Factors Influencing Reaction Rates

It is useful here to recapitulate and place on record the different factors influencing reaction rate. These are ... 

Temperature—Temperature is the most important factor influencing reaction rate as shown in the Arrhenius equation, which describes the appropriate relationship indicating temperature as an exponential term... 

Temperature is a direct measure of the heat energy available at release (Edwards and Lawrence, 1993). Temperature is the most important factor influencing reaction rate as shown in the Arrhenius equation. In practice an increase in temperature of 10°C will increase a specific reaction rate by two to four times depending on the energy of activation (CCPS, 1995a). 

Our knowledge of these factors influencing reaction rates is quite elementary in many respects. Direct study is difficult in many instances, because of the complex nature of the hydrous metal oxide surface. Much of our current understanding comes from using indirect methods. One useful approach is to make small changes in reactant structure, then examine the effect on reaction rate. Another approach is to systematically examine how reactant concentration, medium composition (pH and ionic strength), and the presence of other adsorbing solutes influence reaction rate. 

Learning Goal Five major factors influence reaction rate ... 

By contrast, in the same sediment slurries, sulfonated indigoes, such as indigo-5,5 -disulfonate, were found to be much less reactive, with half-lives of one or more days (Equation 3.42). For a series of such redox indicators, it was found that structure type was the primary factor influencing reaction rates, and that reduction potentials of the indicators would not be useful for predicting reactivity in reducing sediment-water systems. 

Bacteria found in lakes use dissolved oxygen gas to metabolize organic contaminants and use them as a food source, thus removing the contaminants from the lakes. When lake temperatures increase significantly, the rate of this chemical decontamination decreases. Remember your study of gases and explain this apparent violation of the factors influencing reaction rates studied in this chapter. 

Concentration molecules must collide to react. A major factor influencing reaction rate is reactant concentration, because a reaction can occur only if the reactant molecules collide. The more molecules present in the container, the more frequently they collide, and so the more often they react. Thus, reaction rate is proportional to the... 

It has been reported that the principal factor influencing reaction rate is the rate of shear energy application per gram J [31, 35], which is the average shear stress times shear rate (see also Section II.B). Certain other studies... 

Alginate impression materials are chemically reactive mixtures. AH factors that influence reaction rates are, therefore, important in the use of these materials, ie, correct proportioning temperature of the water, powder, and mixing equipment and spatulation rate and duration. 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

Before we can go on to consider the actual use that may be made of Hammett plots, it is necessary to provide some physical justification for influencing reaction rates and equilibria. 

For both catalytic and stoichiometric reactions, each step of the process taking place on the metal can be influenced by the nature of ligands, cations, anions, or solvent. The effects of these factors on reaction rate, selectivity, stereoselectivity, etc. cannot be easily predicted, because each step can be influenced in different ways. The reader is referred to the literature cited below. 

Many different approaches have been reported in the last decade toward a better understanding of the medium factors that influence reaction rates. Fundamental studies have been devoted to probe the reaction at a microscopic level in order to obtain information on the nature of several specific solvent-solute interactions on S Ar and to attempt a description of these effects quantitatively. Recent works have shown the wide applicability of a single parameter scale such as the Ex(30) Dimroth and Reichardt37, as well as other multi-parameter equations. 

Kinetics is the study of the factors which influence reaction rates. Enzyme-catalysed reactions are subject to the same principles of rate regulation as any other type of chemical reaction. For example, the pH, temperature, pressure (if gases are involved) and concentration of reactants all impact on the velocity reactions. Unlike inorganic catalysts, like platinum for example, there is a requirement for the substrate (reactant) to engage a particular region of the enzyme known as the active site. This binding is reversible and is simply represented thus ... 

The mechanism of activation of 8.137 in Fig. 8.13,a was shown to be an intramolecular nucleophilic attack by the phenolate ion at the carbonyl C-atom. Consequently, the rate of reaction increased linearly with pH, plateauing beyond 8-9. The major factor influencing the rate of reaction was the acidity of the leaving ROH, with tm values at pH 10 and 25° ranging from 290 d... 

The mechanisms by which liquid crystals influence reaction rates need further study. Presumably, one important factor is the effect of ordering on entropy changes of various steps of the reaction. Bacon (44) has considered this factor in connection with the ability of thermotropic liquid crystals to increase the polymerization rate of phenylacetylene. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

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