Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium

20 Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium

Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch020

T. W. NEWTON and F. B. BAKER University of California, Los Alamos Scientific Laboratory, Los Alamos, Ν. M.

The experimental observations on the actinide oxidation -reductionreactions are described, and the empirical results are tabulated. The rate laws have been interpreted in terms of net activation processes, and these have been tabulated togther with the associated activation parameters—ΔF*, ΔΗ*, and ΔS*. An electrical analog is described which has been useful in interpreting complicated rate laws. Empirical corrections have been found between the formal entropies of the activated complexes and their charges, and for sets of similar reactions, between the hydrogen ion dependence and ΔF°, between ΔF* and ΔF°, and between ΔΗ* and ΔΗ°. The kinetic and physical evidence for binuclear spe cies is discussed.

"ranium, n e p t u n i u m , p l u t o n i u m , a n d a m e r i c i u m are r e a s o n a b l y stable ^

i n aqueous solutions as ions w i t h o x i d a t i o n states r a n g i n g f r o m 3 +

to 6 + . I n a c i d solutions the 3 + a n d 4 + states exist as h y d r a t e d cations s u c h as U

3

+

and U

4 +

. Am

4 +

has n o t b e e n d e t e c t e d , p r e s u m a b l y because

i t is too u n s t a b l e w i t h respect to d i s p r o p o r t i o n a t i o n . T h e h i g h e r o x i d a t i o n states exist as o x y g e n a t e d " - y F ions s u c h as U 0

2

+

and U 0

2

2 +

.

This situa

t i o n leads to m a n y possible reactions a m o n g t h e a c t i n i d e ions a n d w i t h other o x i d i z i n g o r r e d u c i n g agents.

T h e s t u d y of t h e k i n e t i c s of these

reactions is i m p o r t a n t because i t aids i n u n d e r s t a n d i n g the m e c h a n i s m s of o x i d a t i o n - r e d u c t i o n reactions a n d t h e factors w h i c h d e t e r m i n e t h e i r rates. I n a d d i t i o n , i n f o r m a t i o n is p r o v i d e d w h i c h is u s e f u l i n t h e d e s i g n of s e p a r a t i o n processes a n d a n a l y t i c a l p r o c e d u r e s . 268

Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

20.

N E W T O N

A N D

B A K E R

Oxidation-Reduction

269

Reactions

T h i s subject, or parts of i t , has b e e n r e v i e w e d p r e v i o u s l y b y a n d S e a b o r g ( 3 5 ) , H i n d m a n ( 2 6 ) , R a b i d e a u et al. (69), Rabideau (52).

Katz

and Newton and

T h e present r e v i e w is justified b y the l a r g e a m o u n t of

w o r k w h i c h has a p p e a r e d since 1959. T h e l i t e r a t u r e has b e e n s u r v e y e d to S e p t e m b e r

1966.

The

recent

w o r k ( s i n c e 1959) is discussed i n this section a n d o r g a n i z e d a c c o r d i n g to the elements i n v o l v e d . T h e q u a n t i t a t i v e observations are s u m m a r i z e d i n T a b l e I.


Reactions of Uranium Ions E x c h a n g e R e a c t i o n s . M a s t e r s a n d S c h w a r t z (43)

s t u d i e d the U ( I V ) -

U ( V I ) exchange i n a c i d p e r c h l o r a t e solutions. A t l o w U ( I V )

concentra-

tions i t w a s c o n c l u d e d t h a t the exchange i n v o l v e s the U ( I V ) - f =

2U(V)

e q u i l i b r i a , w h i l e at h i g h e r U ( I V )

U(VI)

concentrations, i n agree-

m e n t w i t h a n e a r l i e r suggestion b y R o n a ( 7 5 ) , a p a t h h i g h e r o r d e r i n U ( I V ) is i n v o l v e d .

A q u a n t u m y i e l d of about 0.01 was e s t i m a t e d for

the exchange i n d u c e d b y u l t r a v i o l e t l i g h t . T h i s exchange has also b e e n s t u d i e d i n aqueous H C l - e t h a n o l solutions (44) ( 2 ) . G o r d o n a n d T a u b e (17)

a n d i n sulfate solutions

h a v e s h o w n that the s l o w U 0

2

2 +

-solvent

o x y g e n exchange is c a t a l y z e d b y U ( V ) . O x i d a t i o n s of U ( I V ) . T h e reactions of P u ( I V ) (53)

a n d of C e ( I V )

( 3 ) w i t h U ( I V ) w e r e f o u n d to b e s i m i l a r i n that b o t h a p p e a r to i n v o l v e a n a c t i v a t e d c o m p l e x of the c o m p o s i t i o n

[U(OH) M]

or P u ) i n spite of the fact t h a t C e ( I V )

is p r e d o m i n a n t l y C e O H

P u ( I V ) is P u

4 +

in 1-2M H C 1 0

4

2

6 +

w h e r e M is C e 3 +

and

solutions.

T h e r e a c t i o n of T l ( I I I ) w i t h U ( I V ) has g i v e n anomalous results. It w a s first r e p o r t e d b y H a r k n e s s a n d H a l p e r n ( 2 5 ) to be first o r d e r i n b o t h U

4 +

and T l

3 +

a n d b e t w e e n inverse first a n d inverse second order i n H . +

I t was p r o p o s e d t h a t a 2-electron o x i d a t i o n p r o d u c e d U ( V I ) a n d T 1 ( I ) i n a single step. It w a s also n o t e d that the r e a c t i o n is m o d e r a t e l y i n h i b i t e d b y A g ( I ) , a n d that the u s u a l second-order plots d i d not pass t h r o u g h the o r i g i n . Jones a n d A m i s ( 3 3 ) , i n c o n n e c t i o n w i t h t h e i r s t u d y of the r e a c t i o n i n m e t h a n o l - w a t e r m i x t u r e s , r e p o r t that t h e i r d a t a i n w a t e r s o l u t i o n c h e c k a c c e p t a b l y those of H a r k n e s s a n d H a l p e r n . H o w e v e r , L o v e et al. (41),

i n connection

w i t h the effect of t a r t a r i c a c i d o n the r e a c t i o n ,

r e p o r t rate constants w h i c h are less t h a n h a l f those of H a r k n e s s a n d H a l p e r n under identical conditions.

T h e y h a v e also s h o w n catalysis b y

F e , b u t since the e a r l i e r results w e r e i n d e p e n d e n t of the i n i t i a l r e a c t a n t concentrations, i r o n c o n t a m i n a t i o n cannot a c c o u n t for the Wear

(96)

discrepancy.

has s t u d i e d the r e a c t i o n over e x t e n d e d ranges of

concentrations

reactant

b u t reports rate l a w s w h i c h are inconsistent w i t h his

observations.


270

LANTHANIDE/ACTINIDE CHEMISTRY

Shastri et al. (79)

r e p o r t that the o x i d a t i o n of U ( I V ) b y N p ( V ) is

complicated b y further reaction between U ( I V ) and N p ( I V ) .

Prelimi-

n a r y results u s i n g the analogous o x i d i z i n g agent, P u ( V ) , also

showed

c o m p l i c a t e d k i n e t i c s (62). S u l l i v a n et al. (87)

N p ( V I ) , o n the other h a n d , was s h o w n b y

to give w e l l - b e h a v e d k i n e t i c s ; the rate is first p o w e r

i n the reactant concentrations a n d inverse first p o w e r i n the h y d r o g e n ion concentration. T h e o x i d a t i o n of U ( I V ) b y H 0 2

2

( 4 ) , l i k e that b y 0

2


p r o b a b l y a c h a i n r e a c t i o n since i t is i n h i b i t e d b y C u ( I I )

( 24),

P l a u s i b l e c h a i n carriers are U ( V )

a n d O H . S o b k o w s k i (82)

significantly h i g h e r rate constants

b u t suggested

involved.

H i s mechanism

requires

that U ( V )

is most

and C o (II). reported

that a c h a i n is not disproportionation

be

faster t h a n its r e a c t i o n w i t h H 0 . 2

2

F e d e r o v a a n d K a n e v s k i i (15)

h a v e r e p o r t e d rates of o x i d a t i o n of

U ( I V ) b y S 0 " , C 1 0 " , a n d C K V , a n d the effects of F e ( I I ) a n d V ( V ) 2

8

2

2

o n the latter r e a c t i o n (14).

G o r d o n and F e l d m a n (19)

rate constants

for

U ( I V )-halogenate

Andrews

h a v e d e t e r m i n e d the rate l a w , b u t not the a c t i v a t i o n

(20)

various

h a v e also g i v e n

reactions.

