Oscillating reactions - a phenomenon studiable by DTA?

iWrnwchin~ica

0

205

Acta. 29 (1979) 205-209

Elsevier Scientific

Publishing Company,

Amstcrdnm

- Printed in the Netlwrlands

REACTIONS - A PHENOMENONSTUDIABLE BY DTA?

OSCILLATINS

E. Koch and B. Stilkerieg Institut

fiir Strahlenchemie

im MPI fUr Kohlenforschung,

Stiftstr.

34-36

D-4330 Mu1heim a. d. Ruhr INTRODUCTION The fascinating sophisticated

phenomenon of chemical

interchange

of numerous reactions;

SOV-ZHABOTINSKIIreaction,there The aim of our first thermal characteristics with respect tried

to represent

catalytic

bursts

meters E = 67.2 dependence for

‘studies

effects

in

reaction

field

was

showing unimolecular and reactant acid

rates

of Ferroine

in an isothermal

and non-isothermal energy of z 100 kJ/mol-

to

behaviour.

find

corresponding

(time=min),

non-iso-

way (Fig.

Kijrbs (4)

of auto-

to activation

Using reactant by Field

the oscillation 1)

has para-

the A value showing a

as proposed during

(l-3).

reactions

- as a series

concentration.

and Ferroine

rather

steps

than to study particular

on the oscillatory

the U.V. absorption

vation

this

kJ/mol and lg A = 10.56

KBr03, KBr, malonic

studied

in the case of the BELOU-

the system - due to temperature

of catalyst

is caused by a.very

are 14 or even more particular

of this

to their

oscillations

amounts

(5),

we

at h = 550 nm

in a net acti-

resulting

.

2LOsec

.283

276

Fig.

1

293

303

Tin

K

U.V. extinction at 550 nm for4heating rate 2.0 K/min; KBr = 0.053; KBrO = 0.1; Ferroine = 5x10 ; malonic acid = 0.375 M; 1 M sulfun -2 acid

DTA studies. Using DTA equipment spezialized we recorded varied rate.

ca.

100 DTA curves

such as concentrations Although

measured

part

were.partially

of

the

reactions

with features of

bromate,

solutions

high,

for

there

in stirred

depending

bromide,

was oscillating, were no direct

solution

(6),

on the parameters

catalyst

and heating

and the calorimetric

enthalpies indications

206

to the oscillations ations

in rate

the lag of our equipment must smooth the fluctu-

since

(kinetic

The influence

cell

constant

Z 0.18 min-l).

of the bromate concentration

on the overall

shows -a step increase from small values ratio of bromide and bromate (0.05 1.1; Fig.

reaction

enthalpy

beginning

at an equimole-

cular

2).

passing

a maximumof Z 500 kJ/mol for demonstration

of the oscillations

crease/The acid

reason

‘chanistic tion

considering

the less

cf.

a tendency

of cooperative

ref.

processes

reactions

Fig.

3).

centration

for

the overall

of. bromous

heat. we tried

of the mechanistic

increasing

coordinates

from the region

small S, M Z M2_order)

reaction

1' S z SI), A + B = C; periodical of mixed-order

conse-

M X M2). x 10m4) leads

of the catalyst

to a continuous

and even to the zone of higher-order

reactions

consecutive

con-

decrease

to the second-order

of zone

when cat =

3)_

the heating

plots

to

of KBr and KBr03; M Z M

A and B) and ending in the region

CeCl3,6H20 (1:25

Increasing

a me-

bromate con-

S and M due to .a movement from the autocatalytic

confusing

while

termination

steps,

hand, we have found that an increase

l-25 x IOm3 M (Fig.

de-

exothermic

bimolecular

For Ferroine,

zone (equimolarity

(S Z SI,

On the other

the formation

data of particular

then passing through the AB zone (formal cutive

the

is a subsequent

is stronly

exothermic

(process A preferred;

move to the first-order change of processes

(1))

for

based on the corrected shape indices S and the reac-

M(7-8;

centration.causes

step,

which reduces

activation

peak analysis

type indices

there

through

between bromate and bromide is not. For even

of HBr02 may prevail

Before

bromate),

(B process;

A-reaction

BrOi concentrations,

reaction

(0.1

which is appropriate

is that the autocatalytic

via the BrO; radicals

the concurrent higher

a composition

After

rate

from 0.3 to 4.3 K/min partially

in the mechanistic

leads

to

diagram (Fig. 4). Obviously, the change

of the bromate concentration from 0.1 t.1 (standard) to 0.12 I1 (catalyst: Ferroine) course-

causes

the line

of increasing

rate

to show a reversing

For the moment, the question remains of how many steps have to be

set up to give chani sms , it

such a phenomenon_ Considering

is obvious

ing first-order, i4 values if

that concurrent

the heating

rate

concentration,

it was confirmed the ceric

oscillations

salt,

two-reaction

the prior

me-

of which be-

type, would lead to decreasing

is increased. activation

that there

wherein EB is minimum and nearly 68 kJ/mol for

possible

reactions,

the later of second-order

From the trend of the overall

tions,

heating

independent

80 kJ/mol for

may be favourably

energy,

EB, with catalyst

is a range (10T3 to 1O-4 M) of this Ferroine).

performed.

