On the mechanism of the thermal decomposition of barium perchlorate

COMBUSTION AND FLAME. 17, 125-129 (19711

125

On the Mechanism of the Thermal Decomposition of Barium Perchlorate P. W . M . J a c o b s , * AND

F

Solymosi and J. Rasko

Gas Kinetics Research ( r o u p , Hungarian Academy of $~:iences, Szeged. Hungary

In view of :ontr':dictory opinions expre:~sed i~l the literature concerning tlze mechanism of the thermal decomposition of barium pe~chlor,ate, additional experiments o a the decomposition of barium perchlorat¢, chlorate, and chlorite have been performed. It is shown that neither of t~,o pwviously proposed mechanisms is in accord with all the experimental data, and a modified mechanism is proposed. Whiile the possible role of partial melting cannot be excluded completely, the suggcsted mechanism consists of the following soli,:l-ph:tse reactions: CIO2 ~ CIO.~ + O,

(l)

ClO~. + 0 ~ C I 0 ~ ~- 0 : .

(2)

00;

~ CI0~ + 0 .

(31

CIO7 ~ C10- + O.

(4)

ClO- ~ Cl- + 0 .

(5)

0+0+M--02+M.

(6)

The induclion period, the discontinuity at ap )r~ximately 50 % decomposition, the effect of added CI-. and the dta and tga data cr,n all be explained using this meehanis n.

Introduction The thermal decomposition of barium perchlorate has been investigated previously by Solymosi and Braun [I, 2] and by AcUeson and Jacobs [3]. The most significant features of the reaction are as follows. In the presence of gaseous products (i.e., oxygenL plots of ~x (fraction decornposecl~ against t (fitae) show a sharp discontinuity at a = 0 . 5 2 [ I - 3 ] . This discontinuity can be prevented b'., pumping off the oxygen [3]. Addition of B~.CIz to the perch~orate before decomposition [2. 3] (a) removes the induction period and :b) reduces * Department of Chemistry. The Universit/of We+tern Ontario. Lon.'~on. Ontario. Canada

the value of :, at whic]~:the discontinuity occurs. Solymosi and Braun [1] proposed that the rapid reaction occurs in the melt, or at least in a partially molten phase, the discontinuity being caused by the slower rate of reaction when the reaction mixture freezes. Ho.wever. Acheson and Jacobs [3] observed that the product (BaC121 always assumed the original shape of the reactant, irrespective of whether this was a pellet or a single crystal, so that complete melting does not occur. Partial melting at a reaction interface would not be excluded by these observations. However, the discontinuity in ~(t) can be removed by pumping off the products [3]. and it is difficult to imagine how the removal of gaseous products Copyright I.(')~971 by The Combustion Institute Published by American ElsevierPublishing Company. It]c.

12,~ coukl either prevent freezing or cause the solidpha~'e reaction rate at ~ = 0.52 to become equal to the liquid-phase reaction rate. Acheson and Jacobs 1.3] proposed that (a) Ba(CIO,~)2 decomposes by successive removal of oxygen atoms and (b) Ba(CIOz) :, as it is formed, undergoes disproportionation to Ba(CIO3) z and BaCI2. Thug the discontinuity was explained by complete decomposition into Ba(CIOz)z, which had itself become transformed into Ba(CiO3),. thus giving a mole ratio of chlorate to chloride at ~ = 0.5 of 2 : I. The slightly higher value of a of 0.52 was attributed to partial decomposition of chlorate before complete decomposi!ion of the perchlorate. The new data to be described show that the dta and tga curves of the residue from the decomposition of Ba(CIO.Oz at ", = 0.53 and of a 2 : 1 mixture of Ba(CIO3)z and BaCi: are quite different, thus contradietir, g Acheson and Jacobs" hypothesis concerning the discontinuity. Modifications to their mechanism. at least with respect to (b) above, are therefore clearly required.

P. W. M. JACOBg, F. SOI..YMOSI, and J. I ~ S K O

Vgr~n ¢"*" *"" ="=~ ~"'~r'~ ~=--

100[ 7.$ t

]

t.,, ~'~

60

120

1~

~0 time(rain)

Figure I. Pressure-time curves for the deeompositioml of (l) Ba~CIO~)~; (2) Ba CIC~):+ BaCI: (mole ratio 2: I~: (3) BatUIOa)= + BaCI: (45 :'55 mole %); (4) residue from the decomposition o," r~a(CIOa): interrupted at ¢~=0.54. Temperature = 464~C. The oxygen content of the samples was practically the slme in each experiment.