Gordon

and

e n e r g y for the U ( I V ) - B r r e a c t i o n . T h i s r e a c t i o n is c a t a l y z e d b y F e ( I I I ) 2

a n d b y M n ( I I ) , b u t not affected b y P b ( I I ) , C u ( I I ) , N i ( I I ) , n o r C o ( I I ) . R y k o v a n d c o - w o r k e r s (77)

g i v e rate l a w s a n d a c t i v a t i o n parameters for

the U ( I V ) b r o m a t e r e a c t i o n i n p e r c h l o r a t e solutions. D i s p r o p o r t i o n a t i o n of U ( V ) . T h i s r e a c t i o n w a s r e i n v e s t i g a t e d

(59)

w h e n i t was o b s e r v e d that it is g r e a t l y i n h i b i t e d b y U ( V I ). A m o d e r a t e l y stable U ( V ) - U ( V I ) 7370A. (e —

complex

27M cm." ). _ 1

was f o u n d w i t h a n a b s o r p t i o n b a n d

at

R a t e d a t a w e r e e x t r a p o l a t e d to zero U ( V I )

1

concentrations, a n d a c t i v a t i o n parameters w e r e d e t e r m i n e d w h i c h are b e l i e v e d to b e m o r e r e l i a b l e t h a n the p o l a r o g r a p h i c a l l y p r e v i o u s l y d e t e r m i n e d ones (32).

A m o r e recent, c o n t r o l l e d p o t e n t i a l m e t h o d

gives results at 2 5 ° C . i n better agreement w i t h the

(63)

spectrophotometric

ones. Reductions

of

U(VI).

Cr(II)

w a s f o u n d to react r a p i d l y w i t h

U ( V I ) to g i v e a green b i n u c l e a r i n t e r m e d i a t e ( 5 5 )

w h i c h is p r o b a b l y

analogous to the substance w h i c h forms w h e n C r ( I I ) a n d N p ( V I ) C r ( I I I ) a n d N p ( V ) are m i x e d (89, 9 0 ) . to U ( V I ) , U ( I V ) , a n d C r ( I I I )

w i t h a n a p p a r e n t h a l f - t i m e at

r a n g i n g f r o m 4 to 8 m i n . G o r d o n (18) from U 0

2

2 +

to C r ( H 0 ) 2

6

3 +

or

T h e intermediate decomposes 0°C.

has s h o w n that transfer of o x y g e n

is efficient i n this r e a c t i o n .

U ( V I ) has b e e n f o u n d to c a t a l y z e the V ( I I ) - V ( I V ) a n d the V ( I I I ) F e ( I I I ) reactions (58, 60). T h e catalysis is c a u s e d b y the r e d u c t i o n of U ( V I ) b y V ( I I ) or b y V ( I I I ) , f o l l o w e d b y the r a p i d o x i d a t i o n of the U ( V ) p r o d u c e d b y V ( I V ) or b y F e ( I I I ) r e s p e c t i v e l y . T h e V ( I I ) - U ( V I ) r e a c t i o n shows s i m p l e s e c o n d - o r d e r k i n e t i c s essentially i n d e p e n d e n t of


20.

NEWTON

AND BAKER

Oxidation-Reduction

271

Reactions

the h y d r o g e n i o n c o n c e n t r a t i o n . T h e V ( I I I ) r e a c t i o n , d e p e n d s o n t h e h y d r o g e n i o n c o n c e n t r a t i o n a p p r o x i m a t e l y to the —1.8 p o w e r . D e t a i l e d analysis of this d e p e n d e n c e indicates consecutive rate d e t e r m i n i n g r e actions a n d a b i n u c l e a r i n t e r m e d i a t e .

Reactions of Neptunium Ions Reductions of N p ( V ) . S h a s t r i et al. (78) h a v e s t u d i e d t h e N p ( V ) - I ~ r e a c t i o n i n aqueous H C l solutions of ca. 3 M . T h e e m p i r i c a l rate l a w w a s r e p o r t e d to b e - d [ N p ( V ) ] / < f t = fc[Np( V ) ] - [ r ] [ H ] - . T h e u n e x p e c t e d N p ( V ) d e p e n d e n c e m i g h t b e caused i n p a r t b y the fact that t h e i o n i c strength a p p a r e n t l y w a s n o t constant.


0

8 6

1 5 5

+

2

6 1

A p p e l m a n a n d S u l l i v a n ( 1 ) f o u n d a t w o - t e r m rate l a w to b e neces sary to describe t h e r e d u c t i o n of N p ( V ) b y V ( I I I ). T h e p r o p o s e d m e c h a n i s m : V ( I I I ) + N p ( V ) ~> V ( I V ) + N p ( I V ) , V ( I I I ) + N p ( I V ) * ± V ( I V ) + N p ( I I I ) , a n d N p ( I I I ) + N p ( V ) -> 2 N p ( I V ) is n o t i n strict a c c o r d a n c e w i t h t h e data. A m i s p r i n t i n the authors' T a b l e I V has b e e n n o t e d : values for fc s h o u l d b e m u l t i p l i e d b y 100. N

T h e r e d u c t i o n of N p ( V ) b y C r ( I I ) has b e e n s t u d i e d recently b y T h o m p s o n a n d S u l l i v a n (94). T h e r e a c t i o n is c o m p l i c a t e d b y t h e c o n secutive-competitive reaction between C r ( I I ) a n d product N p ( I V ) . T h i s latter r e a c t i o n has b e e n s t u d i e d separately (93). Oxidations of N p ( V ) . D u k e s a n d S i d d a l l (12, 81) h a v e u s e d s o l vent extraction techniques to s t u d y the kinetics of the o x i d a t i o n of N p ( V ) b y H N 0 c a t a l y z e d b y H N 0 , a n d b y V ( V ) , i n H N 0 solutions. T h e f o r m e r r e a c t i o n is i n t e r e s t i n g i n that t h e rate is i n d e p e n d e n t of H N O o c o n c e n t r a t i o n a b o v e 5 Χ 1 0 " Μ a n d of N 0 ~ c o n c e n t r a t i o n a b o v e 2 . 4 M . A h i g h l y p r o t o n a t e d N p ( V ) is suggested as the active i n t e r m e d i a t e . T h e V ( V ) r e a c t i o n approaches e q u i l i b r i u m at rates first p o w e r i n N p ( V ) a n d V ( V ) a n d about s e c o n d p o w e r i n H N 0 . 3

2

3

δ

3

3

S u l l i v a n (91) f o u n d t h e rate l a w for t h e N p ( V ) - C r ( V I ) r e a c t i o n to s h o w i n h i b i t i o n b y N p ( V I ) a n d to r e q u i r e t w o consecutive r a t e d e t e r m i n i n g steps. I t appears that C r ( V ) , f o r m e d i n t h e first step, c a n react w i t h either N p ( V I ) to regenerate the reactants or w i t h N p ( V ) to give C r ( I V ) , w h i c h reacts r a p i d l y to g i v e final p r o d u c t s . T h e t e m p e r a ture d e p e n d e n c e of the rates w a s f o u n d not to agree w i t h the A r r h e n i u s equation. Reductions of N p ( V I ) . T h e rate l a w f o u n d b y Z i e l e n et al. (100) for the H 0 - N p ( V I ) r e a c t i o n is analogous to that m e n t i o n e d a b o v e f o r the N p ( V ) - C r ( V I ) r e a c t i o n . I n this case, t h e r a d i c a l H 0 p r o b a b l y forms i n t h e first step a n d reacts w i t h either N p ( V ) to regenerate t h e reactants or w i t h N p ( V I ) to g i v e p r o d u c t s . 2

2

2


272

LANTHANIDE/ACTINIDE

S h e p p a r d (80)

has s t u d i e d the r e d u c t i o n of N p ( V I ) b y V ( I I I )

f o u n d the k i n e t i c s to be c o m p l i c a t e d b y the subsequent reaction.

CHEMISTRY

T o minimize complications

f r o m this source, rate

w e r e e s t i m a t e d u s i n g d a t a f r o m the i n i t i a l 2 0 - 3 0 %

and

V(IV)-Np(VI) constants

of r e a c t i o n .

Our

c a l c u l a t i o n s s h o w that the error i n s u c h a n estimate c a n range f r o m

4%

for a n i n i t i a l V ( I I I ) / N p ( V I ) ratio of 2 a n d 2 0 % c o m p l e t i o n u p to 1 5 % for a n i n i t i a l r a t i o of 1 a n d 3 0 % c o m p l e t i o n .

T h e r e p o r t e d rate constants

are i n p o o r agreement w i t h the A r r h e n i u s e q u a t i o n . H i n d m a n et al. (28, 29, 30) the N p ( I V ) — N p ( V I )

h a v e i n v e s t i g a t e d the effect of D 0 2

on

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

T h e p r e v i o u s l y d e t e r m i n e d d e p e n d e n c e i n H 0 ( 30 ) c o u l d be i n t e r p r e t e d Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch020

2

i n terms of either consecutive or p a r a l l e l r a t e - d e t e r m i n i n g reactions

(51).

W e h a v e n o w u s e d the n e w e r d a t a a n d a least-squares p r o c e d u r e to c o m p a r e the t w o m e c h a n i s m s .

I n H 0 , consecutive 2

reactions fit the d a t a

better t h a n p a r a l l e l reactions; the r o o t - m e a n - s q u a r e a n d 3 . 8 1 % respectively. and 4.18%.

deviations are 3.63

I n D 0 the c o r r e s p o n d i n g deviations are 7 . 7 9 % 2

It is c o n c l u d e d that, u n l i k e the analogous

U(IV)-Pu(VI)

r e a c t i o n ( 51 ), there is no strong e v i d e n c e for consecutive reactions a n d a b i n u c l e a r i n t e r m e d i a t e . T h i s r e a c t i o n has b e e n r e i n v e s t i g a t e d b y R y k o v a n d Y a k o v l e v (76),

w h o r e p o r t h i g h e r rate constants u n d e r

comparable

conditions.

