J. C. HINDMAN, J. C. SULLIVAN AND DONALD COHEN
3278
TABLE I1 INTERPRETATION OF EFFECTS OF ADDEDSOLUTES Substance
Temp., 'C.
102
Ao Amin Amax Slope
Intercept
po
y
CiHjI
20 38.6 25 70 20 0 . 3 5 3 . 6 0.32 -78 37.0 25 CzHjBr 0 31.8 20 60 9 . 4 .35 2 . 5 .47 -78 34.5 20 60 5.7 .45 1 . 7 .54 n-C3H7Bra 25 34.0 22 60 7 . 2 .38 2 . 1 .51 20 37.0 25 56 n-C4HgIb .49 1 . 7 .47 6.4 C&Ib 0 56.0 36 95 9 . 2 .42 2 . 3 .43 -46 57.5 95 a Data of Rowland and Libby, ref. 3. Data 011 the effects of free halogen on the retention were taken from the work of Levey and Willard, ref. 6.
VOl. 76
tion and mean free path are quite gratifying. It is clearly desirable to test the preceding diffusion equation in a. more nearly appropriate system where there is less uncertainty regarding the initial act. Suitable data are, unfortunately, scarce and such a test has thus far been completed only for the photolysis of tribromide ion with bromide ion as the solute17and Mn(I1) as scavenger. The primary process is Bra-
4- hv
+Brz-
+ Br
and the effect of the solute in preventing recombination is probably Br
followed by
significant. T o simplify this evaluation let i_t be assumed that PI = P2 = 1, replace n;/2 by &/I, and employ the reduced parameters PO = Ro/d, y = L/d. From the preceding considerations it follows that
Mn(l1)
+ Br- --+ Br2-
+ Br2- +Mn(Il1) + ZBr-
We find for this reaction po 'v 0.6, y 'u 0.5. Details of this and other related work will be published. Acknowledgments.-The authors wish to thank - h ( 1 - Tt',(o)) = - A O ) / ( & , ~- &,in) = Dr. C. E. McCauley for helpful discussions and ( d . G P O Y 1-1 Dr. R. M. Noyes for his detailed criticism of this Combining these results with the slopes of Figs. 4, paper. This research was supported in part by the 5 , viz. d / a p ~ / ywe obtain the values of the reduced AEC under contract At(11-1)-38. (17) X C. Rutenberg and H. T a u b e , THISJOURKAL, 73, 4426 parameters which are listed in the last two columns of Table 11. In view of the uncertainties involved (1951) the range of values obtained for the initial separa- NOTREDAME,INI~IASA
[COXTRIBYrIOY FROM THE CHEMISTRY IlIVISIOh,
ARGOXXEr\jATIOSAI,
LABORATORY]
Kinetics of Reactions between Neptunium Ions. The Neptunium(1V)-Neptunium(V1) Reaction in Perchlorate Solution 1 3 J. ~ C.
HIKDMAN, J. C.SULLIVAN AND DONALD COHEN RECEIVED JANUARY 14, 1954
+
An investigation of the mechanism of the reaction Kp(1V) Np( V I ) F? 2Np( V ) has been undertaken in perchlorate media. The rate law for the forward reaction is given by: - d ( I f ~ + ~ ) / = d t k ~ O [ S p +[Np02++] ~l [H+]-z. At 25" and p = 2.00, AH* is 24.6 kcal.; A S * is 17.8 cal./deg. and A F * is 19.3 kcal. At 25" and p = 1.00, k10 is k1° is 2.69 mole 1.-' rnin.-l; 3.13 mole 1.-l min.-l. The effect of ionic strength has been measured. Tmo alternative mechanisms for the reaction are postulated.
Few quantitative investigations have been made of the kinetics of reactions in which neptunium ions participate. Huizenga and Magnussonl have reported that the oxidation of N ~ + ~ ( a qby . ) Fe+3 (aq.) appears to proceed by a mechanism involving a hydrolyzed species of neptunium(1V). The exchange reaction between N ~ + ~ ( a q and . ) NpOz' (aq.) has been found to be a complex reaction involving two different paths depending on the acid concentration range.2 Although the disproportionation reaction of neptunium(V) to give neptunium(1V) and -(VI) has been reported3 a t high acid, the experimental conditions make it difficult to interpret the results in any detail. To further extend our information in this field, the investigation of the reaction between N ~ + ~ ( a qand . ) NpOz++(aq.) was undertaken. Details o€ this investigation are reported in the present communication. (1) J. R. Huizenga and L . B. Magnusson, THISJ O W R K A L , 73, 3202 ( 1 93 1). ( 2 ) J. C. Sulliran, D. Cohen a n d J. C. Hindman, i b i d . , in press (1954). ( 3 ) R. S j u b l u n i atid J. C Hindman, ibid., 73, 1 7 4 1 (1921).
