Reaction rates concentration effects

The high ionic concentration at the micellar surface may result in an ionic strength effect on reaction rate. Salt effects in water, however, are generally smaller for ion-molecule reactions than for reactions which involve an increase or decrease of charge, and they should be approximately zero in the... 

This methodology can be used for the calculation of local reaction rates and effectiveness factors in dependence on gas components concentrations, temperature and porous catalytic layer structure (cf. Fig. 9). The results can then be used as input values for simulations at a larger scale, e.g. the effective reaction rates averaged over the studied washcoat section can be employed as local reaction rates in the ID model of monolith channel. 

In conclusion of Section 6.3 we wish to stress that the elastic attraction of similar defects (reactants) leads to their dynamic aggregation which, in turn, reduces considerably the reaction rate. This effect is mostly pronounced for the intermediate times (dependent on the initial defect concentration and spatial distribution), when the effective radius of the interaction re = - JTX exceeds greatly the diffusion length = y/Dt. In this case the reaction kinetics is governed by the elastic interaction of both similar and dissimilar particles. A comparative study shows that for equal elastic constants A the elastic attraction of similar particles has greater impact on the kinetics than interaction of dissimilar particles. 

Figure 4 shows that higher concentrations of seed latex decrease the reaction rate, at constant mixed emulsifier concentrations, while Figure 5 shows that the smallest latex particle gave the lowest reaction rate. This effect is explained by the decreased availability of emulsifier to create micelles, since the small seed latex particle or high seed latex concentrations adsorb more of the surfactant, thus removing it from the water phase. 

The interactions between organic molecules and the pore walls of similar size are very strong (Type I adsorption isotherms) and zeolites may be considered as solid solvents (11, 15, 45). The reactants concentration in zeolite micropores is therefore considerably higher than in the gas phase with a significant positive effect on the reaction rates. This effect is all the more pronounced as the reaction order is greater, favoring more the bimolecular over the monomolecular reactions. 

The reaction rate is effectively controlled by the rate of dissociation of H2O2, and the induction period is determined by the rate of build-up of this species. Since H2O2 is the least reactive chain centre the partial stationary state procedure of Semenov [60] may be used, in which a differential equation is set up for the H2 O2 concentration, and stationary state equations for the other species. Thus... 

Effect of Reactant Concentration on Reaction Rate (70). Effect... 

During oxidative degradation, a concentration gradient always develops at a film surface. Inasmuch as the depth profile depends on permeabilities and reaction rates, the effect is more noticeable in photooxidations than in thermal oxidations. An unusually marked skin effect observed in photooxidized polypropylene has been ascribed (14) to the action of chronophores located at or near the surface. 

Lastly, practically in any kinetic experiment there are some uncertainties in its conditions and results. This concerns such factors as mixing of reactants, regimes of gas flow in different zones of the reaction system, wall effects, axial and radial temperature gradients, spatial distribution of reaction rates, concentrations, pressure, etc. A lot of effort must be applied even to measure the reactant conversion and the main product distributions at the exit of the reactor, needless to say anything about concentrations of active intermediates and concentration distributions inside the reactor. Moreover, up to now there is no clear understanding about which measured parameters are the most informative and should be preferred. 

The rate constant A is a composite parameter, k = ELk, where E is the effectiveness factor, L the concentration of active sites on the surface of the catalyst, and k the actual rate constant of the transformation of the adsorbed species. The effectiveness factor which can attain values from zero to one is a measure of retardation of the reaction by diffusion of reactants or products into or out ofthe pores of the catalyst. For our purpose it should have a value of one or near to one and with careful experimentation this can be achieved. According to Thiele (14) the effectiveness factor is a function of reaction rate and effective diffusion coefficient. Both these parameters depend on the structure of the reacting compound and therefore the effectiveness factor will tend to change with the nature of the substituents. The effect of structure on reaction rate is more critical than on diffusion coefficient and if the reactivity within the series of investigated compounds will vary over some orders there is always danger of diffusional retardation in the case of the most reactive members of the series. This may cause curvature of the log kva a plot. 

Thus, the availability of more enzyme molecules to catalyze a reaction leads to the formation of more ES and a higher reaction rate. The effect of enzyme concentration on the rate of a reaction is shown in I Eigure 10.5. As the figure indicates, the rate of a reaction is directly proportional to the concentration of the enzyme—that is, if the enzyme concentration is doubled, the rate of conversion of substrate to product is also doubled. 

Effect of contact on reaction rate. An effect similar to a concentration increase occurs if a solid reactant is very finely divided, which essentially increases concentration by increasing the surface area at which reactions can occur. We don t think of flour as an explosive substance, but a spark can set off tbe explosion of flour suspended in the air. Dust explosions are a hazard in grain storage and coal mines. 

To produce geochemical rate models, rates determined by reactor experiments must be converted into rate equations that summarize how the rate varies with solution composition, temperature, and other rate-determining variables. If the rates are determined at near-equilibrium conditions, the rate data must be fit to an equation that takes into account both the forward and reverse rate. Most geochemical rate experiments are designed to measure rates for far-from-equilibrium conditions where the reverse reaction rate is effectively zero. These experimental rates can be fit to a simple equation that relates the rate to the product of the concentration (w, molal) of each reacting species raised to a power (n). 

Concentration effect on reaction rate Temperature effect on reaction rate Surface area effect on reaction rate Chemical equilibrium conditions Simultaneous forward and reverse reaction Rate forward reaction = rate reverse reaction Reaction system is closed ... 

Equations (20A2) and f20.13i must be solved for a given rate expression to get the actual relationship between the indicated parameters. However, several generalizations are possible. The reactor volume and conversion change in the same direction. As one increases, so does the other as one decreases, so does the other. As the reaction rate increases, the volume required decreases, and vice versa. As the reaction rate increases, the conversion increases, and vice versa. It has already been discussed how temperature, pressure, and concentration affect the reaction rate. Their effect on conversion and reactor volume is derived from their effect on reaction rate. Example 20.2 illustrates these qualitative generalizations. 

It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

If certain species are present in large excess, their concentration stays approximately constant during the course of a reaction. In this case the dependence of the reaction rate on the concentration of these species can be included in an effective rate constant The dependence on the concentrations of the remaining species then defines the apparent order of the reaction. Take for example equation (A3,4.10) with e. The... 

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