Reduction-oxidation chemistry half-reactions

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

They are the basis of many products and processes, from batteries to photosynthesis and respiration. You know redox reactions involve an oxidation half-reaction in which electrons are lost and a reduction half-reaction in which electrons are gained. In order to use the chemistry of redox reactions, we need to know about the tendency of the ions involved in the half-reactions to gain electrons. This tendency is called the reduction potential. Tables of standard reduction potentials exist that provide quantitative information on electron movement in redox half-reactions. In this lab, you will use reduction potentials combined with gravimetric analysis to determine oxidation numbers of the involved substances. 

Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at the surface of or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The electrode at which oxidation occurs is termed the anode. The electrode at which reduction occurs is termed the cathode. The complete system, with the anode connected to the cathode via an external conductor, is often called a cell. The individual oxidation and reduction reactions are called half-reactions. The individual electrodes with their half-reactions are called half-cells. As we shall see in this chapter, the half-cells are often in separate containers (mostly to prevent contamination) and are themselves often referred to as electrodes because they are housed in portable glass or plastic tubes. In any case, there must be contact between the half-cells to facilitate ionic diffusion. This contact is called the salt bridge and may take the form of an inverted U-shaped tube filled with an electrolyte solution, as shown in Figure 14.2, or, in most cases, a small fibrous plug at the tip of the portable unit, as we will see later in this chapter. 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

Electrochemistry is the area of chemistry concerned with the interconversion of chemical and electrical energy. Chemical energy is converted to electrical energy in a galvanic cell, a device in which a spontaneous redox reaction is used to produce an electric current. Electrical energy is converted to chemical energy in an electrolytic cell, a cell in which an electric current drives a nonspontaneous reaction. It s convenient to separate cell reactions into half-reactions because oxidation and reduction occur at separate electrodes. The electrode at which oxidation occurs is called the anode, and the electrode at which reduction occurs is called the cathode. 

The catalytic cycle of each flavoenzyme consists of two distinct processes, the acceptance of redox equivalents from a substrate and the transfer of these equivalents to an acceptor. Accordingly, the catalyzed reactions consist of two half-reactions a reductive half-reaction in which the flavin is reduced and an oxidative half-reaction, in which the reduced flavin is reoxidized. This review summarizes the chemistry of simple flavoprotein reductases, dehydrogenases, (di)thiol oxidoreductases, oxidases, and monooxygenases (Table 1) (5 0) This grouping provides a good appreciation about what type of common mechanisms can be distinguished and what type of substrates can be converted. Information on the chemistry of complex flavoenzymes can be found in the Further Reading section. 

When the valence-band hole and the conduction-band electron are both trapped by an appropriate oxidation or reduction half reaction, singly oxidized and reduced species are formed on the surface of the photocatalyst. Because these adsorbed intermediates can move about the surface, either migrating closer together or diffusing away from each other, before back-electron transfer occurs, novel chemistry is likely to ensue. It is a unique characteristic of photoelectrochemical catalysts that both oxidized and reduced species are produced on the same surface. As a result,... 

D) Acids do not spontaneously spit out a proton Despite our way of writing ionization equilibria as shown on the next page, acids do not give up a proton unless a base comes by to take the proton away. The reactions as drawn in the table should be considered half-reactions, just as the reactions in the electromotive series were half-reactions for balancing oxidation-reduction reactions in general chemistry. 

SLO follows an ordered, bi-uni mechanism, in which linoleic acid (LA) binds and reacts prior to O2 encounter [8], which has permitted a variety of steady-state and single-turnover studies into chemistry on SLO. The kinetic mechanism can be divided into a reductive half-reaction, described by the rate constant kcat/ffM(LA), and an oxidative half-reaction described by the rate constant fecat/KM(02). On the reductive half-reaction, SLO binds LA (ki), then the Fe +-OH cofactor abstracts the pro-S hydrogen from C-11 of LA (k2), forming a substrate-derived radical inter-... 

The occurrence of protonated bridging amides in 8 may be analogous to the behaviour of three-iron ferredoxins that undergo protonation upon reduction, presumably at bridging sulfides. The correspondence between Fe-NR and Fe-S chemistry extends to the redox activity of the core ligands, where the two-electron reduction of the N-N bond and the four-electron oxidative formation of the N=N bond are fully equivalent to sulfur-based half-reactions prevalent in... 

This codification logically defines all possible constructions, not merely those currently known in chemistry, because all possible mathematical combinations of /-hsts may be generated (there are 40 possible /-lists for half-reaction products i ). The half-reactions are divided by type into RH and RF(==RZ andRIl) and into related polarity types, and . Half-reactions of polarity are oxidative constructions, and essentially nucleophilic, while those of polarity are reductive, essentially electrophilic (although polarity definition does not require mechanistic... 

The potentials are for the half-reaction written as a reduction, and so they represent reduction potentials. We will use the Gibbs-Stockholm electrode potential convention, adopted at the 17th Conference of the International Union of Pure and Applied Chemistry in Stockholm, 1953. In this convention, the half-reaction is written as a reduction, and the potential increases as the tendency for reduction (of the oxidized form of the half-reaction) increases. 

For an element exhibiting several different oxidation states in aqueous solution, we must consider a number of different half-reactions in order to obtain a clear picture of its solution chemistry. Consider manganese as an example aqueous solution species may contain manganese in oxidation states ranging from Mn(II) to Mn(VII), and equations 7.42-7.46 give half-reactions for which standard reduction potentials can be determined experimentally. 

Electrochemistry is ranked by teachers and students as one of the most difficult curriculum domains taught and learnt in secondary school chemistry (cf. Davies, 1991 Griffiths, 1994). For that reason, in this chapter, we primarily discuss this domain at the secondary level but also make connections to the tertiary level. In many chemistry curricula and textbooks, it is common to divide electrochemistry into two topics redox reactions (oxidation and reduction) and electrochemical cells (galvanic and electrolytic). The usual rationale for this distinction is that students need an understanding of oxidation-reduction to apply it to electrochemical cells. This analytical distinction, based on differences in the location of the half reactions, is used throughout the chapter. 

Consequently, these two distinct categories of reaction types may be summarized in the shape of tabular forms depicting the range of structures arrived at by specific nucleophile— electrophile combinations as given in Table-1 below. Thoughtfully, each side of the Table-1 may be regarded as a half-reaction belonging to either oxidation-reduction or acid-base chemistry. 

According to the authors, the terms haif-reaction or oxidation-reduction reaction can be used for this eiectron exchange reaction. The term haif-reaction, which is often used in this document, stresses the fact that if a redox coupie reacts, e.g., in the direction of oxidation, at least one other couple must react in the direction of reduction. This term is derived from redox-chemistry in solution-based reactions, however it is still of educational interest for electrochemists given that at least two half-reactions are always occurring simultaneously, one at the anode and the other at the cathode. 

Remember that in the laws of physical chemistry, stoichiometric numbers are algebraic coefficients. The general equation that is obtained as a result can equally be applied to a half-reaction occurring either in the direction of oxidation (Ve = -i-n) or reduction (Vg = - n) R provided that the reaction rate is also taken in algebraic terms following the direction of the redox reaction. 

NMN is basically half of the NAD+ molecule nicotinamide ribose phosphate. NADP+ is NAD+ bearing a phosphate group at C3 of the ribose group attached to the adenine. The redox chemistry is the same in all three forms of the coenzymes. NAD+ is the form most frequently employed for biochemical oxidation reactions in catabohsm and NADP+ (in its reduced form NADPH) is the form usually employed for biochemical reduction reactions in anabohsm. NMN is employed infrequently. 

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