Electricity oxidation-reduction reactions

Many chemical reactions can be understood in terms of the transfer of electrons between the species involved. These reactions are called oxidation-reduction, or redox reactions. Because electrons also move in generating and delivering electricity, oxidation-reduction reactions provide a connection to electrical systems. 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

The battery acts as an electron pump, pushing electrons into the cathode, C, and removing diem from the anode, A. To maintain electrical neutrality, some process within the cell must consume electrons at C and liberate them at A. This process is an oxidation-reduction reaction when carried out in an electrolytic cell, it is called electrolysis. At the cathode, an ion or molecule undergoes reduction by accepting electrons. At the anode, electrons are produced by the oxidation of an ion or molecule. 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

In theory, any oxidation-reduction reaction can be set up in a cell to do electrical work. The amount of reversible1 work is easily calculated. If, during the discharge of a cell, a quantity of electricity Q flows through the external circuit at a constant potential, the amount of electrical work, n e, produced is given by... 

Electron-transfer reactions occur all around us. Objects made of iron become coated with mst when they are exposed to moist air. Animals obtain energy from the reaction of carbohydrates with oxygen to form carbon dioxide and water. Turning on a flashlight generates a current of electricity from a chemical reaction in the batteries. In an aluminum refinery, huge quantities of electricity drive the conversion of aluminum oxide into aluminum metal. These different chemical processes share one common feature Each is an oxidation-reduction reaction, commonly called a redox reaction, in which electrons are transferred from one chemical species to another. 

Due to participation in oxidation-reduction reactions the reducing or inflammable gases affect the stoichiometricity of oxide and, consequently the concentration of stoichiometric defects which usually control the dope electric conductivity of adsorbent [26, 67, 85, 86, 90]... 

One easily demonstrated electrical characteristic of moist soil is seen in the production of electricity when two different metals, namely, copper and zinc, are inserted into it. This is not unexpected because any salt-containing solution adsorbed in media, such as paper or cloth, and placed between these same two electrodes will cause a spontaneous reaction that produces electricity. The source of this flow of electrons is an oxidation-reduction reaction, represented as two half-reactions as shown in Figure 9.1 for copper and zinc. 

Figure 9.1. Oxidation-reduction reactions that, when coupled, produce an electric current.

Oxidation-reduction reactions involve electron transfer typically by direct collision between chemical species in solution. In electroanalytical chemistry, electron transfer occurs, but through electrical conductors rather than by direct collision. 

What are oxidation-reduction reactions How are they involved in the interconversion of chemical and electrical energy ... 

Figure 10. Cyclic voltammetric response at the NPyeCME for the oxidation/ reduction reaction of benzyl alcohol (32 mM)/C10 in aqueous 4.1 mol NaOCl (A) and nonaqueous CH2CI2 (B) solutions at a scan rate of 50 mV/s. (C) Cartoon for the NPyeCME. Inset (A) corresponds to an enlarged version of the oxidation part without (a) and with (b) benzyl alcohol. In order to marntam the electrical conductivity, 0.1 M tetrabutylammonium bromide (TBAB) is added into the CH2CI2 solution.

If a chemical reaction can make electricity it should not be surprising to learn that electricity can make a chemical reaction. Using an electric current to cause a chemical reaction is called electrolysis, a technique widely used to win elements from their compounds. For example, pure sodium metal (Na) and chlorine gas (CI2) are obtained by passing electricity through molten sodium chloride (NaCl). The study of the interplay of electricity and oxidation-reduction reactions is called electrochemistry. 

The electrochemical treatment of contaminated groundwater technology uses direct electrical current applied between two immersed electrodes to produce oxidation-reduction reactions in aqueous solutions. Positively charged metal ions are attracted to the negatively charged electrode (the cathode), where they are reduced. 

Electrochemistry is the study of chemical reactions in which electricity plays a role. Some electrochemical reactions generate electricity as the reaction proceeds, while in other cases the opposite occurs—electricity drives the reaction. In either case, electrochemical reactions involve the transfer of electrons, which are the negatively charged particles surrounding an atom s nucleus. Reactions in which electrons are transferred (or appear to be transferred) from atom to atom are called oxidation-reduction reactions. 

