a kinetic study of the chemical oscillating system comprising mannitol-acetone-bromate-h2so4
TRANSCRIPT
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8/11/2019 A Kinetic Study of the Chemical Oscillating System Comprising Mannitol-Acetone-bromate-H2SO4
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Indian Jo
urn
al of Chemistry
Vo l. 4 1A, March 2002 ,
pp
. 532-536
A kinetic
st
udy of
th
e chemical oscillating
system compris
in
g mannitol-ace
ton
e
bromate-H
2
S0
4
Shi-G ang Sh en' , Han-Wen Sun, Jin-Hu an Shan
& JL n -li L
iu
Collcge
of
Ch emical
and
Environm
en
tal Sc ience Hebei
University. Baod ing 071002. P. R. China
Rl'Cl il'ed
24
Ma .l 1; rel'ised 16 OClUbl.'r 2001
A new type of chemical osc
ill
at in g system comprising
lll
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NOTES
533
Tab le I- The in
iti
al co
nc
entra
ti
on range of
th
e reacta
nt
s a
nd
tempera ture range
in th
e osc
ill
ating system (Concentration unit : mol dn'-
')
IAc tlo
0. 17-4.0
1.02
1.
02
1.
02
1.
02
1.02
.
[MAlo
IBrO
,'
lo
0.01 88 0.06
75
0.009-0.05 I 0.0675
0.
01
88
0.Q3 8-
0.1
35
0.01 88
0.0675
0.0188 0.0675
0.01 88
0.0675
[Mn
2
+1o
[H
2
S0
4
1
0
T K
0.0025 2.58
298
0.0025 2.58 298
0.0025 2.58
298
0.0001 25- 0.0250
2.58 298
0.0025
1.
30- 4.
10
298
0.0025
2.58 293-323
Time,s
Fig. I-The osci11ating curve of the potential with time [System: [Actlo = 1.02 mol dm'), [MAlo = 0.0188 mol dm') mol dm ,
[8r0.
1'10=
0.0675 mol dm , [Mn
2
+10 = 0.00250 mol dm
3
, [H
2
S0
4
1
0
= 2.58 mol dm
3
, temp.=298 0.1 K].
and the system exhibited a periodic oscillation
between
brown and
pale yellow. n the latter stage,
the solution colour became light, and the oscillating
cycle did not change much during the whole
oscillating process. With time, the oscillating
amplitude began to
show
a trend of
slow
increase, and
after a period
of
stable amp litude, the amp litude
began to
decrease gradually
until
the
oscillation
stopped.
Th e effect
of
temperature
and
the apparent activation
parameters
The mannitol-acetone-bromate-H
2
S0
4
chemical
oscillating system is very sensitive to temperature
changes. When the temperature increased, the
induction period tieS), oscillating cycle tp(s), and
oscillating life tt( s) decreased regularly.
A very good linear relationship was obtained
when fitting In(t
;
t
s
), In(tpt
s
) and In(tt s) with
I K
(shown in Fig. 2) , the linear correlation
coefficient
s
are greater than 0.
995,
and the
corresponding
lin
ear
equations are:
In(t; s) = E/RT
+
A;
In(tp s) = -Er/
RT
+ AI'
In(tl s) =
-E /RT
+ A,
in
which A;, A
I
and A, are the intercept
of
the line,
E;/R
,
Ep/R and E,
/R
are the slopes
of
the fitting line,
respectively. Compared with the Arrhenius equation
In (k) = EAIRT + A, (;-', tp l, and
t,
-' are very similar to
E
-5 A
c
-6
=
-7
3.
05
3.10 3 .15 3 .20 3 .25 3 .
30
3 .35
3.40
3.45
Fig. 2- The plot
of
In (f1s) with
r lK
A:
In
(tj l
s
) with
r lK:
8 :
In(tp
l
s
) w
ith
r K; C:
In(t, s) with
r
K.
[S
yste
m: [A
c
t]
o =
1.
02
mol dm-\ [MAlo = 0.0188 mol dm
3
,
[8r03'
]0= 0.0675 mol dm .