Thermogravimetrie and differential thermal analyses (Figs. 2 and 3;) show a similar pattern. In both types of expt:riments the behavior of the residue obtained by interrupting the decomposition of Ba(CIO¢): at :~ = 13~.53resembles that of a 1 : 1 mixture of Ba(CIO.~): + BaCI, much more than it does a 2:'1 mixture of Ba(CIO~)2 + BaCI z. Further experiments c,,n the stability of barium chlorile confirmed former findings 15] the,t,

t=

Experimental Res¢lts In a n attempt to resolve these contradictory viewpoints additionai experiments on the decomposition of barium perchlorate, barium chlo:'ate, and barium chlorite have been performed. The materials used ,vere all of Analar purity from BDH. except in some comparative experiments. In Fig. I the decomposition curves of p,re Ba(CIO,tJz. Ba(CIO~.) z + BaCI 2 fir, the mole ratio of 1:1). Ba(CIO~), + BaC1, (in the mole ratio of 2 : 1). and that for the residue obtained by interrupting the decomposition of BaiCIO~)2 at ~: =0.54 are shown, From these curw:s it can be seen clearly that the behavior of the residue is similar to that of the BaICIO.~)2 + BaCI z mixture, while that of Ba(CIO~), + BaCI z is fundamentally different. The effect of BaC~lz on the decomposition of the perchlorate is also quite evident.

~._..--....---

~

l ~°'~

r - . ~ . ~ - .~

I I _ I

I/ v,,~/'\ ~c

~...~.

.s2~ /~.~.

Figure 2. Dta curves for (I) pure Ba(CIO~)~: (2) Ba(CIO~): ÷ BaCI: (mole ratio I : I1,: (3) BaiCIO.o2 -)- Ig:aCl: (mole ratio 2 : I ); (4) residue fi'om the decomposition of Ba(CIO4): interrupted at ~ = 0.53 '. iS) BaICIOj.):, obtained by Zinov'cv and Chudinov~= [4]. The r~ea~ing rate for curves I-,~ was ~,/rnin.

DECOMPOSITION OF BARIUM PERCHLORATE II.

,.....

I

.

1

127 the amount of the chlorate, in agreement with former work r2], never exceeds 3 - 5 % at any stage of the decomposition. These experiments were performed with twice-recrystallized BDH material as well as; Ba(CIO,, h from Fluka and from Riedel, but the amount of chlorate present at the discontinuity ( ~ 0 . 5 2 ) was again only about 3-5%.

~

l.

~-

~-~

lomg(~) '~o"c ~

/.~

~

do

-c

Figure 3, Weight-loss curves for (1) Ba(ClO.0:: (2) Ba(CIO4):~-gBaCI z (mole ratio I : l ) : (3) Ba(CIO~):+ BaCI2 (mole ratio 2 : I): ~4) residue from the decomposition of Ba(CIO,d,, interrupted at ~ = 0.53.

above 190'C, Ba(C102L, is transformed into BaCI2 and oxygen in an explosive reaction. The presence of a large amount of BaCI z increases the minimum explosion temperature: however, even a mixture of Ba(CIO2: h + BaCI., in 1:1 mole ratio explodes at 250°C after 120 sec. In order to check the amount of Ba(CIOah formed during the decomposition of the perchlorate, the decomposition of pure Ba(CIO~,h was interrupted at various stages of the reaction and Ihe solid products were analyzed chemically. The results arc shown in Table 1. These data show that, although Ba(CIOs), is formed in the decomposition of Ba(CIO4)2,