Reactions of Plutonium Ions O x i d a t i o n s of P u ( I I I ) .

D u k e s (13)

reaction in H C l , H C 1 0 , and H N O 4

H

has s t u d i e d the P u ( I I I ) - H N 0

2

solutions. T h e rate laws i n the first

t w o acids w e r e essentially the same a n d w e r e e x p l a i n e d b y p o s t u l a t i n g NO

+

+

P u (III)

=

NO +

the presence of NOf it m a y be N 0 2

+

4

Pu (III) =

C l e v e l a n d (7) P u (III) by X e 0

3

P u ( I V ) to be the r a t e - d e t e r m i n i n g step.

In

a faster r a t e - d e t e r m i n i n g step is a p p a r e n t l y p o s s i b l e ; N0 " + 2

N0

2

+

Pu(IV).

has r e p o r t e d p r e l i m i n a r y results o n the o x i d a t i o n of at 30 ° C . i n p e r c h l o r a t e solutions of 2 M i o n i c strength.

S o m e sort of X e ( V ) is p r o d u c e d i n the r a t e - d e t e r m i n i n g step, w h i c h t h e n reacts r a p i d l y w i t h five a d d i t i o n a l P u ( I I I ) to give P u ( I V ) . f r o m the c o m p e t i t i v e o x i d a t i o n of P u ( I V )

Interference

was m i n i m i z e d b y w o r k i n g

at h i g h P u ( I I I ) / P u ( I V ) ratios. T h e slow oxidation by C l P i s h a r o d y (45,

46).

2

has b e e n d e s c r i b e d b y M a z u m d a r a n d

U n f o r t u n a t e l y , n e i t h e r the p r e s e n t a t i o n of the d a t a

n o r its i n t e r p r e t a t i o n is clear.

T h e rate l a w is i n c o r r e c t l y d e r i v e d f r o m

the p r o p o s e d m e c h a n i s m , w h i c h i n t u r n violates the p r i n c i p l e of m i c r o scopic reversibility. R e d u c t i o n s of P u ( I V ) .

R a b i d e a u a n d K l i n e h a v e s t u d i e d the r e d u c -

t i o n of P u ( I V ) b y the f o r m a l l y s i m i l a r r e d u c i n g agents V ( I I I )

(73)


and

20.

N E W T O N

Ti(III)

A N D

(71).

dependences.

Oxidation-Reduction

B A K E R

273

Reactions

T h e s e analogous reactions s h o w different h y d r o g e n The Ti(III)

r e a c t i o n is almost e x c l u s i v e l y inverse

p o w e r w h i l e for V ( I I I ) , terms inverse first p o w e r a n d inverse

ion first

second

p o w e r i n a c i d c o n c e n t r a t i o n are i m p o r t a n t i n the rate l a w . I n c h l o r i d e solutions S n ( I I ) s u r a b l e rates ( 7 2 ) .

reduces P u ( I V )

at c o n v e n i e n t l y m e a -

A f t e r a l l o w a n c e was m a d e for c h l o r i d e c o m p l e x i n g

of the reactants i t was c o n c l u d e d that a c t i v a t e d complexes c o m p o s e d of P u ( I V ) , S n ( I I ) , a n d f o u r a n d five c h l o r i d e ions are i n v o l v e d . F e ( I I ) reacts w i t h P u ( I V ) at rates w h i c h s h o w inverse first p o w e r


h y d r o g e n i o n d e p e n d e n c e (54).

I n c h l o r i d e solutions a t e r m first p o w e r

i n c h l o r i d e b u t zero p o w e r i n h y d r o g e n i o n becomes i m p o r t a n t .

Thus,

f o r this r e a c t i o n , C I c a n r e p l a c e O H i n the a c t i v a t e d c o m p l e x . R e d u c t i o n s of P u ( V I ) . R a b i d e a u a n d K l i n e (70)

s t u d i e d the r e d u c -

t i o n of P u ( V I ) b y T i ( I I I ) for c o m p a r i s o n w i t h the p r e v i o u s results for the analogous r e a c t i o n of V ( I I I )

T h e r a t e - d e t e r m i n i n g step p r o -

(67).

duces P u ( V ) , w h i c h is r a p i d l y r e d u c e d b y T i ( I I I ) to P u ( I V ) . T h i s is i n contrast to most other r e d u c i n g agents w h i c h react s l o w e r w i t h P u ( V ) than with P u ( V I ) .

Since the p r o d u c t ,

Pu(IV),

is also r e d u c e d

by

T i ( I I I ) , a c o m p e t i t i v e - c o n s e c u t i v e system results. A m e t h o d for t r e a t i n g the d a t a was d e v e l o p e d w h i c h u s e d a c o m p u t e r p r o g r a m m e d for n u m e r i c a l i n t e g r a t i o n . U n l i k e the analogous V ( I I I ) r e a c t i o n , the rate l a w w a s f o u n d to h a v e o n l y a single i m p o r t a n t t e r m , inverse i n h y d r o g e n

ion

concentration. R a b i d e a u a n d M a s t e r s (74)

f o u n d that the o x i d a t i o n of S n ( I I )

by

P u ( V I ) is v e r y s i m i l a r to w h a t t h e y o b s e r v e d for P u ( I V ) . A v e r y i m p o r tant feature is t h e i r e v i d e n c e

that a t w o - e l e c t r o n process is i n v o l v e d .

A l t h o u g h the r e d u c t i o n of P u ( V )

is s l o w e r t h a n that of P u ( V I ) , n o

P u ( V ) was formed. T h e net r e a c t i o n for the r e d u c t i o n of P u ( V I ) to P u ( V ) b y

Fe(II)

is q u i t e s i m p l e ; i n spite of this a c o m p l i c a t e d t h r e e - t e r m h y d r o g e n i o n d e p e n d e n c e was f o u n d ( 5 6 ) . sphere

A m e c h a n i s m w h i c h i n v o l v e s b o t h outer-

a n d i n n e r - s p h e r e a c t i v a t e d complexes

sphere complexes

are s u p p o r t e d b y e v i d e n c e

is f a v o r e d .

The

for consecutive

inner-

reactions

and a binuclear intermediate.

Reactions of Americium (V) T h e r e d u c t i o n of A m ( V ) b y H 0 2

b y Z a i t s e v et al. (99).

reactants, b u t i t is c o m p l i c a t e d H 0 . 2

2

2

has b e e n s t u d i e d i n 0 . 1 M H C 1 0

4

T h e rate is p r o b a b l y first o r d e r i n e a c h of the b y some r a d i o l y t i c d e c o m p o s i t i o n

of

T h e t e m p e r a t u r e d e p e n d e n c e was d e t e r m i n e d , b u t the a c i d d e -

p e n d e n c e w a s not.


274

LANTHANIDE /ACTINIDE CHEMISTRY

C o l e m a n (11) yr.) A m

2 4 3

has u s e d a s a m p l e of the r e l a t i v e l y l o n g - l i v e d

(7951

to s t u d y the d i s p r o p o r t i o n a t i o n of A m ( V ) w i t h o u t the exces

sive r a d i o l y s i s w h i c h m a d e e a r l i e r results difficult to i n t e r p r e t ( 9 8 ) .

In

2M

in

p e r c h l o r a t e solutions t h e rate w a s f o u n d to b e second

A m ( V ) a n d a p p r o x i m a t e l y 2.5 p o w e r i n a c i d c o n c e n t r a t i o n .

power

T h i s latter

d e p e n d e n c e differs f r o m those of the analogous reactions of U ( V ) and P u ( V )

(66)

concentration.

(59)

w h i c h are p r e d o m i n a n t l y first p o w e r i n h y d r o g e n i o n

T h e t e m p e r a t u r e d e p e n d e n c e was d e t e r m i n e d at a single

h y d r o g e n i o n c o n c e n t r a t i o n ; so a c t i v a t i o n parameters for the i n d i v i d u a l rate d e t e r m i n i n g reactions cannot b e e s t i m a t e d w i t h reasonable p r e c i s i o n Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch020

w i t h o u t m a k i n g f u r t h e r assumptions.

Interpreting the Kate Laws T h e rate constants a n d e m p i r i c a l rate l a w s s h o w n i n T a b l e I d e s c r i b e the e x p e r i m e n t a l observations, b u t t h e y n e e d further i n t e r p r e t a t i o n i n o r d e r to extract the m a x i m u m a m o u n t of i n f o r m a t i o n f r o m t h e m .

Adopt

i n g the l a n g u a g e of the absolute r e a c t i o n rate t h e o r y (16),

assume

that f o r e a c h r a t e - d e t e r m i n i n g step the reactant species

we

are i n q u a s i -

e q u i l i b r i u m w i t h the a c t i v a t e d c o m p l e x a n d that the rate of this step is p r o p o r t i o n a l to the c o n c e n t r a t i o n of the a c t i v a t e d c o m p l e x .

