M. M. MARKOWIT'Z, D. A. BORYTA, AND H. STEWART, JR.
2282
The Differential Thermal Analysis of Perchlorates. VI.
Transient
Perchlorate Formation during the Pyrolysis of the Alkali Metal Chlorates'
by Meyer M. Markowitz, Daniel A. Boryta, and Harvey Stewart, Jr. Foote Mineral Company, Research and Engineering Center, Exton, Pennsylvania (Received A p r i l 2 , 1964)
The initial chemical changes in the pyrolysis of the five alkali metal (M) chlorates under 1.502, a slowly rising temperature program involve the reactions: (a) MC1O3 -.t MC1 and (b) hfC103 0.75hIC104 0.25MCl; subsequent phenomena are associated with the decompositions of the more thermally stable perchlorates, (c) MC1o4 + SIC1 20a. The extent of conversion of ;\tClOa per reaction b increases in the order Li (40%) < Na (6770) < E(,Rb, Cs (87%) a t the expense of reaction a. The increasing order of thermal stabilities of these metal chlorates follows the decreasing polarizing powers of X f though the stability differences among I(c103, RbC103, and CsC103 are rather small. Each of the MClO3 compounds appears to possess a congruent melting point in contrast to the corresponding Mc10.1salts which, except for LiC104,undergo simultaneous fusion and rapid decomposition. The catalytic effects of MnO2 additions to MC103 salts result primarily in the acceleration of reaction a. The general trends in thermal stability among compounds of the type MC10, (x = 1, 2, 3, 4) as a function of M and ~t:are discussed briefly.
-
Introduction Though the alkali metal (M) perchlorates have been extensively studied by application of differential thermal analysis (d.t.a.)2 , 3 and of thermogravimetric analysis (t.g.a.),3,4the corresponding chlorates appear not to have received such systematic treatment. In the present investigation, therefore, the pyrolysis relationships among the five h1c103compounds as revealed by d.t.a. and t.g.a. are presented. Of interest is the observation that for each hIC103 salt only the initial portion of the thermal decomposition sequence involves the chlorate anion which disappears by virtue of the reactions
+
n/Ic103+>IC1 1.502 MCIO3 +0.75MC1O4 0.25MC1
+
(1) (2)
Thus, the succeeding pyrolytic processes refer primarily to the breakdown of MClO, as per MC104 +nlcl
+ 202
(3)
The extent of reaction 1 was found to increase with decreasing atomic weight of ? t l in the order Li > Na > K, Rb, Cs; the converse being true for reaction 2 . The Journal of Physical Chemistry
+ +
+
As a group, the MC1O3 compounds were found to exhibit long temperature intervals between liquefaction and measurable rates of decomposition. Consequently, these chlorates, unlike the corresponding MClO, analogs with the exception of LiC104,6-7 effectively possess congruent melting points.
Experimental Equipment and Procedures. D.t.a. experiments to about 800" were performed using equipment previously described8 in conjunction with calibrated chromel-alumel thermocouples shielded in concentric close-fitting quartz tubes. Where visual observation (1) Preaented a t the 147th National Meeting of the American Chemical Society, Philadelphia, Pa., April 5-10, 1964. (2) S. Gordon and C . Campbell, A n a l . Chem., 27, 1102 (1955). (3) M . M . Markowitz, D. A. Boryta, and R. F. Harris, J . P h y s . Chem., 65, 261 (1961). (4) G. G. Marvin and L. B. Woolaver, I n d . Eng. Chem., A n a l . Ed., 17,474 (1946). (5) M . M .Markowitz, J . P h y s . Chem., 61, 505 (1957). (6) 11.M. Markowitz, ibid., 62, 827 (1958). (7) A. E.Siniohen, ibid., 65, 1093 (1961). (8) M. M. Markowitz, D. A. Boryta, and H . Stewart, Jr., Inorg. Chem., 2, 768 (1963).
