J. Phys. Chem. 1992, 96,4620-4626
4620
this calculation (using DHaHco = 16 kcal/mol) are listed in Table I. Comparison with Previous Studies. Shinohara and Nishils have examined photodissociation of acrolein at 193 nm and found the average translational energy for HCO to be 10.1 kcal/mol (Tt = 3400 K). Their measurement is greater than our Pl,o(16) Doppler-broadenedtranslational energy determination by ca.60%. The reason for this discrepancy is unclear, but high electron energies (20-40 eV) and long extraction times before fragment detection (ca. 20 ps) in their experimental procedure may contribute to this discrepancy. Our direct experimental observations may be more sensitive to the molecular energy measurement. Using a CW CO laser, Lin and co-w~rkers'~ measured an average vibrational energy of 2.7 f 0.7 kcal/mol for CO from acrolein photolyzed at 193 nm. They describe a statistical model where a methylketene (CH3CH=C=O) intermediate leads to CO + CH3CH products. Their vibrational energy for CO is in reasonable agreement with our value (4.1 f 0.6 kcal/mol). Mechanism of Pbotodbociatioh The scenario presented above is in accord with the following mechanism for the photofragmentation of acrolein. After UV excitation to the S 2 ( m * ) state?3 acrolein internally converts to the highly vibrationally excited ground state. Dissociation from this state is consistent with the reactivity observed for a number of other polyatomic photofrag mentation (e.g., H2C0, NCNO, CH2C0).7-9~21 Next, the excited molecule fragments to HCO and CHzCH radicals followed by (33) Walsh, A. D. Trans. Faraday SOC.1945, 41, 498.
secondary HCO decay to CO and H. On the basis of these observations, we propose the following mechanism for the dissociation of acrolein at 193 nm CHp=CH-C,
/p H
-[
[HCO]*
HCO + CH2=CH
-
CO
+
1'
H
where the asterisk denotes V,R,T excited-state species. V. Conclusions (1) The low-pressure photolysis of acrolein at 193 nm yields CO photofragments with the following characteristic vibrational, rotational, and translational temperatures: T, = 1650 f 310, T,(v=O) = 2750 f 280, and T,[Pl,o(16)] = 1380 f 220 K. (2) Energy disposal into formyl and vinyl radicals is compared to a statistical model that predicts equal energy partitioning to both fragments. A model with outlined that predicts CO vibrational and rotational energy distributions by averaging over the HCO internal energy distribution. (3) A two-step mechanism involving C-C bond rupture followed by secondary dissociation of HCO, in qualitative agreement with experimental observation and theory, is proposed for acrolein photodissociation. Recently, studies34on the photodissociation of propynal at 193 nm were completed. This analogue of acrolein should provide an interesting comparison to the energy disposal presently determined. (34) Lessard, P. C. Ph.D. Thesis, University of California, Davis, 1991.
Second Law Assessment of Behavior Differences In Formation of Several Sulfur-Oxygen Compounds of Lanthanum, Cerium, and Praseodymium E.I. Onstott Chemistry and Laser Sciences Division, Los Alamos National Laboratory and University of California, Los Alamos, New Mexico 87545 (Received: July 18, 1991; In Final Form: December 30, 1991)
Normalized experimental data for disproportionationand concomitant 12(g)oxidation of SO?-(cr) at 675-775 K in dehydrated Ce2(S03)2S04.4H20(cr),admixed with Ce02(cr) substrate, were used to evaluate chronological changes and differences in Gibbs energy, enthalpy, and entropy of formation for several product compounds. Several sequential reactions not observed previously were quantified. Intracrystal disproportionation of 2Ce2(S03)2S04(cr)by reaction with Ce02(cr) at 675-7 15 K to yield CeO(SO,)(cr) + Ce20(S04)S04(cr)+ Ce20(S203)S04(cr)was demonstrated. The labile S2032-(cr)reacted at higher T with 12(g)+ H20(g) to yield SO,"(cr) + 2HI(g) + '/2S2(g). At 605-690 K in the short time of 0.2 min, partially dehydrated La2(S03)2S04(cr)disproportionated to yield S042-(~r) + 1/2S02(g)+ '/4S2(g) that were absorbed by the La202S04(cr)substrate to yield a second solid product of 1/2La20(S203)S04(~r) by S2(g)oxidation of S02(g). At 660-800 K the only solid product of disproportionation and 12(g)oxidation and hydrolysis of dehydrated Pr2(S03)2S04-4H20(cr) was Pr20(S04)S04(cr).Formation of S2032-(~r) was not observed and was not prominent because of a better fit of SOq2in the solids to match the smallest Pr3+cation. This behavior is an effect of the lanthanide contraction. The best crystal fit of S2032-for the pair of La3"' and Ce3+seemed to be in Ce20(Sz03)S04(cr),with 02-(cr) being better accommodated in the available space.
Introduction In a recent paper some thermochemical reactions of La, Ce, and Pr sulfur-oxygen compounds in the presence of 12(g),H20(g), HI(g), S02(g). and S2(g) were quantified by a Gibbs energy matching method for the gas-phase reactivity.' In this paper the second law is utilized to evaluate several reaction pathways by Gibbs energy changes, entropy changes, and entropy of formation differences. Bridge-bonding of SO:- in the solid phase2 prevented I- solids formation. Sz032-formation was not precluded. Onstott, E. I. J . Phys. Chem. 1991, 95, 2520-2525. (2) Fahey, J. A. Crystal Structure of Lanthanum Oxysulfate, Proceedings 12th Rare Earth Research Conference, Vail, CO, July 1976; University of Denver, Denver Research Institute: Denver, CO, 1976; Vol. 2, pp 762-771. (1)
Stoichiometry of Relevant Gas Reactions The temperature region of interest is 600-800 K. S2(g) and 02(g) can react to yield SO(g) that is unstable with respect to disproportionation to 1/2S02(g) I/.,S2(g). S02(g) is the most stable oxide. It is partially converted to SO3@ with e x a s O,(g). H2S(g) formation is suppressed by 12(g)when H20(g) is present. From JANAF table^,^ the AJ for SO formation at 700 K is 4.96 J K-l mol-l. For SO2formation AJ = -73.39 J K-'mol-', and for SO3 formation AJ is -167.39 J K-' mol-'. AJ for disproportionation of SO 1/2S02+ '/4S2 is -41.81 J K-l mol-l.
+
-
(3) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed. J . Phys. Chem. Ref. Data 1985, 24, Suppl. No. 1.