Often, rapid

e q u i l i b r i a p r e c e e d the a c t u a l r a t e - d e t e r m i n i n g step; these c a n b e

added

to the a c t u a l a c t i v a t i o n process to give a net a c t i v a t i o n process w r i t t e n i n terms of the p r i n c i p a l species i n the s o l u t i o n : m A + nB + p C + . . . =

[Act. Complex] * + q L + r M + . . .

a n d the rate w i l l be p r o p o r t i o n a l to [ΑΓ[Βπσρ... [L]*[Mr...

'

w h e r e A , B, C, are i n i t i a l reactants, a n d L , M , are p r o d u c t s of the p r e equilibria.

S i n c e the net a c t i v a t i o n process m u s t f o r m a l l y b a l a n c e , the

c o m p o s i t i o n of the a c t i v a t e d c o m p l e x c a n b e d e t e r m i n e d f r o m the rate law.

T h i s f o r m u l a t i o n has the a d v a n t a g e t h a t the a c t u a l reactant species

n e e d n o t b e k n o w n ; i n fact these species cannot b e d e t e r m i n e d f r o m the rate l a w alone. T h i s discussion shows

that the e m p i r i c a l rate l a w s m u s t b e

re-

expressed, w h e r e necessary, as a c o l l e c t i o n of terms w h i c h are a p p r o p r i a t e functions of the concentrations of species a c t u a l l y present i n the solutions. T h e o b s e r v e d n o n i n t e g r a l d e p e n d e n c e s s h o w n b y some of the s t o i c h i o m e t r i c concentrations

m a y be caused b y

(a)

equilibria i n w h i c h an

a p p r e c i a b l e f r a c t i o n of one of the reactants is present as m o r e t h a n one species; c o m p l e x f o r m a t i o n a n d h y d r o l y s i s are examples of s u c h e q u i libria, (b)

m o r e t h a n one k i n e t i c a l l y i m p o r t a n t a c t i v a t e d c o m p l e x ,


or

20. (c)

N E W T O N

A N D

B A K E R

Oxidation-Reduction

275

Reactions

changes i n the p e r t i n e n t a c t i v i t y coefficient ratios. T h i s last effect is

u s u a l l y c o n s i d e r e d to be s m a l l at constant i o n i c s t r e n g t h for a m o n g ions of the same s i g n (54).

reactions

F u r t h e r d a t a p e r t i n e n t to this p o i n t

are c l e a r l y n e e d e d . A l t h o u g h the rate l a w for a r e a c t i o n does n o t give its d e t a i l e d m e c h a n i s m , i t does give the c o m p o s i t i o n

of the a c t i v a t e d complexes

at the

h i g h e s t barriers as s h o w n a b o v e , a n d i n a d d i t i o n , the p a t t e r n of paths for the r e a c t i o n .

B y p a t t e r n of paths, as contrasted to m e c h a n i s m ,

we

m e a n the v a r i o u s w a y s i n w h i c h reactants get to p r o d u c t s w i t h o u t c o n s i d e r i n g i n t e r m e d i a t e s w h i c h are i n r a p i d e q u i l i b r i u m w i t h the reactants Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch020

or w h i c h react r a p i d l y to g i v e p r o d u c t s . If o n l y a single a c t i v a t e d c o m p l e x is k i n e t i c a l l y i m p o r t a n t , o n l y one p a t t e r n is p o s s i b l e : reactants ~* p r o d ucts. I f m o r e t h a n one a c t i v a t e d c o m p l e x is i n v o l v e d , the possible p a t terns of paths are analogous resistors c a n b e c o n n e c t e d .

to the possible w a y s i n w h i c h e l e c t r i c a l I n this a n a l o g the resistors c o r r e s p o n d

to

k i n e t i c b a r r i e r s ( a c t i v a t e d complexes ), the junctions c o r r e s p o n d to i n t e r m e d i a t e s , a n d the t e r m i n a l s c o r r e s p o n d to reactants or p r o d u c t s . F o r t w o a c t i v a t e d complexes there are t w o d i s t i n g u i s h a b l e p a t t e r n s ; for three a c t i v a t e d complexes f o u r patterns are g e n e r a l l y d i s t i n g u i s h a b l e ; for f o u r a c t i v a t e d complexes ten patterns, a n d so on. T h e first three of these sets of patterns are s h o w n i n F i g u r e 1.

T h e patterns h a v e

been

classified a c c o r d i n g to the n u m b e r of a c t i v a t e d complexes a n d the n u m b e r of i n t e r m e d i a t e s i n v o l v e d .

N o t e t h a t n o n e of these patterns

k i n e t i c a l l y d i s t i n g u i s h a b l e f r o m t h e i r m i r r o r images

are

(22).

A n i m p o r t a n t feature of these e l e c t r i c a l analogs is that w h e n

the

steady-state a p p r o x i m a t i o n is v a l i d the r e c i p r o c a l s of the i n d i c a t e d r e sistances are the analogs of

fc[A] [B] [C] ' m

n

1

. . . , where A , B, C , . . .

are the i n i t i a l reactants, a n d k is the effective rate constant for the f o r m a t i o n of the a c t i v a t e d c o m p l e x d i r e c t l y f r o m the i n i t i a l reactants, e v e n i f i n t e r m e d i a t e s are i n v o l v e d . T h i s means that the o v e r - a l l rate l a w is f o u n d b y c o m b i n i n g the i n d i v i d u a l rate terms a c c o r d i n g to the rules for

com-

b i n i n g the analogous r e c i p r o c a l resistances. T h e a c t i v a t i o n parameters, A F * , A f f * , a n d A S * for the v a r i o u s net a c t i v a t i o n processes are d e t e r m i n e d f r o m the values a n d

temperature

coefficients of the effective rate constants u s i n g the equations b y the absolute r e a c t i o n rate t h e o r y

provided

(16).

T a b l e I I lists the net a c t i v a t i o n processes, patterns of paths, a n d a c t i v a t i o n parameters d e r i v e d f r o m the d a t a i n T a b l e I.

Empirical Correlations Hydrogen Ion Dependences.

T h e hydrogen ion dependences given

i n T a b l e I a n d i n d i c a t e d b y the net a c t i v a t i o n processes i n T a b l e I I are


276

LANTHANIDE/ACTINIDE

Table I.

CHEMISTRY

Observed Rate Laws, Rate k,!MH , 25°C. (in M and sec.) + a

Reactant

Rate Law

Oxidations of U(IV), ~d[U(IV)]/ât = U (V I) k [ U ( I V ) ] ο·»» [ U ( V I ) ] 0.92 [ H ] -s.oa Np(IV) * [ U ( I V ) ] [ N p ( I V ) ] [H ]-2.s Np(V) ^[U(IV)][Np(V)][H ]-2 Fe(III) * [ U ( I V ) ] [Fe(III)] [ H ] - * Pu(VI) * [ U ( I V ) ] [ P u ( V I ) ] [Η*]" · Pu(IV) *[U(IV)] [Pu(IV)] [ H ] - ^ 3 V(V) ^[U(IV)][V(V)][H ] Np(VI) fc[U(IV)] [Np(VI)] [H ]-097 +

+

+

+

8 1

1

2

+


+

+

τη (in) Ce(IV) H 0 Br 0 C10 C10 " S 0 2" BrO 2

2

2

2

3

2

2

8

a

*[u(iv)]

[ T i ( i i i ) ] [η*]-*·»» * [ U ( I V ) ] [ C e ( I V ) ] [Η ]"ΐ· fc[U(IV)] [ H 0 ] [H ]-^i(approx.) * [ U ( I V ) ] [ B r ] [Br"]»[H ]-2 *[U(IV)][0 ][H ]-i ^[U(IV)][C10 -][H ] i * [ U ( I V ) ] [C10 -] [Η ]-ο· ft[U(IV)][S 0 *-][H ] * [ U ( I V ) ] [Br0 -] [Η ]°· or [ U ( I V ) ] [ B r O - ] ( k + k [ H ] 2 ) +

9

2

+

2

+

9

+

2

+

3

+

+

2

2

1 3

+

8

+

3

3

4 3

0

+

2

Oxidations of U(V), -d[U(V)]/dt = U(V) ^[U(V)]2[H ]o-82 Fe(III) *[U(V)][Fe(III)] V(IV) *[U(V)][V(IV)][H ]-

2

Np(V) Reductions of U(VI), ~d[U(VI)]/dt = V(II) * [ U ( V I ) ] [ V ( I I ) ] [Η ]°· V(III) * [ U ( V I ) ] [ V ( I I I ) ] [Η*]" · Sn ( II ) k [ U ( V I ) ] [Sn ( I I ) ] [ H C l ] Reductions of Np(V), -d[Np(V)]/dt = Cr(II) *[Np(V)][Cr(II)][H ]^ Np(III) * [ N p ( V ) ] [ N p ( I I I ) ] [Η+] · V(III) [Np(V)][V(III)][H ]o +