Discussion The two endotherms at approximately 295 ° and 370°C in the dta traces for Ba(CIO4)2 in Fig. 2 correspond to well-known phase transitions, Of chemical interest, therefore, are the endotherms and ¢xotherms summa:rized in Table 2, where a ( ) indicates a broad exothcrm, usually with multiple peaks. The dta trace for pure Ba(C104)2 has a pronounced endotherm at 48~°C, This could indicate melting, a third phase transition, or an endothermic chemical process. The case against melting has been summarized in the introduction. The dta trace (1) in Fig, 2 shows that this endotherm and the ¢xothcrm at 500°C overlap, and this fact, together with the size and shape of the endotherm, indicates thalt it corresponds to the first stage in the thermal decomposition of Ba(CIO,0,. The changes in AH: (298°K) as successive oxygen atoms are removed from Ba(CIO4)2 to give BaCI, are +11.1, +23.5, -17.9. and -29.5 kcal/mole, corresponding to a net exothermicity of AH = - 1 2 . 8 kcal/ mole. The first steps in the decomposition of

Table I. Ba(CIOa) a Colatent of the Ba(CIO~):, Decomposed to Different :Stages Temperature : 464~C Ba(CIOa): (BDHD Ba(CIOs) 2 found, ?i,

~t = 0.075 0.81

0.18 0.87

0.3 2.85

0,44 3.58

0,52 3.76

0,59 3.91

0.74 3.39

1.0 , 1.33

Ba(ClOa): (Riedel) Ba(ClOj): found, ~

:c = 0.08~ 3.03

9.21 5.30

0~33 5,45

0,50 4.42

0.53 2,12

0.66 2.74

0.83 1.79

1.0 2.11

BaICIOah, BDH recry,~.tallized 2 x : BalCIO~): formed at ~ = 0,54 is 4,11 ~i,. Ba(CIO~) z. Fluka : B,a(CiO~): formed at :~ = 0.53 is 2.82'~o. Ba(CIO,):. BDH: B;I{CIOs) z formed at ~ = 0.52 is 5.09 ?i.. In this last experiment the amount of Ba(CIOa): used was !0 rag: in all other experiments it was 100 rag.

P. W. M. JACOP,tS.F. SOLYMOSI+and J. RASKO

128 Table 2. Information Derived from dta of Ba(OO.)= aad of Ba(CIO~). + BaCl= Mixtcres

(A - -

indicatesa broad or extensiveexothetm.) Reactan~

Endotherm

Exothctms

(1) Ba(CIO+): (2) Ba(CIO=~:+ BaCI+(I : 1) f3) Ba(CIO~): + BaCI~ (2 : I) (4) Residt~cfrom decomposition of Ba(CIO.02 interrupted at ~t= 0.53

485 408

500 480. 520---450. 505 - -

407

478.520--

C!O2 and CIO~" are both clearly endothermic (Fig. 2), and thus we are able to identity the 408 + and 485~C peaks (Table 3). The dta trace of Ba(C1Oa)z, (3) in Table 2, indicates, in agreement with former work [6], that Ba(CIO3)2 is mog'e unstable than Ba(CIO+), itself, thus disproving Acheson and Jacobs' hypothesis [3] about the discontinuity. In confirmation, the CIO~" content at x-~ 0.52 is ~ 4 mole % and never exceeds 5.5 mole % in the isothermal reaction. It is thus s~fe to assume that CIO~" decomposes soon after it is formed. The 485°C endotherm in (1) then includes both CIO2--* CIO~ and L I O ~ - , CIO[. Solymosi and Bansfigi [5] have shown that Ba(CIO2)2 decomposes rapidly above 200+C with oxygen evolution, in preference to the

disproportionation reaction, and the present work confirms that this reaction occurs even in the presence of BaCI 2. The Ba(CIO2)2 thus also decomposes as it is formed, and, being exothermic, its decomposition is responsible for the rapid change from endothcrmic to exothermic occurring in curves (I) and (3) of Fig. 2. The variation in the peak temperature is caused by the differ:ent conditions under which the Ba(CIG2): is formed: in (I) from pure Ba(CIO,,)2, in (21 from a 1:1 mixture of Ba(CIO+)2 + BaCt;. and in (3) from Ba(CIO3) 2. The final exothe~rm, which is not resolved in (1) but occurs at 505-520°C in (2). (3), and (4). is due mainly to the decomposition of Ba(CIO)2. The main features of the mechanism of the decomposition of Ba(CIO+) 2 are thus established. In essence, these are the successive removals of O atoras from C10~, and although it is possible to correlate the dta ¢ndotherms and exotherms with these steps (Table 3), it must be stressed tL at the various stages overlap. In an isothermal ~xperiment, the four stages will all be going en simultaneously. In particular. the complete ~'emoval of CIO,~ by ct = 0.52, as envisaged by Acheson and Jacobs [3], is not attained, nor ~s there any accumulation of CIO~ as suggcsl,'d by these authors. The

Tabie 3. Identificationof the dta Peaks in Table :Z (The numbers in the column headed "'Reactant" refer to Table ! and Fib'. 33 DTA Peak. C

Reactant

Nature

Principal Cl'cm ~al Prpccss

485

I

Endotherm

C[O• ~ CIO i + ~O:

408407

34}

Eadotherin

CIO; ~ CIC,~ + .[O..

