Chemical reaction rates forward reactions

Equilibrium — A system has reached equilibrium when the properties of the system (such as volume, pressure, color, composition, etc.) no longer change with time. For a chemical equilibrium (involving one or more reversible chemical reactions), the forward and reverse directions of each reaction proceed at equal rates and the system has reached a state of minimum -> Gibbs energy. See also -> reversibility. 

The concentrations of initial and final products S and D are maintained at constant values. So, there are two independent variables Xand Y. The klf and klh denote the forward and backward chemical reaction rate constants, respectively. The overall affinity characterizes the thermodynamic state of the chemical system, and is found form... 

In all three reaction equations above, k 10 and k 10 represent the chemical reaction rate constants in the forward and backward directions, respectively, which are independent of the electrode potential. For electrochemical reactions (4-XII) and (4-XIII), the... 

Every textbook of physical chemistry describes that chemical equilibrium is a state where the forward and reverse reactions are taking place at exactly the same rate, and that the equilibrium constant K is the ratio of the forward-rate constant Iq to the reverse one Iq. This universally accepted principle of great importance in chemistry had been conventionally justified on the basis of kf and Iq determined far from equilibrium since the reactions cannot be followed by conventional chemical methods in the state of equilibrium. This crucial relationship between chemical reaction rate and equilibrium was first proved by the use of radioactive tracers. 

Complex chemical mechanisms are written as sequences of elementary steps satisfying detailed balance where tire forward and reverse reaction rates are equal at equilibrium. The laws of mass action kinetics are applied to each reaction step to write tire overall rate law for tire reaction. The fonn of chemical kinetic rate laws constmcted in tliis manner ensures tliat tire system will relax to a unique equilibrium state which can be characterized using tire laws of tliennodynamics. 

Activation Processes. To be useful ia battery appHcations reactions must occur at a reasonable rate. The rate or abiUty of battery electrodes to produce current is determiaed by the kinetic processes of electrode operations, not by thermodynamics, which describes the characteristics of reactions at equihbrium when the forward and reverse reaction rates are equal. Electrochemical reaction kinetics (31—35) foUow the same general considerations as those of bulk chemical reactions. Two differences are a potential drop that exists between the electrode and the solution because of the electrical double layer at the electrode iaterface and the reaction that occurs at iaterfaces that are two-dimensional rather than ia the three-dimensional bulk. 

Chemical reactions involving gases carried out in closed containers resemble in many ways the H20(/)-H20(g) system. The reactions are reversible reactants are not completely consumed. Instead, an equilibrium mixture containing both products and reactants is obtained. At equilibrium, forward and reverse reactions take place at the same rate. As a result, the amounts of all species at equilibrium remain constant with time. 

Figure 8-8 shows the analogous situation for a chemical reaction. The solid curve shows the activation energy barrier which must be surmounted for reaction to take place. When a catalyst is added, a new reaction path is provided with a different activation energy barrier, as suggested by the dashed curve. This new reaction path corresponds to a new reaction mechanism that permits the reaction to occur via a different activated complex. Hence, more particles can get over the new, lower energy barrier and the rate of the reaction is increased. Note that the activation energy for the reverse reaction is lowered exactly the same amount as for the forward reaction. This accounts for the experimental fact that a catalyst for a reaction has an equal effect on the reverse reaction that is, both reactions are speeded up by the same factor. If a catalyst doubles the rate in one direction, it also doubles the rate in the reverse direction. 

The backward reaction tends to increase the resistance to mass transfer. If the backward reaction rate is very small compared with the forward reaction rate, the transfer rate is at its highest value. Then, as the backward reaction rate is increased, the transfer rate begins to decline. When the backward reaction rate approaches infinity, the chemical reaction exerts no influence on the mass transfer and the system behaves as if no chemical reaction is involved. 

How does the increase of the scan rate affect the ratio of peak currents (backward/forward) in a cyclic voltammetric experiment involving a redox process followed by a chemical reaction ... 

Like phase changes, chemical reactions tend toward a dynamic equilibrium in which, although there is no net change, the forward and reverse reactions are still taking place, but at matching rates. What actually happens when the formation of ammonia appears to stop is that the rate of the reverse reaction,... 

Chemical reactions reach a state of dynamic equilibrium in which the rates of forward and reverse reactions are equal and there is no net change in composition. 

A catalyst speeds up both the forward and the reverse reactions by the same amount. Therefore, the dynamic equilibrium is unaffected. The thermodynamic justification of this observation is based on the fact that the equilibrium constant depends only on the temperature and the value of AGr°. A standard Gibbs free energy of reaction depends only on the identities of the reactants and products and is independent of the rate of the reaction or the presence of any substances that do not appear in the overall chemical equation for the reaction. 

In chemical reactions, the kinetic parameters k and k are constant for given conditions (of temperature, etc.). Hence, the same step will be rate determining in the forward and reverse directions of the reaction (provided that the reaction pathways are the same in both directions). 

Chemical equilibrium The state reached in a reversible reaction when the forward reaction is proceeding at the same rate as the reverse reaction. 

In an EC mechanism the ratio of the forward and backward reaction rates is decisive for k/ d in , the chemical follow-up reaction has no influence here, so that for a sufficiently rapid electron transfer step the limiting current remains diffusion controlled.)... 

If the forward and reverse reactions are nonelementary, perhaps involving the formation of chemical intermediates in multiple steps, then the form of the reaction rate equations can be more complex than Equations 5.33 to 5.36. 

The electrochemical mechanism was rejected by Salvago and Cavallotti [26] on the basis that it does not explain several features of electroless deposition of ferrous metals it does not account for the isotopic composition of the H2 gas evolved it does not explain the effect of the various solution components on reaction rate and it does not account for the homogeneous decomposition of very active solutions or the fact that they can give deposition on insulating surfaces. These authors put forward a chemical mechanism, involving various hydrolyzed nickel species, which they claim explains the observed behavior of the system ... 

Most chemical reactions do not progress completely from reactants to products. Instead, the net reaction stops in the forward direction when equilibrium is established. Analysis of the contents of the reaction vessel would show a constant concentration of monomers and polymer once equilibrium is reached. This situation is actually a dynamic equilibrium, where the monomers are forming polymers at the same rate as the polymers depolymerize to monomer. Therefore, at equilibrium, the net concentrations of any one species remains constant. The amount of monomer converted into polymer will be defined by the equilibrium constant, K. This constant is the ratio of the concentration of the products to the reactants, with each concentration raised to the stoichiometric coefficients in the balanced equation. For Eq. 3.5 ... 

However, some sets of reactants can undergo both a forward and a reverse reaction under the same set of conditions. This circumstance leads to a state called chemical equilibrium. Before we take up equilibrium, however, we have to learn about the factors that affect the rate of a chemical reaction. 

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... 

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