1

05

[H ]-i-V[V(IV)] ) +

k [ N p ( V ) ] .8β [ Γ ] 1-55 [ H ] - 6 ! +

2

Oxidations of Np(IV), -di[Np(IV)]/dt = Fe(III) *[Np(IV)] [Fe(III)] [ H ] " Np (V) k [ N p ( I V ) ] ι.δ [ N p ( V ) ] °·* [ H ] " + &'[Np(V)] [H ] Reduction of Np(IV), -d[Np(IV)]/dt = Cr(II) fc[Np(IV)] [Cr(II)] [ H ] " Oxidation of Np(III), -d[Np(III)]/dt = Fe(III) Jfc[Np(III)] [ F e ( I I I ) ] [ H ] " ^ Oxidations of Np(V), -d[Np(V)]/dt = +

X X Χ Χ Χ

10~ 10 2.3 ΙΟ" ΙΟ" ΙΟ"

Χ Χ Χ Χ

ΙΟ" 10" 1 0 " (k ) Ι Ο " (fc )

2

3

4

2

3

3

1

1

Q

2

2

2

4

4

1.1 4.3 3.0 6.0 1.6 1.7

Χ X Χ Χ Χ Χ

ΙΟ 10* 10" ΙΟ" 10" ΙΟ"

1

5

3

1

2

(*')

1 3

5.7 Χ Ι Ο " 6.45 Χ 1 0 " 2

3

+

2

4

1

+

I"

5

6

7.4 X 1 0 2.8 Χ 1 0 " 1.7 X 1 0 ~

8

+

fc'[Np(IV)]

X 10

7

( M e a s u r a b l e at 2 5 ° C .

0 1

1

X (k +

Χ ΙΟ" Χ ΙΟ" Χ 10"

2.4 X 1 0 5.0 X 1 0 2.5 X 1 0

+

+

2.1 5.0 8.0 12.8 3.1 31.5 2.0 22.0 3.9 8.7 0.8 5.3 2.4 2.9 19.0 7.2 2.6 2.1 5.3

2

8

+

+

1.1 X I O - (k>) 5

1 2 7

4.3

+

6.8 Χ 1 0


2

20.

N E W T O N

A N D

B A K E R

Oxidation-Reduction

Reactions

277

Constants, and Activation Energies E, kcal./mole


a

M

Réf.

11.0

2.00 1.00 1.00 1.02 2.00 2.00 1.00 2.00 2.90 2.00 2.00 2.00 0.50 0.50 1.00

27.2

2.00

43 79 79 5 51 53 62 87 25 3 4 20 24 15 19 34 76

9.0

2.00 2.00 2.00

59 60 60

38.0 16.0 33.0 23.6 19.1 24.8 13.0 18.8 22.5 15.4 16-19 22.4

Comment

Evidence for binuclear intermediate Preliminary result

In H S Q 2

4

92

Extrap. t o O M U ( V I ) Estimated from the effect of Fe(III) and V ( I V ) on V ( I I I ) - U ( V I ) reaction Preliminary result

2.00 2.00 7.00

58 60 48

Evidence for binuclear intermediate k value is for 1 M C I " and 7 M H

0.20 2.00 3.00 1.00 3.00 3.00

94 27 1

35.0 37.4 18.2

1.00 1.20

31 85

18.0

1.00

93

15.0

2.00

61

7.8 22.1 18.0

6.4 15.2

28.3

+

78 Corrected for back reaction Exchange


278

LANTHANIDE/ACTINIDE

CHEMISTRY

T a b l e I. k,lMH , 25° C. (in M and sec.) + a

Reactant

Rate Law 1 6

+

V(V) N0 --N0 Np(VI) 2

^[Np(V)][Cr(VI)][H ]1 + &'[Np(VI)][H ]-i-V[Np(V)] *[Np(V)][V(V)][H ] fc[Np(V)] [ H ] ^ * [ N p ( V ) ] [Np(VI)] [ H ] +

Cr(VI)

3

+

+

2 3

4

+

0 1 3

Reductions of Np(VI), -d[Np(VI)]/dt = V(III) * [ N p ( V I ) ] [V(III)] [ Η * ] " · Np(IV) Jfe[Np(VI)] [ N p ( I V ) ] [ H * ] " * fe[Np(VI)][H Q ] [H ]-i 1 + *'[Np(V)]/[Np(VI)]


1

H

O

2

Oxidations of Pu(III), -d[Pu(III)]/dt = Pu(VI) fc[Pu(III)] [Pu(VI)] [H ]° Pu(V) Jt[Pu(III)] [Pu(V)] [Η ]·

1.9

Pu ( IV) HN0 2

97

k [ P u ( III ) ] [Pu ( IV ) ] ^°· [ H ] k [ P u (III)] [ H N 0 ] [HCl] X(l + fc'[N0 ]) 8

+

0 8 3

2

1

(*')

2.7 4.5 Χ 10~

+

+

5

1

2

4

+

2

2.0 1.0 X 10" 9.0 Χ 1 0

2.45 Χ 1 0 4.9 X 10" 8.9

47

2

4.3

1 2 8

2

1.0 X 1 0 3.0 Χ 10" 3

1

3

Oxidations of Pu(III), -d[Pu(III)]/dt = Xe0 k [ P u ( III ) ] [ X e 0 ] [ H ] ° Cl fc[Pu(III)][Cl ]o[HCl]o(?) 0 [ P u ( III ) ] [ 0 ] ^ [ H ] X(/c[S0 "] + fc'[S0 \] )

1.6 7.0 6.0 1.9

X Χ Χ X

10~ 10" 10 10» (*')

Reductions of Pu(W), -d[Pu(IV)]/dt = Fe(II) fc[Pu(IV)] [Fe(II)] [Η ]~ · V(III) fc[Pu(IV)] [V(III)] [ H ] " Ti(III) fc[Pu(IV)] [Ti(III)] [ H ] Pu(IV) ^[Pu(IV)] [H ]~ Sn(II) Jfc[Pu(IV)] [Sn(II)] [ H ] o [ C l " ]

4.7 6.2 6.6 5.4 1.7

Χ Χ Χ X Χ

10 10 10

3

+

3

2

2

2

2

4

+

2

2

2

0 1

2

3

4

+

0

+

92

1 4 3

+

2

+

1 0 9

3 1 4

+

Reductions of Pu(V), -d[Pu(V)]/dt = Pu(V) fcfPuiV)] ^] Fe(II) fc[Pu(V)] [Fe(II)] [ H + ] 2

+

+

Reductions of Am(V), -d[Am(V)]/dt = Am(V) &[Am(V)] [H ] H 0 &[Am(V)][H 0 ][H ]? 2

2

+

2

2

5

2

+

1 3

1 1

ÎO"*

10

1

3

2.24 1.1 X 1 0 1.66 Χ 1 0 6.7 Χ 1 0 2

03

3β

1 4 6

3

1

5.7 Χ 10" 4.1 Χ 10"

F o r solutions w i t h μ < 1 M , this is the h y p o t h e t i c a l v a l u e for 1 M H w i t h the i o n i c strength listed. a

1

1

1

1

+

6

3

8.0 Χ 10" 3.0 Χ 1 0

11

Reductions of Pu(VI), -d[Pu(VI)]/dt = V(III) Jfc[Fu(VI)] [ V ( I I I ) ] [ H ] " Ti(III) fc[Pu(VI)] [ T i ( I I I ) ] [Η+]" · Fe(II) *[Pu(VI)] [Fe(II)] [Η ]-°· Sn(II) le [ P u ( V I ) ] [Sn(II)] [ H ] o [ C l ]

2

1 9

2

+

4 3

i n a solution


20.

NEWTON

AND

BAKER

Oxidation-Reduction

279

Reactions

Continued

M

Réf.

11.6

2.00

91

k ' = 7.6 Χ 10"

11.7

2.00

12

Values are for H N 0

12.0 10.6

2.90 3.00

81 9,10

Values are for 2 . 9 M H N O Values apply to 0°C. exchange

19.0 25.5

2.00 2.00

80 28, 29, 30

12.4

3.00

99

5.4 —

1.00 1.00

68 64, 65

5.0 -6.0

2.00 varied

37 13

2.00

7

1.00 2.00

45,46 49,50

Values for 1 M H C l [ 0 ] in M

2.00 2.00 1.02 1.00 2.00

54 73 71 64, 65 67

Values are for 1 M C I "

1.00 1.00

66 62

Preliminary result

16.1 10.9 7.5 10.6

2.00 2.00 2.00 2.00

67 70 56 74

Evidence for binuclear intermediate Evidence for a 2-electron process

14.0 13.0

2.00 0.10

11 99

Value extrapolated to 25°C. Values apply to 0 . 1 M H C l


kcal./mole

19.6 21.2 17.3

—

20.0

19.6

—

8,

Comment

1

3

solution s

F r o m reverse reaction and equilib rium quotient Exchange

Values for 30°C.

2


280

LANTHANIDE/ACTINIDE CHEMISTRY

Pattern

No. of Kinetically Distinguishable N o . of Activated Complexes Intermediates

1- 0

1

2- 0

2

2-1

2

Electrical Analog

ρΛΑΛΛΛΛΛι oA k> w w w

3

3-1-a

3

A

R2

± + 1 Ri R2

- W W W - < >

R i "4" Ro 4" Ro

ι

ρΛΛΑΛΛι

Ο-λΛΛΛΛ»

1

—O

3

ΚΛΛΛΛΛ

1

3

-=r-

111

+

R

2

+ Ro

RI + · 2

3

ΚΛΛΛΛΛΛ^λΛΛΛΛΛτ

3-2

R1 +

2

3-0

3-1-b

Ri

0-VVWV\AA-0

0 - A W V


Rate L a w

3

ι

Figure

1.

