1993
Dissolution of Anhydrous CrC13 in Aqueous Solutions
Catalyzed and Uncatalyzed Dissolution of Anhydrous Chromic Chloride in Aqueous solutions' Hendifar,2 W. F. Libby,* Depadment of Chemistry, University of California, Los Angeles, California 90024
and George L. Zimmerman Department of chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19070 Revised Manuscript Received May 6, 1974)
(Received October 9 , 1973;
Publication CQStS assisted by the U. S.Air Force Office of ScienrificResearch and the University of California, Los Angel.%
The Cr(I1)-catalyzed rate of solution of CrC13 in aqueous media is consistent with a mechanism analogous to that proposed by Taube and Myers3 for the Cr(I1)-catalyzed dissociation of CrC12+ into CrC12+and C1-, but after about 40 catalytic cycles, the rate of solution returns to a value somewhat larger than the uncatalyzed rate possibly as a result of the diffusion of Cr2+into the CrCl3 crystals. The uncatalyzed rate of dissolving of CrC13 is very low, increases with the total surface area, is independent of the nature of the cation but depends on the anion, depends little on acidity, and has an activation energy of 11.7 f l kcalfmol and a large activation entropy. Perhaps surface defects with water entering to coordinate gradually the surface Cr3+ ions over a particular region to free C1- ions as well as Cr (H2Ok3+constitute a possible mechanism. Insufficient data are a t hand to settle the matter.
I. Introduction The purpose of this work was to study quantitatively the rate of solution of anhydrous chromic chloride in aqueous solutions as function of the concentration of H+, C1-, Br-, and other selected ions, temperature, and CrC13 particle size in tha presence and absence of Cr(1I). Taube and Myers3 had studied the homogeneous reaction C ~ C I +~ +CP+
cr2+ + CIand demonstiated that one of the chloride ions from the chromic dichlioride ion was transferred to the chromous as the electron transfer occurred to form the nonlabile CrC12+ ion. It seemed reasonable that the known fact that chromous ion catalyzes the dissolving of chromic chloride in aqueous solutions should involve analogs of this reaction. It was our purpose to test this point. C;CI~+
+
11. Experimental Procedure A. Makpials. 1. Ferrous ammonium sulfate (Baker AR grade used without further purification) solution was prepared and standardized according to a common procedure.4a Diphenylamine was used as an i n d i ~ a t o r . ~ ~ , ~ A sample of anhydrous chromic chloride was ground and introduced into a series of sieves with the following opening~:~!'(a) 0.295, 0.246, 0.147, 0.124, .061 mm; (b) 0.250, 0.149,0.074; (IC) 0.38,0.18,0.117,0.104,0.084mm. The surface area (BET with N2) was measured for four particle sizes i2nd the results were as follows. -.IParticle size, mm
Surface area, m2/g
Size X area
0.25
0.095 0.063
0.117
0.35 0 .50
0 ,084
0.64
0,0538
0.218 0.18
0.0595
The sieves were arranged nn series from the largest to the smallest openings. The sieves were shaken by a sieve shaker until a suitable sample of each particle size was collect-
ed. TheoCrC13 samples were identified by the size of the finest sieve they could pass. Thus, 0.124-mm samples will actually contain a distribution of sizes between 0.061 and 0.124 mm. We know little about the distribution of sizes within a given sample however. Microscopic examinations of particle sizes 0.38, 0.18, and 0.084 mm showed the particles to be platelike crystals. X-Ray diffraction patt!erns of the above three particle sizes show no change in the structure of CrC13 due to grinding. 2. A solution of 0.01 M chromous chloride was prepared by reduction of chromic chloride in a modified Jones reductor containing amalgamated zinc. 3. The amalgamated zinc was prepared according to the procedure given by Kolthoff and SandelL8 B. Apparatus. The nitrogen used for purging was passed through a column maintained at 200O containing metallic copper obtained by H2 reduction of CuO wire. The spectrophotometer cell had quartz windows and an inlet and outlet for purging with nitrogen. Two sets of bottles were used in this work. The first set, used for the measurement of the uncatalyzed rate, was made of 25-mm tubing, 20 cm long, capped with a ground joint. The second set was especially made for the measurement of the catalyzed rate of solution. This set has an inlet and outlet; nitrogen could be introduced to flush out the residual air. A constant-temperature bath was made according to the design of Hildebrand, et aL9 The temperature was maintained at 32.8 rt 0.10,42.5, and 50.8. C. Procedures. 1. Analysis. Chromium in solution was determined according to a slight modification of the procedure given in Kolthoff and Sandell.lo To 1 ml of CrC13 solution was added 2 ml of 0.1 M AgN03 followed by 1.5 g of KzSzO8 or (NH&SzOa and 5 ml of 6 N HzSO4. The mixture was diluted to 50 ml and boiled vigorously for 45 min to 1 hr. The pale yellow solution was titrated with 0.1 N ferrous ammonium sulfate using diphenylamine as an indicator. A t the end point the color I
The Journal of Physical Chemislry, Vol 78 No 20, 1974
Hendifar, Libby, and Zimmerman
1994
"
'49 e
TiME (hrs) Figure 1. Dissolution of
400 600 TIME ( h r s )
200
0
CrI& in IO-' MHCl at 32.8' Figure 3.
