Electron transfer, in redox reactions

Even though some oxidation numbers found using these rules are averages, the rules are still useful for monitoring electron transfers in redox reactions. 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

The use of the reaction term in place of other terms - like interaction (1) or relationship (2) - is very frequent [23, 24] and is often associated to inadequate understanding of the concept expressed by the other term. The confusion (3) between the verbs to transfer and to transform [23] generates difficulties in the understanding of the very nature of chemical reactions (a transformation) or some of their aspects (e.g. electron transfer in redox reactions). At assessment level, it is difficult to guess whether the perceptions or mental images that students have formed correspond to the meaning concerned, or remain blurred because of inadequate distinction between the meanings of two different terms. 

This mechanism involves the formation of a chemical bond between the two reacting species, which may act as a bridge for the transfer of the electron. This is the less common method of electron transfer in redox reactions involving L-ascorbic acid. We have already seen that reaction of iron (in) results in the formation of an intermediate blue compound and it is likely that this is an inner-sphere electron transfer. 

Why is it particularly easy to apply Marcus theory to electron transfers in redox reactions such as that shown below ... 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Redox reactions are more complicated than precipitation or proton transfer reactions because the electrons transferred in redox chemishy do not appear in the balanced chemical equation. Instead, they are hidden among the starting materials and products. However, we can keep track of electrons by writing two half-reactions that describe the oxidation and the reduction separately. A half-reaction is a balanced chemical equation that includes electrons and describes either the oxidation or reduction but not both. Thus, a half-reaction describes half of a redox reaction. Here are the half-reactions for the redox reaction of magnesium and hydronium ions ... 

The structure of HRP-I has been identified as an Fe(IV) porphyrin -ir-cation radical by a variety of spectroscopic methods (71-74). The oxidized forms of HRP present differences in their visible absorption spectra (75-77). These distinct spectral characteristics of HRP have made this a very useful redox protein for studying one-electron transfers in alkaloid reactions. An example is illustrated in Fig. 2 where the one-electron oxidation of vindoline is followed by observing the oxidation of native HRP (curve A) with equimolar H202 to HRP-compound I (curve B). Addition of vindoline to the reaction mixture yields the absorption spectrum of HRP-compound II (curve C) (78). This methodology can yield useful information on the stoichiometry and kinetics of electron transfer from an alkaloid substrate to HRP. Several excellent reviews on the properties, mechanism, and oxidation states of peroxidases have been published (79-81). 

Secondly, instead of a pure and simple electron transfer, the redox reaction can be coupled to a chemical reaction in such a way that the electron transfer takes place either after incorporation of the substrate or an intermediate into the inner coordination sphere of a metal ion ( inner-sphere electron transfer), by formation of a charge transfer complex, or in form of a hydrogen or hydride atom abstraction, respectively. In these cases the reaction between redox catalyst and substrate does not directly depend on the difference of the two standard potentials (see Sect. 2.3). 

In inorganic compounds or complexes with variable valence system, a redox reaction may be set up by intramolecular or intermolecular electron transfer processes (redox reactions). 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

Potentiometric sensor is based on a redox reaction that occurs at the electrode-electrolyte interface in an electrochemical cell. If a redox reaction Ox + Ze Red takes place at an electrode surface, it is called a halfcell reaction. In the above reaction. Ox is the oxidant. Red is the reduced product, e in the electron, and Z is the number of electrons transferred in the reaction. At thermodynamic quasiequilibrium conditions, the Nernst equation is applicable and can be expressed as ... 

Write the equation for a reaction that is not a redox reaction. Are electrons transferred in this reaction ... 

Voltaic cell A system in which two half-reactions for a redox reaction are separated, allowing the electrons transferred in the reaction to be passed between them through a wire. 

The distinctive chemical environment of estuarine and coastal sediments is determined to a large extent by electron transfer, or redox reactions, in which a... 

In the primary reactions of photosynthesis, the light-induced charge separation and stabilization involves the generation of a radical cation and of radical anions at the various steps of electron transfer. In the reaction centers, specialized pigment molecules and redox components act as primary electron donors and electron acceptors. They are embedded in the protein matrix and provided with very specific interactions that seem to be responsible for the spectral and redox properties of the pigments as well as for the efficiency and specificity of electron transfer. 

---