0022-365419212096-4620%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4621
Several Sulfur-Oxygen Compounds of La, Ce, and Pr
TABLE I: Effect of Temperature, Reactant Valuea, and Reaction Times on Normalized Gas Quotients and Gibbs Energy Changes for Disproportioartion (2A(Ce)) of Dehydrated C e 2 ( S 0 3 ) ~ , . 4 H 2 and 0 Concomitant Oxidation (l(Ce)) of 5032-on a Reactive CeOl Substrate T,
time,
K
min 2 8 4 8 2 4 2 2 1
675 675 675 675 675 675 715 715 715 715 715 730 745 745 745 745 745 745 745 745 775 775 775
5 2 5 1 2 1 2 2 2
5 2 1 1 1
yieldb mol fracn
reactants"
I2
H20
1.432 1.379 1.753 1.847 1.770 1.828 1.323 1.603 1.621 1.713 1.475 1.436 1.413 1.675 1.384 2.25 1 1.575 2.123 1.804 2.006 1.777 1.393 1.757
2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
CeO, 0.835 0.835 0.723 0.723 0.125 0.125 0.835 0.723 0.603 0.603 0.125 0.603 0.835 0.835 0.723 0.723 0.603 0.603 0.603 0.125 0.835 0.723 0.125
(1) 0.0326 0.0365 0.0216 0.0300 0.0830 0.1200 0.0254 0.0209 0.0950 0.199 0.138 0.138 0.345 0.329 0.163 0.274 0.254 0.252 0.210 0.150 0.391 0.283 0.245
(2A) 0.472 0.894 0.433 0.518 0.416 0.550 0.879 0.750 0.295 0.349 0.745 0.593 0.590 0.659 0.797 0.690 0.697 0.741 0.745 0.797 0.579 0.680 0.725
gas quotients for E-6 (2B)
unreacted mol fracn
SO,20.495 0.069 0.546 0.452 0.501 0.330 0.095 0.229 0.586 0.452 0.117 0.269 0.065 0.012 0.040 0.036 0.049 0.007 0.045 0.053 0.030 0.037 0.030
I2
0.00944 0.01280 0.004 26 0.008 12 0.056 7 1 0.1156 0.004 85 0.003 23 0.055 16 0.204 0.1227 0.111 2 0.41 1 7 0.393 2 0.1428 0.3130 0.282 2 0.280 6 0.2143 0.123 9 0.435 9 0.295 0 0.241 1
(1) 0.00956 0.014 81 0.01020 0.01006 0.01 141 0.010 16 0.01296 0.01397 0.021 48 0.021 70 0.014 63 0.01921 0.025 66 0.024 10 0.01991 0.022 73 0.022 33 0.021 68 0.021 06 0.01973 0.035 58 0.030 87 0.029 29
A-Gfor 2S032-,J
(2'4) 0.1253 0.1968 0.1181 0.1341 0.1114 0.1321 0.1955 0.1751 0.0917 0.0902 0.1630 0.1393 0.1152 0.1264 0.1688 0.1380 0.1417 0.1482 0.1547 0.1701 0.1111 0.1377 0.1490
(1) 51 104 54521 51 481 51 600 50217 51 517 51 654 50813 45 663 45 538 50 232 47 986 45 379 46 152 48515 46 876 47 094 47 468 47819 48 627 42 992 44818 54452
(2'4) 23314 18 253 23 954 21 780 24 635 22 721 19420 20 720 30719 28 596 21 565 23 929 26 773 25618 22 035 24 536 24 207 23 655 23 119 21 910 28 314 25 550 24535
"I2 and H 2 0 were mol/(mol of (=0.254.8 mmol) at start. C e 0 2 in column five was the mole fraction vs Ce2(S03)2S04-4H20 in the starting binary mixture. SOj2-consumed. All calculations were on the basis of 1 mol of except for the normalized A,G values for two SO,2-.
Formation of HI requires A$ = 7.88 J K-' mol-I, and H 2 0 requires A$ = -54.47 J K-'mol-I.
Stoichiometry of Solid-Phase Reactions The anion chemistry can be indexed by substitution of SO$or S2- or S20j2- or SO," or a combination for 02-in La203(cr), Ce203(cr),Ce02(cr), and Pr203(cr). La203(cr)reacts with aqueous SO2and H2SO4 at 295 K to yield insoluble oxidesulfitesulfatehydratesof variable compositi~n.~.~ La202S04(cr)as a recycle reagent reacts with S02(aq) at 295 K to yield similar products. At 365 K an insoluble product of the reaction of S02(aq) with La202S04(cr)was La;(S03)2,25(S0,)o,754.3H20(cr).5 Pr202SOl(cr), which is stable in air to 1375 K, reacts in S02(aq)at 340 K to yield insoluble binary mixtures of the reactant + Pr2(S03)2S04.4H20(cr).6 Ce20#04(cr), although it exists,' is not a useful recycle reagent because of the propensity of S042-(~r) to oxidize Ce3+to Ce4+ at >lo00 K to yield 2Ce02(cr) 2S02(g). Ce02(cr) in S02(aq) at 377 K gives a binary mixture of unreacted CeO,(cr) and Ce3+ as Ce2(S03)2S04-4H20(cr).1 At 605-690 K some of the S032-(~r)in La2(S03)2S04(~r), partially dehydrated, admixed with La202S04(cr),is oxidized by 12(g)in the presence of H20(g) to yield S042-(cr),and 2HI(g) is a hydrolysis product.' The reaction is La2(S03)2S04(cr)+ 212(g) + 2H20(g) + La202S04(cr) 2La20(S04)S04(cr)+ 4HI(g) (l(La))
+
-
Pr3+at 660 K follows the same pathway. Ce3+in combination with Ce02(cr) reacts to a lesser extent at 675 K, following the pathway of placement of 02-(g) into the crystal phase by hy(4) Onstott, E. I.; Bowman, M. G.; Michnovicz, M. F.; Hollabaugh, C. M. Modification of the Sulfur Dioxide-Iodine Thermochemical Hydrogen Cycle With Lanthanum Sulfites and Sulfates. Proceedings ofrhe 5rh World Hydrogen Energy Conference, Toronto, Ontario, Canada, July 1984; International Association of Hydrogen Energy: Coral Gables, FL, 1984; Vol. 2, pp 433-437. (5) Haas, N. S.;Peterson, E. J.; Onstott, E. I. Dilanthanum Dioxymonosulfate as a Recycle Reagent and Recycle Substrate for the Sulfur DioxideIodine Thermochemical Hydrogen Cycle. Inr. J . Hydrogen Energy 1990,15, 397-402. ( 6 ) Onstott, E. I.; de Bruin, D. Inr. J . Hydrogen Energy 1988, 13, 1-6. (7) Peterson, E. J.; Foltyn, E. M.; Onstott, E. I. J. Am. Chem. SOC.1983, 105, 7572-7573.
