Thermal Dissociation of Sulfur Dioxide and the Dissociation Energy of

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THERMAL DISSOCIATION OF SULFUR DIOXIDE

Oct. 5, 1954

lene concentrations and short contact times used in these studies, together with an analytical method yielding virtually continuous analyses, have made it possible to observe the conditions which obtain

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during the formation of the reactive surface layer; whereas the usual experimental procedures observe only the steady state conditions. PHILADELPHIA, PENNSYLVANIA

[COXTRIBUTION FROM DEPARTMENT O F METALLURGY, MASSACHUSETTS INSTITUTE OF

TECHNOLOGY]

Thermal Dissociation of Sulfur Dioxide and the Dissociation Energy of SO and S, BY GEORGEST. PIERREAND JOHN CHIPMAN RECEIVED MARCH10, 1954

+

The r a t i o j = Fe+++/(Fe+++ Fe++) in lime-iron oxide slags in equilibrium with SO2 or SOZ-CO mixtures is used to determine the partial pressure of 0 2 in the gas a t 1550’. The products of dissociation of SO2 a t this temperature are principally SO and 0 2 . The data lead to a decision between possible values for the dissociation energies of SO and SZ in favor of 5.146 and 3.6 e.v., respectively. The free energy of formation of SO from SZ and OZa t 298.16 and 1823”K.,respectively, is -18.6 and -20.6 kcal./mole.

Introduction The dissociation products of sulfur dioxide a t high temperatures include Sz, Oz, SO, SO3, S and 0. Equilibrium concentrations under given conditions are unknown because of major uncertainties in the

chose the latter. The situation for Sz is similarly uncertain and i t appears that the two values for so and the three values for SZ are equally probable. Six different values for the heat of formation of SO from SZand 0 2 are given in Table 111.

TABLE I SUMMARY OF THERMODYNAMIC PROPERTIES AT 1823OK. S

0

S2a

soa

0 9

-(F’ - E”o)/T

co

SOP

44.453 42.539 61.66 55.369 59.82 69.20 53.369 (H” - E”a)/T 5.222 5.080 8.49 8.025 8.21 12.01 7.739 47.620 70.15 SO 49.675 63.394 68.03 81.21 61.108 a Extrapolated from 1500°K. using C, data. Extrapolated from 1800°K. using C, data.

heats of formation of Sz and SO from the atoms. The importance of these gases in many pyrometallurgical processes, in particular their reactions with slags containing oxygen and sulfur, has led to this study. Data will be presented which establish the oxygen pressure a t 1550’ in pure SO2 and in several ,502-CO mixtures which lead to a clear choice among the proposed values for the uncertain heats of formation. Spectroscopic and Thermochemical Data.-Data on the thermodynamic properties of all the gases under consideration are presented in Table I. The data for each gas except COS have been taken from “Selected Values of Chemical Thermodynamic Properties.”‘ The values for COS are those given by Cross.2 In Table I1 are summarized data on some heats of formation. The several heats of formation of SO and Sz from the atoms arising from different interpretations of the spectroscopic measurements are reported. According to G a y d ~ n if, ~the predissociation limit of 41,520 cm.-’ for SO is due to a 3~ state arising from normal atoms, then the dissociation energy is 5.146 e.v.; however, if i t goes to O(3P) and S(lD) then the dissociation energy is 4.002 e.v. Gaydon favored the first interpretation while Herzberg4 (1) “Selected Values of Chem. Therm. Prop.,” Series 111, Natl. Bur. of Std., Loose Leaf, 1947-1951. (2) P. C. Cross, J . Chem. Phys., 3 , 825 (1935). (3) A. G. Gaydon, “Dissociation Energies and Spectra of Diatomic Molecules,” John Wiley and Sons, Inc , N. Y . , 1947. I n a private communication Professor Gaydon states that the listed value, 5.184 e.v., should have read 5.148. (4) G. Herzberg, “Molecular Spectra and Molecular Structure. I. Diatomic Molecules”; 1st Edn., Prentice-Hall Book Co., N. Y., 1939; 2nd E d . , D . Van Nostrand Co., Inc., New York, N. Y., 1950.

COz

COSb

SOaa

60.76 11.84 72.60

65.75 12.36 78.11

75.53 17.62 93.15

TABLE I1 SUMMARY OF DATAON HEATSOF FORMATION E.v./

Reaction

s

mole

+ 0 = so

2s =

s:,

co

+ +

1/202

92.3 -118.7 76 - 83 -101 -117.96

-

-3.3 -3.6 -4.4 -5.115

0 2 =

-

so2

=

AHOm.la, Method kcal./ of mole determ.

-

-4.002 -5.146

2 0 = On 1/2s2

AHen Kcal./ mole

cog

co + ‘/nS1 = cos

85.74

Reference

Spect.

a,b,c

Spect.

a.bAe.p

Spect.

-86.36

66.767

-67.636

21.01

-21.78

(Et:

Calor.

{Et:

f.e.h.i i,k,l,o f

f ,m

son + 1 / 2 0 1 = sos - 22.70 -23.47 Calor. f A . G. Gaydon, “Dissociation Energies and Spectra of Diatomic Molecules,” John Wiley and Sons, Inc., New York, N. Y., 1947. b G . Herzberg, “Molecultx; Spectra and Molecular Structure. I. Diatomic Molecules, 1st Ed., Prentice-Hall Inc., Co., New York, N. Y., 1939; 2nd Ed., D. Van Nostrand Co.. Inc.. New York. N. Y.. 19.50. E. V. Martin, Phys. Reo., 41, 167 (1932). E. Olsson, 2. Physik, 100,656 (1936). G. M. Naude and A. Christy, Phys. Rev., 37, 490 (1931). f “Selected Values of Chern. Therm. Prop., Circ. 500,” Natl. Bur. Std., 1952. 0 E. D. J. R. Eastrnan, B u r . M i n e s Inform. Circ., 6454 (1931). Eckman and F. D. Rosgini, J . Research Natl. B u r . StandJ. B. Ferguson, THIS JOURNAL, urds, 3, 597 (1929). 40, 1626 (1918); 41, 69 (1919). i G . Preuner and W. Schupp, 2. physik. Chem., 68, 157 (1909). A. R. Gordon, J . Chem. P h y s . , 3, 336 (1935). K. K. Kelley, U.S. B u r . of Mines Bull., 406 (1937). E . Terres and H . Wesemann, Angew. Chem., 45, 795 (1932). P. Bix and G. Herzberg, J . Chcm. Phys., 21, 2240 (1953). H. Braune, S. Peter and V. Nevelling, 2. Nuturforsch, 6a, 32 (1951). PP. Goldfinger, W. Jeunehomme and B. Rosen, Nature, 138, 205 (1936). Cf. W. Nernst and H. von Wartenberg, 2. Elektrochem., 9, 626 (1903); 2. anorg. Chem., 56, 320 (1908). %

47ss

GEORGE ST.

TABLE rrr CALCULATED X'ALUES FOR AH"o FOR THE REACTION: '/poi= so, BASEDO N SPECTROSCOPIC MEASUREMENTS REPORTED I N TABLE I1

+

AH;, kcal ./m