KINETICS OF THE OXIDATION OF HYDRAZOBENZENE BY IODINE AND TRIIODIDE

The oxidation of hydrazobenzene (AH2) to azobenzene by iodine in aqueous ethanol solution was found to be very rapid and in order to render the kinetics measurable it proved necessary to reduce the concentration of free iodine by complexing with iodide. This revealed a parallel oxidation path, much slower than for iodine, involving the triiodide ion. The overall rate law thus is: −d[AH2]/dt = k1[AH2][I2] + k2[AH2][I3−] where the rate constants, in 60 volume% ethanol, are given by, k1 = 4.2 × 1011 exp [−10,000/RT] M−1 sec−1and k2 = 2.5 × 1011 exp [−16,400/RT] M−1 sec−1. These and other features of the kinetics, including the inverse dependence of the rate constants on the ethanol concentration and the absence of effects due to added quinone and various metal ions, are interpreted in terms of a simple bimolecular mechanism for each path. Some measurements of the thermodynamics of triiodide formation in aqueous ethanol also are reported.

CATALYTIC ACTIVATION OF MOLECULAR HYDROGEN BY RUTHENIUM (III) CHLORIDE COMPLEXES

Ruthenium(III) chloride, in aqueous HCl solution, was found to activate H2 homogeneously and to catalyze the reduction by H2 of Ru(IV)and Fe(III). The kinetics of these reactions were examined and, in each case, the rate law was found to be −d[H2]/dt = k1[H2][RuIII] where k1 = 4.0 × 1014 exp [−23,800/RT] M−1 sec−1. The mechanisms of these reactions are discussed and compared to those of other homogeneously catalyzed reactions of hydrogen. A special feature of the present system is the resistance of the catalytic species itself (i.e. RuIII) to reduction by H2.

CATALYSIS OF THE CIS-TRANS ISOMERIZATION OF AZOBENZENE BY ACIDS AND CUPRIC SALTS

The catalytic effects of various acids and metal salts on the cis-trans isomerization of azobenzene in aqueous ethanol were examined kinetically. The effect of perchloric acid is apparently due to H+ ions and is interpreted in terms of a catalytic mechanism involving the conjugate acid of azobenzene. The much greater catalytic effect found for hydrochloric acid is attributed to an additional path involving catalysis by the undissociated acid. Acetic acid was found to be inactive. Of the metal salts examined only those of Cu++ showed pronounced activity which was attributed to co-ordination with the azo group. Kinetic evidence was obtained for a catalytic path involving Cu++ and H+ simultaneously.

CATALYTIC ACTIVATION OF MOLECULAR HYDROGEN IN SOLUTION BY CHLORORHODATE(III) COMPLEXES

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CATALYTIC ACTIVATION OF HYDROGEN IN AQUEOUS SOLUTION BY THE CHLOROPALLADATE(II) ION

The kinetics of the reduction of ferric chloride by molecular hydrogen in aqueous solution, in the presence of chloropalladate(II), were examined. The latter acts as a homogeneous catalyst for the reaction. The rate-law was found to be,[Formula: see text]where[Formula: see text]The mechanism of the reaction is discussed.

THE PRINCIPLE OF EQUIVALENCE CHANGE IN OXIDATION–REDUCTION REACTIONS

The principle of equivalence change predicts that oxidation–reduction reactions between 1-equivalent oxidants and 2-equivalent reductants (or vice versa) will, in general, be slow, since they must proceed either through termolecular paths or through the formation of unstable intermediates. In this paper, the kinetics and mechanisms of a number of reactions of this type are examined and an attempt is made to assess the validity of the considerations on which this principle is based. Among the reactions considered are (1) electron transfer between metal ions; (2) oxidation of metal ions by oxygen; and (3) reduction of metal ions by hydrogen. In each of these cases it is found that the principle of equivalence change has only limited validity and that a number of other factors are important in determining the relative rates and mechanisms of reactions of different equivalence type. Among these are the formation of stabilized intermediate complexes between oxidant and reductant and the possibility of unstable intermediates acting as carriers in chain reactions. In reactions of thallium(I) or thallium(III) with 1-equivalent metal ions, thallium(II) is formed as an intermediate. Some of these reactions are not as slow as expected, apparently because of favorable entropies of activation. Several of the reactions examined proceed simultaneously through bimolecular and termolecular paths, the latter being favored because of lower activation energies.

KINETICS OF THE OXIDATION OF MERCURY(I) BY THALLIUM (III) IN AQUEOUS SOLUTION

The kinetics of the oxidation of mercury(I) by thallium(III) in aqueous perchloric acid solution, i.e. Hg(I)2 + Tl(III) → 2Hg(II) + Tl(I), have been examined. The rate law was found to be of the form −d[Hg(I)2]/dt = kexp[Hg(I)2][Tl(III)]/[Hg(II)] where kexp is inversely dependent on the concentrations of H+ and of ClO4−. The rate-determining step of the reaction appears to be a 'two-electron transfer' between a mercury atom, formed by the dismutation of Hg2++, and a hydrolyzed thallium ion, i.e. Hg + TlOH++ → Hg++ + Tl+ + OH−. The rate constant, k, of this reaction is given by k = 1016±2 exp[−14000 ± 3000/RT] liters mole−1 sec.−1.H+ retards the reaction by opposing the hydrolysis of Tl+++, while the effect of ClO4− appears to be due to its complexing with Hg2++, Cl− and Br− catalyze the reaction probably by complexing with Hg++, thus displacing the Hg2++dismutation equilibrium, [Formula: see text], to the right and increasing the concentration of Hg atoms. The kinetics and mechanism of the Tl(I)–Tl(III) isotopic electron exchange reaction and of other electron transfer processes in solution are considered in the light of these observations.