drolysis, and simultaneous exchange of S 0 4 2 - ( ~ rfor ) 02-in Ce02(cr) Ce2(S03)2S04(cr)+ 212(g) + 2H20(g) + Ce02(cr) Ce20(S04)S04(cr)+ CeO(SO,)(cr) + 4HI(g) (l(Ce))
-
Reaction 1 for La, Ce, and Pr is concomitant with 12(g)catalyzed S2(g) S02(g) (without disproportionation that yields SO&cr) a valence change) where RE is La3+ or Pr3+ or Ce3+ (special conditions): (RE)2(SO3)2SO4(cr) (RE)20(S04)2(cr) + %s2(g) + f/2so2(g) (2A) This reaction can proceed also in the absence of 12(g)and H20(g) for La, Ce, and Pr.59798 S02(g) from reaction 2A can react spontaneously and sequentially with HI($) from reaction 1 and return 12(g)and H20(g) to the gas phase along with additional S2:
+
+
-
-
+ f/4so2(g) )/2H2O(g) + 7212h) + l/s2(g) (2B) Gibbs energy changes for (2B) as written are3 at 600 K, -16.48 kJ mol-! at 650 K, -15.70 kJ mol-l; at 700 K, -14.88 kJ mol-'; at 750 K, -15.47 kJ mol-l; at 800 K, -13.15 kJ mol-l. The extent of reaction is controlled by the mole fraction of I2 in the gas. Near equilibrium can be calculated for the computer limit of E-6 for the extent of reaction 2B to normalize gas quotients for the reference base. At 600 K A,G(ref (2B)) = -0.0168 J mol-', and at 800 K A,G(ref (2B)) = -0.0138 J mol-'. Intracrystal disproportionation without gas evolution is a possible second mechanism. For La (also Pr) the equation is 2La2(S03)2S04(cr)+ La202S04(cr) 2La2O(SO,),(cr) + La20(S2O3)SO,(cr) (3A(La))
-
-
For Ce with Ce02 substrate 2Ce2(S03)2S04(cr)+ Ce02(cr) Ce20(S04)2(~r) + Ce20(S203)S04(cr)+ CeO(SO,)(cr) (3NCe.l) At higher temperatures S2032-(cr)can further react: S2032-(cr) + I2(g)
+ H2O(g)
-
S042-(Cr) + 2 H W + Y2S,(g) (3B) (8) Onstott, E. I. Inr. J . Hydrogen Energy 1990, IS, 255-258.
4622 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Onstott
TABLE II: Effect of Temperature, Reactant Values, and Reaction Times on Normalized Cas Quotients and Cibbs Energy Changes for Disproportionation(ZA(Pr)) of Dehydrated Prz(S03)zS04~4Hz0 and Concomitant Oxidation (l(Pr)) of S032-on a PrZO+O4Substrate yieldb
T,
K
660 660 660 660 700 720 720 770 780 800 770 770
time, min 0.2 0.2 0.2 10 10 10 40 10 10 10 10 10
- reactantQ I,
H20
4.13 5.56 2.24 1.84 1.59 1.70 1.77 1.75 1.00 2.03 2.34 3.20
2.03 1.88 2.03 2.03 2.03 2.03 2.03 2.03 2.03 2.03 2.03 2.13
Pr202S04 0.760 0.674 0.426 0.426 0.426 0.426 0.426 0.426 0.426 0.426 0.265 0.065
mol fracn (1) 0.0768 0.0729 0.0605 0.1227 0.0957 0.1583 0.1941 0.309 0.303 0.294 0.1963 0.259
(2A) 0.541 0.525 0.271 0.483 0.31 1 0.480 0.634 0.680 0.647 0.656 0.746 0.691
unreacted mol fracn SO? I, 0.382 0.0548 0.402 0.05723 0.669 0.0309 0.394 0.1259 0.593 0.0587 0.362 0.1401 0.172 0.2024 0.011 0.3309 0.OSc 0.3107 0.05' 0.2801 0.058 0.1699 0.0Y 0.2415
A.G for 2SOl2-, J
gas auotients Tor E-6 (2B) (1) 0.008 15 0.008 125 0.011 31 0.008 8 15 0.018 35 0.019 28 0.017 25 0.029 91 0.033 55 0.039 67 0.026 84 0.028 94
(2A) 0.1342 0.1346 0.0821 1 0.1 192 0.08941 0.1173 0.1384 0.1339 0.1308 0.1345 0.1577 0.1410
52812 49190 51915 46541 47277 48604 44932 44030 42932 46327 45359
22008 27380 23346 28103 25663 23675 25741 26380 26692 23 115 25082
"I2 and H 2 0 were mol/(mol of S032-)at start. P r 2 0 2 S 0 4was the mole fraction vs Pr2(S03)2S04.4H20in the starting binary mixture. bS0,2consumed. CEstimateddatum. See ref 6 and Table I for other details of experiments and calculations. TABLE 111: Effect of Temperature, Reactant Values, and Reaction Times on Normalized Cas Quotients and Cibbs Energy Changes for Disproportionation(ZA(La)) and Concomitant Oxidation (l(La)) of Dehydrated Lanthanum(II1) Oxide-Sulfite-Sulfate-Hydrates on a Reactive LaZO2S0,Substrate yieldb
T, K 605 615 660 660 660 675 680 690 660 660 660
time, min 0.2 0.2 0.2 10 125 0.2 0.2 0.2 135 120 135
I, 2.44 2.09 2.10 1.80 1.84 2.11 2.22 2.01 2.77 2.08 1.32
reactants' H20 La202S04 3.86 0.429 3.86 0.429 3.86 0.429 3.86 0.429 3.86 0.429 3.86 0.429 3.86 0.429 3.86 0.429 4.79 0.609 4.24 0.502 3.74 0.244
mol fracn (1)
0.167 0.199 0.317 0.274 0.271 0.251 0.200 0.204 0.126 0.231 0.147
(2A) 0.414 0.305 0.464 0.626 0.635 0.347 0.410 0.430 0.774 0.654 0.753
unreacted mol fracn SO3,0.419 0.496 0.219 0.10' 0.094 0.402 0.390 0.366 0.1OC 0.115 0.10'
ArG for 2S032-, J
gas quotients for E-6 (2B)
I, 0.0962 0.1148 0.2009 0.1669 0.1641 0.1187 0.0794 0.0781 0.0279 0.1669 0.0606
(1) 0.004608 0.006396 0.01075 0.009 168 0.009 101 0.01495 0.01446 0.016 18 0.008295 0.009160 0.007998
(2A) 0.08639 0.06822 0.08853 0.1124 0.1137 0.07457 0.086 11 0.08920 0.1308 0.1124 0.1374
(1) 54120 51668 49741 51 504 51 572 47171 47900 47323 52587 51732 52993
(2.4 24638 27462 26608 23978 23858 29141 27726 27730 22328 23654 21782