DIFFERESTIAL THERMAL ANALYSIS OF
2283
PERCHLORATES
of the test sample was desired at temperatures up to about 350") both sarnple and alumina reference, contained in test tubes, were heated in a stirred siliconefluid bath (550 Fluid, Dow Corning Corp., Midland, Mich.) in accordancle with a d.t.a. method detailed earlier.6 Complementary t.g.a. runs to about 700" were made by procedures developed in other s t u d i e ~ .I~n ~order ~~ to avoid interpreting small weight losses due to volatile impurities as indicative of the onset of sample deconiposition, the t.g.a. reaction templeratures reported here correspond to thle loss of 2% of the total observed weight loss, Le., the 2(% decomposition temperature. Each d.t.a. and t.g.a. experiment was performed with a 1-g. sample under a dry, flowing argon atmosphere at a linear heating rate of 4"/min. Materials. LiC103 was prepared by double decomposition of Li2S04and Ba(C103)2in aqueous medial2; after filtration of the precipitated Ba804 and evapora-. tion of water from tihe filtrate, the solid product obtained was dried for 2 days at 100" and 1 p pressure Because of the extremely hygroscopic nature of the anhydrous material, it was stored over PzO, and only handled in a drybox under a dry inert atmosphere. NaC103 and KC103 were used as the commercially available reagent grade compounds after drying a t 110". RbCIOB and CsC103 were made from the respective chlorides by reaction with equivalent quantities of SaC103 in water solution; the initial crops of sparingly soluble RbC103 and CsC103 crystals were washed, recrystallized once from water, and dried a t l10".13 All the chlorates were analyzed for C1- contents gravimetrically by precipitation as AgCl and for C103- contents as additional C1- after reduction with aqueous SO2. Anal: Calcd. for LiC103: C103-, 92.3. Found: C103-, 92.0; Cl-, 0.02. Calcd. for SaC103: CX03-, 78.4. Found: (>IO3-, 78.4; 61-, 0.00. Calcd. for KC103: C103-, 68.1. Found: ClOa-, 68.1; C1-, 0.00. Calcd. for RbC103: C103-, 49.4. Found: C103-, 49.4; C1-, 0.00. Calcd. for CsC103: c103-, 38.6. Found: c103-,38.6; C1-, 0.00. All materials were ground to pass through a 325-mesh sieve. Reagent grade R9n02, of 0.72 m.2//e. specific surface area, was dried at llOo,was analyzed for total manganese by the bismutliate method and for active oxygen by oxidation of H2Cz04,and was back-titrated with 0.1 N KMn04. Anal Calcd.: A h , 63.2; active 0, 18.40. Found: nfn, 62.9; active 0, 18.44. The densities of RbC103 and CsC1O3 as determined by a gas pycnometer method were found to be 3.184 and 3.626 g./cc., respectively, at room temperature.
Experimental Results D.t.a. Experiments with Pure Chlorates. The d.t.a. curves obtained for the pure RIC103 salts are presented in Fig. 1. The pertinent data derived from these thermograms are sumniarized in Table I. Table I : D.t.a. Temperatures for MClO3 Compounds Onset of rapid hl.p., deoompn., ' C . , Ti, "C., Td,
Cryst. trans.,
nlclos
O C .
LiClQ3 NaClOs KC101 RbClOj csc103
M.P.,
MClOa
MClOa
AT = Td - T f
129 263 357 342 388
367 465 472 480 483
138 202 115 138 95
111 ... , . .
323 305
"C.,
MCI
607 800 768 721 644
&l
4
2 W
I I-
O X
W
t I-
a
I -1
I. Markowits, D. A. Boryta, and G. Capriola. J . Chem Educ., 38, 96 (1961). (12) A. Potilitsin, J . Russ. I'hys. Chem. SOC.,16, 840 (1853). (13) J. W. Retgors, 2. p h y s i k . Chem., 5 , 449 (1890).
Volume 68, Number 8 Augwrt, 1964
2284
R/I. hf. MARKOWITZ, D. A. BORYTA, AKD H. STEWART, JR.
