SECONDARY PROCESSES I N T H E PHOTOCHLORINATION OF CARBON MONOXIDE AND HYDROGEN‘ HUGH S. TAYLOR Department of Chemistry, Princeton University, Princeton, New Jersey Received May 86, 1988
Since the publication of the Second Report of the Committee on Photochemistry considerable progress has been made towards a final and quantitative formulation of the secondary processes both in the hydrogen-chlorine combination and in the reactions, thermal and photochemical, of phosgene synthesis and decomposition. These several reactions have been the principal objective of the researches of Bodenstein and his school. At the present time Bodenstein (6) is occupied with the publication of the definitive conclusions of this long series of investigations and is attempting to incorporate within the framework of those conclusions, or reject for reasons ascertained, the auxiliary data that have accumulated from the investigations of other workers, notably Rollefson (15, 19,20, 21), Ritchie (17, 18), Norrish (16), Allmand (l),and others. A discussion of the earlier work (3, 7, 8, 9, 10, 15, 22, 23, 24, 26) was included in the Second Report. I. THE PHOSGENE REACTIONS
For the photoreaction a t room temperatures and pressures over 100 mm. the kinetic expression is
d[CoC1zl = K J X . [Clz][C0]1’2 dt This equation is derivable from the Bodenstein reaction scheme:
+ E = 2C1 C1 + CO + M = COCl + M COCl + M = CO + C1+ M COCl + Clz = COClZ + c1 COCl + c1 = co + Cla C1 + wall = 1/2 Clz Clz
(1) (2)
(3) (4)
(5)
(5’)
Contribution No. 9 t o the Third Report of the Committee on Photochemistry, National Research Council. 789
790
HUGH
8. TAYLOR
Equations 2 and 3 lead, on the Bodenstein interpretation, to an equilibrium expressed by the equation :
KCOCl = ~~~l[col/[coc~l The first five reactions, with the assumed equilibrium, yield the kinetic expression for KI, which is obeyed by experimental results, under the given conditions, except in the beginning of the reaction and towards the end when reaction 5' becomes important with low concentration of COCl. The same chain-ending process (5') becomes important also a t higher temperatures and lower concentrations ( < 90 mm.). At high temperatures the kinetic expression then becomes d[coCl2l/dt
dabs.
[cl2][co]
The reaction constants KI and KZ are related by the following equations to the several constants of the individual reaction steps: Ki
=
k4 _ _ I _
k:"Kcoci
,.
Ka=-
k4
k6'KCOCl
By reason of the additional investigations of Bodenstein, Brenschede, and Schumacher (4, 5), Bodenstein (6) has rejected the interpretation by Rollefson (19, 20), which makes use of Cla as an intermediate, and maintains his contention that the COCl equilibrium exists in spite of reaction 4 in which this intermediate is steadily consumed. The thermal formation and decomposition between 350" and 45OoC. yield the kinetic expression d[COCl2]/dt =
Kth
[c12]a'2[[co]- K~h[cl~]1'2[coc12]
The reaction scheme pertaining to this is Cl2 @ 2C1 (equilibrium; Kcl, = [C1]z/[C12]) C1
+ CO @ COCl (equilibrium;K C O C=~ [Cl][CO]/[COCl]) ~ COCl + Clz = C0C12 + C1 (formation) COCl2 + C1 = COCl + Cla (decomposition)
The reaction constants Kth and
Klh
are then given by the equations
The numerical data for the equilibrium and reaction constants Bodenstein, Brenschede, and Schumacher (6) have recently completed a calculation of the numerical data for the several individual reactions and
PHOTOCHLORINATION OF CARBON MONOXIDE AND HYDROGEN
791
for the equilibria involved. Their results are summarized in the following. For the equilibrium between chlorine molecules and atoms the accurate data of Giauque and Overstreet (13) are available
+
log Kc1, = 57156 3.820 4.571T
For the equilibrium COCl F! co
+ c1
both the heat of reaction and the constant must be so chosen that log Kcocl = log ks - log kz Further, log ka must be sufficiently greater than log k4 so that the assumption of practical equilibrium in spite of reaction 4 can be maintained. By trial, the equation obtained was
For reactions 4 and 5 the data are given in the form of equations log k =
-4.571T E + log 21+ 1/2 log T - log f
where E is the activation energy, Z1the collision yield for T = 1°K., and! is the steric factor. I n these equations E and f are both adjustable. lOgk4 =
---4.571 2612 + 1/2 log T + 10.101 - 3.871 T
log k6 =
-4.571T -+ 1/2 log T + 10.106 - 0.976
The equilibrium constant Kcocl yields the value 5676 cal. for the heat of formation of COC1. The heat of formation of phosgene from CO Cla is 26,100 cal. With these two data and the value of 2612 cal. from log kd the expression for k p becomes
+
+
+
log kd' = - 23036 1/2 log T 10,110 - 0.171 4.571T With these several equations the calculated values for the overall reactions, photochemical and thermal, from room temperatures to 450'C. agree excellently with the measured values. Bodenstein sees in this concordance the best and most convincing support for the reaction schemes assumed and for the equilibrium,
co + c1= COCl
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HCGH S. TAYLOR
nhich Ilollcfson (19) especially has questioned. The heat of formation of COC1,5676 cal., is materially lower than the value of 10 kg-cal. originally estimated, the higher figure justifying the objection of Rollefson. With the newer numerical data, Bodenstein is of the opinion that the several reactions are described very satisfactorily and, he believes, in final and definitive form. TI. T H E HYDROGES-CHLORINE
PHOTOREACTION
The reaction sequence in the hydrogen-chlorine combination is, it is quite generally agreed, the Ncrnst chain mechanism, with the chains normally terminated by interaction of atomic hydrogen with oxygen impurities. The reaction scheme thus becomes
+ E = 2C1 CI + H2 = HCl + H - 800 cal. H + HC1 = H, + C1 + 800 cal. H + Cl2 = HCI + C1 H + 0%+ M = HO2 + M Clz
(1) (2) (2’)
(3) (4)
The numerical data for the several reactions are still subject to final revision but, according t o Bodenstein, the most reliable data now available are obtained from the following equations for velocities, the units being in moles per liter per second. log k , = 5750 4.571T log k2’ =
+ 1/2 log T + 10.47 - 0.92 = 6.40 a t 288°K. + 1/2 log T + 10.48 - 1.39 = 6.55 a t 288°K.
