Reaction spontaneous direction

We have seen that the value of an equilibrium constant tells us whether we can expect a high or low concentration of product at equilibrium. The constant also allows us to predict the spontaneous direction of reaction in a reaction mixture of any composition. In the following three sections, we see how to express the equilibrium constant in terms of molar concentrations of gases as well as partial pressures and how to predict the equilibrium composition of a reaction mixture, given the value of the equilibrium constant for the reaction. Such information is critical to the success of many industrial processes and is fundamental to the discussion of acids and bases in the following chapters. 

EXAMPLE 12.7 Predicting the spontaneous direction of a redox reaction under standard conditions... 

We need to determine which reaction occurs as reduction and which occurs as oxidation when this battery is operating in its spontaneous direction. Then we can use Equation to determine E ° for the nickel half-reaction. 

What is the E for the following redox system and in which direction will the reaction spontaneously... 

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. 

Note also that if infinitesimally small currents are allowed to flow (see Fig. 7.175) through the galvanometer G in one or another direction, (i.e., when the electrode reactions occurring at both electrodes are reversed from their spontaneous direction), the polarity of the electrodes remains unchanged. Thus the sign of the electrode potential remains in this convention invariant, irrespective of whether the electrode processes proceed in the spontaneous or reverse direction, and thus are written as... 

As seen from Fig. 10, the non-exponential drop in the intensity of fluorescence for Nh-inverse kinetic isotope effect is observed as distinct from the process of spontaneous deactivation, for which a normal isotopic effect (td > ih) is observed. Note that the possibility of the abnormal isotopic effect for electron tunneling reactions follows directly from the theoretical concepts set out in Chap. 3, Sect. 6. The mean values of the parameter / obtained from experiments with various concentrations of CC14 proved to be p = (0.240 0.010) M 1 for Nh d8 and j = (0.205 0.010) M l for Nh. As the effect of the nuclear motion on W R) must be reflected more in the value of ve than in that of ae it seems natural to connect the difference observed between the values of P for Nh and those for Nh-d8 with the change in the parameter ve. At the value of ae 1 A typical of tunneling reactions, the difference observed in the values of P corresponds to an approximately 2.5-fold increase of ve upon naphthalene deuteration. With an increase in temperature from 77 to 140 K, the parameter / remained virtually unchanged, although the time, t, for spontaneous deactivation was markedly reduced. Thus, tunneling reaction (14) proceeds via a non-activated mechanism. 

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