The crystallographic transition and fusion phenomena any release of chlorine; no chlorine was found to be (except for the melting of CsC103) were also observed evolved from any of the chlorate melts at the temperavisually as the d.t.a. traces were recorded, thereby tures indicated in Table 11. The cooled, solidified permitting identification of the processes relating to residues were pulverized and analyzed, yielding the these endothermic breaks. Each of these phase data of Table 11. Clearly, these compounds show a changes was found to be reversible on three sequentially considerable degree of stability in the liquid phase repeated heating and cooling cycles with no change in near their melting points. This behavior is in ma,rked the determined invariant temperatures, thus suggestcontrast to the heavier RIC104 salts where fusion and ing constancy of composition despite the exposure of rapid thermal decoinposition are concomitant occurthe chlorates to temperatures slightly above their rences. Thus, congruent melting points can beascribed melting points. The ( p a ) polymorphism found for to each of the pure chlorates of Table 11. It is to be ~ LiC103 at 111' has been reported p r e v i ~ u s l y l ~ - ~anticipated that phase studies of anhydrous salt and tends to substantiate the nonexi~tence'~~~5 of ansystems, akin to those performed with LiC104, are other transition ( y p) said to occur a t 440.16 The entirely feasible with MC103 compounds as com(p a ) phase changes determined for RbC103 (323") p o n e n t ~l 9. ~ ~ ~ ~ ~ ~ ~ and for CsC103 (305") apparently have not been reported in the literature heretofore. Table I1 : Analyses of Residues of MCIOI Salts Heated for It is of interest to note that the d.t.a. curves of Fig. 2 days Above Melting Point under Flowing Argon 1 show that the decomposition region of each of the hlC103 salts consists of a pair of exotherms. In the M.p., Temp. of % instances of KCIOI, RbC103, and CsC103 good temMClOa, heating, Yo % MC101, MClOa O C . MClOs" M C P by diff.a perature separation is found between the two successive breaks, whereas for LiC103 and YaC103, there is LiC103 129 99 54 135 0.06 0.40 NaC10; 263 300 99.49 0 02 0.49 clearly some overlap of the processes giving rise to KC102 357 99.56 0.38 0.06 370 these thermal effects." Resolution of the nature of RbC103 342 370 98.83 0.38 0.80 these pairs of exotherms will be made subsequently 85.12 4.11 10.77 CSC103 388 400 in this paper. a Weight yo. The residues from the d.t.a. runs with LiC103 were found to be alkaline to phenolphthalein; the reaction products from the other chlorates were neutral to this The partial decomposition of MClOs to hfC104 indicator. Thus, it seems likely that a portion of the (reaction 2) shown in Table I1 is in accord with the LiC103decomposes as per known behavior of MC103compounds.20-22 4LiC103--+2Liz0 2C1, 502 (4) Interrupted D.t.a. Runs for Pure Chlorates. The chemical reactions taking place during the thermal However, in all instances, the final endotherms cordecompositions of the chlorates were determined by respond closely to the reported melting points for the perforniing d.t.a. runs to the peak temperatures of respective metal chlorides, thereby establishing re-+
-+
-+
O C .
+
+
action 1 as the ultimate, preponderant decomposition state for the pure chlorates. Constant Temperature Experiments with Pure Chlorates. The d.t.a. curves of Fig. 1 show long temperature intervals between R/IC103fusion and the onset of rapid chlorate decomposition as indicated by the start of the first exotherm (cf. Table I, column 5 ) . This behavior would therefore suggest that for all practical purposes the chlorates are kinetically stable a t their melting points and probably to some temperature beyond before appreciable rates of thermal decomposition are achieved. Accordingly, 4-5-g. samples of these materials mere heated for 2 days under dry, flowing argon at temperatures slightly above their respective melting points. The effluent gas stream was bubbled through KI solution to ascertain The Journal of Physical Chemistry
(14) A. N. Campbell and J. E. Griffiths, Can. J . Chem., 34, 1647 (1956). (15) L. Berg, 2. anorg. allgem. Chem., 155, 311 (1926); 166, 231 (1927); 181, 131 (1929). (16) C . A. Kraus and W. M . Burgess, J . Am. Chem. Sac., 49, 1225 (1927). (17) M. M. Markowits, D. A. Boryta, and H. Stewart, Jr., J. Chem. Eng. Data, in press. (18) 31. M. Markowitz and R. F Harris, J . Phys. Chem., 6 3 , 1519 (1959). (19) M. M. Markowitz and D. A. Boryta, ibid., 65, 1419 (1961). (20) T. W. Clapper, W. A. Gale, and J. C. Schumacher, "Perchlorates: Their Properties, Manufacture, and Uses," J. C. Schumacher, Ed., Reinhold Publishing Corp., New York, N. Y., 1960, pp. 71-100. (21) C. C. Addison, "Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry," Supplement 11, Longmans, Green and Co., London, 1956, Part I, pp. 538-538. (22) C. E. Otto and H. S. Fry, J . A m . Chem. SOC.,46, 269 (1924).
