Kinetics and Thermodynamics of Chemical Reactions

In this section we will briefly outline some of the conventional approaches of classical physical chemistry to the thermodynamics and kinetics of chemical 

A (i = 1,2. n) denote the chemical substances in the reaction volume before their chemical transformation (initial products), are their stoichiometric coefficients, Bj (j = 1,2. m) are the chemical substances after chemical transformation (final products), and Pj are their stoichiometric coefficients. The words initial and final are rather relative the reaction is assumed to be reversible. For the sake of convenience the left side substances are always called initial products, and the right side substances are called final products.  

Stoichiometric coefficients indicate the relation between the numbers of appearing and disappearing molecules of different chemical species. For instance, in the reaction 

There are two main ideas in the foundation of classical chemical thermodynamics and in chemical kinetics the notion of dynamic equilibrium and the law of mass action. Historically, formulating the mass action law Van t Hoff proposed that the reaction rate was determined by the concentrations of reacting molecules [5]. The elementary acts of chemical transformations in forward and backward reactions can proceed independently. According to the notion of dynamic equilibrium, the chemical equilibrium is established when the rates of forward and backward reactions become equal. 

By definition, the rate of a chemical reaction, V, is determined by the number of chemical transformations in the unit of reaction volume in a unit of time, i.e., V = d /dt. Mass-action law postulates that the reaction rate is proportional to the concentrations of reacting substances, [AJ or [B ], 

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These experiments suggest that the factors affecting the yields from plasma reactors are not always the most obvious ones, and much remains to be learned about the molecular processes that occur. The difficulties involved in the study of kinetics and thermodynamics of chemical reactions in high-temperature plasma tails are formidable, but even a crude understanding might permit a degree of control over plasma reactors that does not at present exist. 

Guliaev, G.V, Polak, L.S. (1965), Kinetics and Thermodynamics of Chemical Reactions in Low-Temperature Plasma, Polak, L.S. (ed.), p. 72, Nauka (Science), Moscow. 

As a chemical reaction, it is necessary that the effect of temperature, pressure and gas component on reaction is considered from the viewpoint of kinetics and thermodynamics of chemical reaction. 

Steric effects refer to the impact of nonbonded interactions on the kinetics and thermodynamics of chemical reactions. At a qualitative level, it is relatively straightforward to identify substituents that exert large or small steric effeas just through... 

The kinetics and thermodynamics of the reaction, and of possible side reactions, need to be understood. The explosive potential of chemicals liable to exothermic reaction should be carefully appraised. 

Common reaction rate v. temperature characteristics for reactions are illustrated in Figure 6.5. To avoid runaway conditions (Fig. 6.5a) or an explosion (Figure 6.5c), it may be essential to control the rate of addition of reactants and the temperature. The kinetics and thermodynamics of the reaction, and of possible side reactions, need to be understood. The explosive potential of chemicals liable to exothermic reaction should be carefully appraised. 

Linear free energy relationship (LFER) — For various series of similar chemical reactions it has been empirically found that linear relationships hold between the series of free energies (-> Gibbs energy) of activation AG and the series of the standard free energies of reactions AGf, i.e., between the series of log fc (k -rate constants) and log K (Kt - equilibrium constants) (z labels the compounds of a series). Such relations correlate the - kinetics and -> thermodynamics of these reactions, and thus they are of fundamental importance. The LFER s can be formulated with the so-called Leffler-Grunwald operator dR ... 

This dynamic world is governed by the laws of chemistry and electrochemistry, physics and physical chemistry, metallurgy and corrosion science. This world is a meeting point for the kinetics and thermodynamics of chemical and electrochemical reactions, crystal nucleation and growth with diffusion and migration processes, mechanical and corrosion processes in lead alloys, as well as a number of phenomena of efficient transformation of chemical energy into electrical energy and vice versa. 

The chemistry audit examines the whole reaction scheme. The potential to use different solvents, catalysts or operating conditions should be considered. Ideal operating conditions and those conditions that promote byproduct formation should be determined, such as temperature of operation or residence time. Check if the chemical reaction rate is inhibited in any way. Some knowledge of the kinetics and thermodynamics of the reaction is essential. 

In principle one can treat the thermodynamics of chemical reactions on a kinetic basis by recognizing that the equilibrium condition corresponds to the case where the rates of the forward and reverse reactions are identical. In this sense kinetics is the more fundamental science. Nonetheless, thermodynamics provides much vital information to the kineticist and to the reactor designer. In particular, the first step in determining the economic feasibility of producing a given material from a given reactant feed stock should be the determination of the product yield at equilibrium at the conditions of the reactor outlet. Since this composition represents the goal toward which the kinetic... 

Much of this chapter will be a review for those who have had courses in chemical kinetics. In this chapter we will also review some aspects of thermodynamics that are important in considering chemical reactors. For students who have not had courses in kinetics and in the thermodynamics of chemical reactions, this chapter will serve as an introduction to those topics. This chapter will also introduce the notation we will use throughout the book. 

This is the fun (and frustration) of chemical reaction engineering. While thermodynamics, mass and heat transfer, and separations can be said to be finished subjects for many engineering apphcations, we have to reexamine every new reaction system from first principles. You can find data and construct process flowsheets for separation units using sophisticated computer programs such as ASPEN, but for the chemical reactors in a process these programs are not much help unless you give the program the kinetics or assume equihhrium yields. 

Kinetic properties (rates of chemical reactions) and thermodynamic properties (equilibrium constants, energy, entropy) are described by a large number of different mathematical relations, which are usually just presented for the student to memorize. Part of the reason for this is the complexity associated with a full treatment of these properties these subjects are taught in graduate chemistry and physics courses at every major university, and multivariate calculus is needed to formulate a rigorous treatment. Unfortunately, simple memorization does not provide much intuition. 

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