'I2 and H 2 0 were mol/(mol of S032-) at start. The La202S04was the mole fraction in the starting binary mixture. bS032-consumed. 'Estimated datum. See refs 4-6 for other details. Calculations Gas quotients for reactions 1,2A, 2B, and 3B were normalized to the reference of near equilibrium by computer adjustment of the I2 pressure, as described above. ArG values for reactions 1 and 2A were normalized with yields at 1.000. These results are given in Tables 1-111. Reaction enthalpy changes were calculated from the T'variation of selected gas quotients that showed the best conformance with reaction stoichiometry for a wide temperature difference. Results of A$! for reactions 1, 2A, and 3B are given in Table IV. Second law A$ data were calculated from the equation AJ = (A$!- A,G)T'. Experimental values are listed in Tables V-VII. Other details of calculations are discussed in the following sections. Values for A@ of some solid reactants and products for disproportionation and oxidation are listed in Tables VIII-X. Data for differences in AfHof solid reactants vs products are given in Table XI. Entropy Constraints for Disproportionation
Thermochemical data for L a 2 0 3 , La202S04,Ce2O3, Ce02,and Pr203have been evaluated and serve as reference sources."' Of special interest in this paper are reactions of compounds in which crystal anion reactivity of 02-, SO;-, S2032-, and SO>- precludes (9) Grizik, A. A.; Abdullina, I. G.; Gardifzhanova, N. M. Russ.J . Inorg. Chem. 1974, 19, 1412-1413. (10) Gschneidner, K. A., Jr.; Kippenhan, N. A.; McMasters, 0. D. Thermochemistry of the Rare Earths, Part I; IS-RIC-6; Rare Earth Information Center: Iowa State University, Ames, IA, 1973. (11) Holley, C. E., Jr.; Huber,E. J., Jr.; Baker, F. B. The Enthalpies, Entropies and Gibbs Energies of Formation of the Rare Earth Oxides. In Progress in the Science and Technology of the Rare Earths; Eyring, L., Ed.; Pergamon Press: New York, 1968; pp 343-433.
TABLE I V Enthalpy Changes of Oxidation, Reactions 1 and 3B, and Disproportionation, Reaction ZA, from Cas Quotient Data Normalized for E-6 (Near Equilibrium) in Reaction 2B rare data earth table La
111
Ce
I
Pr
I1
T,, K 605 605 605 605 605 675 675 675 745 745 660 660 660
T,,
K
reaction
660 660 690 690 690 745 745 745 '775 775 800 800 800
660 800
1 2A 1 3B 2A 1 3B 2A 1 2A 1 2A 1 3B
660 800
2A
gas quotients TI T2 0.004608 0.08639 0.004608 0.1026 0.08639 0.01006 0.1444 0.1341 0.02566 0.1152 0.008 150 0.1342 0.008 125 0.1370 0.1346
0.01075 0.08853 0.01618 0.1420 0.08920 0.02273 0.1788 0.1380 0.03558 0.1111 0.03967 0.1345 0.03967 0.2037 0.1345
kJA&, mol-^ 1SOj251.13 1.48 51.29 13.27 1.31 48.69 12.76 1.71 52.30 -5.80 49.62 0.07 49.72 12.44 4.023
I- formation in the solids by reaction with HI(g). The entropy change in a chemical reaction from the second law is A,.!? = (A,.H - 4 G ) T L . The same information can be obtained from AJ = A$[products - reactants]. Additional information can be derived from two reactant solids vs reactant gases: AJ = A$(cr)[solid product - solid reactant] + A$(cr)[solid product 2 - solid reactant 21 + A$(g)[sum of product gases - sum of reactant gases]. For reaction 2A(La) h f i ( ~ r ) [ L a ~ ( S O ~-) ~La20(S04)2] S0~ = A$(g)['/2S02 + '/4S2] - AJ. Equation 2A(La)) in the formation mode is La20(S04)2(cr)+ 1/2S2(g) 1/202(g) LadS03)2S04(cr); AJ = Mer) [La2(S03)2S04- La20(S04),I
+
-
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4623
Several Sulfur-Oxygen Compounds of La, Ce, and Pr
TABLE V Calculated Entropy Valua from Normalized Experimental Data for Disproportionation (2A(La)) and Concomitant Oxidation of S O : (l(La))and/or Sz0:- (3B(La)) in Dehydrated Lanthanum Compositions on a Reactive LazO+304 Substrate (Enthalpy Data Given in Table IV) La202s04
T, time, starting K min 12" mol fracn 605 615 660 660 660 675 680 690 660 660 660
0.2 0.2 0.2 10 125 0.2 0.2 0.2 135 120 135
2.44 2.09 2.10 1.80 1.84 2.11 2.22 2.01 2.77 2.08 1.32
0.429 0.429 0.429 0.429 0.429 0.429 0.429 0.429 0.6 19 0.502 0.244
J
A$, J
(1) 54120 51668 49741 51504 51 572 47171 47900 47323 52587 51732 52993
(2A) 24638 27462 26608 23978 23858 29141 27726 27730 22328 23654 21 782
A$(solids) (2A)' A$(solids) ( [ L ~ ~ ( s o ~ ) -~ sso(ref o~ JANAF) [ L ~ ~ O ( S ~ O J-S Ouo(ref ~ JANAF)
K-'mol-'
(1) 79.83 82.54 80.23 77.15 77.04 81.85 79.01 79.85 75.50 76.80 74.89
(2A) -36.11 -40.12 -36.09 -32.10 -31.92 -39.04 -36.14 -36.145 -29.60 -31.61 -28.78
+ '/2SJ 239.561 240.35' 242.701 242.70
La20(S04),I
['/202
1239.02 + 0.54 '243.55 [241.88 0.82 237.89 237.71 + 4.76 245.62 [242.98 0.54 [243.54 + 0.51 235.39 237.40 234.56 8.14
"Oxidant and catalyst, mc '(mol of S032-)at start. bAJ = (A&A$ solids reaction 2A) f mismatch.