concentration. Under these

(E

-

conds-ii

207

kJ/mol 500-DH

X LOO_

Ferroine

X

300200-

IOO-

(KE3r concentration)

Fig. 2

0.15 M

K!3r03 -

Overall reaction enthalpy for different KBr03 concentrations \\

25-

T

‘i,

)$rder \ \ \

22-

103.M ZOklllmol-K

line

Ia16-

12m = 1.5 Klmin

10-

123.10-3

0.2

Fig. 3

Mechanistic

103M

I

0.6

I

Mechanistic

I

1.2

1.0

-

\

\

x Ferroine. Karol. 0; M V----- + Ferroine. KSrO: 0.12 M *----0 Cerlum. K&O, OlM Numbers: Heatmg rote \

! a5 0

1.0 shape index ___c

Fig. 4

I

0.8 shope index S -

points vs. catalyst concentration

I 30

I

0.f.

1.5

points vs. heating rate

1.2

208 Mechanism: A+C=B A+B=2B+D D=C+E

m = 1.38 K/min ICI, = 0.053 [AlO = 0.1

E 49.3 117.6 58.5

16.7 92.7 175.3

E/Ig A 7.06 5.97 6.97

359K

323K fig.

AH

5.

Computer reconstructionof an individualDTA curve by the use of

a three-step

mechanism

TABLE:. Activation

data

and

other

quantities

for the particularreactionsof the

three-stepmechanism (cf. Fig. 5) Time unit: min lKBrl = 0.053 rd Activationenergies used:O El = 49.3 E2 = 79.7 E3 = 41.8 kJ/mol 21 experimentswith cat = Ferroine; CKBr031,= 0.04, 0.06, 0.075 or 0.10 $1 (0.10 in 5 cases) Heating rates: O-6-1.6 K/min Correlation: R = 0.970 -+ 0.036 step -AH kJ/mol 19 A k298 1 2 3

7.59 i-0.34 13.43 T 0.61 5.17 z 0.38

0.097 0.33 0.008

8 - 130 6- 80 15 - 197

7 experimentswith cat = CeC13-6H20;IKBr03Jo= 0.06, 0.10, 0.12 or 0.14 Heating rates: 0.5 - 1.5 K/min Correlation:R = 0.964 2 0.044 step -AH kJ/mol 19 A k298 1 z

7.57 + 0.43 12.93 5.18 T 0.39 0.32

z

0.092 0.008 0.105

415 42 72 - 238 133

209

of the DTA curves by computer.

Reproduction

All curves (9) and compared

were

both evaluated

with

a computer-generated

out 50% of all curves simple

bimolecular

better

than 0.99

340 K. The

obtained

of BORCHARDT and DANIELS

by the method

may

curve

(LO). bJe found

be reproduced

that ab-

by the assumption. of a

rate law since correlation coefficients, then, were including

remainder

the total measuring

of the curves

which

range

mostly

from about

involved

275 to

special

condi-

tions due to catalyst concentration, heating rate or bromate concentration, could

be reproduced

by the following

three-step

in many cases

mechanism

(Fig, 5 and Table), 1.

Bimolecular initiating

2.

Autocatalytic (initiating) step

3.

Bromide-regenerating

A = BrO;;

shows,

of SKRABAL

there

A+C=B

D=

catalyst;

will often

and WEBERITSCH

son. The enthalpies

when the

be a good fit when

our fitted

data

on the

much more complicated tely rate-determing be a too simple

is

heating

required. rate

steps of three

steps, however,

This

indicates groups

for EI the

since

the

catalyst

show enormous

and a certain

that

dependence

of

the true

mechanism is

assumed

are approxima-

of reactions.

to take the five-step

model

using

product(s).

mean value B no material for compari-

and that the three reactions

concept

a true non-isothermal

fit

B)

(cf. (12)) and for E2 the E

used for the individual

best

A)

C+E

C = Br-; E = final

of our AB-simulations, while for step 3 we found variations

(= process

A + B = 28 f D (= prokess

step

B = HBr02,- D = oxidized

As the table value

step

OREGONATOR

effects

It seems even to mechanism

cannot

(12) as

be explained

by it. REFERENCES 1 2 3 4 5

6 7 8

1: 11 12

B.J. Field and R.M. Noyes, J. Chem. Phys. 60 (1974) 1877-1884 D. Edelson, R.J. Field and R-M. Noyes, Int. J. Chem. Kinetics 7 (1975) 417-432 I. Lamprecht and B. Schaarschmidt, Thermochim. Acta 22 (1978) 257-266 E. Kdriis, Nature 251 (1974) 703-704 R.J. Field, Chemie in unserer Zeit 7 (1973) 171-176 E. Koch, Chem. Ing.-Techn. 37 (1965) 1004-1010 E. Koch, Non-isothermal Reaction Analysis, Academic Press, London, 1977, espec. Chapters 4 and 6 III E. Koch and B. Stilkerieg, Thermochim. Acta 17 (1976) 1-15 H.J. Borchardt and F. Daniels, J. Am. Chem. Sot. 79 (1957) 41-46 E. Koch, Therm. Anal. Proc. Int. Conf. (5th), Kyoto, 412 E_ Koch and B. Stilkerieg, Thermochim. Acta, in press; J. thermal Analysis, in press R.M. Noyes, R.J. Field and E. Kiiros, J. Am. Chem. Sot. 94 (1972) 8649-8663