Electrochemistry is the study of the relationship between electrical energy and chemical change. It involves oxidation-reduction reactions to produce electricity, or electricity to cause an oxidation-reduction reaction. 

Thus, if we start with our reactants and products under standard conditions and allow the reaction to proceed to equilibrium, an amount of energy AG° becomes available for external work. In the context of doing external electrical work, an oxidation—reduction reaction can generate a standard electromotive force AE° given by... 

Virtually all energy transductions in cells can be traced to this flow of electrons from one molecule to another, in a downhill flow from higher to lower electrochemical potential as such, this is formally analogous to the flow of electrons in a battery-driven electric circuit. All these reactions involving electron flow are oxidation-reduction reactions one reactant is oxidized (loses electrons) as another is reduced (gains electrons). 

THE ELECTRICITY OF A BATTERY COMES FROM OXIDATION-REDUCTION REACTIONS... 

So we see that with the proper setup it is possible to harness electrical energy from an oxidation-reduction reaction. The apparatus shown in Figure 11.8 is one example. Such devices are called voltaic cells. Instead of two containers, a voltaic cell can be an all-in-one, self-contained unit, in which case it is called a battery. Batteries are either disposable or rechargeable, and here we explore some examples of each. Although the two types differ in design and composition, they function by the same principle two materials that oxidize and reduce each other are connected by a medium through which ions travel to balance an external flow of electrons. 

Disposable batteries have relatively short lives because electron-producing chemicals are consumed. The main feature of rechargeable batteries is the reversibility of the oxidation and reduction reactions. In your car s rechargeable lead storage battery, for example, electrical energy is produced as lead dioxide, lead, and sulfuric acid are consumed to form lead sulfate and water. The elemental lead is oxidized to Pb2+, and the lead in the lead dioxide is reduced from the Pb4+ state to the Pb2+ state. Combining the two half-reactions gives the complete oxidation-reduction reaction ... 

Batteries are electrochemical cells. Where would we be without batteries A battery is needed to start a car. Batteries power flashlights, move toys, and make watches work. Jewelry with lightbulb designs can use tiny batteries. A battery provides an electric current through oxidation-reduction reactions in which the flow of electrons is directed through a wire. The force of the electrons through the wire is measured in volts. 

These laws (determined by Michael Faraday over a half century before the discovery of the electron) can now be shown to be simple consequences of the electrical nature of matter. In any electrolysis, an oxidation must occur at the anode to supply the electrons that leave this electrode. Also, a reduction must occur at the cathode removing electrons coming into the system from an outside source (battery or other DC source). By the principle of continuity of current, electrons must be discharged at the cathode at exactly the same rate at which they are supplied to the anode. By definition of the equivalent mass for oxidation-reduction reactions, the number of equivalents of electrode reaction must be proportional to the amount of charge transported into or out of the electrolytic cell. Further, the number of equivalents is equal to the number of moles of electrons transported in the circuit. The Faraday constant (F) is equal to the charge of one mole of electrons, as shown in this equation ... 

Many oxidation-reduction reactions may be carried out in such a way as to generate electricity. These cells are known as voltaic (older term galvanic) cells. In principle, any spontaneous, oxidation-reduction reaction (aqueous) can be set up to generate electricity by the following requirements ... 

An oxidation-reduction reaction that is not spontaneous, for which the calculated cell potential is negative, may be induced by electrolysis. This reaction can be due to an external electrical potential to force electrons into the couple undergoing reduction and to extract electrons from the couple undergoing oxidation. The minimum external potential required for electrolysis is the value of the calculated cell potential for the reaction. 

Batteries are a practical application of the galvanic cell in that an oxidation-reduction reaction generates an electric current. A battery that has an enormous impact on our lives is the automobile battery, shown in Figure 10.3. 

For oxidation-reduction reactions in aqueous solution under an externally applied electrical potential, or in its absence, one can write O for the oxidized species and R for the reduced species, and the half-cell reaction can be written as... 

This is the principle of the voltaic cell, in which an oxidation-reduction reaction is used to produce electricity. 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

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