2285
DIFFERENTIAL THERMAL ANALYSISOF PERCHLORATES
the first exotherms of Fig. 1, quenching the sample holders in ice water, and then analyzing the final solids. The results obtained from these experiments are given in Table 111. From these data it may be seen that in each instance the first break is a composite of the two simultaneously occurring reactions represented by eq. 1 and 2 ; and for the cases of LiC103 and ?LTaC1O3,the overlapping of the first and second exothermic breaks also indicates the OCcurrence of appreciable perchlorate decomposition (reaction 3) within the temperature ranges covered by the first exotherms. However, the extent of overlap cannot be estimated from these d.t.a. experiments although some quantitative measure can be estimated from the t.g.a. rum to be discussed subsequently. Nevertheless, it is clear that the second exotherniic breaks in the d.t.a. patterns of Fig. 1 refer primarily to the pyrolysis of 1,he initially formed h4C104 compounds. It is pertinent to note from Table I11 that about 87% of the KGlOa, RbC103, and CsC108 engage in reaction 2. The extents of decomposition as per reaction 1 based on the chemical analyses contained in Table I11 are in good agreement with the weight loss data, as are the chlorine balances. These results would thus support the absence of any appreciable quantities of hypochlorites or chloritcbs as intermediates in the chlorate deconipositions. However, independent weight loss and chlorine balance checks after the d.t.a. runs were not possible with LiC103 due to considerable sample splattering.
Table I11 : MClOa Salts Heated to Peak Temperatures of First Exotherms on D.t.a. Curves, 4"/iktin. Heating Rate from Ambient % % Residue composition, -----mole %-
LiC103* NaC1O3 KClOi RbClOi CSCIO~ e
%
%
%
MClOa
MCl
MCIOra
0.9 2.6 0.9 1.6 1.3
68.2 47.3 33.9 32.9 34.0
30.3 50.1 65.2 65.5 64.7
By difference.
MClOs decomposing t o MC1
MClOs decomposing t o 0.26 MC1
1.50%
MClOi
Peak temp., OC.
57.8 30.5 12.1 11.0 12.4
40.2 66.9 87.0 87.4 86.3
425 542 566 563 560
+
+ 0.75
Also contains 0.6 mole yo LizO.
Because the formation of MCIOl in R!tC103 melts Is not accompanied by any change in sample weight but still represents a mode of MC103 decomposition, the d.t.a. technique tends to give a more complete
- LiC103
P3-
---
NaCIO3
._._
KC103
2-
.......,,... RbC103 1-
-c+c
1
CSCIO,
Figure 2. T.g.a. curves for pure alkali metal chlorates.
picture of the events taking place than does the t.g.a. approach. T.g.a. Experiments with Pure Chlorates. The t.g.a. patterns obtained for the pure 14c103salts are presented in Fig. 2. The initial weight losses must be due to reaction 1, whereas the succeeding decrements characterize MClOl pyrolysis (reaction 3). The pertinent t.g.a. data are summarized in Table IV.
Table IV : T.g.a. Results for Pure MClOp Compounds % of weight
Temp., OC.
MClOa
loss based on conversion to MC1
total weight loss
LiC108 NaC108 KClOs RbClOa CSClOa
102.5 100.8 100.9 100.7 100.7
384 474 502 513 489
of 270 of
Inspection of the t.g.a. curves for LiClO, and NaC103 shows marked decreases in rates of weight loss a t about Volume 68, Number 8 August, 196.4
14. 14.MARKOWITZ, D. A. BORYTA, A N D H. STEWART, JR.