Various

All Ri are of the form k i [ A ] < [ B ] * [ C ] ' m

n

l

f

patterns

of

3

1

R2

R3

1 , 1 , 1

paths

. . . , where A , B , C , etc. are initial

reactants.

n e a r l y a l l consistent w i t h the p r o p o s i t i o n that the g a i n or release of h y d r o g e n ions i n a n a c t i v a t i o n process w i l l l i e b e t w e e n zero a n d the t o t a l n u m b e r g a i n e d or released i n the c o r r e s p o n d i n g o v e r - a l l process.

For

e x a m p l e , R e a c t i o n s 7-12 a n d 14-19, i n T a b l e I I , a l l i n v o l v e the o v e r a l l release of f o u r h y d r o g e n ions, w h i l e the a c t i v a t e d complexes

are a l l

f o r m e d w i t h the p r i o r release of f r o m one to three h y d r o g e n ions.

Con

s p i c u o u s exceptions to this g e n e r a l i z a t i o n are s h o w n b y the o x i d a t i o n of U(IV)

by V 0

2

+

and by C 1 0

3

a n d b y R e a c t i o n s 2, 3, 5, 27, 28, a n d 29.

T h e s e latter exceptions illustrate the t e n d e n c y of a c t i v a t e d complexes to h y d r o l y z e a n d to r e d u c e a n o t h e r w i s e h i g h charge.

I n this c o n n e c t i o n

no r e l i a b l e e v i d e n c e has b e e n f o u n d for a n a c t i v a t e d c o m p l e x w i t h a net c h a r g e greater t h a n 6 + . It is reasonable to suppose t h a t the s m a l l e r the d r i v i n g force for a r e a c t i o n the m o r e the a c t i v a t e d c o m p l e x w i l l r e s e m b l e the p r o d u c t s . T h i s


20.

N E W T O N

A N D

Oxidation-Reduction

B A K E R

Reactions

281

s u p p o s i t i o n is b o r n e out f a i r l y w e l l b y the class of reactions i n w h i c h a n a c t i n i d e ( I V ) i o n is o x i d i z e d to the ( V ) s t a t e — R e a c t i o n s 7-18 i n T a b l e I I . F o r the five reactions for w h i c h AF°

is greater t h a n —1 k c a l . / m o l e , three

out of a total of f o u r h y d r o g e n ions are released i n the net a c t i v a t i o n process.

F o r AF°

less t h a n —2 k c a l . / m o l e , the c o r r e l a t i o n is m a r r e d o n l y

b y the three reactions for w h i c h b o t h one a n d t w o h y d r o g e n ions are r e l e a s e d i n simultaneous a c t i v a t i o n processes.

This supposition ratio

nalizes the fact that the d i s p r o p o r t i o n a t i o n reactions of U ( V ) a n d P u ( V ) are p r e d o m i n a n t l y first p o w e r i n the h y d r o g e n i o n c o n c e n t r a t i o n , w h i l e the analogous

d i s p r o p o r t i o n a t i o n of A m ( V )

is b o t h s e c o n d

and third


power. T h e E n t r o p i e s of the A c t i v a t e d C o m p l e x e s . lation h a d been noted previously (52) a c t i v a t e d complexes

A n approximate corre

b e t w e e n the f o r m a l entropies of

a n d t h e i r charges.

D a t a o b t a i n e d since that t i m e

e n a b l e this c o r r e l a t i o n to b e e x a m i n e d for a c t i v a t e d complexes c a r r y i n g charges

f r o m 0 to 6 + .

V a l u e s c a l c u l a t e d f o r S * o m i e x are g i v e n i n C

P

T a b l e I I a n d p l o t t e d i n F i g u r e 2. T h e d i s c o r d a n t p o i n t 21

corresponds

to the m i n o r p a t h for the V ( I I I ) - N p ( V I ) r e a c t i o n a n d has a n u n c e r t a i n t y of at least 16 e.u.

T h e u n c e r t a i n t i e s for points 8 are a b o u t 13 e.u. a n d are

c o r r e l a t e d s u c h that i f the true v a l u e of one is l a r g e r t h a n i n d i c a t e d , t h a t for the other is s m a l l e r . It is a p p a r e n t that a l t h o u g h charge is a n i m p o r tant factor i n d e t e r m i n i n g the e n t r o p y of a n a c t i v a t e d c o m p l e x ,

other

less o b v i o u s ones are i m p o r t a n t also. F r e e E n e r g i e s of A c t i v a t i o n . It is of interest to c o n s i d e r a set of s i m i l a r reactions a n d d e t e r m i n e the effect of the free energy changes o n t h e i r rates. R e a c t i o n s 7 - 1 8 are the largest s u c h set of a c t i n i d e i o n r e a c tions for w h i c h q u a n t i t a t i v e d a t a are a v a i l a b l e . I n this set a n a c t i n i d e ( I V ) ion, M

4 +

, is o x i d i z e d to the c o r r e s p o n d i n g MO > i o n b y a reactant w h i c h L

+

does n o t u n d e r g o a d r a s t i c change i n structure. A p l o t of A F * VS. AF° for these reactions is g i v e n i n F i g u r e 3.

T h e n u m b e r of h y d r o g e n

ions

released i n the v a r i o u s net a c t i v a t i o n processes range f r o m one to three a n d are i n d i c a t e d b y the n u m b e r s i n parentheses. A l l 15 points f a l l w i t h i n 3.5 k c a l . / m o l e of a s t r a i g h t l i n e w i t h a slope of 1/2, i r r e s p e c t i v e of the n u m b e r of h y d r o g e n ions released. A slope of 1 / 2 is r e q u i r e d i f the v a l u e for a r e a c t i o n a n d its reverse are to f a l l o n the same straight l i n e . T h e r e l a t i o n s h o w n here suggests that a measure of the i n t r i n s i c rate of a r e a c t i o n , c o r r e c t e d for its d r i v i n g force, is g i v e n b y A F * = AF°.

A F * — 0.5

T h i s amounts to t a k i n g the average A F * for the r e a c t i o n i n the

f o r w a r d a n d reverse d i r e c t i o n s . tion: k

12

=

— RT l n / ) .

(fc fci K /) n

2

1 2

1 / 2

(42)

V a l u e s for these

U n d e r conditions where M a r c u s ' rela is a p p l i c a b l e , Δ Ρ * — i ( A F * u +

AF*

" i n t r i n s i c " a c t i v a t i o n free energies

2 2

are

g i v e n i n T a b l e I I I . T h i s t a b l e shows some patterns of r e l a t i v e r e a c t i v i t y as w e l l as some a p p a r e n t anomalies.

F o r e x a m p l e , the r e l a t i v e i n t r i n s i c


282

LANTHANIDE/ACTINIDE

Table II.

T h e r m o d y n a m i c Q u a n t i t i e s of

Process no Μ—Ο 1.

bonds formed or

U0 U0 Pu Pu Np Np Np Np Np Pu0 Pu0 Pu0 Pu0 Pu Pu Pu Pu Np0 Np0 2

4 +

4 +


3 +

4.

2

2

2

4 +

+

2

7.

9.

10.

U U Am Am Am Np Np Np Np U U U 4

+

4

Pu Pu U U U U U U U U U U Np 4

4

4

2 +

4- 2 H 0 = U 0 4- N p 0 4- 4 H 4- H 0 = [ * ] 4- H 4- 2 H 0 = 2 A m 0 + 4H + H 0 = [*]"> 4-H + H 0 = [*] + 2H 4- 2 H 0 = 2 N p 0 4- 4 H + H 0 = [*] 4- 2 H 4- H 0 = [ * ] 4- 2 H 4- H 0 = [ * ] + 3H + 2HoO = U 0 4- P u 0 4- 4 H 4- H 6 = [*] 4- H 4- H 0 = [ * ] 4- 2 H

2

2 +

2 +

2+

2

2

+

2 +

2

2.00

2-1

2.00

1-0

1.00

1-0 2-l(?)