450 47~

;}

Exothcrm

CIO; ~ CIC + !O:

Exothcrm

CIO- -+ CI + [O. " "

Not resolved 520 505 530

i I

129

DECOMPOSITION OF nARIUM PERCHLORATE

clearest evidencc for the proposed mechanism lies perhaps in curve (3) of Fig. 2, which shows the one endothermic and two exothermic processes expected for the decomposition of Ba(CIOs)z. The difference between an experiment carried out at a constant heating rate and an isothermal experiment must be kept clearly in mind. Dta has the advantage of separating out the successive steps, but if an isothermal run is carried out at, say 490°C, the decomposition of CIO,~, CIO~, and CIO~ will occur very rapidly, whereas that of CIO- will ve noticeably slower than the other reactions (Fable 3). The isothermal ~,(t) curves show two marked characteristics: the initial reaction is acceleratory, :nd there is a marked discontinuity at = = 0.52. Both these features are affected by adding BaCI z to the reaction mixture, The acceleratory period is rauch reduced (even removed if the temperature is high enough), and the discontinuity occurs at lower values of g. The accelerztory period we as:~ociate with the decomposition of CIO~" to CIO~ : (:102 ~ CIO~ + O,

(!)

CIO2 + O ~ CIO~ + 02.

(2)

Since the CIO~" is more unstable than 0 0 2 and its decomposition also produces O atoms, a chain process is initiated, accounting for the acceleratory kirw,tics. The activation energy [3] is about 7') kcal/mole, which i5 reasonably close to that expected if the initiation process were rate determining. The bond dissociation energy for the fi:,s;on o f a CI-O bond is 64 kcal/ mole. Eventually. a steady state is set up. As CI- accumulate,=, the reaction C~- + 0--, CIO-

(51

becomes competitive with reaction - 1 , and at about 50% decomposition, w~.th a molar ratio CIO2 CI- of approximately 1:1 (with only small amounts of CIO~ and, perhaps, CIO~ present), the kinetics become dominated by the slow exothermic decomposition of C I O - . This reaction is reversible, and thus the decomposition is greatly enhanced by pumping off the oxygen [3]. (The effect of the accumulating gaseous products is to slow down the diffusion of O atoms away from the surface of the reactant, as air and nitrogen have similar inhibitory effects [3].) Thus the discontinuity at ~ =0.52 and its removal by pumping are explained. Finally, since CIO~ is more unstable than CIO2, addition of CI- to the reactant removes the induction period because the CI- ions m o p up the O atoms, thus preventing the reverse reaction - 1, which is the cause of the initial slowness of the decomposition of Ba(CIO,d2. It is known that the disproportioaation reaction 4C10~ ~ 3CIO~. + CIoccvrs in both the potassium and barium salts [6], and this implies that reaction - 1 must occur. References |. SOL'IMOSLF., and BRAUN, G¥.. dcla Chim. ,4cad. Sci. Hung.. 52, I (1967). 2. SOLYMO$1,F.. Acta Chim. AcalL Sci, Hung.. 57. 35 (196S). 3, ACHESOS. R. J., and JACOaS, P, W. M.. Can. J Chem.. 47. 303| (1969). 4. Zl~ov'Fv. A. A.. and C,UDI~OVA. L. T.. Z. Ne,rg. Khim.. L 1772 (1956). 5. SOLYMOSI.F.. and BANS~.GI.1 . . 4 e t a Chim..4cad SoL Hung.. 56, 337 (1958). 6. SOLYMOSLF~, and B~,NSA(;I,T.. At'ta Chim. ,Icad. St'i. Hung.. 50, 251 (1968).

(Recx,ived April, 1971)