Kinetics Changing Reaction Speeds

Changing the equilibrium conditions by having condensate in the sample due to water in the carrier gas or the diffusion limitation of the condensate in larger particles changes the reaction speed. Although the kinetics of the reaction and the diffusion of the condensate are not the process Imitating steps they have an effect on the overall reaction rate as described above. 

If all pathways are Irreversible, the composition of the products depends on the relative speed along the pathways it is kinetically controlled. These speeds are determined by entropy changes and by the stereochemistry of the transition states concerned the latter is often related to the stereochemistry of the reactants in a complex way, and has still to be elucidated for many reactions. On the other hand, if each pathway of reaction is reversible, and if a true equilibrium is established, the composition of the products is quite independent of the mechanism of reaction, being determined solely by the relative thermodynamic stabilities of the constituents, even though the speed of the individual reactions may be very different. A study of such reversible reactions is therefore suitable for assessing the relative stabilities of isomeric compounds. 

Relaxation processes can be very fast, and measurements are limited only by the speed at which one can perturb the system and by how fast the detection system can follow the changes. Reaction kinetics on the microsecond time scale are normally feasible. 

Stirred suspensions of droplets have proven to be a popular approach for studying the kinetics of liquid-liquid reactions [54-57]. The basic principle is that one liquid phase takes the form of droplets in the other phase when two immiscible liquids are dispersed. The droplet size can be controlled by changing the agitator speed. For droplets with a diameter < 0.15 cm the inside of the drop is essentially stagnant [54], so that mass transfer to the inside surface of the droplet occurs only by diffusion. In many cases, this technique can lack the necessary control over both the interfacial area and the transport step for determination of fundamental interfacial processes [3], but is still of some value as it reproduces conditions in industrial reactors. 

The RHSE has the same limitation as the rotating disk that it cannot be used to study very fast electrochemical reactions. Since the evaluation of kinetic data with a RHSE requires a potential sweep to gradually change the reaction rate from the state of charge-transfer control to the state of mass transport control, the reaction rate constant thus determined can never exceed the rate of mass transfer to the electrode surface. An upper limit can be estimated by using Eq. (44). If one uses a typical Schmidt number of Sc 1000, a diffusivity D 10 5 cm/s, a nominal hemisphere radius a 0.3 cm, and a practically achievable rotational speed of 10000 rpm (Re 104), the mass transfer coefficient in laminar flow may be estimated to be ... 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Kinetics is the study of the speed of reactions. The speed of reaction is affected by the nature of the reactants, the temperature, the concentration of reactants, the physical state of the reactants, and catalysts. A rate law relates the speed of reaction to the reactant concentrations and the orders of reaction. Integrated rate laws relate the rate of reaction to a change in reactant or product concentration over time. We may use the Arrhenius equation to calculate the activation... 

Equation (4.20) permits one to establish various trends of the flame speed as various physical parameters change. Consider, for example, how the flame speed should change with a variation of pressure. If the rate term j follows second-order kinetics, as one might expect from a hydrocarbon-air system, then the concentration terms in Co would be proportional to P2. However, the density term in n(=XJpcp) and the other density term in Eq. (4.20) also give a P2 dependence. Thus for a second-order reaction system the flame speed appears independent of pressure. A more general statement of the pressure... 

If the speed of the second reaction determines the measured speed of the whole change, then the reaction is purely ter-molecular in the kinetic sense. 

Previously to any kinetic experiment, we have studied the influence of external and internal diffusion by changing the stirring speed and the particle size respectively. It has been found that for more than 1000 r.p.m. and particle size < 0.25 mm the reaction is not controlled by either external or internal diffusion. 

However, the reverse process, in going from speed to distance, involves integration of the rate equation (6.2). In chemistry, the concept of rate is central to an understanding of chemical kinetics, in which we have to deal with analogous rate equations which typically involve the rate of change of concentration, rather than the rate of change of distance. For example, in a first-order chemical reaction, where the rate of loss of the reactant is proportional to the concentration of the reactant, the rate equation takes the form ... 

By stoicheiometry we understand the calculus of changes in composition that take place by reaction it corresponds to kinematics in the analogy with continuum mechanics for its provides the framework within which chemical motions must take place, irrespective of the forces that bring them about. By kinetics we understand the relations that govern the speed of the composition changes and this bears some analogy to the dynamics of continua. Just as the latter can only be built on a proper understanding of the kinematics, so the analysis of stoicheiometry must precede that of kinetics. 

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