2.00 2.00

+

2

3 +

3 +

6+

1-0

2.00

1-0

2.00

2-0

1.02

2 +

6+

4H

+

4H

+

+

5+

1-0

1.00

+

2

4- 2 H 0 == [ * ] 2

+

+

+

2

4- 3 H

+

+

2

+

+

6+

2

2

3 +

2-0

+

+

+

3+

2

2

H

+

+

3

+

2

3 +

4- F e

2.00

+

2

3 +

4 +

4 +

2-0

+

+

2

+

+

2

4+

2

2

3 +

2.00

+

+

5+

2

4 +

3 +

1-0

+

+

3+

2

3 +

4 +

+

3.00

+

4+

2

3 +

+

+

4+

2

2 +

+

+

2

2

2

1-0

+

4- P u 0 4- 2 H 0 = 2 P u O o 4- 4 + PuOo 2 + 4- 2 H 0 = [ * ] 3 4- 3 H 4- U 0 4- 2 H 0 = 2 U ( V 4- 4 H + UO> + 2 H o O = [*] * 4- 3 H 4- U 0 4- 2 H 0 = [ * ] 4- 3 H 4- C e O H 4- H 0 = U 0 4- C e 4- C e O H 4- H 0 = [ * ] 4- H + Pu 4- 2 H 0 = U 0 4- P u 44- P u + H 0 = [*] + 2H + Fe 4- 2 H 0 = U O , 4- F e 44- F e + H 0 = [*] 4- H 4- F e + H 0 = [*] 4- 2 H 4- F e + 2H 0 = Np0 4- F e 2

2.00

+

+

2

+

+

4+

2

2 +

2 +

2

2

2 +

2 +

2

2

+

2

2 +

9

+

2

5+

2

2

2

2

1-0 +

2

3+

2

2

2

4 +

4 +

Np

2 +

2 +

2 +

4 +

16.

+ Np0 4- N p 0 4- A m 0 4- A m 0 + Am0 4- N p 0 4- N p 0 4- N p 0 4- NpOo 4- P u O , + PuO., 4- P u O >

2 +

+

+

2 +

i+

4 +

4

1.00

M

4 +

4 +

+

2

4 +

4 +

1-0

3 +

2 +

2

2

2.00

+

5+

+

3-2

2 +

2

4 +

4 +

4 +

15.

+

4 +

+

3+

3 +

4 +

4 +

14.

2

3 +

2

+

1.00

4+

4 +

2

2-0

1-0

+

+

4+

+

+

2

2.00

+

+

2 +

2

4 +

13.

2

4 +

4 +

12.

2 +

2 +

2

4 +

11.

2 +

2-0

2 +

2

formed from 4 +

8.

2 +

4 +

3 +

M0

4 +

2

3 +

6.

+

4+

2

2 +

2

+

2

2 +

2.00

3 +

2

3 +

4 +

5.

5+

3 +

3 +

1-0

3 +

2

2 +

3 +

3.

3 +

2 +

μ, M

3 +

4+

2 +

4 +

2c.

2 +

2 +

4 +

+

2

2 +

4 +

2b.

2 +

Pattern"

broken

+ V = U0 4- V 4- V == [ * ] + Fe = Pu + Fe + Fe + H 0 = [*] + H + Cr = Np + Cr + Cr + H 0 = + H + Cr + H 0 = [*] 4- 2 H + Fe = Np + Fe + Fe + H 0 = [*p 4-H + Fe = Pu0 + Fe + F e = [*] + Fe = [*] + Fe 4- H 0 = [ * ] + H + Pu0 = Pu 4- P u 0 + Pu0 = [*] + Pu = Pu + Pu + Pu 4- H 0 = [ * ] « 4 - H + Np0 = NpOo + Np0 + Np0 = [*] +

2 +

2

2.

2

CHEMISTRY

2

4+

+

2 +

+ 3H

+

4- 4 H

+


20.

N E W T O N

A N D B A K E R

Oxidation-Reduction

283

Reactions

O v e r - a l l Processes and N e t Activation Processes AF,


tcal./mole

AH,

AS,

kcal./mole

e.u.

-7.3 14.9 -4.9 15.1 -13.0 16.7 17.7 -14.2 13.7 -3.3 13.3 12.4 13.4 -1.6 15.3 0.0 13.4 0.0 14.0

-14.0 7.1 -4.0 19.1 -11.9 16.2 20.6 -15.3 14.6 -13.3 4.4 8.6 9.4 8.7 13.6 0.0 7.4 0.0 10.6

-24.0 -26.1 3.0 13.0 4.0 -1.6 9.8 -4.0 3.0 -34.0 -30.0 -12.6 -13.4 +34.0 -6.0 0.0 -20.4 0.0 -12.0

-12.9 16.0 -12.9 11.6 11.4 -9.2 19.2 18.5 19.2 -7.8 16.6 16.0 5.8 26.6 11.9 26.8 26.2 -24.0 12.2 -9.2 15.4 -4.4 17.0 16.0 -0.7 19.1

1.7 18.2 8.2 20.0 20.0 7.5 24.7 24.3 30.9 6.9 17.6 21.3 19.0 37.8 26.7 37.5 37.7 -6.0 14.0 16.2 24.3 20.2 18.0 24.1 26.0 34.6

49.0 74.0 68.0 28.0 30.0 56.0 18.4 19.0 39.0 49.0 3.4 17.6 44.0 38.0 49.6 36.0 38.6 62.0 6.0 85.0 30.0 83.0 3.0 27.0 90.0 52.0

S* comp. e.u.

b

AF*;

kcal./mole

Réf.

d

-70

18.6

58

± 1

-82

17.6

54

± 1.4 ± 3.3

-93 -82

23.2 24.2

93(f)

± 0.3

-87

20.8

61

± 1.6 ± 2 ± 2

-84 -67 -50

15.0 14.0 15.0

56

± 1

-106

16.1

68

± 4

-127

13.4

37(f)'

± 3

-29

14.0

8, 9,10,

± 1

-69

22.4

87

± 13 ± 13

-42 -40

18.0 17.8

ll(r)

± 1 ± 5 ± 5

-62 -61 -25

23.8 23.1 23.8

28, 29, ί 76

± 0.4 ± 1.2

-87 -72

20.5 19.9

61

± 9

-40

23.7

66

± 1.7 ± 1.5

-28 -25

20.8 20.2

43 59

± 2

-116

24.2

3

± 2

-118

20.0

12

± 9 ± 2

-130 -106

19.2 18.2

5(r)

-69

19.4

31

± 0.4

±

e

1


284

LANTHANIDE/ACTINIDE

CHEMISTRY

Table I I .

Process 17.

0

Np + Np + 2H 0 = Np0 + Np + 4H N p + N p + 2 H 0 = [*] 5 + 3 H 1-0 18. Pu + Pu + 2H 0 = Pu0 + Pu + 4H Pu + Pu + 2H 0 = + 3H 1-0 19. U + Tl + 2H 0 = U 0 + TP + 4H U + T l + H 0 = [*] 6 + H 2-0 U + Tl + H 0 = + 2H M0 formed from M 20. Pu0 + Ti + H 0 = Pu0 + Ti0 + 2H Pu0 + T i + H 0 = [*] + H 1-0 21. Np0 + V + H 0 = Np0 + V0 + 2H Np0 + V = Np0 + V + H 0 = [*] + H 2-0 22. Pu0 + V + H 0 = Pu0 + V0 + 2H Pu0 + V + H 0 = [*] + H 2-0 Pu0 + V + H 0 = [*]« + 2 H 23. U0 + V + H 0 = U0 + V0 + 2H U0 + V + H 0 = [*] 4- H 2-1 U0 4- V 4- H 0 = [*] + 2H 24. P u + T i 4- H 0 = P u + T i 0 + 2 H P u + T i + H 0 = [*] + H 1-0 25. Pu + V + H 0 = Pu + V 0 + 2H P u + V 4- H 0 = [*] + H 2-0 P u + V 4- H 0 = [*] + 2H Other Reactions 26. Np0 4- V 4- 2 H = N p + V 0 + HUO Np0 + V = [*] 1-0 27. Np0 4- U = N p + U 0 N p + U + 2 H 0 = N p 4- U 0 + 4H Np0 + U + H 0 = [*] + 2H (note °) N p + U 4- H . , 0 = [*] + 2H 28. Np + Np0 = Np0 + Np Np + Np0 + H 0 = [*] + 2H (note ') 2 N p 0 4- H = [*] 29. U 4- U 0 = U0 + U 2U + U 0 + 2 H 0 = [*] + 4H 1-0 30. Np0 + H 0 = N p 0 4- H 0 4- H Np0 4 - H Q = [*] + H 2Np0 + H 0 = [*] 4- N p 0 + H 2-1 31. Np0 + HCr0 "= Np0 + Cr( V) Np0 + H C r 0 " 4- 2 H = [*] 4- H 0 2-1 2 N p 0 + H C r C V 4- 4 H = [*] + Np0 + 2H 0 Reactions involving Cl~, net rate determining reactions not known 32. P u 4- F e 4- C I " = [*] 33. U0 4- V + CI" = [*] 34. N p Q 4- N p Q 4- C P = [*] 4 +

4 +

4 +

4 +

4 +

2

4 +

2

4 +

+

2

+

2

2 +

2

3 +

+

2

2.90

+

2

2+

1.00

+

+

3 +

4 +

2.00

+

+

2

M

+

3 +

2

3 +

4 +

3 +

+

+

2

4 +

4 +


Pattern

3+

2 +

3 +

2 +

3 +

2

2

2 +

2

2

2

2 +

3 +

3 +

2 +

3 +

2 +

3 +

2 +

3 +

2

2 +

3 +

2 +

3 +

2 +

3 +

2

2

3 +

2

4 +

3 +

4 +

3 +

4 +

3 +

4 +

3 +

4 +

3 +

2

2

2

2

2

2

2

+

+

4 +

4 +

2

2

+

2

2

2 +

6+

4

2

2

+

+

+

2+

2

3+

2

2 +

2.00

2

3+

2 +

3.00

2 +

5+

2

0.14

+

+

2 +

+

1.20

+

2

2+

+

2 +

+

+

+

2

+

2 +

+

4 +

4

4 +

2

2 +

2

2

2

+

2

1.00

4 +

4+

2

2

2 +

+

2

2

2 +

+

+

3+

2 +

2 +

2

2

+

+

2

+

2

2

2

3+

2 +

3.00

+

3 +

2

+

2

2

2 +

6+

4 +

2.00

+

4 +

4 +

1.02

+

+

2

4 +

2.00

+

2 +

4 +

4 +

2

2 +

+

5+

2

+

+

6+

2

2

2 +

+

3 +

4+

4 +

+

6+

2

2.00

+

3 +

2

2.00

+

+

3+

3 +

4 +

2 +

4+

2

1.00

+

+

+

+

2

+

2 +

4+

2

3 +

2

+

4+

2

2

+

4 +

2 +

+

2

2

+

2

2

2

+

4 +

+

2

4+

2

2 +

2

2

2

2+


2.00 2.00 3.00

20.