4950 4.57111
log k3 = - _ 2550 _ 4.571T log kq
=
+ 1/2 log T + 10.50 - 0.78 = 9.01 at 288’K.
- 4.571T __ 500 + 1/2 log T
+ 10.42 - 1.60 - log$
In the last expression the datum -1.60 represents the logarithm of the number of three-body collisions (moles per liter). The value of logf varies with M according to the best evidence. Bodenstein assigns the following values: logf = -1.37 for M = Clg, I&, 0 2 ; logf = -0.77 for M = HC1; - 1.07 for M = Hz Cl, mixture. and log f The value E, = 5750 cal. is obtained from Hertel’s value for the temperature coefficient for 10’ = 1.37. Hence, since Ez - E2, = 800 cal.,
-
+
PHOTOCHLORINATION O F CARBON MONOXIDE AND HYDROGEN
793
EzTbecomes 4950 cal. From a comparison of H
+ HC1 = Hz + C1
H
+ Ciz
with =:
HC1
+ C1
(in para-hydrogen) and the variation of lizI/kswith temperature, E21 E3 = 2400 cal. and so E3 = 2550 cal. From Hertel's data (14) it follows O2 M were temperature-indealso that E3 would be 2060 cal. if H pendent, To reconcile the two data for E3 me therefore can set Ea equal t o 500 cal. The absolute value of log k2 = 6.40 at 288'K. comes from a comparison, by Brenschede and Schumacher (4, 5 ) , of
+ +
eo + e1 + c1, = COClZ + c1 with C1
+ Hz = HCl + H
Hence log f = - 0.92. The absolute value of log k4 = 8.60, in the mean, was obtained by Bodenstein from analysis of data by Frankenburger and Klinckhardt (12) and by Bates (2) on peroxide formation from atomic hydrogen. From data of Ritchie (18), with experiments in which both water and hydrogen chloride were formed, log k3 - log k4 = 0.41 and hence log ka = 9.01. The steric factor corresponding is then log f3 = -0.78. From Hertel's data already discussed log k3 - log k2r = 2.46. Hence log at 288'K. is 6.55 and log fir = -1.39. The uncertainty in the values for log k is estimated by Bodenstein to be not greater than 0.3. k21
IEEFERESCES
(1) ALLMAND:3 . Chem. SOC.1937, 1878, and earlier papers. (2) BATES: J. Chem. Phys. 1,467 (1933). (3) BODENSTEIN: 2. physik. Chem. 130, 422 (1927). (4) BODEXSTEIN, BRENSCHEDE, AXD SCHUMACHER: Z. physik. Chem. B28,81 (1935). (6) BODENSTEIN, BRENSCHEDE, AND SCHUMACHER: z. physik. Chem. B36, 382 (1937). (6) BODENSTEIN, BRENSCHEDE, AKD SCHUMACHER: 2. physik. Chem., in press (1938). (7) RODENSTEIN, LENHER,.4KD WAGNER:Z. physik. Chem. B3, 459 (1929). (8) BODENSTEIN AND ONODA: Z. physik. Chem. 131, 153 (1927). AXD PLAUT: Z. physik. Chem. 110, 399 (1924). (9) BODENSTEIX (IO) BODENSTEIN AND SCHENK: Z. physik. Chem. B20, 435 (1933). (11) BODENSTEIN AND WINTER:Sitzber. preuss. Akad. Wiss., Physik.-math. Klasse 1936, 2-18, (12) FRANCKEA-BURGER AND KLINCKHARDT: Z. physik. Chem. B16, 421 (1932). (13) GIAGQUE A N D OVERSTREET: J. Am. Chem. SOC.64, 1731 (1932). (14) HERTEL:Z. physik. Chem. B16, 325 (1932).
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HUGH 8. TAYLOR
(15) LENHERAND ROLLEFSON: J. Am. Chem. SOC.62, 500 (1930). (16) WORRISH AND RITCHIE:Proo. Roy. SOC. (London) A140, 713 (1933). AND NORRISH: h o c . Roy. SOC. (London) A140, 99, 112 (1933). (17) RITCHIE J. Chem. SOC.1937, 857. (18) RITCHIE: (19) ROLLEFSON: Trans. Faraday SOC.27, 465 (1931). J. Am. Chem. SOC. 66, 579 (1934). (20) ROLLEFSON: Z. physik. Chem. B27, 472 (1937). (21) ROLLEFSON: (22) SCHULTZE: Z. physik. Chem. B6, 368 (1929). Z. physik. Chem. 129, 253 (1927). (23) SCHUMACHER: J. Am. Chem. SOC. 62, 3132 (1930). (24) SCHUMACHER: (25) SCHUMACHER: 2. angew. Chem. 40, 613 (1936). (26) SCHUMACHER AND STIEGER: Z. physik. Chem. B13, 157, 169 (1931).
