Contributions of Svante Arrhenius to reaction kinetics.

Among chemical reactions in the gas phase or in solution that require no external agent such as light or ionizing radiation to bring them about, the concept of reaction order is well understood as the index of the concentration of the reactant that leads to a parameter proportional to the reaction rate. Where the reaction is occasioned by light absorption and is a straightforward conversion of one species into another, the progress of the reaction is controlled simply by the rate of light absorption by the molecules of reactant. However, one can cite circumstances in which the log of the concentration of the reacting species will fall linearly with time and there is then a temptation to describe the photochemical process as being a first-order reaction. The propriety of so doing deserves comment.If one considers a simple photochemical reaction system in which the reactant molecule A absorbs at the wavelength of the exiting light whereas the product molecule B does not, assuming that a parallel and uniform beam of light makes normal incidence on a rectangular cell, it is possible to produce a simple expression for the rate of light absorption. If there are Q quanta per second entering the cell, of path length l, then the number of quanta per second absorbed within the cell is given by Q (1 -10 -εcl ), where ε is the molar decadic absorption coefficient of A at the wavelength of irradiation and c denotes the concentration of A. Consequently the number of moles of A caused to react in unit time will be φQ(1 -10 -εcl )/N A , where φ is the quantum yield for the photochemical interconversion of A to B and N A is Two extreme situations are readily identified. One occurs when εcl > 2, so that well in excess of 99% of the light entering the cell is absorbed within it. Thus the number of moles of A reacting in unit time is essentially φQ/ N A , which is independent of the concentration of A. That is, provided that the concentration of A is much greater than (ε l) ᎑1 , the rate of the photochemical conversion is constant. Consequently, the amount of A in the cell will now decrease linearly, and if the cell contents are well stirred, the concentration of A will fall off in that manner.The other extreme is when εcl << 1, and only a small fraction of the light entering the cell is absorbed within it. In this case the factor (1 -10 ᎑εcl ) is well approximated by 2.303 εcl, so that the number of moles of A reacting in unit time is given by 2.303 φQεcl /N A , which is directly proportional to the concentration of A. Consequently, the amount of A in the cell will decrease exponentially.This shows that, as a consequence of these differences in experimental conditions, this photochemical conversion may fit either of two regimes. In the first, the amount of A in the cell decreases as it would for a reaction of zero order; but in the second, it falls as for a firstorder reaction. Since the manner of the decrease of the reactant concentration is so totally a function of the reaction conditions, it makes no sense to speak of a reaction...

The decomposition of ammonia in the presence of hydrogen, catalyzed by evaporated iron films, was followed using a mass spectrometer. With reaction mixtures of small ammonia content, initial rates of decomposition to hydrogen and nitrogen, without apparent loss of the latter to the catalyst, gave an activation energy of 38.8 kcal/mole and were proportional to pK&sp,Fp",.Reaction mixtures having a higher ammonia pressure reacted with the iron catalyst to form the nitride Fe4N, confirmed by X-ray diffraction analysis. Activation energies for the formation 38, decomposition 38.6, and hydrogenation 14 kcal/mole, of the nitride phase were obtained. From this information and the kinetic data, desorption of nitrogen can be identified as the rate-determining step in ammonia decomposition.Where iron nitride was formed, the influence of temperature on the rate of production of nitrogen showed three distinct regions corresponding to decomposition of ammonia on iron, on a surface which is becoming increasingly nitrided, and finally on an iron nitride surface. The results obtained are discussed in relation to the anomalous behaviour reported for the bulk catalyst no. 954 used by Love and Emmett.4 922

In the radiolysis of aqueous solutions containing N20 and nitrite ion, the yield of reducing radicals appears to increase with the solute concentrations. This increase is much greater than the decrease in G H ~ and therefore cannot be wholly attributed to increased scavenging within the spur of the reducing radicals which would otherwise have combined with another radical. The results are consistent with an initial yield, of G about unity, of a species which, if it were the excited water molecule H20* would have a half-life of about 10-6-10-7 sec, which can react with N20 to produce N2 and whose relative rate constant with NO; and N20 is comparable with that of the solvated electron.

The Bramsted-Bjerrum equation has been derived by a formulation which does not assume a thermodynamic equilibrium of reactants with a transition state or use activity coefficients. Around an ion, Boltzmann distributions of all species are assumed so that the result, like that of the TST method, can be valid only for slow reactions. In its simplest form there is a lack of symmetry in the equations which does not arise if approached by the diffusion model, with rate constant assumed much smaller than encounter number. These equations also demonstrate a weakness in previous work on rates of diffusion-controlled reactions of ions.