)
=
m-CP-(aq)
+ AHext
(5)
I'itzer' has shown that all A S j n t values for n series of similar acids a r t substantially the same so we write
+ HiO(1iq) AH2
(2)
(6)
Because the entropies of ionization of the three monochlorophenols are very nearly the same we also write AS,,,(o-CP) = AS,,t(m-CP) = AS,x,(P-CP) (7)
It can be shown that several different models for
Results of these experiments, listed in Table 11, solute-solvent interactions lead to A H e x t proporwere extrapolated to zero concentration to give tional to A S e x t . 6 We therefore also conclude that A H 2 0 =: -7.54 f 0.07 kcal./mole. AH,,t(o-CP) = AH,,t(m-CP) = AH,,t(p-CP) (8) .
HEATSOF
TABLE11 XEUTRALIZATION OF m-CP( LIQ)
Moles m-CP,'g50 nil.
M of NaOH
and that BY
NaOH( AQ)
AH2 (kral./mole)
AH,nt(o-CP) < AHidm-CP)
< AH,nt(P-CP)
(9)
Thus observed acid strength differences for these compounds are due to differences in A H i n t . A H i n t is the energy required to break one mole of 0-H bonds. The 0-H bond strengths in simi.00841 lar molecules are roughly proportional to the squares .00922 of the 0-H stretching frequenciesS6 It8has been ,01448 found by Ingold* and others that the 0-H stretch.02244 ing frequency is greater for p-chlorophenol than Combination of AHIO with AHo = 13,500 cal./ for m-chlorophenol, in accord with the order of A H i n t mole for ionization of water4 gives A H i o = 5286 values we have found. Interpretation of the speccal./mole for the ionization of aqueous m-chloro- trum of o-chlorophenol is complicated because of phenol as indicated by the equation intramolecular hydrogen bonding but the maximum is a t a still lower frequency than that for m-chlorom-CP(aq) = H+(aq) m-CP-(aq) AH? (3) phenol as expected from the order of A H i n t values. Bordwell and Cooper5 have determined the ther- There is no indication that intramolecular hydrogen modynamic ionization constant of m-chlorophenol bonding is of much importance to the thermody(as) at' 25' to be 8.32 X 1O-lo. We calculate the namics of dissociation of aqueous o-chlorophenol, standard free energy of ionization of m-chloro- at least as compared to 0-H bond breaking and phenol (aq) to be AFiO = 12390 cal./mole and solate-solvent interactions. comhine this value with our AHiO to obtain A S i o = Most of the many more or less successful at-23.8 cal./mole deg. tempts to correlate or explain acid strengths in Thermodvnamic data for the ionization of all terms of molecular structure have involved applithree mon&hlorophenols in aqueous solution are cation of theories of resonance, inductive effects, listed in Table 111. etc., to predict relative values of what we have called A H i n t . It generally has been implicitly TABLEI11 THERMODYNAMICS OF IONIZATIONOF AQUEOVS CHLORO- assumed that A H e x t , A S e x t and A S i n t contribute only insignificantly to differences in acid strength. PHENOLS Pitzer' has shown that it is ordinarily proper to AHio ASiO K x 10'0 (oal./mole) (cal./mole dea.) ignore A S i n t but our work on nitrophenols' and 0-c e 33.4 4636 -23.5 methyl substituted phenolss has shown that it m-CP 8.32 5286 -23.8 is sometimes entirely incorrect and misleading to p-CP 4.18 ignore AHextand ASext. It therefore is interesting 5800 -23.5 to note that the monochlorophenols are the only Previous investigations of other phenolslq6have class of phenols so far studied for which considerashown that interpretation of acidity of these com- tion of AH;,, values alone is sufficient to predict pounds must be based on consideration of what we relative ionization constants correctly and by the ea11 internal and external contributions to the ther- right reasoning. modynamic functions. By external contribution I n the case of the monochlorophenols it is inwe mean solute-solvent interactions and by in- ductive and field effects which cause the observed ternal contribution we mean differences in enthalpy variation in bond strengths and hence in A H i n t , and entropy within the acid molecule and its anion. AHiO and K . We therefore write Acknowledgment.--We are grateful to the NaAS? = ASxnt ASex$ (4) tional Science Foundation and the Alfred P. Sloan ___Foundation for support of this mid related re(4) IT. M. Papee, W. .I. Canaday and K.J. Laidler, Can. J . Chem., 34. 1677 11956). search. 0 00139
0 0593
,00250 .00437
,0593 ,0988 ,0988 .0988 ,0988 .I236
-7.52 -7 52 -7.47 -7.44 -7.42 -7.44 -7.48
+
+
( 5 ) F. (1952).
G. Rordwell and G. D. Cooper, J . A m . Chem. Soe., 74, 1058
(6) L. G.Hepler and W. F. O'I-Iara, J . Phys. Chem., 65,811 (1961).
(7) K. S. Pitrer, J . Am. Chem. Soc., 69, 2365 (1937). (8) Ii. U. Ingold, Can. J . Chem., 38. 1092 (1960).