Dissolution of CrCI3 in
--
I.'[
;:$ ?m
1.0
0
- .a
,104
I
d
800
1000
MHCl at 32.8'.
"
\
-"-.
Y
0
u
.6
2 2 4 0 0
.2 0 Figure 4.
0
200
400
600
Dissolution of CrC13 in lo-' MHCl at 42.5'.
600
TIME (hrs 1 Figure 2.
200 400 TIME (hrs.)
Dissolution of CrC:Iq in lo-* MHCl at 32.8'.
changed from a deep red to a pale green. A few drops of concentrated H3P04 was used to sharpen the end point. 2. Preparation of CrCl2. Chromous chloride was prepared by the reduction of chromic chloride in a modified Jones reductor. In order to exclude air, nitrogen was introduced into the system 1.0 flush out the residual air. 3. Measurement of the Uncatalyzed Rate. About 1.00 g of the salt of a given particle size was placed in a sample bottle and 70 ml of He1 or other aqueous solutions as indicated was added. No attempt was made to exclude air. The bottle was placed in a water bath shaking continuously for about 100 hr. A 1-ml sample was withdrawn through a pipet, the tip of which was attached by a small piece of gum rubber tubing to an 0.8-ml fine-fritted glass Biichner funnel which served to prevent any solid CrC13 from entering the pipet. Samples were taken and analyzed about every 100 hr for each of the particle sizes. Light was excluded intentionally. 4. Measuremeiit of the Catalyzed Rate. One gram of chromic chloride was placed in the special bottle, and 50 ml of KCl solution was added to the salt. The bottle was flushed out with nitrogen for about 5 hr. It was then sealed at both ends and placed in the inert-atmosphere box. A known volume of 0.01 A4 CrClz solution was added to the solution in the glove box, The bottle was removed from the box and placed in the water bath. The bottle was left in the water bath first for 1 hr. It was then placed in the air lock of the glove box, and the air was flushed out with nitrogen for 30 min. A small sample was taken for analysis, and the The Journal of Physical Chemisiry, Vol. 78, No. 20, 1974
bottle was returned to the water bath for a longer time (about 3 hr). This procedure was repeated at different intervals.
111. Results A. T h e Uncatalyzed Rate. The uncatalyzed rate was first studied with particle sizes 0.061, 0.074, 0.124, 0.125, 0.147, 0.149, 0.246, and 0.295 mm in HC1 concentrations of and M a t 32.8' with the results in Figures 1-3. Second, with particle sizes 0.38, 0.18, 0.117, 0.104, and 0.084 mm, at temperatures of 42.5 and 50.8O for various concentrations of HCl, NaC1, KCI, KBr, and He104 solutions, the rate was studied with the results shown in Figures 4-7. These results indicate that (1) the rate is very slow and over a period of 1000 hr shows no signs of changing when something less than 10% has dissolved, (2) the rate increases with total surface area exposed (Table I, Figure 8), (3J the activation energy is low a t about 11.7 f 1 kcal/mol (Table 11), (4) Na+ and K+ give the same rates (Table 111), (5) a tenfold increase (0.01 to 0.1 M ) in NaCl or KC1 concentration decreases the rate 17% a t 42.5' with 0.084-mm (0.64-m2/g) powder (Table III), (6) H+ behaves nearly the same as Na+ and K+ at 42.5' but increases the rate slightly as its concentration is increased at 32.5' (Table IV), (7) Br- rates are 21% larger than C1- rates a t 42.5' and 0.01 M; at 0.1 M , they are 22% larger (Table V), and (8) the rate is increasing somewhat as the concentration of HClOl is decreasing (0.01 to 0.1 M ) at 42.5' (Figure 7). B. T h e Catalyzed Rate. In the case of the catalyzed rate of solution of CrC13 in 0.1 M HC1 the rate was studied for the particle sizes 0.072, 0.125, and 0.245 mm, using two dif-
Dissolution of Anhydrous CrC13 in Aqueous Solutions
1995
0
--. E
8
-
m
u
L
6
0
0
E 4 u 2
0
200
400 630 TIME (hrs.)