+
243.26 243.521 244.051 242.70 242.70 242.70
+
+
La202S041
[SO2 + I/*S21
1409.66 - 3.85
405.811
[415.17 - 3.59 407.7 1 407.43 3.87 421.15 [415.88 - 2.21 [417.69 - 1.92 403.57 406.88 402.13 9.45
411.581 411.58 41 1.58 413.15 413.671 415.771 41 1.58 41 1.58 41 1.58
+
+
A,G)TI. 'Experimental value + mismatch. dExperimer
11 value
(includes
TABLE V I Calculated Entropy Values from Normalized Experimental Data for Disproportionation (?A(Pr)) and Concomitant Oxidation of SO$- (l(Pr)) and/or S 2 0 j - (3B(Pr)) in Dehydrated Praseodymium Compositions on a Reactive Pr2OZSO4Substrate (Enthalpy Data Given in
Table IV)
T, time, K min 660 660 660 720 770 800 770 770
0.2 0.2 10 40 10 10 10 10
I," 4.13 5.56 1.84 1.77 1.75 2.03 2.34 3.20
Pr202S04 starting mol fracn (1) 0.760 0.674 0.426 0.426 0.426 0.426 0.254 0.065
52 786 52812 51 915 48 604 44 932 42 932 46 327 45 359
A$(solids) (2A)' (2A) 22048 22008 23346 23675 25471 26692 23 115 25082
(1) 70.54 70.50 71.86 70.47 70.66 70.51 68.85 70.11
(2.4) -33.41 -33.35 -35.37 -32.88 -33.43 -33.37 -30.02 -30.26
ASo(ref JANAF) [P~zO(SO~)ZI [ ' / 2 0 2 + '/ZS21
[239.19 [239.13 241.16 [241.81 [244.58 [245.92 241.71 241.41
+ 3.28 + 3.34
+ 1.31 + 3.70
+ 3.26 + 3.32 + 6.13 + 6.43
242.471 242.471 242.47 245.511 247.841 249.241 247.84 247.84
A$(solids) ( l)d [Pr20(S04),- ASo(ref JANAF) Pr202S041 [SO2+ 1/2021 [402.41 - 3.84 [402.31 - 3.74 405.84 - 7.27 [407.94 - 3.30 [412.42 - 3.37 [414.66 - 2.97 407.20 1.85 407.55 + 1.50
+
398.571 398.571 398.57 404.641 409.051 41 1.791 409.05 409.05
'Oxidant and catalyst, mol/(mol of S 0 3 2 - ) . bA+S= (A,H - A,G)T'. 'Experimental value + mismatch. dExperimentalvalue (includes A$ reaction 2A solids) A mismatch. TABLE VII Calculated Entropy Values from Normalized Experimental Data for Disproportionation (2A(Ce)) and/or (3A(Ce)) and Concomitant Oxidation of Sod-, Reaction 1(Ce), and/or Sz03z-,Reaction 3B(Ce), in Dehydrated Cerium Compositions on a Reactive Substrate of CeOz (Enthalpy Data Given in Table IV) T, time, K min 675 2 675 8 675 4 675 8 675 4 715 2 715 2 715 1 715 5 715 2 730 5 745 1 745 2 745 1 745 2 745 2 745 2 745 5 745 2 775 1 775 1 775 1
Ito 1.432 1.379 1.753 1.847 1.828 1.323 1.603 1.621 1.713 1.475 1.436 1.413 1.675 1.384 2.251 1.575 2.123 1.804 2.006 1.777 1.393 1.757
CeO, starting mol fracn 0.835 0.835 0.723 0.723 0.125 0.835 0.723 0.603 0.603 0.125 0.603 0.835 0.835 0.723 0.723 0.603 0.603 0.603 0.125 0.835 0.723 0.125
A,G, mol-' (1) (2A) 51 104 23314 54521 18253 51 481 23 954 51 600 21 780 51 517 22721 51 564 19420 50813 20 720 45 663 30719 45 538 28 596 50 232 21 565 47 986 23 929 45 379 26 773 46 152 25618 48515 22035 46 876 24 536 47 094 24 207 47 468 23 655 47819 23119 48 627 21 910 42 992 28 314 44818 25 550 54 452 24 535 J
J K-I mol-I
(1) 68.56 63.49 68.00 67.82 67.95 63.95 65.13 72.33 72.51 65.94 67.66 69.80 68.76 65.59 67.79 67.50 67.00 66.52 65.44 70.18 67.82 55.39
(24 -29.47 -21.97 -30.42 -27.20 -28.59 -22.38 -24.20 -38.18 -35.21 -25.38 -28.09 -31.35 -29.80 -24.99 -28.34 -27.90 -27.16 -26.44 -24.82 -32.12 -28.55 -27.25
ArS(solids) (2A, 3.4)' [Ce2(SOd2SO4 (dispropn prod)] 236.05 r228.55 + 1.66 236.00 233.78 235.17 [231.05 + 1.08 [232.87 - 0.74 [246.85 - 1.57 [243.88 + 1.40 [234.05 - 1.92 237.55 [241.59 + 5.09 240.04 + 6.04 [235.23 - 1.76 238.59 238.15 237.41 236.69 - 3.22 [235.06 - 1.591 [243.94 + 4.14 240.37 239.06 - 4.25
PSo(ref JANAF) ApS(so1ids) ( l)d ASo(ref JANAF) [O,] ['/IO, + i/2S~1[CeOSO, - Ce02l [SO]+ '/20,1 230.21 243.26 [398.29 + 1.84 400.131 230.211 385.73 400.13 230.2 1 243.26 [398.68 + 1.45 400.13) 230.21 243.26 395.28 400.13 230.21 243.26 396.80 400.13 390.24 232.13] 404.50 393.24 232.131 404.50 [414.42 + 2.87 245.281 417.29'1 245.281 [411.63 + 5.66 417.291 404.50 395.23 232.131 400.98 245.98 232.80 405.44 246.681 [407.69 - 0.56 407.131 246.68 407.131 [405.10 + 2.03 246.68 397.12 233.471 407.13 233.47 407.13 246.68 402.68 246.68 401.94 233.47 407.13 407.13 246.68 400.70 233.47 246.68 233.47 407.13 399.51 233.47 407.13 246.68 396.80 248.081 [411.47 - 2.42 409.051 248.08 234.81 409.05 405.55 + 3.50 234.81 391.81 409.05
'Oxidant and catalyst, mol/(mol of starting S03"). bA$ = (A,H - A,G)Tl. CExperimentalvalue f mismatch. dExperimental value (includes contribution of reactions 2A, 3A) f mismatch. 'Reference is SO, + 1/2S2to show CeOSlO, product.