[Mn
2
+]
o = 0.00250 mol dm-
3
, [H
2
S0
4
]O
= 2.58 mol dn'-' ]
the reaction rate constants,
but
E
Ep and
E
should be
con-esponding to
the activation parameters, which are
called the apparent activation
parameter
in thi s paper,
and their values are E = 45 . 14 kJ mol , Ep = 60.20 kJ
mol , and E 67 .30 kJ mol , respectively. Th e results
are in good
agreement
with the inves
ti
gations of An
et al.
6
who obtained practically the apparent energy
of activation (Ep=61.03kJ mol ) for the
BZ
oscillating
system containing H
2
S0
4
, BrO}', lactic acid, acetone
and Mn
2
+.
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534
I
NDI N
J CHEM. ,
SEC
A,
M R
CH 2002
Tli
e /j
e lS
/,tlie reactallt cOllcelltratioll
It
is found from the experiment that both
th
e
induction period tieS ) and th e osc illation cycle tp(s) of
the chemical osci lIati ng reaction are affected by th e
concentrati ons
of
th e reac tants; In (t/s) and In (t,,/s)
have good
lin
ear relationship with
In
([A ct]om
or'
dm'},
In
([MAl
o
mor'
dm' ),
In
([BrOJ' ]o mor' dm\
In([Mn2 ]o mo
r'
dm\ and
In
(
[H
2
S0
4
]0
mor'
dm' ) in
th e concen tration range as shown in Table
I.
Their
lin
ear relationsh ips can be expressed as:
In(t;ls) = ai+bi In ([Act]o
mor'
dm' ) +Ci In ([M
A]o
mo
r'
dm' ) +d
i
In[Br0
3'
]0 mor ' dm
3
)
+ ei InClMn 2 ]u
mor'
dm' ) + f I n [ H S mol dm' )
In
(t
i s )
,=
ap+b
p
m
~ m +C
p
In
([MA]o
mol dnl') +d
p
In[BrO, ]0 mol dnl' )
+ e
p
In ([Mn
2
]0
mor ' dm' ) + fp In[H
2
S041o mol dm\
I
Based on
th
e expe
ri
rnental data (w
hi
ch are
compose d
of
37 experimental data, eac h represents
the average of three parallel experim ental results), th e
correlati on coefficie
nt
s in
th
e above two formulae can
be
determined
by
plural
lin
ear fitting: ai = 6.88, b
i
=
-0.48, Ci = - 1.1 5, d
i
= 1.03, ei = -0.18, fi
=
-2.94; a
p
=
5.10, b
p
= -0.55, c
p
=
-1.48 , d
p
=
0.93, e
p
=
0.70,
1 1 =
- 1.28. The plural linear correlation coeffici ent s are
0.999 and 0.998, respect ively. Therefor
e, th
e
relationship of
th
e induc
ti
on pe
ri
od
ti eS)
and
oscillating cycle tp(s) with
th
e initial co
nc
entration
of
th
e reactan ts can be ex pressed a
s:
t (s )=969[ Act ]
O,OA8
[M A]0, ' ,'5
B
1'0
3
]0
OJ
[M n
2
]o,O'8
S
~ \
m o l dm
'J
)'
72
s
tp(s)= o , o s s A [ B r 0 J i ) ~
[M n2 ]o070[1-I
2
S04h ' 2s (mol
dm
) 6 8
s
It
can be seen from the above relationship , that
increas
in
g IAc
t]
o, IMA10 and
can
in
crease
th
e rate a
nd
sho
rt
en induction pe
ri
od t
ieS)
and
oscillation cyc le tp(s);
In
creas
in
g IMn
2
+Jo can sho
rt
en
th e induction period tieS) and elongated th e oscillating
cyc le tp(s);
In
creas ing [BrO, ' ]0 can elongate th e
induction period t
ieS)
and osc ill ating cyc le tp(S) .