2286
430 and 512O, respectively, corresponding to aggregate weight losses of 51.2 and 24.1Yc of the total final weight losses. I n the case of LiC103, it was shown by isothermal decomposition experinients17 and by chemical analyses of residues from interrupted t.g.a. runs of LiC1O3I7that the initial steps in the pyrolysis sequence of LiC103 can be closely represented by the over-all equation 8LiC103+5LiC1
+ 3LiC103 + 602
(5)
Reaction 5 is characterized by 50% of the weight loss corresponding to complete conversion of the LiC103 to LiCl (reaction 1). This is then consistent with the observed t.g.a. behavior for LiC103 (Fig. 2) which indicates that above 430°, the resulting weight loss is due to the decomposition of LiC104 (reaction 3). By analogy it is felt that the decrease in rate of weight loss found for XaC1o3 is due to the slower decomposition of the more thermally stable NaC104 formed earlier in the temperature program by the composite reaction 5NaC1o3
--f
3NaC104
+ 2KaC1 + 1.502
?\IC103 salts gave only chlorine-free oxygen upon decomposition. Thermal Behavior of 00 Mole MC103-10 Mole MnOz Mixtures. I n every instance the effect of additions of MnOz to MC103 is an appreciable lowering of the thermal stability of the RIC103 compound, Except for LiC103-iVInO2, the d.t.a. patterns of these mixtures (Fig. 3) show decomposition to become rapid soon after following a transition temperature (vix., RbC103-Mn02 and CsC103-Mn02) or a fusion temperature (vix., XaC108-Mn02 and KC103-Mn02). Such behavior would indicate that for the heavier RfC1o3 salts, those processes resulting in enhanced freedom of movement of the ionic lattice units facilitate catalytic decomposition by MnOz. The t.g.a. results (Fig. 4) demonstrate slow decomposition to start for each of the MC103-Mn02 mixtures, again with the exception of the LiC103-Mn02 sample, a t some temperature below the transition or melting points. It appears likely then that the observed fusion temperatures in the presence of Mn02 refer to mixtures of MC103 with the decomposition products formed up to that point. How-
(6)
It is doubtful if eq. 5 and 6 have exact stoichiometric significance in representing the chemical changes occurring during the initial heating of these chlorates; rather, such equations appear to be reflections of the relative average rates of the direct chlorate decomposition route (eq. 1) and of the disproportionation path (eq. 2). On this basis, it may be seen that for LiC103 the ratio, average rate reaction l/average rate reaction 2 , is approximately 1, for SaC1O3, it is l j 4 , and for K, Rb, and csc103, it is 1/9, Assuming that these rate ratios hold, then it can be computed that for LiC103, the pair of exotherms in Fig. 2 overlap such that about 187c of the LiC104 originally formed decomposes by time the peak temperature is reached; for xaClO3, about 19% of the NaC104 formed decomposes in the region of decomposition overlap. Alternations in slope are not evident in the intermediate stages of decomposition in the t.g.a. curves for K, Rb, and CsC103 due to the occurrence of reaction 1 to only a rather small degree in a region of rapidly increasing reaction rate. The data of Table IV show quantitatively that ?\IC1 is the major reaction product from the pyrolysis of a chlorate. However, the high weight loss found for LiC103 also betokens the occurrence of reaction 4 to the extent of 3.8%. Direct determination of the chlorine gas evolved from decomposing LiC103 showed reaction 4 to occur to the extent of 4.5%. While XaC103 evolved a slight trace of chlorine, the other The Journal of Physical Chemistry
.
,
,
MOLE
J
herm is characteristic of a reversible crystallographic transition for C S C I . ~ ~ However, ,*~ this endotherm could be a composite of exothermic CsC104 decomposition and the endothermic CsCl phase change. The t.g.a. curve for IJiC1O3-MnO2 manifests a decrease in rate
2MnO2 3 Mn203
+ 0.502
(7)
Faint traces of chlorine gas were determined in the gases evolved from the iSaC103-Mn02 and KC1O3Mn02 r n i ~ t u r e s ~ * - none ~ ~ ; from the RbC1O3-h!tnOz and CsC103-Mn02 samples. The LiC103-Mn02 sample indicated 4,3y0decomposition of LiC103 to Li20 (reaction 4) based on chlorine release. Each of the total weight losses determined by t.g.a. for the MC103MnOz mixtures, except for LiC1Os-XnO2, could be accounted for by reactions 1 and 7. Apparently reaction 4 does not appear to be appreciably affected by the presence of MnOz.
Discussion of Results Because the disproportionation of A!Ic103 compounds to perchlorates (reaction 2) does not involve any change in weight of the sample, the d.t.a. method is more suitable to gauging the thermal stabilities of the chlorates than is the t.g.a. method which initially at least is principally a measure of reaction 1. As evidenced by many other oxyanions salts of the alkali metal^,^^-^^ the thermal stabilities of the MC103 ~~
(23) C. D. West, 2. Krist., 88, 94 (1934). (24) G. Wagner and L. Lippert, 2 . physilc. Chem., J I B , 273 (1936). (25) NI. M . Markowite, D. A. Boryta, and H. Stewart, Jr., unpublished results. (26) A. D. Mah, "Thermodynamic Properties of Manganese and I t s Compounds," V. S. Bureau of Mines Report of Investigations 5600, 1960. (27) T. E;. Moore, M . Ellis, and P. W. Selwood, J . A m . Chem. Soc.. 7 2 , 856 (1950). (28) J. M. Geidis and E. G . Rochow, J . Chem. Educ., 4 0 , 78 (1963). (29) 0. Bostrup, K. Demandt, and K. 0. Hansen. ibid., 39, 573 (1962). (30) J. W. Mellor, "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. I , Longmans, Green and Co., London, 1922, p. 360. (31) R . K. Osterheld and M. &I. Markowitz, J . Phgs. Chem., 6 0 , 863 (1956).