- ASo(g)[I/zS2 + ' / 2 0 2 ] . If the two bracketed terms have the same value, then A$ = 0. Any mismatch can be equated to -A$ of the reaction pathway. One 02-in La202S04(cr)is labile, but Sod2does not decompose rapidly at lo00 K to yield 2Ce02(cr) + S02(g) by S042-(cr)oxidation.' Ce02(cr)as the substrate provides labile O"(cr) for the anion exchange to yield CeOS04(cr) at 675-775 K but does not oth-
erwise alter the anion chemistry very much. Calculated values for (AH- AG) T I for reaction 2 A for La in Table V with the least variation at 605-690 K were as follows: at 605 K,-36.1 1 J K-I mol-I; a t 660 K, -36.09 J K-I mol-I; at 680 K,-36.14 J K-' mol-I; a t 690 K, -36.145 J K-l mol-'. For Pr, with sufficient catalysis and time of reaction, values of A$ in Table VI were as follows: at 660 K,-33.41 and -33.35 J K-I mol-'; at 770 K,-33.43 J K-' mol-I; at 800 K,-33.37 J K-l mol-'. The averaged difference of A,S(La) - A,S(Pr) was -2.73
4624
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Onstott
TABLE VIII: Calculated Gibbs Energy Values from Experimental Data for La, Product Compositions on a Reactive La20fiOI Substrate (A& Values Affected by Substrate) AfC(cr), kJ mol-' substrate T, time, starting ArG, kJ K min Iz" mol fracn (1) (2.4) La20(S04),I La02S041 L a 2 0 2 S 0 4 La20(S04), La20(S203)S04 600 605 615 660 660 660 675 680 690 660 660 660 700
-2040.66' 0.2 0.2 0.2 10 125 0.2 0.2 0.2 135 120 135
2.44 2.09 2.10 1.80 1.84 2.11 2.22 2.01 2.77 2.08 1.32
0.429 0.429 0.429 0.429 0.429 0.429 0.429 0.429 0.619 0.502 0.244
54.120 51.668 49.741 5 1.504 5 1.572 47.171 47.900 47.323 52.587 51.732 52.993
24.638 27.462 26.608 23.978 23.858 29.141 27.726 27.730 22.328 23.654 21.782
-166.06 -1 69.16 -169.56 -166.94 -166.82 -170.06 -1 71.24 -171.46 -165.28 -166.62 -164.74
-495.50' -500.19 -495.89' -491.43 -491.25 -488.02 -496.68' -496.12' -488.76 -490.92 -487.8 1
-2023.18
-2519.07
-2023.18
-2514.43d
-2023.18
-2514.1d
-2011.52'
"Oxidant and catalyst, mol/(mol of S032-)at start. bFrom equilibrium data for La202S04.9 ' L a 2 0 ( S 2 0 3 ) S 0 4is the substrate product from La20,S04 Y2S2(g) SOz@)(Table V), which is later converted to La20(S04), in reaction 3B. dAfter the reorganization energy is expended in (3B) and only SO:- remains.
+
+
TABLE I X Calculated Gibbs Energy Values from Experimental Data for Pr2 Compositions on a Reactive Pr202S04Substrate (Values of A& Affected by Substrate) T, K 600 660 660 660 700 720 720 770 780 800 770 770
time, min 0.2 0.2 10 10 10 40 10 10 10 10 10
12" 4.13 5.56 1.84 1.59 1.70 1.77 1.75 1.oo 2.03 2.34 3.20
substrate starting mol fracn 0.760 0.674 0.426 0.426 0.426 0.426 0.426 0.426 0.426 0.254 0.064
A,G(cr), kJ mol-' A,G, kJ (1) (2'4) 52.786 52.812 51.915 46.541 47.277 48.604 44.932 44.030 42.932 46.327 45.359
22.048 22.008 23.346 28.103 25.663 23.675 25.741 26.380 26.692 23.115 25.082
[pr2(so3)2so4Pr*O(SO4)2I -165.02 -164.96 -166.31 -1 72.18 -170.26 -168.28 -1 7 1.46 -172.54 -173.38 -168.89 -170.85
LPr2O(So4)2
-
Pr202S041 -488.31 -488.22 -490.53 -496.26 -491.08 -487.77 -48 8.3 8 -488.99 -488.40 -484.73 -487.67
Pr202S04 -2067.34' -2049.85'
Pr20(S04)2 -2538.16
-2038.18'
-2010.02'
-2498.42
"Oxidant and catalyst, mol/(mol of SO3,-) at start. 'From equilibrium data for L a 2 0 2 S 0 4 9and Pr2O3.I0
TABLE X Calculated Gibbs Energy Values from Experimental Data for Ce(III)Ce(IV) Product Compositions on a Reactive Ce02 Substrate (Reactions 1, 2A. 3A, and 3B Were Active) A,G(cr), kJ mol-' substrate starting A,G, kJ T, time, [Ce2(S0J2S04[CeOS04K min (dispropn prod)] CeO2l Ce202S04 C e 0 2lo (1) (2A) mol fracn
'
600 675 675 675 675 675 700 715 715 715 715 715 730 745 745 745 745 745 745 745 745 775 775 775 800
2 8 4 8 4
1.432 1.379 1.753 1.847 1.828
0.835 0.835 0.723 0.723 0.125
51.104 54.521 51.481 51.600 51.517
23.314 18.253 23.954 21.780 22.721
-166.74 -161.68' -167.38 -165.98 -167.94
-489.67 -481.18 -489.93 -488.41 -490.45
2 2 1 5 2
1.323 1.603 1.621 1.713 1.475 1.436 1.413 1.675 1.384 2.251 1.575 2.123 1.804 2.006 1.777 1.393 1.757
0.835 0.723 0.603 0.603 0.125 0.603 0.835 0.835 0.723 0.723 0.603 0.603 0.603 0.125 0.835 0.723 0.125
5 1.654 50.813 45.663 45.538 50.232 47.986 45.379 46.152 48.515 46.876 47.094 47.468 47.819 48.627 42.992 44.818 54.452
19.420 20.720 30.719 28.596 21.565 23.929 26.773 25.618 22.035 24.536 24.207 23.655 23.119 21.910 28.314 25.550 24.535
-163.95' -165.25' -1 75.19d -1 73 .06d -166.09 -168.69 -1 72.02f -170.86 -169.28 -169.78 -169.44 -168.79 -168.25 -1 68.14 -174.29' -171.5 -170.51
-480.98 -483.12 -498.25' -496.26' -484.54 -488.33 -49 1.56, -489.63 -483.70 -487.82 -487.98 -488.36 -488.71 -485.09 -492.37f -486.65 -477.20
5
1 2 1 2 2 2 5 2 1 1 1
'
-2040.50 -2018.69
-962.27 -946.84 -946.84
-2011.42
-941.70
-1998.60
-932.50 -932.50
-1990.05
-926.37
-1982.93
-921.26
"Oxidant and catalyst, starting 12/S03,-. From equilibrium data for L a 2 0 2 S 0 4 9and Ce2O3.I0 'Reaction 3A with C e 0 2 participation. dReaction 2A predominated. eCeO(S203)(cr)by CeO,(cr) + '/,S,(g) + S02(g). ,Best conformance in reactions 1 and 2A.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4625
Several Sulfur-Oxygen Compounds of La, Ce, and Pr
TABLE XI: Calculated Enthalpy of Formation Values from Experimental Data for Lanthanum, Cerium, and Praseodymium Oxide-SulfiteSulfate Compitions ArH(cr), kJ mol-'
T. K 605-690 615-145 660-800
RE La Ce
Pr
* *
(2A) (RE)2(S03)zS04] 2.19 129.4 3.42 130.0 0.05 126.1
(1) 102.42 91.38 99.34
(RE)202S04a (RE),O(SO,),
(REj202S04] -494.14
-2241.15 -2243.29 -2268.34
La datum -494.44
-2735.89 -2131.33 -2162.18
(RE)2(S03)2S04 CeOSO, -2865.09 -2861.33 -2889.48
-1584.8
'From equilibrium data for La20zS049and RE2O3.I0bFor reaction l(Ce), ArH(cr)[CeOS04- CeO,] = -499.14 kJ.
