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Subsurface Processes
Published in Stephen M. Testa, Geological Aspects of Hazardous Waste Management, 2020
Chemical reactions are accompanied by changes in energy. The amount of energy is expressed in terms of the gain or loss during the reaction. Not all of the energy change is available to do useful work, but is lost as heat or to the disorder of the system (entropy). The available useful energy is the free energy. A negative free energy means that energy is released during the reaction (exergonic). A positive free energy means that energy is required for the reaction to proceed (endergonic). The reaction can be reversible in that the product of the reaction can re-react to form the initial reactants. In an exergonic reaction, the reaction products will build up and the reactants will be used up until a balance, or chemical equilibrium, occurs in which the rate of the forward reaction equals the rate of the backward reaction. A large negative free energy means that the reaction favors the products, and if equilibrium is reached, then little of the reactants will remain. A small negative free energy means that the reaction favors the reactants and, if equilibrium is reached, then both reactants and products will remain. This field of study which investigates chemical equilibrium is called thermodynamics.
Chemical Reaction Thermodynamics, Kinetics, and Reactor Analysis
Published in Debabrata Das, Debayan Das, Biochemical Engineering, 2019
For an endothermic reversible reaction, Keq increases with temperature, whereas for an exothermic reaction, Keq decreases with increasing temperature. The parameter Keq varies with temperature, which is further given by the van’t Hoff equation as follows: () logKeq2Keq1=ΔH2.303R(1T1−1T2)
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
A reaction that proceeds by a mechanism involving more than a single reaction path or step is termed a complex reaction. Unlike elementary reactions, the mechanisms of complex reactions differ considerably from their stoichiometric equations. Most industrially important reactions are complex reactions, the mechanisms of which can often be determined by assuming that the overall reaction consists of several elementary reaction steps. The resulting overall rate expression is then compared with the experimental data, and the procedure is repeated until a desired degree of agreement is obtained. Each of the elementary reaction steps may proceed reversibly, concurrently, or consecutively. A reversible reaction is one in which conversion of reactants to products is incomplete at equilibrium because of an increasing influence of the reverse reaction as the forward reaction approaches equilibrium. For a reversible reaction of the type A+B⇔krkfD+E
Catalysts used in biodiesel production: a review
Published in Biofuels, 2021
A catalyst is a substance that increases the chemical reaction rate without being consumed by the reaction itself. Theoretically, the catalyst is practically consumed in one stage and regenerated at a later stage, and this operation is continuously repeated without imposing a permanent change on the catalyst. Accordingly, the catalyst in a given reaction can be recycled unchanged at the end of the reaction. Catalysts change the speed of a chemical reaction that can be thermodynamically carried out. Therefore, they cannot perform reactions that are not thermodynamically feasible. Basically, a catalyst is considered a chemical compound capable of applying an accelerating effect on the reaction rate and a directional effect on the reaction progression which is thermodynamic in nature. In a reversible reaction, the catalyst evenly affects the rate of forward and backward reactions. Therefore, the equilibrium constant of the reaction is the same whether in the presence of a catalyst or without it. When there are several mechanisms available for the reaction, the catalyst must be selected. In principle, the catalyst should increase the ratio of the desired material to the unwanted material. Although ideally catalysts remain unchanged during the reaction, this is inaccurate in practice, since the catalyst itself is a reactive substance that undergoes irreversible physical and chemical changes during the reaction, reducing its ability to function. Over time, this reality may be vividly observed since the catalyst enters into billions of reactions [5]. In general, the catalysts used in the transesterification of vegetable oils and animal fats can be classified into three groups – homogeneous, heterogeneous and enzymatic catalysts [12] – as shown in Figure 1.