KINETICS OF THE OXIDATION OF URANIUM(IV) BY MOLECULAR OXYGEN IN AQUEOUS PERCHLORIC ACID SOLUTION

The kinetics of the oxidation of uranium(IV) by molecular oxygen in aqueous perchloric acid solution were studied. Over a considerable range of conditions, the results are fitted approximately by the rate law:−d[UIV]/dt = k[UIV] [O2]/[H+], where k ≈ 2 × 1014 exp[−22,000/RT]sec.−1. The reaction is catalyzed by Cu++ and inhibited by small amounts of Ag+ and Cl−. The results are interpreted in terms of a chain reaction mechanism involving UO2+ and HO2 as chain carriers.

EFFECTS OF COMPLEXING ON THE HOMOGENEOUS REDUCTION OF MERCURIC SALTS IN AQUEOUS SOLUTION BY MOLECULAR HYDROGEN

The effects of various complexing agents on the homogeneous reduction of mercuric salts by molecular hydrogen in aqueous solution were determined. In all cases the kinetics suggest that the rate-determining step is a bimolecular reaction between a mercuric ion or complex and a hydrogen molecule, probably leading to the formation of an intermediate mercury atom. The reactivity of various mercuric complexes was found to decrease in the following order: HgSO4 > Hg++ > HgAc2, HgPr2 > HgCl2 > HgBr2 > Hg(EDA)2++. Addition of anions such as OH−, CO3=, Ac−, Pr−, and Cl−, in excess of the amounts required to form stable mercuric complexes, was found to increase the rate. An interpretation of these effects is given.

EFFECT OF COMPLEXING ON THE HOMOGENEOUS CATALYTIC ACTIVATION OF HYDROGEN BY CUPRIC SALTS

The effects of various organic and inorganic complex-forming reagents on the homogeneous catalytic activation of H2 by Cu++ in aqueous solution have been determined. The catalytic activities of the cupric complexes decrease in the order: butyrate, propionate > acetate > SO4− > Cl− > H2O (i.e. the uncomplexed Cu++ ion) > glycine, ethylenediamine. The mechanism of the activation process is discussed.

KINETICS OF THE CUPRIC ACETATE CATALYZED HYDROGENATION OF DICHROMATE IN AQUEOUS SOLUTION

In aqueous solution, cupric acetate was found to act as a homogeneous catalyst for the reduction of dichromate by hydrogen, i.e.[Formula: see text]

The paper describes a kinetic study of this reaction. Rates were determined at temperatures between 80° and 140 °C. and hydrogen partial pressures up to 27 atmospheres. The rate is independent of the dichromate concentration but varies directly with the partial pressure of hydrogen and is nearly proportional to the concentration of cupric acetate. The activation energy is 24,600 calories per mole. Cupric acetate, apparently acting as a true catalyst, activates the hydrogen through formation of a complex with it. An extension of the mechanism proposed earlier for the reaction of cupric acetate itself with hydrogen also accounts for the kinetics of the dichromate reaction.

KINETICS OF THE HOMOGENEOUS REACTION BETWEEN CUPRIC ACETATE AND MOLECULAR HYDROGEN IN AQUEOUS SOLUTION

Cupric acetate was found to react homogeneously with molecular hydrogen in aqueous solution according to the following equation:[Formula: see text]The paper describes a kinetic study of this reaction. Rates were determined at temperatures between 80 and 140 °C and hydrogen partial pressures between 6.8 and 34.0 atm. The reaction was found to be of second order, the rate being proportional to the concentrations of cupric acetate and molecular hydrogen. It was established that the rate was independent of the surface of the reaction vessel, the cuprous oxide product and of the concentrations of sodium acetate and acetic acid in the solution. The reaction has an activation energy of 24200 cal. per mole. The kinetic results are discussed and a mechanism is proposed. This appears to be one of the few known homogeneous reactions of molecular hydrogen in solution.

THE POTENTIOMETRIC TITRATION OF CARBONATE SOLUTIONS CONTAINING URANIUM

Potentiometric titrations with hydrochloric acid were carried out on standard sodium carbonate solutions containing varying amounts of uranyl nitrate. The results confirmed the fact that uranium is present in such solutions as the complex ion [Formula: see text]. It was found that only the free CO3−− is titrated with H+ up to the first end point at pH = 8. 2. The complex ion is very stable and is decomposed only on further addition of acid when the complexed CO3−− along with the HCO3− in the solution is converted to carbonic acid before the second end point. The pH at which the second end point occurs is lowered from its normal value of 4.0 in the presence of uranium. This effect is attributed to hydrolysis of the UO2++ ion. The necessary corrections for determining carbonate and bicarbonate in the presence of uranium are given.