NEWTON AND BAKER

Oxidation-Reduction

285

Reactions

Continued AF, kcal./mole


13.5 28.7 4.5 24.0 -43.0 19.7 19.7 -18.7 14.7 -17.9 16.5 15.7 -12.7 17.1 18.3 6.9 16.6 18.3 -20.3 15.0 -14.3 15.7 15.3

AH, kcal./mole 41.0 46.9 28.3

—

-12.5 24.7 20.5

(-9.4) 10.3 -9.0 32.0 13.0 -2.6 15.6

— 16.0

AS, e.u.

S* comp. e.u.

94.0 63.0 ± 2 80.0

-72

22.0

27

—

21.8

64,65

—

102.0 17.0 ± 4 3.0 ± 4 (31) -14.7 31.0 52.0 -9.0 32.0 -5.0

— 32.0

b

AF*; kcal./mole

-88 -102

Ref.

25(f)

± 1.3

-88

24.0

70

± 16 ± 6

-26 -59

25.4 24.6

80(f)

± 4

-81

23.4 24.6

67(r)

± .9 ± .5

-62 -52

13.1 14.8

60

± 2

-125

25.2

71

± 3 ± 2

-128 -112

22.8 22.4

73(f)

-81

22.6

I

-41 -174

26.3 18.4

79

-67 -30

16.0 24.2

85

-118

25.1

75 100

17.7 22.1 (-0.3) 16.7 6.0 17.2 21.6

3.8 12.9 (67) 6.0 68.0 5.0 21.0

-8.8 18.2 -3.8 9.7 24.4 23.2 0.0 16.0 24.2 0.0 25.1 8.4 16.6 17.0

15.3 14.6 -5.8 36.0 32.2 15.2 0.0 19.9 17.6 0.0 32.8

-24.0 -12.3 -7.0 87.0 26.2 -27.0 0.0 31.1 -22.2 0.0 26.0

11.8 13.3

-16.1 ± -12.2

+35 -2

12.4 —

16.8 17.0

13.4 11.4

-11.7 -18.7 ± 3

-16 -31

— —

91

14.2 14.3 13.6

14.4 10.6 9.0

0.6 ± 5 -12.4 ± 1 - 1 4 . 0 ± 10

-98 -39 -18

— — —

54 58 8,9,10

—

—

± 2.6

± 1.7 ± .07 ± 2


286

L A N T H A N I D E / A C T I N I D E

C H E M I S T R Y

Table II.

Process 35.

Pattern

μ, M

2-0

2.00

2-0

2.00

2-0

4.00

0

PuQ + S n + 4C1- = [*]* Pu0 + S n + 3 C 1 " = [*] P u + S n + 5C1- = [*] P u + S n + 4C1" = [*] U + Br0 " = [*] U + Br0 - + 2H = + H 0 2

2

36. 37.

2 +

2 +

2 +

2 +

4 +

2 +

4 +

2 +

4 +

+

+

2+

3+

3

4 +

+

3

2

See Figure 1. Formal entropy of the activated complex, S* comp. = AS* + Σ S° reactants — Jj S° other products in net activation process. "Average value for forward and reverse reactions, A F * = A F * — 0.5 AF°. A F ° , Aif°, and AS° were calculated from data in (31) for actinide ions and from data in Ref. 36 for other ions. Where necessary values were estimated using correla tions given in Ref. 36.


a

6

d

rates of r e d u c t i o n of P u 0 U

> Pu

4 +

4 +

> V

3 +

>

2

Ti . 3 +

2 +

b y v a r i o u s r e d u c i n g agents is F e T h e sequence for P u

i n v e r s i o n of the first t w o m e m b e r s . and V

3 +

Np0

, and U 0

2

+

with U

-V0 4 +

2 +

4 +

2

2 +

3 +

> Pu

W h e n the t w o couples,

, i t is seen t h a t N p 0

2

>

3 +

is the same e x c e p t for

, are c o m p a r e d i n t h e i r reactions w i t h P u 0

, and V

2 +

2

2 +

U

4 +

-U0

, Pu , N p 0 4 +

2

2

2 +

+

,

reacts s l o w e r t h a n e x p e c t e d

+

reacts faster t h a n e x p e c t e d w i t h U 0

2

2 +

.

It is clear t h a t

f u r t h e r d a t a are n e e d e d . T h e d i m e n s i o n s of the rate constants f r o m w h i c h the A F * values w e r e c a l c u l a t e d are sec." , M sec." , or M 1

1

2

sec." , d e p e n d i n g o n w h e t h e r the 1

h y d r o g e n i o n d e p e n d e n c e is - 1 , - 2 , or - 3 , r e s p e c t i v e l y . T h i s means that the use of different c o n c e n t r a t i o n units w o u l d not c h a n g e the A F * values for the processes i n w h i c h one h y d r o g e n i o n is released b u t w o u l d c h a n g e those f o r three t w i c e as m u c h as those for t w o h y d r o g e n ions released. T h e o b s e r v a t i o n is that the A F * values, b a s e d o n m o l e / l i t e r , a l l f a l l n e a r the same straight l i n e i n spite of the different h y d r o g e n i o n d e p e n d e n c e s . T h i s suggests

either a c a n c e l l a t i o n of

effects,

or that the c h o i c e

of

m o l e / l i t e r f o r t u i t o u s l y makes the c o r r e c t i o n to a c o n c e n t r a t i o n i n d e p e n d e n t basis s m a l l .

T h e c o r r e c t i o n necessary to p u t a l l the a c t i v a t i o n

free energies o n the same basis is difficult to estimate since it i n v o l v e s the t r a n s l a t i o n a l c o n t r i b u t i o n to the entropies of the v a r i o u s solutes. R e a c t i o n s 2 0 - 2 5 i n v o l v e the o x i d a t i o n of V or T i to V 0 or Ti0 ( ? ) b y a c t i n i d e ( V I ) or ( I V ) ions, M 0 or M . E x a m i n a t i o n of A F * VS. A F ° shows t h a t the V - U 0 R e a c t i o n , 23, is a n o m a l o u s l y fast, b u t t h a t the other A F * values are c o r r e l a t e d w i t h A F ° as before, i r r e s p e c t i v e of the n u m b e r of h y d r o g e n ions released i n the net a c t i v a t i o n processes. 3 +

2 +

2

3 +

2

2 +

3 +

4 +

2 +


2 +

20.

Oxidation-Reduction

NEWTON AND BAKER

287

Reactions

Continued AF, kcal./mole 12.2 12.7 13.1 13.6 18.0 18.7

AH, kcal./mole

S* comp. e.u.

AS, e.u.

14.6 14.0 24.1 26.9 27.6 22.8

b

8.0 5.5 4.4 7 37.0 44.7 32.1 ± 0.3 13.9 ± 0.3

AF*, kcal./mole C

Ref. 74(f)

+28 +11 12.0 6.6 -10.0 -45.0

72 76

Estimates of the uncertainties are given for the entropy values o n l y ; those for ΔΗ"* m a y be estimated b y m u l t i p l y i n g b y T ; those for A F * are generally very m u c h smaller. ( r ) indicates recalculated from the o r i g i n a l data. See o r i g i n a l reference.


e

1

s

40

35

Ο

30

Ο

20 360Q35

0 -20 ω -40

34

Ο

36

Ο

30

- θ -

31

Cl

d 33

^ 3 1

O23 23

*co°-60

28

3£»

-80

?

V

16

19

6

32

-!00

9

14

-120 15^

-140

^5

Figure 2. Formai entropies vs. charge for the activated complexes described in Table II. S* complex = AS* + Σ S reactants — Σ S products other than activated complex 0

The numbers refer to the

0

Table


288

LANTHANIDE /ACTINIDE

I

30 Il (3) 18 (3)

Φ

'7(3)_

12 (3)

Y)

9 (2)

I

&

20[ 7 (Ik

σ ο * «


CHEMISTRY

Ό

, (2Î

13 (lî

0

Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium

Recommend Documents