Chemical reaction kinetics difference between homogeneous

Chlorobenzonitrile and adrenaline, our second example, both give electrode products that are unstable with respect to subsequent chemical reaction. Because the products of these homogeneous chemical reactions are also electroactive in the potential range of interest, the overall electrode reaction is referred to as an ECE process that is, a chemical reaction is interposed between electron transfer reactions. Adrenaline differs from/ -chlorobenzonitrile in that (1) the product of the chemical reactions, leucoadrenochrome, is more readily oxidized than the parent species, and (2) the overall rate of the chemical reactions is sufficiently slow so as to permit kinetic studies by electrochemical methods. As a final note before the experimental results are presented, the enzymic oxidation of adrenaline was known to give adrenochrome. Accordingly, the emphasis in the work described by Adams and co-workers [2] was on the preparation and study of the intermediates. 

Homogeneous catalysis does not really raise any special questions. Its problems are the general ones of deciding what chemical reactions are possible between molecules of different kinds what the activation energies will be and what general kinetic laws they wrill follow. But there is one piece of chemistry to which reference should here be made, and that is the widespread influence of acids and bases upon reactions which, as far as conventional chemical equations go, do not appear to demand them. 

In other words, reactants exist everywhere within the pores of the catalyst when the chemical reaction rate is slow enough relative to intrapellet diffusion, and the intrapellet Damkohler number is less than, or equal to, its critical value. These conditions lead to an effectiveness factor of unity for zerofli-order kinetics. When the intrapellet Damkohler number is greater than Acnticai, the central core of the catalyst is reactant starved because criticai is between 0 and 1, and the effectiveness factor decreases below unity because only the outer shell of the pellet is used to convert reactants to products. In fact, the dimensionless correlation between the effectiveness factor and the intrapeUet Damkohler number for zeroth-order kinetics exhibits an abrupt change in slope when A = Acriticai- Critical spatial coordinates and critical intrapeUet Damkohler numbers are not required to analyze homogeneous diffusion and chemical reaction problems in catalytic pellets when the reaction order is different from zeroth-order. When the molar density appears explicitly in the rate law for nth-order chemical kinetics (i.e., n > 0), the rate of reaction antomaticaUy becomes extremely small when the reactants vanish. Furthermore, the dimensionless correlation between the effectiveness factor and the intrapeUet Damkohler nnmber does not exhibit an abrupt change in slope when the rate of reaction is different from zeroth-order. 

It was concluded on the basis of carbonylation reaction kinetics measurements (26) that in spite of the obvious physical differences between heterogeneous (3% Rh/C) and homogeneous (RhCl3 /CH3I) catalysts (5), a similar chemical reaction mechanism is probably operative (Fig. 1). 

The principle of most electrochemical methods is based upon the competition between transport to and from the electrode and creation or disappearance via chemical reactions. Indeed the electrons are supplied at, or taken from, the solution/electrode interface whereas the substrate and products evolve in the solution volume adjacent to the interface. Thus the principle of the direct electrochemical methods consists in opposing the rate of transport, usually by means of diffusion-migration, to that of chemical reactions. Extraction of the sought information then requires the resolution of the pertinent transport-reaction, time and space dependent equations. A more serious limitation of direct electrochemical methods is that the interfacial nature of the electron exchange limits the time scale domain accessible, when the kinetics have to be studied in conditions matching those encountered in homogeneous chemical situations. Indeed, because of the potential difference between the electrode and the solution, an interfacial charged... 

---