Th
e j
illl
ctions of reacta
nt
s
Keep ing other reaction conditions consta
nt
, water
was added in place of acetone. It was found th at
adding potassium bromate solution produced large
quantities
of
Br2. The system colour changed to
brown and gave off a browil gas. After th e induction
peri od,
th
e potenti al decreased witho
ut
go in g up , and
th
e brown colour did not disappear, and no osc illating
react ion took p
la
ce. The ex istence
of an
induction
period demonstrates that
th
e accumu lati on process
of
HBrOz ex ists
in th
e osc illating
t c m . In
acidic
med ium, under th e cata lytic condit ion
of
Mn
2 , an
oxidation-reduction react ion takes place between
BrO, ' and MA produc
in
g Br2
wh
ich causes
th
e
solution colour to change to brown. The existence
of
large amount
of
Br2 preve
nt
ed
th
e accumulation of
HBr0
2. The brown colour
di
sappeared quick ly after
adding acetone and the osci llation took place . When
nitrogen was rapidly bubbled through
th e sys tem with
no acetone added, some osci Iations were observe
d.
So one
of th
e main functi on of acetone is to eliminate
excess ive of Br2 and to produce Br' simultaneously,
I e.,
Br' c
an
be oxidized
by
BrO
,'
,
wh
ich is favourable
fo
r
th
e accumulati on
of
1-1 Br02, so this is an
osc
ill
a
ti
on switch.
In
creas
in
g
th
e amo unt
of
acetone
increased
th
e acc umulati on, and
th
e induc tion period
became shorter. This accelerated
th
e whole oscillating
process, and the osci Iati ng cycle became sho rte
r.
Keeping the oth er conditions constant , adding
small amount of bromoacetone shortened th e
induction period . It indicates th at th e accumulati on of
bromoacetone is very important dur
in
g
th
e induction
period. Bromoacetone was partly oxid ized
by
Mn
'+
to
produce Br', which was fav ourabi e for
th
e
accumulat ion of HBr02 and shorte ned
th
e induction
period. The reaction process is as fo
ll
ows:
Mn
3
++CH,COC I-I
2
Br--;; Mn
2
+fBr'+Oxidation
Pro duct I .. . (2)
In
th e same way, water was added in th e system
re
pl
ac
ing MA. The so lution colour changed to pink
after adding bromate solution. The potenti al did not
decrease
af
ter th e first increase and no osc illating
reaction took place. Then MA wa s added agai n,
th
e
so luti on colour changed to brown, and th e osc ill
ati
ng
ph eno menon took place.
It
indi cated that Mn
2
+ was
regenerated through reduction
of
Mn
3
+
by
MA , i.e.,
Mn ]+ + MA
--;; . .
Mn
2
++ Oxidation product
II
... (3)
If without MA, [Mn' +] (or pote
nti
al) did not
decrease , no osc
ill
at in g reac tion could take plac
e.
Moreover, if
MA
wa s added again after
th
e osc illati on
stopp ed,
th
e oscill a
ti
on reac tion took place again. Th is
indicates th at MA was cons
um
ed in the reaction and
could not be rege nerated, w
hi
ch ac ted
as th
e reaction
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NOTES
535
substrate. MA played the role
of
the reductant and
participated in the formation of Br2 at the same time.
So with the increase in the initial concentration of
MA, the rate of regeneration of Mn
2
+ through the
reduction of Mn
3
+ by MA increased, and the induction
period t;(s) and oscillating cycle tp(s)
became
shorter. .
On replacing
Mn
2+ by water, though no oscillation
occurred, the solution colour changed to light-brown
gradually. This indicates that though there is no Mn2+
as catalyst, Br0 3 can partly oxidize MA.
This
is
because alcohol is more easily oxidized than acid
6
.
7
.
This reaction shown as processes E in the following
section is the side reaction
of
the oscillating reaction,
consumes part
of
MA. In the presence
of
Mn2+,
Br0
3
and MA reacted rapidly to produce Br2, so that the
colour of the solution changed rapidly to brown. Here
Mn
2
+ plays the catalytic role, which is shown n
mechanism
in
the following section.
