Electrochemical reaction, spontaneous

Electrochemically, a spontaneous reaction generates a positive cell potential, Scell Thermodynamically, a spontaneous reaction has a negative change in free energy, AG. Thus, a reaction that has a negative change in free... 

Electrochemical reactions have many practical applications. Some are spontaneous, and others are driven uphill by applying an external potential. In this section, we present practical examples of spontaneous redox processes. We describe externally driven redox reactions in Section 19-1. 

C19-0123. A cell is set up using two zinc wires and two solutions, one containing 0.250 M ZnCl2 solution and the other containing 1.25 M Zn (N03)2 solution, (a) What electrochemical reaction occurs at each electrode (b) Draw a molecular picture showing spontaneous electron transfer processes at the two zinc electrodes, (c) Compute the potential of this cell. 

This discrepancy arises primarily from the fact that spontaneous liquid flows will always develop in any hquid even without artificial stirring (e.g., under the action of density gradients caused by local temperature or concentration fluctuations). This phenomenon has been termed natural convection. Electrochemical reactions reinforce natural convection, since the concentrations of substances involved in the reaction will change near the electrode surfaces, and also since heat is evolved. Gas evolution attending the reactions has a particularly strong effect on naturaf convection. 

Many naturally occurring substances, in particular the oxide films that form spontaneously on some metals, are semiconductors. Also, electrochemical reactions are used in the production of semiconductor chips, and recently semiconductors have been used in the construction of electrochemical photocells. So there are good technological reasons to study the interface between a semiconductor and an electrolyte. Our main interest, however, lies in more fundamental questions How does the electronic structure of the electrode influence the properties of the electrochemical interface, and how does it affect electrochemical reactions What new processes can occur at semiconductors that are not known from metals ... 

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 ... 

A spontaneous electrochemical reaction is reported in magnesium-TCNQ salts. See Gutmann, F. Herman, A. M. Rembaum, A. J. Electrochem. Soc. 1967, 114, 323. 

Electric energy is then used to force non-spontaneous electrochemical reactions to occur. 

With the arrangement shown above, the reaction proceeds spontaneously, in which electrons move from left to right and X ions from right to left so that the electroneutrality is maintained. This type of reactions which take place in an electrochemical manner is called electrochemical reaction. A device like the one shown above, which permits a spontaneous electrochemical reaction to produce a detectable electric current, is termed a galvanic cell. As shown in the above figure, oxidation occurs in one half-cell and reduction occurs in the other half-cell. The electrode at which oxidation occurs is referred to as the anode, while the electrode at which reduction occurs is termed cathode. 

Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Under normal environmental conditions, the thermodynamically stable states of most of the metallic elements are the cations, rather than the metal itself. This is the reason that considerable energy (and expense) must go into the extraction of a metal from its ore. However, once the metal is won and put into use, it tends to spontaneously revert back to its more stable form. To do so, the metal must lose electrons, and this requires the presence of an electron acceptor or oxidizing agent. Oxygen, of course, is the most prominent of these, but hydrogen ions and the cations of any more noble metal1 are also very common promoters of corrosion. 

Fig. 12.21. The cathodic and anodic Tafel lines at two different electrode reactions. At l/corr there is a steady state. No net current passes, but there is a steady production of A (e.g., H2) and B+ (the metal, corroding) in a net electrochemical reaction that appears to be chemical (no net current flows to an outside circuit). Note the difference in these diagrams from the Evans-Hoar diagrams, which show corrosion is spontaneous and drives itself. The difference is similar to that between an electrochemical reaction and a fuel cell. (Reprinted from J. O M. Bockris and S. N. M. Kahn, Surface Electro Chemistry, Fig. 8.1, p. 747 Plenum, 1993).

An example of an irreversible process is a spontaneous chemical reaction or electrochemical reaction. 

Since the overall electrochemical process is thermodynamically spontaneous, the free energy change associated with the electrochemical reaction is directly converted to dc electrical energy. 

An electrolytic cell is similar to a voltaic cell except the electrochemical reactions involved do not occur spontaneously but require the input of current from an external source. Wires connected to each end of a battery and submerged in a suitable electrolyte can represent an electrochemical cell. As with voltaic cells, the creation and/or removal of ions at the electrodes facilitates the transfer of current into and out of solution. If the electrolytes in solution are redox-inert within the stability field of water (e.g., Na and Cf) and the voltage is over 1.2 volts, the hydrolysis of water may transfer current at the electrodes ... 

Although corrosion is a serious problem for many metals, we will focus on the spontaneous electrochemical reactions of iron. Corrosion can be pictured as a short-circuited galvanic cell, in which some regions of the metal surface act as cathodes and others as anodes, and the electric circuit is completed by electron flow through the iron itself. These electrochemical cells form in parts of the metal where there are impurities or in regions that are subject to stress. The anode reaction is... 

Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating ID nanostructures in relatively large quantities. It is well known that at critical micellar concentration (CMC) surfactant molecules spontaneously organize into rod-shaped micelles [315c]. These anisotropic structures can be used immediately as soft templates to promote the formation of nanorods when coupled with appropriate chemical or electrochemical reaction. The surfactant needs to be selectively removed to collect the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by using surfactants such as Na-AOT or Triton X of known concentrations [238, 246]. 

Thus, in Refs, and 209-214) been suggested that electrochemical reactions of cathodic reduction and the processes of spontaneous dissolution of metals in aqueous media proceed via an intermediate stage of formation of hydrated electrons. This means that the electrons leave the electrode and go into the solution and then the hydrated electrons react in the solution bulk, reducing, for instance, the proton donors. The theoretical and experimental argumentation of these authors, as regards the possibility of the mechanism itself, was disputed by other researchers... 

In this section, you will examine electrolysis, the process in which electrical energy is used to cause a non-spontaneous chemical reaction to occur. In the second part of the chapter, you will learn about the process that is the reverse of electrolysis—electrochemical reactions that can be used to produce electricity. 

In contrast to potentiometry that operates under zero current conditions, other electroanalytical methods impose an external source of electricity to the sample solution, to induce an electrochemical reaction that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions or organic compounds that can either be reduced or oxidized electrochemically. 

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