Volume 68, Number 8
August, 1.964
2288
salts are seen from the d.t.a. data presented in Table I to follow the order Li < Xa < K < Rb < Cs. Nevertheless, as found for MC104 c o m p o u n d ~ , ~the ~ ~range 4 of thermal decomposition temperatures for rapid pyrolysis is quite small despite the large range of cation sizes in proceeding from Li+ to Csf (empirical ionic crystal radii in A.35: Li+, 0.78; Na+, 0.98; Kf,1.33; Rb+, 1.49; Csf, 1.65). The sequence found for the MC1O4 salts at a higher heating rate and with larger sample sizes3 is Li < Na < Cs < K < Rb, which corresponds to the order of fusion temperatures of these materials under the prevailing heating conditions. A reflection of this fusion effect in accelerating the pyrolyses of perchlorates is readily seen in the instance of KC104.36-37Thus, the first-order liquid phase decomposition of KClO, is about 50 times faster than the corresponding first-order solid phase decomposition. It appears likely that the decompositions of all the perchlorates formed during the heating of the MC103 salts (Fig. 1) stem from the liquid phase due to the formation of low melting MC104-MCl compositions. The observed order of thermal stabilities of the relatively low melting MC103 salts may be most conveniently related to the decreasing polarizing powers of ionic potentials of the cations in passing from Lif to C S + . ~ ~ The fact that large quantities of MC104 are transiently formed as each MCIOssalt isheated substantiates the greater thermal stabilities of the perchlorates in comparison to the corresponding chlorates. This, of course, can also be judged by comparing the t.g.a. and d.t.a. results previously obtained3 for pure 14ClO4 compounds with the present results for the RilC103 salts. High cation polariziizg power appears *to favor direct R4C10D decomposition to MCl (reaction 1) as gauged from Table I11 for LiC103 and NaC1O3; with the other MC103 compounds, there is virtually complete participation (87y0)in the disproportionation reaction (eq. 2) which leads to perchlorate formation. The temperatures a t which these intermediate MC104-R/IC1 mixtures decompose rapidly correspond fairly closely to newly determined d.t.a. decomposition temperatures for the pure MClO, under the same conditions of sample size and heating rate used in the present study. 25 Addition of MnOz tu, MC103 results in a leveling effect relative to the observed thermal stabilities such that differences in stability previously related to polarizing effects seem to be virtually eliminated. Consequently, the t.g.a. curves (Fig. 4) for each of the MC103-MnOz mixtures are quite similar. The d.t.a. curves for these heterogeneous samples (Fig. 3) indicate that the MnOz brings about acceleration of reThe Journal of Physical Chemistry