Experimental The neptunium stock solutions in perchloric acid were prepared electrolytically from a neptunium(V) stock in perchloric acid according t o previously described method^.^ The reaction solutions were prepared volumetrically using standardized solutions of sodium perchlorate, perchloric acid and neptunium(1V). A two-centimeter silica absorption cell was used for the reaction vessel. After spectrophotometric measurement of the density of the neptunium(I\'). band a t 724 mp3 t o give a value for the initial optical density, the neptunium(V1) was added. The progress of the reaction was followed by monitoring the neptunium (IV) band at 724 mp. Small corrections were made for the absorption of the neptunium(V) produced in the reaction The temperature in the thermostatic cell compartment of the C$y recording spectrophotometer was maintained t o fO.l . The acid concentration in the reaction solution was checked after completion of the reaction by titration with standard alkali .z
Results and Discussion Effect of Metal Ion.-The stoichiometry of the reaction with respect to each of the metal ions was determined by experiments in which the concentra(1) D Cuhcu a n d J C H i n d m a n , $bid 74, $679, 4682 (1952)
NEPTUNIUM(IV)-(VI)REACTION IN PERCHLORATE SOLUTION
June 20, 1954
tions of neptunium(1V) and neptuniwn(V1) were varied in the reaction mixture. Graphical analyses showed that the data were most consistent for a bimolecular process. The results of a typical experiment are summarized in Table I. The precision of the data as in all cases reported is given in terms of standard deviation. The apparent rate constant, k', was calculated by the integrated equation for a bimolecular process kl' = ([Np(IV)]
2.303 [Np(VI)]O)t log
O
-
TABLE I THE RATEOF REACTIONO F Np(IV) WITH Np(V1) p = 2.00; [H+] = 1.OOOM; [Np(IV)] = 4.99 X [Np(VI)]O = 1.024 X M Time,
dr14,
mm.
cor.
0 5.00 10.00 15.00 18.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 60.00 70.00 80.00
1.374 1.221 1.070 0.969 ,911 ,875 ,778 ,700 ,628 ,567 ,520 ,467 .438 ,387 ,334 ,273
AT
culated from the relation for a bimolecular reaction k' =
M;
2.39 2.61 2.46 2.49 2.48 2.51 2.57 2.59 2.61 2.60 2.63 2.56 2.65 2.61 2.67
-
(2)
x)t
and K O from the relation kO = k ' [ H + I n
(3)
TABLE I11 EFFECTOF HYDROGEN IONCONCENTRATION ON RATE OF REACTION OF NEPTUNIUM(IV) AND NEPTUNIUM(VI) AT 25" p = 2.00, [Np(IV)]O = [Np(VI)]' = 5.00 X lo-* M kO W+18 k', moles/l.
1. mole-1 m i n - 1
n = 1
n = 2
n = 3
1.994 1.046 0.532 0.227
0.581 2.60 10.47 54.5
1.16 2.69 5.57 12.4
2.31 2.84 2.96 2.81
4.61 2.94 1.58 0.64
TABLE IV STRENGTH ON RATE O F REACTIONO F EFFECT OF IONIC NEPTUNIUM(V) AND NEPTUNIUM(VI) AT 25" [Np(IV)] = [Np(VI)] = 5.00 X M [H+l,
where [Np(IV)]', [Np(VI)]', represent the initial concentrations of reactants, and x the concentration of Np(1V) that has reacted a t time, t . The back reaction can be neglected in the calculations of the forward rate constants since a t equilibrium the concentrations of Np(1V) and Np(V1) are negligible. The results of experiments a t varying neptunium (IV) and neptunium(V1) concentrations are summarized in Table 11. The kI0 values are corrected for the effect of hydrogen ion concentration as discussed in the next section. TABLE I1 ON THE KATE OF EFFECTOF METALIONCONCENTRATION REACTIOSO F NEPTUNIUM(1V) AND NEPTUNIUM(\'I) AT 25" IL = 2.00 [KPUV) la, moles/liter x 103
[NP(VI) lo, moles/liter x 103
[H +]. moles/l.
1. mole-1 mk-1
kio, mole 1. -1 mk-1
2.42 4.85 4.99 6.47
4.89 4.89 10.24 3.07
1.046 1.046 1.000 1.022
2.63 2.60 2.56 2.35
2.88 2.84 2.56 2.45
Effect of Hydrogen Ion.-The hydrogen ion order of the reaction was determined by measuring the rate of the reaction as a function of hydrogen ion concentration a t constant and equal Np(1V) and Np(V1) concentrations. The data are summarized in Table 111. The kl' values were cal-
ki0,
moles/l.
P
0.494 ,502 ,511 ,520 1.046 0.532
0.55 0.55 1.00 1.00 2.00 2.00
Av. 2.56 f 0.08
ki',
X
[NP(IV)I'( [NP(WI'
As can be seen from Table 111, the best fit of the data is given with n = 2. Effect of Ionic Strength.-Very little effect of ionic strength on the reaction 2) (1) - x rate is indicated by the data in Table IV. There is a slight trend in ko with increasing 25" ionic strength.