J K-' mol-', virtually independent of temperature. This number appears to be a crystal structure constant for the lanthanide contraction. For Ce, the A+ data in Table VI1 for T < 730 K were smaller and more variable than data for La and Pr. S203'- formation by an intracrystal mechanism was indicated for most experiments. Only two experiments at 7 15 K for the 0.603 composition gave values reasonably close to values expected for La. Enhanced product stability in the solids could account for this behavior of Ce. Decomposition eq 2A(La,0.603 Ce,Pr) shows that the preferred solid product was (RE)20(S04),(cr)by loss of I/$,(g) I/,O2(g) from the reactant. The preferred loss of gas was 1/2S0z '/4S2, the stoichiometric equivalent of SO(g) disproportionation. The formation-modeequation for the reactant solid of reaction 2A(La,Ce,Pr) is (RE),O(SO4Mcr) + '/2%(g) + '/202(g) (RE)2(S03)2S04(cr).The AJ equation can be written in the reference mode:
+
+
-
A,S(cr)[product - reactant] - A,S = hs0(g)[y2S2+ y202] Comparisons of experimental solids data vs gas reference data are shown in Tables V-VII. Nearly identical values are indicated by the bracketed data pairs for the appropriate JANAF reference column. In Table V for La there are four data pairings that show the small AJ for Laz(S03)zS04(cr)formation. There is more mismatch for Pr in Table VI than there is for La in Table V, showing the larger AJ for formation of Pr2(S03)2S04(cr).The -(La) - AJ(Pr) difference for 600-800 K is -2.68 J K-'mol-' compared to the value above of -2.73 J K-' mol-' calculated directly from second law data. SZ-(cr)formation in Ce compositions by intracrystal disproportionation to yield S2032-(cr)was 2Cez(S03)zS04(cr) CeOz(cr) Ce20(S203)S04(cr)+ Ce20(S04)S04(cr)+ CeO(SO,)(cr) (3NCe))
-
+
The anion disproportionation pathway for the net valence change of 6 was
-
+
+
4S032-(~r) S2032-(~r) 2S042-(~r) 02-(cr)
(3C(Ce))
-+
(3D(Ce))
The substrate contribution was Ce02(cr)
Ce02+(cr) + 02-(cr)
Addition of 4Ce3+(cr) 2S042-(cr)(ions not changed) to eqs 3C(Ce) 3D(Ce)) gives the neutralized eq 3A(Ce). The second 675 K entry and first two 715 K entries in Table VI1 illustrate the close match of experimental vs reference data for loss of O,(g) from the SO:-(cr) reactant to yield the S2032-(cr) product as follows: Ce2(S03)2S04(cr) O,(g) Ce20(S203)S04(cr). The coproducts oxidation equation is Ce2(S03),S04(cr) + Ce02(cr) + 02(g) Ce,0(S04)S04(cr) + CeO(SO,)(cr). The sum of the 02(g) loss reaction and the 02(g) gain reaction is the same as reaction 3A(Ce). Calculated AJ for reaction 3A(Ce) for the first experiment at 715 K in Table VI1 was -84.69 J K-'mol-', and for the second one at 715 K AJ was -83.51 J K-'mol-'.
+
-
-
Entropy Constraints for Concomitant Oxidation
+
Thermochemical data for A+!?= ( A 3 - A,G)T1 for reaction l(La,(Pr,Ce)) are in Tables V-VII.
The following equation for AJ is valid for (1(La)) when there is only one crystal SO," product and one gaseous product of HI: AJ = A$(cr)[La20(S04), - La2(S03)2S04]+ A$(cr)[La20(S04)2 - Laz02S04]+ AS0(g)[4HI - 212 - 2H20] The first-bracketed term is the same as in reaction 2A, so the second bracketed term is experimentally defined. The equations for (1(La)) can be utilized also for (l(Pr)). For (l(Ce)) the term A$(cr)[CeO(S04) - Ce02] replaces the bracketed term for the substrate product in the AJ equation. Additional information can be derived from the formation equation for the substrate reactivity: La202S04(cr) '/202(g) S02(g) LazO(S04)2(cr): AJ = A,S(cr)[La,O(SO& La202S04]- ASo(g)[S02+ 1/202]. If the two bracketed terms are equal, then A+ = 0. Any mismatch can be equated to -AJ of the reaction pathway@). If '/2S2(g) is substituted for 1/20,(g), the oxidation product becomes La20(S203)S04(cr)instead of LaZO(S04)2(cr) * In Table V the AJ values for La for 605,660, 680, and 690 K (0.2 min) showed a minimum at 680 K of 79.01 J K-'mol-' and a peak of 80.23 J K-' mol-' for the maximum yield of HI(g) at 660 K. The oxidation product of SO:-(cr) in reaction 1(La) in the short term contained SO:-(cr) by hydrolysis, but S2032-(cr)also was a product by S2(g) oxidation of SO2@ adsorbed on the substrate, as shown by the small AJ (mismatch) numbers (Table V, last two columns) of the experimental data vs JANAF data. Data in Table V for two experiments at 660 K for 120 and 125 min gave S042-(cr)as the most likely oxidation product. The substrate concentration in these experiments was different than the substrate concentration for the short term experiments. For the short term, in strongly catalyzed Pr experiments at 660 K, AJ in Table VI was 9.7 J K-' mol-' less than for La. For experiments at T = 660,720,770, and 800 K, where the reaction times differed by a factor of 200, AJ was invariant. PrZ0(S2O3)SO4(cr) was not identified as a product by oxidation or by disproportionation, except possibly in the 10-min undercatalyzed experiment at 660 K. A$ of the S2032derivative can be estimated by adding the contribution of hs0(g)['/2S2- 1/202] J K-' mol-' to the value for Pr20(S04)S04(cr). For Ce, oxidation of S03Z-(cr)to yield S042-(cr)in reaction I(Ce) became competitive with reaction 2A at 745-775 K because of the increased reaction rate. Maximum reactivity occurred with the 0.835 Ce02(cr) composition for a 1-min reaction interval. In contrast the 0.723 CeO,(cr) composition produced S20sZ-(cr)in 1 min in reaction 3A(Ce) that was oxidized to SO,Z-(cr) 2HI(g) '/2S2(g) in 2 min in reaction 3B(Ce). The 0.125 Ce02(cr) composition retained S2032-(cr)for 2 min at 745 K, but not as much at 775 K after 1 min.
