Solvent effect on reaction rates in molten salts - Springer Link

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Chemical reactions in molten salt solvents are of great interest at the moment. Experimental data on the subject are presented in some detail, e.g. in Refs. [I-3].
React. Kinet. Catal. Lett., Vol. 46, No. 2,

SOLVENT

EFFECT

ON

REACTION

S.A. Institute

of G e n e r a l

RATES

301-304 (1992)

IN M O L T E N

SALTS*

Kirillov

and

Inorganic

252601,

Chemistry,

Kiev

USSR

Received July 8, 1991 Accepted October 8, 1991

Predominant

solvation

to be c h a r a c t e r i s t i c reactions

~

p~C~B~eHHNX

tion

Chemical

reactions

at the m o m e n t .

of

in some

these

additions as

to

solvents

their The nism

reactions

effect

of

The

considerations This

effect

salt

e.g.

was

the

study

active found

containing

weakly

containing

strongly

are

accelerated

[7]

been

rate

solvents

actions

solvents

has

by t h o s e

[5]

and

particles

was

polarizing

or e v e n

* P r e s e n t e d at E U C H E M C o n f e r e n c e 2-8, 1990, R i o - P a t r a s , G r e e c e

change

salts

definition

rate

of

in Ref.

[6].

and mecha-

the

effect

reactions

showed are

cations,

on M o l t e n

small

and

of to

formation. who

cations

polarizing

and

of m o l t e n

to a n a l y z e

complex

are

insufficiently.

of c h e m i c a l

by D u k e ,

of g r e a t

subject

given

on the

studied

CO~5BaTalJ~Mg

[I]

the

was

B

A classifica-

in Ref.

salt

first

[I-3].

properties

in Ref.

on the on

in R e f s

peaK~MM

are

on the

Dynamic

active

present

solvents

data

suggested

[4].

reactions

solvents

3~eKTpOXMMMHeCKMX

in m o l t e n

determined

a i m of the

salt

electrochemical

~peMMymeCTBeHHa~

was

the m o l t e n

of c h e m i c a l

and

is s h o w n

xapaKTepHa

detail,

it in Ref. were

M

Experimental

electrochemically

molten give

COdeX

KOM~eKca.

presented

chemical

complex

salts.

XI4MMqeCKMX

aKTHBHOFO

interest

for

in m o l t e n

~OKa3aHO,~TO

of a c t i v a t e d

their Salts,

that

when

substituted acid--base

re-

mechanism

[8].

September

Akad4miai Kiad6, Budapest

KIRILLOV: SOLVENT EFFECT

Duke

attributed

cationic

field

ic radii. tivity

The

these that

and the c a t i o n i c suggested

Gibbs

activation

energy

rate

ions d e p e n d s correlation

et al.

between

on the i n v e r s e

cationic

for o t h e r

of s m a l l e r

the

of m o l t e n [9]. They

for e l e c t r o c h e m i c a l

also h o l d s

of r e a c t a n t s

for the c a t i o n s

relation

composition

by F l e t c h e r

This

to the a c t i v a t i o n

is s t r o n g e r

semi-empirical

first

melts

effects

solute

salt

radius

was

that the

reduction

chemical

ion-

reac-

solvents

showed

by

of n i t -

of solvent.

reactions

in

[10,11]. Let us m a k e

Gibbs

activation

tween

the G i b b s

use of the t r a n s i t i o n energy

is c o n s i d e r e d

energies

of a c t i v e

state

theory,

where

to be a d i f f e r e n c e

complex

the

be-

X # and reactants

A. 1

AG*~

=

GX#-

ZGA. 1

This

gas

equation

phase

is,

processes.

strictly

For

l

speaking,

reactions

applicable

in s o l u t i o n s

AG~,

only

to

GX# and

GA. are c o n s i d e r e d to u n d e r g o c h a n g e s due to the i n t e r a c t i o n l of a c t i v e c o m p l e x and r e a c t a n t s w i t h s o l v e n t (solvation)

AG#

If the have

= GX,

solvation

AGsolvx#

media will tionic

- G s o l v X* -

to the gas phase, The solvents

302

(stronger case

solvation

in c o n d e n s e d

and the

ionic

we w i l l

smaller

interactions)

the r e a c t i o n

rate

cawill

increases.

of r e a c t a n t s

< E A G s o l v A. ) i n c r e a s e s 1

is p r e d o m i n a n t ,

in the gas phase,

solvent In this

The p r e d o m i n a n t

complex

IAGI.soiv" A.•

A.I ; AG # for r e a c t i o n s

than

of salt

be e v e r - d e c r e a s i n g .