Moles/l. x 104
5.57 11.1 14.7 17.0 18.3 21.6 24.6 27.1 29.3 31.1 32.9 34.0 35.8 37.8 40.0
3279
mole 1. -1 min. -1
3.54 3.35 3.11 3.16 2.84 2.96
The Effect of Temperature.-In Table V are summarized the data on the temperature coefficient of kIo. From the data in Table V, which are plotted in Fig. 1, the Arrhenius energy of activation, AE, for the reaction was calculated to be 25.2 f 1.6 kcal. using the method of least squares. TABLE V ON THE RATE OF REACTION OF EFFECTOF TEMPERATURE NEPTUNIUM( IV) AND NEPTUNIUM(VI) p = 2.00 [Np(IV)]' = [Np(VI)]' = 5.00 X lo-' M ki', kio Temp., W+I. moles/l.
1. mole min.-'
1,009 1.017
10.01 9.79
....
, . .
1,004 1.013 a
0.609 0.614
mole
1.-1
mk-1
10.19 i 0 . 3 5 10.13 f .31 2 . 6 9 i .23 0.614 zk .03 0.630 =!= .03
O C .
34.8 34.7 25.0" 15.5 15.4
Mean of data for 25" (metal and hydrogen ion effects).
The heat and entropy of activation for the reaction were calculated from the rate expressions for the transition state theory5in the form kT -
klO=eh
__
- A E hS* k' eRT e R = e h
The computed values of A H *
e
-(AH*+RT) RT
= 24.G f
=*
e R
(4)
1.6 kcal.,
( 5 ) S. Glasstone, K . Laidler and H Eyring, "The Theory of Kate Processes," McGraw-Hill Book C o . , Inc., K e w York. N.Y . , 1941, PI>. 197-199.
J. c. HINDMAN, J. c. SULLIVAN
3280 \ '
I
1
I
I
i
AND
DONALD COHEN
Vol. 76
It is of interest to note that the simplest molecularity for the neptunium(V) disproportionation that can be deduced from the stoichiometry of the over-all reaction 5 , i.e. -d [NpOz'] /dl = k [NpOz+] [H+] *
(7)
differs from that found for the analogous uranium (V) r e a c t i ~ n . ~ In~ the ~ latter case the rate law is given as -d[UOp+]/dt
1x
Fig. 1.-Effect
10:
rapid equilibrium
of temperature on kI0.
+
+ 2H20
2Sp0zc(aq.)
+ 4"
(5)
is 5.45 X lo6 a t 25" in 1 M perchloric acid. Using the data for kI0 a t unit ionic strength
+ Np(V1) 2 ~NP(V) kz ki
Np(IV)
the value of kzo = klo/K = 5.74 mole-3 min.-'.
f
0.55 X
*
followed by
A S * = 17.8 =k 2.7 cal./deg. and AF* = 10.3 =k 1.6 kcal. From the free energy, heat and entropy data4 for the over-all reaction (equation 5 ) and the data for the forward reaction given above, AF* = 28.5 f 1.6 kcal., AH* = 17.1 =k 1.6 kcal. and AS* = -38.1 =k 2.9 cal./deg. for the disproportionation reaction of neptunium(V). In these calculations the effect of the difference in ionic strength a t which the data were obtained was neglected. The Disproportionation Reaction.-From the published data4 on the potentials of the neptunium couples, the constant, K , for the over-all reaction X ~ + ~ ( a q . )NpOZ++(aq.)
= k[H+][U02']2
(8)
Why the stoichiometry should be different in two cases is not readily explained. Mechanism of the Reaction.-Based on the served metal ion dependencies and hydrogen dependence, a number of mechanisms can postulated for this reaction. Two of these are S p f 4 ( a q . ) + HzO I_ N p O + + + 2 H +
(6)
SpO++
+ spo*++
the obion be
(9)
[0-Xp-0-Np-01 +4 --+XpOz' NpO+++ (10)
+
and then XpO+'+
+ HzO
Np02'
+ 2H+ rapid equilibrium (11)
Alternatively, we may write
+
S ~ " ~ ( a q . ) 2H20 Xp( OH)+2z 2H rapid equilibrium (12) S ~ ( O H Z ) + S~p O z + + [O-Np-O---H-O-Np-O-H] ---+ 2Np00H" (13)
+
+
+
+'
and NpOOH+2
;\'pOz+
+ H+
rapid equilibrium
(14)
With the data available a t present, preference for either the oxygen transfer or the hydrogen transfer mechanism is a matter of individual insight. LEMONT, ILLINOIS (6) D. M. H. Kern and E. F. Orlemann, THISJOURNAL, 71, 2102
L3
(1949).
(7) H. G. Heal and J. G . N. Thomas, Trans. F n r n d a y Soc., 46, 11 (1949).
