Spontaneous processes oxidation-reduction

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

The skull metal and oxide are first completely burned to oxide by heating in air to 400-500°C. The plutonium metal spontaneously burns and is collected as a green Pu03 powder. This oxide is recycled back as feed for Direct Oxide Reduction. This process is normally 100% efficient with only a small plutonium residue showing up in items such as clean-up rags. 

Galvanic2 Negative Oxidation Zn —> Zn2 + 2 e Positive Reduction Cu2 + 2 e — Cu Spontaneous process Current generated... 

Thus, the tables of standard electrode potentials predict those processes that tend to occur spontaneously if any pair of listed interfacial systems are built into an electrochemical cell that with the lower (algebraically, i.e., more negative) standard potential will spontaneously undergo deelectronation (oxidation), while that with the higher potential (i.e., more positive) will spontaneously undergo electronation (reduction). 

Corrosion is the spontaneous destruction (oxidation, in particular) of a material as a result of its physicochemical interaction with the surrounding medium, for example, with an electrolyte solution. Since we mean here a process, in which the net current is absent, the system solid-solution remains, as a whole, neutral hence, the oxidation of a solid in the course of corrosion must be accompanied by the reduction of other components in the system (i.e., solvent or solutes). 

The energy available from spontaneous cell reactions can be used to power vehicles or generate electricity (Box 12.2). To calculate the standard cell potential for a spontaneous process, we must combine the standard potential of the cathode half-reaction (reduction) with that of the anode half-reaction (oxidation) in such a way as to obtain a positive... 

If a voltaic cell is to run spontaneously, the reduction potential at the cathode must be higher (more positive) than the reduction potential at the anode. This will allow the reaction at the anode to proceed as an oxidation (rather than a reduction). The greater the difference in potentials between the cathode and anode, the greater the cell voltage. Eor E° will be positive for spontaneous processes and negative for nonspontaneous ones. 

Besides oxidation, reduction processes can also take place spontaneously or by chemical treatments when the redox potential of the sorbed cation makes it possible. As an example, the spontaneous reduction of silver ions in silver-mont-morillonite is shown here (Konya et al. 2005). 

The over-all reduction of CO2 to CH4 is expected to be a spontaneous process that goes through the reduction levels of formate, formaldehyde, and methanol with only a limited, perhaps early requirement for activation by ATP. At the lowest reduction stages, extra ATP may even be generated. In the fermentation of methanol by M. barkeri which utilizes only the last reduction step for methane formation (Reaction 12) somewhat more than 1 mole of ATP appears to be generated for each mole of CH3OH oxidized to CO2, judging from cell yields (14). 

MUTAROTATION The a- and /T forms of monosaccharides are readily interconverted when dissolved in water. This spontaneous process, called mutarotation, produces an equilibrium mixture of a- and /1-forms in both furanose and pyranose ring structures. The proportion of each form differs with each sugar type. Glucose, for example, exists primarily as a mixture of a- (38%) and j8- (62%) pyranose forms (Figure 7.11). Fructose is predominantly found in the a-and /J-furanose forms. The open chain formed during mutarotation can participate in oxidation-reduction reactions. 

Electrochemistry is the branch of chemistry that deals with the interconversion of electrical energy and chemical energy. Electrochemical processes are redox (oxidation-reduction) reactions in which the energy released by a spontaneous reaction is converted to electricity or in which electrical energy is nsed to cause a nonspontaneous reaction to occur. Although redox reactions were discnssed in Chapter 4, it is helpful to review some of the basic concepts that will come np again in this chapter. 

Electrochemistry is best defined as the study of the interchange of chemical and electrical energy. It is primarily concerned with two processes that involve oxidation-reduction reactions the generation of an electric current from a spontaneous chemical reaction and the opposite process, the use of a current to produce chemical change. 

A recent XPS study [276] on the interactions between PAN films and silver cations revealed that the more reduced form of the polymer was capable of depositing silver metal from 0.1 M AgNOj. For the intrinsically more oxidized polymer with open-circuit potential greater than the formal potential of silver deposition (0.115 eV), a metal-polymer complex was formed. In a parallel development, it was found that by coupling the metal reduction process in acid solutions with an increase in the intrinsic oxidation state of a N-containing polymer, such as PAN or PPY, and the subsequent re-protonation and reduction of the intrinsically Oxidized polymer in acid media, spontaneous and sustained reduction of precious metals (such as Au and Pd) to their elemental form can be readily achieved... 

Oxidation-reduction reactions are the basis of the branch of chemistry called electrochemistry. Such a reaction may occur spontaneously and produce electrical energy, as in a galvanic cell. If the reaction does not occur spontaneously, the addition of electrical energy may initiate a chemical change, a process called electrolysis. 

In writing the equation this way, we have dropped the subscript cell to indicate that the calculated emf does not necessarily refer to a voltaic cell. Also, we have generalized the standard reduction potentials by using the general terms reduction and oxidation rather than the terms specific to voltaic cells, cathode and anode. We can now make a general statement about the spontaneity of a reaction and its associated emf, E A positive value of E indicates a spontaneous process a negative value of E indicates a nonspontaneous process. We use E to represent the emf under nonstandard conditions and E° to indicate the standard emf. 

The positive value of E indicates that the displacement of silver by nickel resulting from oxidation of Ni metal and reduction of Ag is a spontaneous process. Remember that although we multiply the silver half-reaction by 2, the reduction potential is not multiplied. 

The process of iron being oxidized to make iron(III) oxide (rust) is spontaneous. Which of these statements about this process is/are true (a) The reduction of iron(III) oxide to iron is also spontaneous. (b) Because the process is spontaneous, the oxidation of iron must be fast, (c) The oxidation of iron is endothermic, (d) Equilibrium is achieved in a closed system when the rate of iron oxidation is equal to the rate of iron(III) oxide reduction, (e) The energy of the universe is decreased when iron is oxidized to rust. 

As discussed in Chapter 4, oxidation is the loss of electrons in a chemical reaction, and reduction is the gain of electrons, ooo (Section 4.4) Thus, oxidation-reduction (redox) reactions occur when electrons are transferred from an atom that is oxidized to an atom that is reduced. Redox reactions are involved not only in the operation of batteries but also in a wide variety of important natural processes, including the rusting of iron, the browning of foods, and the respiration of animals. Electrochemistry is the study of the relationships between electricity and chemical reactions. It includes the study of both spontaneous and nonspontaneous processes. 

Zinc metal is oxidized and Cu aq) is reduced in this reaction. These substances are in direct contact, however, so we are not producing usable electrical work the direct contact essentially "short-circuits" the cell. Nevertheless, the driving force for the reaction is the same as that in a voltaic cell, as in Figure 20.5. Because the E°gd value for the reduction of Cu (0.34 V) is more positive than the E°gd value for the reduction of Zn " (-0.76 V), the reduction of Cu aq) by Zn(s) is a spontaneous process. 

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