+
+
-
+
+
Gibbs Energy Changes Differences in A& of products vs reactants were obtained from the conventional equation for experimental A$. For disproportionation, the chemically preferred reactions were (2A(La)), (2A(Pr)) : A,G = A & ( C ~ ) [ L ~ ~ O ( S- O La2(S03)2S04] ~)~ + [!@02 + y4Sz1 Reaction 1(La) for oxidation was tied experimentally to reaction 2A(La) by concomitanceand substrate reactivity to yield oxidation
4626 The Journal of Physical Chemisrry, Vol. 96, No. 11, 1992
end products that varied. The equation for (l(La)) was A,G = A , G ( C ~ ) [ L ~ ~ O ( S - OLaz(S03)2S04] ~)~ + A,G(cr)[product - La202S04]+ A,G(g)[4HI - 2 H 2 0 - 212] In the short term the product associated with the substrate was La20(S2O3)SO4(cr), and in the long term it was La20(S0,)2(cr). For (1(Pr)) the only identified solid product was Pr20(S04),(cr) by Iz(g) oxidation. The ArG equation for La was utilized for Pr. For Ce, the presence of the reactive CeOz(cr) substrate precluded an exclusive SO:-(cr) product of Ce3+;thus CeO(S04)(cr) was a second product if CeO(S203)(cr)did not form at lower T by S,(g) oxidation: CeO,(cr) + S02(g) + '/,S,(g) CeO(S203)(Cr)* The data in Tables VIII-X for differences in A& of reactants and products showed that there were reorganization energy changes which decreased the differences in the long term. HI(g) yield was lowered by reaction 2B. The work for replacing 02-(cr) with S2032-(cr)in La202S04(cr) (Table VIII) to yield La20(S203)S04(cr)was the same in the short term for the 605, 660, 680, and 690 K experiments that showed the constant AJ value in Table V. For the long term, two experiments for 120 and 125 min showed La20(S04)S04(cr) product formation. Work of disproportionation of reaction 2A increased with an increase in T because of the increase in AJ of the gas evolved. Data in Table IX for Pr showed that the reaction time 0.2 min at 660 K was sufficient for minimum work for formation of Pr20(S04),(cr)when Iz(g) pressure was high for catalysis. Longer reaction times with less 12(g)also were beneficial. Reorganization effects were less prominent than for La. For Ce in Table X there was some confounding of the data at 675-7 15 K by the solid-state disproportionation reaction involving Ce02(cr) sustrate reactivity. A more detailed analysis possibly could better quantify mechanistic yields. The difference of -488 kJ mol-' for A&(cr)[SO>- product - substrate] showed that the preferred pathway for many experiments at 745-775 K was the same as that for Pr at 660-800 K. The product was labile SO,,-(cr), and the required work of formation did not change much with a change in T.As expected, data for disproportionation did show variation with a change in T.
-
Enthalpy of Formation A,H data that were calculated for several compositions at 600-800 K are listed in Table XI. Conclusions The positive AJ for reaction 1 showed that the 02-that was available for SO,,-(cr) formation came from H20(g). The negative AJ for reaction 3A(Ce) showed that there was rearrangement of anions to relieve crowding in Ce2(S03),S04(cr)
Onstott by redistribution of 02-(cr) and formation of S2032-(cr) by valence changes in sulfur, plus exchange of substrate 02-for SO,"(cr) product. Experimental A$ for reaction 3A(Ce) was -42.3 J K-' mol-! for 2SO?-(cr) (715 K,0.835 substrate), compared to A S of -33.4 J K-I mol-' for reaction 2A for Pr2(S03)2S04(cr) (660-800 K, 0.760-0.426 substrate), where the pathway was evolution of l/&(g) + '/,SO,(g). No Pr product containing S,O?-(cr) was identified. The disproportionationmechanism for La,(S03),S04(cr) gave the same products as reaction 2A, but '/,SO,(g) were captured by the the evolved '/4S2(g) La,0$04(cr) substrate to yield 2La20(S203)S04(cr), CeOz(cr) substrate (0.603) also captured /4S2(g) 1/2S02(g)evolved by mechanism 2A in two experiments at 715 K. Enhanced solids stability allowed this pathway. A$' of oxidation of SO,"(cr) by I&) plus hydrolysis was closely equivalent to A+ of oxidation by 1/202(g). AJ of oxidation of reactant in reaction 2A was closely equivalent to AJ of oxidation by '/zO,(g) + I/,S2(g) without SO(g) formation. Retention of all reactants and products in the solids in reaction 3A(Ce) resulted in AJ closely equivalent to AJ for O,(g) loss from the reactant and reabsorption by oxidation of solids to facilitate anion rearrangement. Virtually all of the work of disproportionation by mechanism 2A for Pr was entropy change, since A,.H was very small, as shown in Table IV. For this reason the lanthanide contraction allowed the best crystal fit of SO& in Pr,O(SO,)SO,(cr) instead of the La or Ce derivative. The best fit for S20j2- seemed to be in Ce20(S203)S04(cr)instead of LazO(S203)S04(cr)because of the intracrystal disproportionation in reaction 3A(Ce) and the instability of La20(S203)S04(cr)at 660 K. A larger cation is required for best crystal fit of S2032-vs S O P . Normalized data were most useful for determining reaction pathways by the chronological entropy changes of phase change of O,(g) and/or S2(g) by disproportionation,oxidation, and hydrolysis. Disproportionation was the dominant solid-statereaction that activated 12(g)oxidation of SO,Z-(cr) plus hydrolysis to yield HI(g) as a concomitant secondary product. S042-(~r)was the primary product in the solids. S2032-(cr)was a primary product of intracrystal disproportionationfor Ce, but a secondary product for La, and not identified for Pr. Reorganization energy for La was larger than for Ce,showing the preference of La for SO,"(cr) as the end product. High reactivity of S2precluded sulfur condensation as a liquid at 600-800 K. The vapor pressure of S,(g) is 0.OOO 81 bar at 600 K and 1 bar at 882 K.3
+
'I
+
Acknowledgment. The Benefits Program of the University of California provided most of the funds for writing this paper. I thank W. C. Danen and the Physical Chemistry Group for continued support.