Figure 7. Dissolution of CrCI3 in lo-' and particle size 0.084 mm.
gl
TIME (hrs.)
27t
Figure 5. Dissolution of CrCI3 in lo-' MHCl at 50.8'.
P 12
/-/
1
1
-
4
3
2
MHCIO., at 42.5';
/-
//
h 2.51
5
SURFACE A R E A
u
+ 10zM K C I
0
200
Particle size, mm
400 and
lo-*
M KCI, KBr, and
NaCl at 42.5'; particle size 0.084 mm. a
ferent concentrations of the catalyst (Cr2+): 2.38 X and 4.80 X M . The results are given in Figures 9 and 10. These results indicate that (1) after two to four dozen catalytic cycles of the Cr2+ the catalytic rate diminishes dramatically (we call this the "turnover" point), ( 2 ) the process is a surface phenomenon since the rate is higher for finer materiril, (3) the rate increases with increasing concentration of Cr2+ and the total CrCl3 dissolved before turnover is nearly proportional to the square of the initial Cr2+ concentration, (Cr2+)o,(4) there is very rapid initial rate of dissolving over a period of about 1hr when the turnover takes place. After that, the rate of solution is slower but still considerably larger than the uncatalyzed rate. The ratio of the rate of solution after turnover for the catalyzed systems to the rate of the uncatalyzed systems both for the same particle size increases with particle size. IV. Discussion A . T h e tiitcatalyzed Dissolving. The activation energy of the uncatalyzed dissolving of CrC13 in aqueous solutions is about 11.7 kcal/anol (Table 11). This low value taken with the very low rate shows that the reaction has a large negative entropy of activation.
Surface area, m2jF
Rate X 103, g/(l. hr)
0.25 0.35 0.50 0.64
I .79 2.27 2.69 2.82
0.38 0.18 0,117 0.084
TIME (hrs.) Figure 6. Dissolution of CrCI3 in lo-'
(dig)
TABLE I: Surface Area Effectsa
1 0 2 M KBr
LA--
7
.6
Figure 8. Rate of dissolution of CrCI3 vs. surface area.
O10"M HCI * IO"M K B r 0
800
Rates a t 60.8'; HC1 concentration 0.1 M .
TABLE 11: Temperature Coefficient (0.1 M HC1) Temp, OC
Rate X 103, d 0 . hr)
42.5 50.8 32.8 50.8 42.5 50.8 42.5 50.8
1.15 1.79 0.7 2.0 1.49 2.27 1.60 2.82
A H * , kcal/mol
Particle size, mm
0.38 10.8 i 1 0.25 11 f 1
0.18 10.4 f 1 0,084 13.9
1
TABLE 111: Cation Effects-Na+ and K + Concentrations" (NaCl), M
Rate X 108, g/(L. hr)
0.1 0.01
2.6 3.2
(KCl), M
0.1 0.01
Rate X 108, d ( 1 . hr)
2.6 3.2
Temperature 42.5O; particle size 0.084 mm.
The rate increases nonlinearly with surface area (Table I, Figure 8). The nature of cation has little effect and the rates are somewhat faster at lower salt concentrations. The Journal of Physical Chemistry, Voi. 76. No. 20, 7974
Hendifar, Libby, and Zimmerman
1998
TABLE IV: Ciation Effects-H+ Concentration. (HCI), M
Rate X 103,g/(l. hr)
TABLE VI: Rates after Turnover us. Uincatalyzed Rates (32.8’) Rates X 103, g/(L hr)
I
0 .‘L 0 .Ol 0 .I101 a
1.1 0.90 0.80
Temperature 32.8’; particle size 0.25 mm.
TABLE V: Anion Effects-C1-
a
us. Br-a
Salt
Concn, M
Rate X 103, d ( 1 . hr)
KCI KBr KCI KBr
0.1 0.1 0.01 5.01
2.58 3.14 3.15 3.8
Temperature 42.5’;
particle size0.084 mm.
Figure 9.
IO
20 30 40 TIME (hrs )
50
Dissolution of anhydrous CrCI3 in lo-’ MHCI catalyzed by
2.3 X
MCICI:.
0
IO
20 30 40 TIME (hrs.)
50
Dissolution of anhydrous CrCI3 in lo-’ M HCI catalyzed by 4.88 x 10-4 M C X I ~ . Figure 10.
