Noms RATECON8ThNTR
FOR IONTC n1RSOCIATTflN IN
‘I‘A~LF, I AQTTEOI:S 8OLUTIONS
OW
hf A C N E S I U M
MhNoaNous
SULI”ATE! A N D
SULFATE
(250) Degree of
Salt
Magnesium sulfate
Manganoussulfate
Concn. (mole/l.)
diesociation, 6
0.003 .005 ,008 .010 .014 ,020 .001 ,002 .005 ,010 .020
0.851
Mmn rntional nctivity cocfficient
.809
.776 .760 .734 .707 .go2 ,851 .779 .717 ,662
-Tabuchi’s
f*
0.658 .594 .542 .515 .473 .430 .770 ,705 ,604 ,522 .439
12.6 x 106 11.57 10.32 9.21 8.24
ON THE REACTIVITY OF HYDROGEN ATOMS IN AQUEOUS SOLUTIONS BY JOSEPH RABANI Department of Physical Chemiatru. The Hebrew unite rail^. Jerwalem, 1 8 T d
Receited Julu 19. 1961
Hart’ measured the constants ratio kl/kz for the reactions where HC02H denotes both formic acid H + +HOs + HCOzH +Hz + COzH 0 2
13
(1) (2)
and formate ion. Using various concentrations of formic acid and oxygen, it was shown that at low formic acid concentrations the ratio kl/k2 is about 500 a t p H -3. This value was used for the estimation of rate constants218a t other pH values. However, a t the high formic acid concentration (1 iM)this ratio is about 5000 (pH 1.7-1.8).’ This discrepancy is due to the different reactivities of IICOzH and I-IC02- toward hydrogen atoms.4 Using various concentration ratios of formic acid and ferricyanide, the yield of the hydrogen produced by the action of X-rays was measured a t the p1-I range of 1-3.4 In this system, formate and ferricyanide compete for the hydrogen atoms according to reactions 2 and 3. H
+ Fe(CN)s*- +H + + Fe( CN)64-
ki,,
I./molo-sec.
5.74 X 10’ 5.52 5.01 4.43 4.35 4.30
sulfate solutions are of the order of 7.7 and 120 (cc./mole)2, respectively.
(3)
The ratio k3/k2 is pH dependent. At pH 1.3 this ratio is about 3500, falling to 800 a t pH 2.05 and to 300 a t p H 2.55. These results were obtained in solutions of constant (0.047 M ) formic acid concentration, for direct comparison with Hart’s experiments. These results indicate that in the ferricyanideformic acid system, only ferricyanide and formate ion (and not the undissociated acid itself) react with H atoms, a t the pH range investigated. Reaction 2 has to be replaced by (2’). (1) E. J. Hart, J . A m . Chrm. Soc., 7 6 , 4312 (1954). (2) J. H . Baxendale and D. 11. Smithies, 2. phyaik. Chcm. (Frankfurt). 7, 242 (1956). (8) P. Ricsz and E . J. Hart, J . Phye. Chcm., 6 3 , 858 (1959). (4) J. Rabani. P1i.D. Thesis, The Hebrew University, Jerusalem, Israel, 1961.
--
theory-
krl BCC.-’
9.11 8.76 7.95 7.03 6.90 6.87
x
107
2.52 2.31 2.07 1.84 1.65
x
100
H
B i d thcory-
ki. sec. -1
kh, I./molesec.
6.37 X 105 6.47 6.19 5.64 5.82 6.16
10.1 X lo7 10.3 9.8 9.0 9.2 9.8
13.2 x 106 12.63 12.25 12.06 11.72
+ HCOs- +H1 + COz-
2.64 x 109 2.53 2.45 2.41 2.35 (2’)
Kinetic evidence indicates that the oxidation of formic acid in solution by C12,6 permanganate,s some other inorganic ions6 and SOr- radical’ mainly proceeds by the oxidation of the formate ion. The values of k3/k2’ were found to be 12.5 a t pH 1.3, 15.4 a t pH 2.05 and 17.9 a t pH 2.55. Thus, k3/k2’ does not depend on t,he pH, within experimental errors, in agreement with the present interpretation. In Table I. our results for the ferricvanide-formate system are compared with those for the oxygen-formate system.’ TABLEI THEREACTIVITY OF H ATOMSIN FORMIC Acm SOLUTIONS kdktl
ki/kP
kdki
1.71 3950 1.83 5600 2.32 1875 2.35 945 3.12 526 a Computed values.‘
1700 1300 430 385 81
2.3 4.3 4.4 2.4 6.5
PH
Taking into account the statistical weight of these data, a mean value of kl/k3 = 5 is obtained. Since k3/k2 is 3500 in 0.1 N HzS04, it follows that kl/k2 = 17,500 a t the same pH. Using the recenb + X lo6 results of Riesz and Hart3 for k ~ + F e ~ (4.8 1. mole-‘ sec.-l) a t pH 2.1, the following corrected values for the reactivity of H atoms are obtained (Table 11). These “absolute” values are based on gaseous phase data. The numerical values in column 2 of this table represent relative rate constants with respect to 0 2 : k~ + s/kH + ormultiplied by lo4. The values taken from reference 4 were obtained from measurements of hydrogen yield in the ferricyanide-organic solute systems. Much care should be taken in considering the reactivity of hydrogen atoms in aqueous irradiated ( 5 ) J. Thamaen, Acto Chem. Scand., 7 , 682 (1953). (6) J. Halpern and 9. M. Taylor, Diecussions Faradou Soc., 174 (1960). (7) E. J. Hart, J . A m . Chem. Soc., 83, 567 (1961). (8) W. G. Rothsohild and A. 0. Allen, Radiolion Reeearch, 8, 101 (1958). (9) J. H. Baxendale and G. Hughes, 2. phyaik. Chem. (Frankfurt), 14, 306 (1958).
TABLE TI I~P:ACTIVITY O F €1 ATOMS
compourid UCdu. The existelice and formula of this compound werc verified in t,he course of IL study
of the volatilization of cadmium from U-Cd alloys means of a recording effusion balance. Thc by sec. -1) Ref. pressure due to decomposition of UCd,, t'o cadDn 0.4 Gas 3 re'+ 48 2.1 3 mium vapor a,rid a-uranium, measured in the course Fez + 6.7 2.1 8 of that study and later, is the subject, of this iiotc. On 10000 2.1 8 The cxperimenta,l arrangemciit of the recording Fe(Ch')s'2000 1-3 4 effusion balance has been described elsewhere. IICOI 130 1-3 4 HCOyH