M. M. MARKOWITZ, D. A. BORYTA, A N D H. STEWART, JR.
action 1 a t the expense of MC104 formation. For both the pure R4C103 salts and the MC1o3-&ho2 mixtures, &IC1 is the ultimate primary product of thermal decomposition as would be anticipated from the greater thermodynamic stabilities of the MC1 salts as compared with the equivalent oxides. 17v3, The d.t.a. patterns of Fig. 3 emphasize the trend that processes giving rise to increased anion mobility tend to facilitate decomposition. Thus, the MC1O3-MnO2 mixtures are seen to decompose rapidly for each of the i\!tC103 salts, except LiC103, virtually immediately following or concurrent with a crystallographic transition (Le., “Hedvall effect”3Q)or a liquefaction phenomenon. The exothermic natures found for reactions 1, 2, and 3 are consistent with estimates made for the enthalpy changes characterizing these reactions. 3,17 The disproportionation reactions of the lower oxyanions of chlorine (MClO,) to the higher oxyanions (MC1Og,where y = 3, z = 1, 2 ; y = 4,z = 3) may be looked upon as internal oxygenation processes. Thus, for the conversion of MClOa to MClO~,the oxygenation can be represented by the sum of the individual reactions
+ 1.502
(8)
43MClO4
(9)
MClO3 ---) MC1
3MCIO3
+ 1.502
4MC103 --+-3MC104
+ MCl
(10)
It can be seen from the present studies that the extent of occurrence of step 9 appears to be dependent on the nature of M. The results obtained here suggest the degree of internal oxygenation to decrease with increasing polarizing power of M so as to favor 0 2 release (reaction 8). The oxygenation of MC10, compounds to the MC10, state seems to be a more likely occurrence than the oxygenation of MC1 to nmo,.4oV41 (32) W. E. Van Arkel, “Molecules and Crystals in Inorganic Chemistry,’’ 1st Ed., Interscience Publishers, Inc., New York, N . Y., p. 118. (33) R. T. Sanderson, “Chemical Periodicity,” Reinhold Publixhing Corp., New York, N. Y., 1960, pp. 162-166. (34) M. M. Markowitz, J . Inorg. Nucl. Chem., 2 5 , 407 (1963). (35) J . R. Partington, “An Advanced Treatise on Physical Chemistry: The Properties of Solids,” Vol. 3, Longmans, Green and Co., London, 1952, p. 137. (36) A. E. Harvey. M.T. Edmison, E. D. Jones, R. A. Seybert, and K. A. Catto, J . Am. Chem. Soe., 7 6 , 3270 (1954). (37) A. E. Harvey, C. J. Wassink, T. A. Rodgers, and K. H. Stern, A m . N . Y . Acad. Sci., 79, 971 (1960). (38) G. €1. Cartledge, J . Am. Chem. SOC.,5 0 , 2855, 2863 (1928); 5 2 , 3076 (1930). (39) K. Hauffe, “Reaktionen I n und An Festen St,offen,” SpringerVerlag, Berlin, 1955, pp. 583, 594, 629. (40) A. V. Bosch and A. H . W. Aten, J . Am. Chem. Soc., 7 5 , 3835 (1953).
DIFFERENTIAL T H E R M A L AKALYSIS OF
It is instructive to attempt to delineate the trends in thermal stability among compounds of the general type MC10, (z = 1, 2, 3, 4) as a function of both M and z. I n the preseiit study, the conversion of AIC103 to AICIO, in the course of thermal decomposition was demonstrated to occiur for each JIClO, salt. JIClOz and MClO salts are known to form appreciable amounts of rCIC103 at elevatedl temperatures as per the over-all reaction~~~p~~
+ 2MCI 3MCIO2 +2MC1O3 + MCI 3MC10 -+ MC10,
2289
PERCHLORATES
(11)
(12)
Accordingly, it seems clear that foir a given M, the order of thermal stability for lbIC10, compounds is hIC1O4 > MC103 > (31c1o2,WIClo). Significantly, this is also similar to the order of extent of n-bonding for these oxyanions of chlorine, i . e . , C104- > C103- > CIOz- > C10- = 0.43-45Though little information is available relative to the entire families of MC102
and MClO compounds, it appears reasonable that, other factors being equal, for a given value of 2, the thermal stabilities of the compounds RIC10, will be principally determined by the degree of anion polarization and will therefore be in the sequence LiC10, > XaC10, > KC10, > RbCIO, > CsC10,. However, as seen in the instances of the chlorates and perchlorates, the differences in thermal stabilities manifested by the E(, Rb, and Cs salts of the same anion are rather small, thereby suggesting a limit in the effect of the diminution of cation polarizing power toward these anions below the ionic potential of K+. (41) A. Glasner and L. Weidenfeld, J . A m . Chem. Sac., 7 4 , 2464 2467 (1952). (42) R. B. Heslop and P. L. Robinson, “Inorganic Chemistry,” Elsevier Publishing Co., New York, N. Y . , 1960, pp. 380-384. (43) E. L. Wagner, J . Chem. Phys., 37, 751 (1962). (44) E. Cartmell and G. W. A. Fowles, “T’alency and Xlolecular Structure,” Academic Press, Inc., Iiew Yorli, N. Y., 1956, pp. 179, 180. (45) H. G . Palmer, “Valency: Classical and Modern,” 2nd Ed., Oxford University Press, London, 1959, p. 154.
Volume 68, ,Vumber 8
August, 1964