Even H+has little effect, the rate increasing slightly as the H+ concentration is increased from 0.001 to 0.100 M at 3 2 . 5 O . On the other hand, the anions have larger effects. Possibly surface defects which coalesce to free a CrCl3 molecule is the general nature of the mechanism with anions from the solution playing an essential role. There are several possibilities but, we have insufficient data to distinguish between them. B. Mechanism for the Catalyzed Dissolving. The catalysis of the dissolving of CrCls by Cr2+ can be understood as The Journai of Physical Chemistry, Vol. 78,No. 20, 1974
x
Uncatalyzed
M Cr2+
0.072 0.125 0.245
2.3 1.5 1.2
3.0 3.4 2.9
2.3
10-4
x 10-4 M Cr2
4.8
+
1.2 4.7 12
TABLE VII: Turnover Points at 32.8” g of Cr2+/1. in
Particle size, mm
0.074 0.125 0.245
‘ a
0
Catalyzed after turnover Particle size, mm
(Cr2+) = 2.3 x 10-4~
0.8 0.55 0.45
0.1 M HCI
(Crz+)o = 4.8 x 10-4 M
Ratio
2.8 2.2 1.8
3.5 4.0 4 .O
being due to the lability of Cr(I1) complexes and the nonlability of Cr(II1) structures. Thus, CrC13 is slow to dissolve since the C1- ions are held to the surface Cr3’ ions. An approaching Cr2+, however, can be absorbed on the surface and share a C1- with the surface Cr3+. This immobilizes the shared C1- in the coordination sphere of the newly created Cr3+ now in aqueous solution as something like Cr(H20)S C12+. The surface layer of the solid now has a el- vacancy and a Cr2+ ion beneath it. Cr2+ ion is labile and can dissolve taking two C1- ions, thus completing the catalytic cycle. However, before this can happen, two neighboring 61- ions must be freed. This appears to require a certain minimum surface population of Cr2+ ions before the cycle can be completed. After two or three dozen such trips, the Cr2+ seems to disappear, or a t least the catalytic activity does, in a phenomenon we call the “turnover” corresponding to the sharp bend in Figures 9 and 10. The rate after turnover appears to be somewhat larger than the uncatalyzed rate (cf. Table VI), and the ratio of catalyzed rate after turnover to uncatalyzed rate appears to increase with particle size (cf. Table VII). There is much evidence3 derived from studies of oxidation-reduction reactions involving Cr2+ and Cr3+ that, in the activated complex, there is a formation of a bridge between Cr3+ and Cr2+by a ligand associated with the initial Cr3+. In the case under study, the ligand can only be chloride. Taube and Myers3 have observed this bridging mechanism in the case of catalysis of the reaction GrClz+ C r C P + C1- by Cr2+where one ligand from CrC12+ acts as the bridging group between Cr3+ and Cr2+.These authors observed that the only ligands participating in bridging be- e tween Cr3+ and Cr2+ come from Cr3+ and not from the Cr2+or other ions in the solution. At first sight, one might think that the turnover phenomenon was due to oxidation by oxygen which was destroying the Cr2+ and the catalysis causing the “turnover.” However, great care was taken to exclude oxygen, as described above, and this theory could hardly explain the increase of the ratio of the residual rate to that for the uncatalyzed system nor its increase with particle size as seen in Table VI. Another possibility comes to mind, namely, that the CrC13 crystals are impure and contain oxidizing agents. No evidence for such impurities was found by analysis. How-
-
Dissolution of Anhydrous CrC13 in Aqueous Solutions
ever, the amounts required are quite small: 4.6 X mol of Cr2+/g of CrG13 for the 2.3 X lo-* M Cr2+ cases and twice this for the 4.8 X lo-* M cases. This theory, however, also cannot explain the rates after turnover being larger than those ip the uncatalyzed cases nor the variation of the rate ratio with particle size. Thus we seem to! be driven to consider another explanation: the internal dissolving of the surface-adsorbed catalytic Cx2+ ions into the CrC13 crystals by diffusion so as to remove them from action. While waiting for the necessary two labile neighbors to appear to free two 121- ions, the surface-exposed Cr2+ has a chance to diffuse inward and be replaced by a nonlabile Cr3+Cl- on the surface, thus being removed from the catalytic cycle. The simultaneous movement inward of an electron to an underlying Cr3+ neighbor converts it into labile Cr2+ and the outward movement of a C1- to transfer to the empty surface site would accompany this change. The formerly surhce-exposed labile Cr2+ would then have been converted to a nonlabile Cr3+ with a surface C1- bound to it. Tests of the Proposed Mechanism. Observations show that the tol al CrCl3 catalytically dissolved before the “turnover” is
