Spontaneous reaction, criterion

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

Because processes will be spontaneous in closed systems at constant T and P if they lower the Gibbs free energy (dG/dc < 0), we see that the criterion for a spontaneous reaction in a system at constant T and P is... 

The specific examples in Section 14.5 demonstrate that when 1C > 1 the reaction has progressed far toward products, and when K 1 the reaction has remained near reactants. The empirical discussion in Section 14.6 shows how the reaction quotient Q and the principle of Te Chatelier can predict the direction of spontaneous reaction and the response of an equilibrium state to an external perturbation. Here, we use the thermodynamic description of K from Section 14.3 to provide the thermodynamic basis for these results obtained empirically in Sections 14.5 and 14.6. We identify those thermodynamic factors that determine the magnitude of K. We also provide a thermodynamic criterion for predicting the direction in which a reaction proceeds from a given initial condition. 

Since Ecell is defined to be positive for a spontaneous reaction, this equation correctly expresses a decrease in Gibbs function, which is the thermodynamic criterion for a spontaneous reaction at constant T and P. It is evident that if AGreact can be calculated from AG j- data, the potential of a cell arranged for reversible operation can be determined conversely, experimental measurements of Ecen permit calculation of AGreact. Both types of calculations are useful in electrochemical work and, thus, in the analysis of corrosion. 

Now we have a criterion for a spontaneous reaction that is expressed only in terms of the properties of the system and AS ys) and we can ignore the surroundings. For... 

The open circuit potential of copper in a copper ion solution and zinc in a zinc ion solution are represented as Ceq.Cu d e - zn, respectively, in Fig. 3.5. The currents corresponding to the open circuit potentials are the exchange current densities of the electrode reactions. For such a cell, thermodynamics favors the reduction of copper and the oxidation of zinc and the criterion for a spontaneous reaction (AG= — tiFE°) is valid. 

For the transient relaxation of X to the non-equilibrium stationary state AG is not a valid criterion of irreversibility or spontaneous reaction. We shall develop necessary and sufEcient thermodynamic criteria for such cases. 

The first law of thermodynamics provides no universally applicable criterion as to whether a particular reaction will occur or not. However, it could be deduced from it that a chemical reaction is feasible at constant temperature and pressure if the change in enthalpy is negative. Many spontaneous reactions satisfy the above criterion, but there are also many spontaneous reactions with positive values of AH. For example, the enthalpy change for the reaction... 

We now turn specifically to the thermodynamics and kinetics of reactions (5. EE) and (5.FF). The criterion for spontaneity in thermodynamics is AG <0 with AG = AH - T AS for an isothermal process. Thus it is both the sign and magnitude of AH and AS and the magnitude of T that determine whether a reaction is thermodynamically favored or not. As usual in thermodynamics, the A s are taken as products minus reactants, so the conclusions apply to the reactions as written. If a reaction is reversed, products and reactants are interchanged and the sign of the AG is reversed also. 

There is still another basic objection to using the sign of AH as a general criterion fir spontaneity. Endothermic reactions that are nonspontaneous at room temperature often become spontaneous when the temperature is raised. Consider far example, the decomposition of limestone ... 

As pointed out previously, the value of the standard cell voltage, E°, is a measure of the spontaneity of a cell reaction. In Chapter 17, we showed that the standard free energy change, AG°, is a general criterion for reaction spontaneity. As you might suppose, these two quantities have a simple relation to one another and to the equilibrium constant, K, for the cell reaction. 

The decrease in Gibbs free energy as a signpost of spontaneous change and AG = 0 as a criterion of equilibrium are applicable to any kind of process, provided that it is occurring at constant temperature and pressure. Because chemical reactions are our principal interest in chemistry, we now concentrate on them and look for a way to calculate AG for a reaction. 

The thermodynamic function used as the criterion of spontaneity for a chemical reaction is the Gibbs free energy of reaction, AG (which is commonly referred to as the reaction free energy ). This quantity is defined as the difference in molar Gibbs free energies, Gm, of the products and the reactants ... 

As a new criterion for reaction spontaneity, we say AS(totai) must be positive. We must consider the surroundings if we are to understand how the overall extent of energetic disorder increases during a process. 

In practice, the primary objective of chemical thermodynamics is to estabhsh a criterion for determining the feasibility or spontaneity of a given physical or chemical transformation. For example, we may be interested in a criterion for determining the feasibility of a spontaneous transformation from one phase to another, such as the conversion of graphite to diamond, or the spontaneous direction of a metabohc reaction that occurs in a cell. On the basis of the first and second laws of thermod5m-amics, which are expressed in terms of Gibbs s functions, several additional theoretical concepts and mathematical functions have been developed that provide a powerful approach to the solution of these questions. 

In the form in which it has been expressed thus far, the second law is not a statement that can be applied conveniently to chemical problems. We wish to use the second law of thermodynamics to establish a criterion by which we can determine whether a chemical reaction or a phase change will proceed spontaneously. Such a criterion would be available if we could obtain a function that had the following two characteristics. 

The thermodynamic criterion for spontaneity (feasibility) of a chemical and electrochemical reaction is that the change in free energy, AG have a negative value. Free-energy change in an oxidation-reduction reaction can be calculated from knowledge of the cell voltage ... 

The criterion for spontaneity of a reaction is the value of AG, not AG °. A reaction with a positive AG ° can go in the forward direction if AG is negative. This is possible if the term RT In ([products]/[reactants]) in Equation 13-3 is negative and has a larger absolute value than AG °. For example, the immediate removal of the products of a reaction can keep the ratio [prod-ucts]/[reactants] well below 1, such that the term RT In ([products]/[reactants]) has a large, negative value. 

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