Br2 produced was removed in reaction
I)
to
produce B( at the same time. The B( thus formed can
be oxidized by Br03 which is favourable for the
accumulation of HBr02. The reaction process 10 took
place as process A.
At the beginning of the reaction (the induction
period), the concentration of
B (
is very low. The
catalytic process takes place predominately. With the
increase
in
the concentration of Mn2+ , it may cause
Reaction (II) and 10) proceed very fast and more
Br
2
is produced.
This
further causes reaction 1) to
produce more
B(
and then reaction (4) accelerates the
accumulation of
HBr0
2. The
time to reach the critical
value o( the oscillation is very short, so the induction
period t;(s) becomes shorter with the increase in
[Mn
2
+]o.
At the beginning of the oscillation, Mn
3
+
returns to Mn
2
+ through reactions (3) and (2). When
[Mn2+]o is large, though the rate of reaction (3)
becomes faster, every oscillation reaction consumes
large amount of MA, causing [MA] to
become
lower;
and during each cycle the time of
Mn
3
+
changing
to
[Mn2+] becomes longer with the increase in [Mn
2
+]o
;
so the oscillating cycle tp(s)
becomes
longer with the
increase in fMn2+
].
When
Br03
is not in the reaction
sys tem, the solution colour does not change, and no
oscillating phenomenon takes place. If Br03 is added
at the last stage of the oscillating process, the
oscillation restarts. This indicates that in the
oscillating system, Br031 is consumed and not re
formed. Without the catalysis of Mn2+, BrO) can still
oxidize part
of
MA and
consume
part
of
MA ,
(Process E) . So , on increas in g the concentration of
Br03 , more
MA
is consumed. Therefore, with the
increase in
[Br03
], the induction period t;(s) and the
oscillating
cycle
tp(s)
become
longer.
The effect o he radical inhibitor
Keeping other conditions constant, acrylonitrile
was added into a reaction system, the oscillation
stopped immediately. Also on adding ethanol into the
reaction system, the oscillation stopped immediately
after a few oscillations. Acrylonitrile and ethanol are
both radical inhibitors; it indicates that radicals are
involved in the oscillation reaction. It is reported \ J
that the possible radical reaction process may tak e
place as process B.
iscussion o he oscillating mechanism
Based on the above discussions, it is proposed that
the system may undergo the following five
processes
1.12:
Process A:
.. . 4)
HBr0
2
+
B(+ W
HOBr
. .. 5)
... 6)
. .. I )
Process B:
... (7)
... 8)
... 9)
4 M n 2 H ~
4Mn
3
++HOBr+HBr0
1
II ) ,
Process C: Mn
3
+ transformed to Mn
2
+ through reacli ons 3)
and (2)
Process D: B( was rege nerated by reactions ( I) and (2)
Process E: BrO)' oxidized MA through reactions (4) , (5) a
nd
10)
shown as follows:
H O B r M A ~ 1I2Br2
+0xidation
Product
I .. . 10)
The existence of the induction period indicates that
the mechanism is an automatic catalytic process
dominated by Br02 produced in reaction (8) during
Process B. The catalyst is
HBr0
2. B( plays the role
of
kinetic control.
The
multitude
of
[Br'] determined th e
rate
of
the self-catalytic oxidization. When fBr' ] is
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536
INDI AN
J
CHEM., SEC. A, MARCH 2002
large enough, Process A
dominates
, and Process B is
inhibited, which causes the formation rate o f
Br0
2to
become slower.
The
outcome of Process A is to make
Process B dominating by removi ng
B
fro m system.
The proceeding of Process B results in [Mn
3
]
in
Reac tion
(9)
increasing constantly, which causes
Process C to proceed leading to regeneration
of
Mn
2
+.
At the sa me time,
B
is formed through Process D.
These processes go round and round, the oscillating
phenomenon of [B ] or [Mn
3
+]/[Mn
2
+] begins to take
place.
eferences
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Z,
Zhao H X,
Xu
Y T Zang Y R, HI/ax l e
TOlI
gbao
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avel/illg waves ll
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