Free energy redox reactions

Redox reactions that have a positive Gibbs free energy of reaction are not spontaneous, but an electric current can be used to make them take place. For example, there is no common spontaneous chemical reaction in which fluorine is a product, and so the element cannot be isolated by any common chemical reaction. It was not until 1886 that the French chemist Henri Moissan found a way to force the... 

Table 16-5 The standard free energy of reaction, AG , for the main environmental redox reactions...

We start with a simple reversible redox reaction for which we can directly measure the free energy of reaction, Ar<7, with a galvanic cell. This example helps us introduce the concept of using (standard) reduction potentials for evaluating the energetics (i.e., the free energies) of redox processes. Let us consider the reversible interconversion of 1,4-benzoquinone (BQ) and hydroquinone (HQ) (reaction 14-5 in Table 14.1). We perform this reaction at the surface of an inert electrode (e.g.,... 

Table 14.2 Standard Reduction Potentials and Average Standard Free Energies of Reaction (per Electron Transferred) at 25 °C of Some Redox Couples that Are Important in Natural Redox Processes (The reactions are ordered in decreasing 1 h(W) values.)a...

The thermodynamics of the redox couple in Eq. (a) can be described by an electrochemical free energy of reaction, AG° ... 

The free energy of reaction, AG°, is an important parameter in Marcus theory (as in, for example, eq. 4.4). Fortunately, this is an easy quantity to calculate from the redox potentials of the couples involved. For forward ET between ground-state D and A (eq. 4.1), the Nemst equation takes the form... 

F is the Faraday constant (96,485 CVmol), n the charge number, and Erev Qie reversible potential or equilibrium potential of the cell reaction. By convention, the work supplied by a system is negative, which explains the negative sign of equation (2.35). For a given electrochemical redox reaction (2.1), the free energy of reaction is equal to ... 

As the experiments showed, iodide and iodate were formed in a ratio of about 5 1 upon contact of I2 with the basic aerosol materials. Apparently, it is easier for I2 to disproportionate on the surface than it is for it to undergo a redox reaction with ions in the crystal to form Csl alone. Thermochemical data show that the formation of iodide and iodate would result in a lower free energy of reaction than formation of iodide alone. Formation of iodate alone would give a lower iodine potential than formation of iodide and iodate however, iodate formation seems to be limited by reaction kinetics. The extent of I2 reaction with anhydrous CS2O and CS2CO3 is probably limited only by the surface concentration of iodide and iodate which prevents or delays further interaction between I2 and the host crystal. In tests in which saturated aqueous solutions of these compounds were present, no such limitation was observed, nor had it been expected. 

An electron transfer reaction may be separated into two half-reactions or redox couples so that the free energy, AG°, can be separated into AGa and AGg the free energies of reduction of the donor (D) and the acceptor (A), respectively, by... 

The free energy changes of the outer shell upon reduction, AG° , are important, because the Nernst equation relates the redox potential to AG. Eree energy simulation methods are discussed in Chapter 9. Here, the free energy change of interest is for the reaction... 

Reduction Potentials—An Accounting Device for Free Energy Changes in Redox Reactions... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

Spectator ion An ion that, although present, takes no part in a reaction, 279,82-83, 372-373,399 Spontaneity of reaction concentration and, 465-467,475-476q entropy and, 453-458 free energy and, 458-471 pressure effects, 465-467,475-476q process, 451-453 redox, 489-490... 

What Do We Need to Know Already This chapter extends the thermodynamic discussion presented in Chapter 7. In particular, it builds on the concept of Gibbs free energy (Section 7.12), its relation to maximum nonexpansion work (Section 7.14), and the dependence of the reaction Gibbs free energy on the reaction quotient (Section 9.3). For a review of redox reactions, see Section K. To prepare for the quantitative treatment of electrolysis, review stoichiometry in Section L. 

In the ease of the reactive chemisorption the electrode redox potentials assigned to the chemisorption step represent the thermodynamic free energy of adsorption according to AGad - n F Em- This can be visualized by eonsidering the example of the reactive adsorption of an n-aUcanethiolate on a silver electrode surfaee. The reaction is... 

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 ... 

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