Chemical reaction thermodynamically controlled

Tetrose (Section 25 3) A carbohydrate with four carbon atoms Thermochemistry (Section 2 18) The study of heat changes that accompany chemical processes Thermodynamically controlled reaction (Section 10 10) Re action in which the reaction conditions permit two or more products to equilibrate giving a predominance of the most stable product... 

The implication of these studies is of critical importance. Chemists generally think of the product distribution of a chemical reaction being controlled by kinetics or thermodynamics. Under kinetic control, the distribution favors the product that results from crossing the lowest activation barrier. Under thermodynamic control, the distribution favors the lowest energy product. Schreiner and Allen now add... 

In many chemical reactions, new products are formed. Chemical reactions are controlled both kinetically and thermodynamically. In the former case the ratio of the products at any moment equals the ratio of the rate constants of all competitive processes. If one of the competing reactions is reversible, the less stable, kinetically controlled products become more thermodynamically stable after a certain time interval, when the ratio of the products is determined by the ratio of the equilibrium constants. This is called a thermodynamically controlled reaction. 

For thermodynamic reasons, the existing valency state of the fission product atom must be compatible with the prevailing conditions. This is a consequence of the postulate mentioned earlier that the progress of chemical reactions is controlled by the ambient conditions. 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Ceramic—metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. Knowledge of the chemical reaction products and, if possible, their properties are needed. It is therefore imperative to understand the thermodynamics and kinetics of reactions such that processing can be controlled and optimum properties obtained. 

If the gas has the correct composition, the carbon content at the surface increases to the saturation value, ie, the solubiUty limit of carbon in austenite (Fig. 2), which is a function of temperature. Continued addition of carbon to the surface increases the carbon content curve. The surface content is maintained at this saturation value (9) (Fig. 5). The gas carburizing process is controlled by three factors (/) the thermodynamics of the gas reactions which determine the equiUbrium carbon content at the surface (2) the kinetics of the chemical reactions which deposit the carbon and (J) the diffusion of carbon into the austenite. 

The series of reactors and exchangers which methanates a raw syngas without pretreatment other than desulfurization is collectively termed bulk methanation. The chemical reactions which occur in bulk methana-tion, including both shift conversion and methanation, are moderated by the addition of steam which establishes the thermodynamic limits for these reactions and thereby controls operating temperatures. The flow sequence through bulk methanation is shown in Figure 1. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

In many other cases (by a change in experimental conditions, faster chemical reaction) the value of the catalytic current may be governed by the SET rate (see reaction 20). The value of k1 may be found and its variation as a function of the nature of the mediator (with several values for °j) leads by extrapolation (when k2 can be assumed to be diffusion-controlled) to the thermodynamical potential °RS02Ar which is somewhat different from the reduction potentials of overall ECE processes observed in voltammetry. 

Why Do We Need to Know This Material The dynamic equilibrium toward which every chemical reaction tends is such an important aspect of the study of chemistry that four chapters of this book deal with it. We need to know the composition of a reaction mixture at equilibrium because it tells us how much product we can expect. To control the yield of a reaction, we need to understand the thermodynamic basis of equilibrium and how the position of equilibrium is affected by conditions such as temperature and pressure. The response of equilibria to changes in conditions has considerable economic and biological significance the regulation of chemical equilibrium affects the yields of products in industrial processes, and living cells struggle to avoid sinking into equilibrium. 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

A Kinetic Control Versus Thermodynamic Control of a Chemical Reaction... 

Reverse hydrolysis a thermodynamically controlled equilibrium process, in which a free monosaccharide reacts with a nucleophile under exclusion of a water molecule and hence chemically, can be considered a condensation reaction. 

The ability of a chemical to act as a template is frequently attributed to a combination of thermodynamic and kinetic factors. As has been defined by Busch [3] a thermodynamic template binds more strongly to one of the products present in an equilibrium (i.e. a mixture under thermodynamic control) shifting the reaction towards the formation of this specific product which is then obtained in higher yields. In contrast, kinetic templates operate under irreversible conditions by stabilising the transition state leading to the final product. 

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