Spontaneous change redox reactions

In Section 19-3, we developed a criterion for spontaneous change (Ecdi > 0), but we used the criterion only with E° data from Table 19.1 Qualitative conclusions reached with E eU values often hold over a broad range of nonstandard conditions as well. However, when E eii is within a few hundredths of a volt of zero, it is sometimes necessary to determine E eii for nonstandard conditions in order to apply the criterion for spontaneity of redox reactions, as illustrated in Example 19-10. 

Voltage will increase if the concentration of a reactant is increased or that of a product is decreased. Either of these changes increases the driving force behind the redox reaction, making it more spontaneous. 

The reactivities of potassium and silver with water represent extremes in the spontaneity of electron-transfer reactions. The redox reaction between two other metals illustrates less drastic differences in reactivity. Figure 19-5 shows the reaction that occurs between zinc metal and an aqueous solution of copper(II) sulfate zinc slowly dissolves, and copper metal precipitates. This spontaneous reaction has a negative standard free energy change, as does the reaction of potassium with water ... 

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

Galvanic cell A device in which chemical energy from a spontaneous redox reaction is changed to electrical energy that can be utilized for power. 

Redox reactions are more conveniently described in terms of relative electrical potentials instead of the equivalent changes in Gibbs free energy. The electrons in Equation 6.8 come from or go to some other redox couple, and whether or not the reaction proceeds in the forward direction depends on the relative electrical potentials of these two couples. Therefore, a specific electrical potential is assigned to a couple accepting or donating electrons, a value known as its oxidation-reduction or redox potential. This redox potential is compared with that of another couple to predict the direction for spontaneous electron flow when the two couples interact—electrons spontaneously move toward higher redox potentials. The redox potential of species /, ), is defined as... 

In a redox reaction, electrons move spontaneously toward atoms or molecules having more positive reduction potentials. In other words, a compound having a more negative reduction potential can transfer electrons to (i.e., reduce) a compound with a more positive reduction potential. In this type of reaction, the change in electric potential A is the sum of the reduction and oxidation potentials for the two half-reactions. The AE for a redox reaction is related to the change in free energy AG by the following expression ... 

Now consider the free energy of a redox reaction. The standard free-energy change of a reaction is the sum of the free energy of formation of prodncts in their standard states minus the free energies of formation of the reactants in their standard states. Negative valnes of AG represent a spontaneous reaction, and the reaction proceeds as shown below ... 

The change in the Gibbs free energy, AG, is a measure of the spontaneity of a process that occurs at constant temperature and pressure. (Section 19.5) The emf, E, of a redox reaction also indicates whether the reaction is spontaneous. The relationship between emf and the free-energy change is... 

It has recently been observed31 that the completely reduced base form of polyaniline, (leucoemeraldine), spontaneously reduces, and is itself oxidized, when its solution in NMP is mixed with an NMP solution of the completely oxidized base form of polyaniline, (pernigraniline). This reaction may be studied by "titrating" a solution of the leucoemeraldine with a solution of pernigraniline and by following the course of the reaction by the changes in the electronic spectrum of the reaction mixture. When equimolar solutions of the reactants are mixed this redox reaction is found to approach equilibrium after 20 hours at room temperature, to produce the emeraldine base, viz.,... 

Some oxidation-reduction reactions do not occur spontaneously but can be driven by electrical energy. If electrical energy is required to produce a redox reaction and bring about a chemical change in an electrochemical cell, it is an electrolytic cell. Most commercial uses of redox reactions make use of electrolytic cells. 

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