Chemical element couples, redox

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

Among the four elemental components, carbon, nitrogen, and sulfur represent natural chemical redox couples, because each element occurs both in an oxidized and a reduced state. The process of formation of organic matter by photosynthesis constitutes chemical reduction of C, N, and S from their inorganic... 

In the first half-reaction Zn° loses electrons and hy definition is oxidized. Note that the process of oxidation is defined in terms of electron loss, and does not necessarily involve the element oxygen in any form. In the second half-reaction accepts electrons and is said to he reduced. The two chemical species that make up each halfreaction are referred to as a couple (i.e. Zn°/Zn " and Cu +/Cu°) that interconvert by gain or loss of electrons. The separation of full redox reactions into half-reactions is largely conceptual because electrons are directly exchanged among reacting species and thus an oxidation cannot occur without a simultaneous reduction. 

An accumulator is characterized by two voltage values. The first is the nominal voltage, which is determined by the potential for chemical reaction of each element in the redox couple used. For instance, in the case of a lead accumulator, the nominal voltage (operational voltage) per cell is 2.1 volts. In the case of a lithium battery, it is around 3.8 V, depending on the chemistry used. 

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