Factors that Influence the Velocities of Reactions

As a consequence, where there is a change in the total number of moles of gas, the rate of conversion for a reaction at constant volume and temperature will be given by 

From the perfect gas equation, if a gas phase reaction in which Ev, + 0 occurs at constant temperature and pressure, the volume must change, hi this case, if all the components follow ideal behaviour 

It is important to prove that eq. (2.2) defines the rate of conversion in the reaction, that eq. (2.4) defines the velocity of the reaction and that in the expressions (2.5), (2.6), (2.12) and (2.15), the quantities (d[j]/dt)yj, (dP/dt), (dP/dt)yj and dV/dt)j-p are proportional to the velocity of the reaction. It must also be emphasised that, generally, the rate can be defined in terms of any of the reactant or product molecules, provided that the stoichiometry of the reaction is included. In other words, eq. (2.16) is valid 

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FACTORS THAT INFLUENCE THE VELOCITIES OF REACTIONS 2.2.1 Nature of the reagents... 

Enzymes can be isolated from cells, and their properties studied in a test tube (that is, in vitro). Different enzymes show different responses to changes in substrate concentration, temperature, and pH This sec tion describes factors that influence the reaction velocity of enzymes. Enzymic responses to these factors give us valuable clues as to how enzymes function in living cells. 

In addition to catalyst pore structure, catalytic metals content can also influence the distribution of deposited metals. Vanadium radial profile comparisons of aged catalysts demonstrated that a high concentration of Co + Mo increases the reaction rate relative to diffusion, lowering the effectiveness factor and the distribution parameter (Pazos et al., 1983). While minimizing the content of Co and Mo on the catalyst is effective for increasing the effectiveness factor for HDM, it may also reduce the reaction rate for the HDS reactions. Lower space velocity or larger reactors would then be needed to attain the same desulfurization severity. 

These have shown that complete hydrolysis of dichloroethyl sulphide takes place by an irreversible reaction except in the presence of a considerable quantity of hydrochloric acid. The velocity of the hydrolysis may be determined either by measuring the development of acidity or of the quantity of ionised chlorine present (Hopkins), or from the decrease in electrical resistance. This velocity is influenced by various factors, such as the time of contact, the temperature, the water/dichloroethyl sulphide ratio, the quantities of acid, alkali and hydrolysis products present, as well as the degree of dispersion of the dichloroethyl sulphide in the water. 

The views so far presented in this chapter may be summarized as being based upon the primary formation of addition compounds when two or more molecules react, these addition compounds then breaking down to form new molecules. In catalytic reactions, the first stage of the reaction is the same, but. in the second stage, one of the substances formed in the breaking down of the intermediate compound is identical in composition with one of the substances which took part initially in the reaction in the formation of the addition compound. While the experimental evidence is favorable to this view of catalytic reactions in many cases, it may be objected that physical influences may often modify the velocity of the reaction between gases. At present there is no experimental evidence of any kind available to prove or disprove the formation of definite chemical compounds in such cases, but on the other hand, evidence is accumulating that adsorption (or perhaps the solution of a gas or a liquid in a solid) is the important factor here. Just how far phenomena of this nature may be identical with the formation of definite chemical compounds (possibly so-called loose combinations) on a surface is not at present certain, but until direct evidence is obtained that such reactions must be included in a... 

In general the vibrational partition functions are small compared with the rotational, and the latter in their turn with the translational. Consequently the product in the formula for kg is small, that is the concentration of transition complexes is low. The non-exponential factor in the Arrhenius equation is therefore small or, otherwise expressed, the entropy of activation is low. The reaction velocity will only be appreciable in these circumstances if is small, which, for the oxidation of nitric oxide, it proves to be. If E is small enough, the influence of the exponential term is unimportant, and the temperature variation of kg may be determined by such terms in T as the partition functions themselves contain. In the present example the non-exponential term contains an inverse cube of the absolute temperature, which, since E 0, imposes the negative temperature dependence of the reaction velocity. 

Enzymatic reactions are influenced by a variety of solution conditions that must be well controlled in HTS assays. Buffer components, pH, ionic strength, solvent polarity, viscosity, and temperature can all influence the initial velocity and the interactions of enzymes with substrate and inhibitor molecules. Space does not permit a comprehensive discussion of these factors, but a more detailed presentation can be found in the text by Copeland (2000). Here we simply make the recommendation that all of these solution conditions be optimized in the course of assay development. It is worth noting that there can be differences in optimal conditions for enzyme stability and enzyme activity. For example, the initial velocity may be greatest at 37°C and pH 5.0, but one may find that the enzyme denatures during the course of the assay time under these conditions. In situations like this one must experimentally determine the best compromise between reaction rate and protein stability. Again, a more detailed discussion of this issue, and methods for diagnosing enzyme denaturation during reaction can be found in Copeland (2000). 

According to transition-state theory it is possible to consider reaction velocities in terms of a hypothetical equilibrium between reactants and transition state. It follows that the influence of the isotopic composition of the medium on reaction velocity can be considered to be the same as its influence on the concentration of transition states. The kinetic formulation of the problem can thus be replaced by one couched in equilibrium terms, and the equilibrium theory of the preceding section can be applied with a minimum of modification (Kresge, 1964). The rate constant, or catalytic coefficient, (k) for a catalysed reaction can be written as the product of three factors, viz. the equilibrium constant (K ) for the process forming the transition state from the reactants, the transmission coefficient, and the specific rate of transition state decomposition (kT/h). We recognize that the third factor is independent of the isotopic nature of the reaction and assume that there is no isotope effect on the transmission coefficient. It follows that... 

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