(AGsolvx,

of a c t i v e

> ZAGs~

be lower

radii

( ~GA'I l -

AG#

in m o l t e n

media

compared

1

and hence

reactions are l i s t e d

whose

decreases

rates

below:

were

the p r o c e s s

studied

rate.

at least

in t h r e e

KIRILLOV: SOLVENT EFFECT (I)

NO~

>

NO~ + 1 / 2 0 2

[10,12,13]

(2)

C104

>

C1- + 2 02

[11]

(3)

C104

C15

C1- + 2 02

[11]

4NOj-ClO~

~

4.o F +el

[10,14]

5)

NO2 + I/2 0 2

>

NO;

[12,13,15]

6)

ClOy + I-

>

IO~ + el-

[16]

(4)

7)

BrO3

Br$

Br- + 3/2 0 2

[7]

8)

2BrO; + Cr20~-

>

Br2 + 5/2 02 + 2 CrO~-

[17]

9)

P2ot+2

>

2 Po~

E18]

>

NO2 + O 2-

(I0)

No]

NO; + 2 e-

+ 2 No~ + i/2 o 2

[9]

0@8~ I

0m88 I

-o-.._o.i~ 320

154 ~O-..

1901

152

A

E 300

O

180 I

%

6 [ ~

21(]

r

1.3

1.4

1.0

1.4

140

200

100 0.7

0.8

0.9

1.0 1/r,(~ -1)

Fig.

I. Dependence of activation free energies for reactions 5-7 (left) and 8-10 radius of the solvent

(right) on the cationic

303

KIRILLOV: SOLVENT EFFECT

Gibbs

activation

been c a l c u l a t e d cation radius Refs.

energies

and AG # values

of the solvent

[10,11]).

are plotted

(Fig.

gies for all these reactions

complex

signs of their charges. as c h a r a c t e r i s t i c

the inverse I-4,

solvation

ener-

cationic

of the a c t i v a t e d

in addition

contains

to r e a c t i n g

species having opposite

This p h e n o m e n o n

for m o l t e n

see

I, Gibbs a c t i v a t i o n

In other words,

the a c t i v a t e d

have

decrease with d i m i n i s h i n g

i.e. p r e d o m i n a n t

complex takes place.

against

I; for reactions

As is seen from Fig.

radius of solvent,

species

for all these reactions

should be c o n s i d e r e d

salts.

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V.D.

Prisyazhnyi,

S.A. Kirillov:

Ionnye Rasplavy,

3,

82

(1975). 2.

D.H. Kerridge: VB. A c a d e m i c

3.

The C h e m i s t r y

Press,

I.K. Delimarskii: Naukova

Dumka,

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S.V. Volkov:

New York

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Kiev

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Zh., 55,

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S.A. Kirillov:

6.

S.I. Chernukhin, Safronova:

Elektrokhimiya,

7.

F.R.

Duke,

E.A.

8.

F.R.

Duke,

W.W.

9.

A.N.

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126,

1496

S.A. Kirillov,

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(1989).

51, 820

916

M.L.

Chan:

66,

2114

Soc.,

(1962).

83,

1271 (1961).

J. Electrochem. Soc.,

(1979) .

S.A. Kirillov,

L.L.

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Ukr. Khim. Zh.,

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S.A. Kirillov,

L.L.

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React.

287

I.M.

(1982).

Phys.,

J. Am. Chem.

M.H. Miles,

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V.D. Prisyazhnyi,

10.

37,

(in Russian)

1980.

Khim.

Ukr.

vol.

1978.

The C h e m i s t r y

Ukr.

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Kinet.

54,

582

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E. Freeman:

J. Phys.

13.

E. Freeman:

J. Am. Chem.

14.

F. Paniccio,

P.G.

Chem.,

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838

J. Phys. 72,

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Chem.,

77, 1810 (1973).

15.

D.W.

James:

16.

F.R.

Duke:

Fused Salts.

McGraw-Hill,

17.

F.R.

Duke,

J. Schlegel:

J.Phys.Chem.,

18.

J.L. Copeland, A.S. Matculf, B. Hubble: J.Phys.Chem.,80, 236 (1976).

304

J. Phys.

60,

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