Oxidation and Reduction Redox Reactions

The ability to balance a single half-reaction as a bookkeeping exercise does not mean that a single half-reaction can occur on its own. In a redox reaction, oxidation and reduction must both occur. 

In redox reactions, oxidation and reduction processes occur at the same time. The following reaction is an oxidation-reduction reaction. For example,... 

In a redox reaction, oxidation and reduction occur at the same time. 

In redox reactions, oxidation and reduction always occur simultaneously. Oxidation is characterized by the loss of electrons, reduction by the gain of electrons. 

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. 

Redox Reactions Oxidation and reduction processes which affects the stability and persistence of electron acceptors (e.g., oxygen, ferric iron, nittate, etc.) and donors (e.g., organic matter, ferrous iron, sulfide, etc.) often mediated by microbial activity. 

Reactions in which reactants undergo oxidation or reduction are called redox reactions. Oxidation and reduction are indicated conveniently by the oxidation number changes that occur. Oxidation numbers are assigned according to a specific set of rules. A substance is oxidized when the oxidation number of a constituent element increases, and it is reduced when the oxidation number of a constituent element decreases. 

In redox reactions, oxidation and reduction always occur simultaneously. Oxidation is characterized by the loss of electrons, reduction by the gain of electrons. Oxidation numbers help us keep track of charge distribution and are assigned to aU atoms in a compound or ion according to specific rules. Oxidation can be defined as an increase in oxidation number reduction can be defined as a decrease in oxidation number. 

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 ... 

If electrons are liberated, they will not remain as free charges. They will be absorbed somewhere else in a complete reaction. Thus in a complete chemical reaction, oxidation and reduction reactions are coupled as in the case of a galvanic cell where electrons are liberated by an anode and are subsequently absorbed by a cathode. Thus, these coupled reactions are redox reactions. 

Photoreactions that involve transition metal ions, complexes or compounds can usually be classified as (photo)redox (simultaneous oxidation and reduction) processes. A representative non-photoassisted catalytic system is Fenton s reagent that produces HO radicals on reaction of ferrous ions (Fe2 +) and hydrogen peroxide (Scheme 6.287a). Its photochemical counterpart is the photo-Fenton process,1527 in which ferric (Fe3 + ) complexes in aqueous solutions (absorbing over 300 nm) are reduced to ferrous ions (Scheme 6.287b). 

Here s an overview of how it works You convert the unbalanced redox equation to the ionic equation and then break it down into two half-reactions — oxidation and reduction. Balance each of these half-reactions sepeirately and then combine them to give the balanced ionic equation. Finally, put the spectator ions into the balanced ionic equation, converting the reaction back to the molecular form. (For a discussion of molecul lr, ionic, and net-ionic equations, see Chapter 7.)... 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

Redox flow batteries, under development since the early 1970s, are stUl of interest primarily for utility load leveling applications (77). Such a battery is shown schematically in Figure 5. Unlike other batteries, the active materials are not contained within the battery itself but are stored in separate tanks. The reactants each flow into a half-ceU separated one from the other by a selective membrane. An oxidation and reduction electrochemical reaction occurs in each half-ceU to generate current. Examples of this technology include the iron—chromium, Fe—Cr, battery (79) and the vanadium redox cell (80). 

Ox and Red are general symbols for oxidation and reduction media respectively, and n and (n-z) indicate their numerical charge (see Section 2.2.2). Where there is no electrochemical redox reaction [Eq. (2-9)], the corrosion rate according to Eq. (2-4) is zero because of Eq. (2-8). This is roughly the case with passive metals whose surface films are electrical insulators (e.g., A1 and Ti). Equation (2-8) does not take into account the possibility of electrons being diverted through a conductor. In this case the equilibrium... 

Any redox reaction can be split into two half-reactions, an oxidation and a reduction. It is possible to associate standard voltages x (standard oxidation voltage) and (standard reduction voltage) with the oxidation and reduction half-reactions. The standard voltage for the overall reaction, °, is the sum of these two quantities... 

Redox reaction A reaction involving oxidation and reduction, 86-87, 97-99q... 

Oxidation is electron loss reduction is electron gain. Oxidation and reduction occur together in redox reactions. 

Half-reactions express the two contributions (oxidation and reduction) to an overall redox reaction. 

Assuming that the enzymatic reaction is highly enantioselective, then even after only four cycles the enantiomeric excess will have reached 93.4% whereas after seven catalytic cycles the enantiomeric excess is >99% (Figure 5.3). This type of deracemization is really a stereoinversion process in that the reactive enantiomer undergoes stereoinversion during the process. One of the challenges of developing this type of process is to find conditions under which the enzyme catalyst and chemical reactant can coexist, particularly in the case of redox chemistry in which the coexistence of an oxidant and reductant in the same reaction vessel is difficult to achieve. For this... 

Table 16-4 Examples of redox reactions, consisting of an oxidation and reduction half reaction...

Seven chemical reactions were identified from the chemistry syllabus. These chemical reactions were selected because they were frequently encountered during the 2-year chemistiy course and based on their importance in understanding concepts associated with three topics, namely, acids, bases and salts, metal reactivity series and inorganic chemistry qualitative analysis. The seven types of chemical reactions were combustion of reactive metals in air, chemical reactions between dilute acids and reactive metals, neutralisation reactions between strong acids and strong alkalis, neutralisation reactions between dilute acids and metal oxides, chemical reactions between dilute acids and metal carbonates, ionic precipitation reactions and metal ion displacement reactions. Although two of the chemical reactions involved oxidation and reduction, it was decided not to include the concept of redox in this study as students had only recently been introduced to ion-electron... 

In any redox reaction, some species are oxidized and others are reduced. To reveal the number of electrons transferred, we must separate the oxidation and reduction parts. This is most easily accomplished by dividing the redox reaction into two half-reactions, one describing the oxidation and the other describing the reduction. In half-reactions, electrons appear as reactants or products. 

After oxidation and reduction half-reactions are balanced, they can be combined to give the balanced chemical equation for the overall redox process. Although electrons are reactants in reduction half-reactions and products in oxidation half-reactions, they must cancel in the overall redox equation. To accomplish this, multiply each half-reaction by an appropriate integer that makes the number of electrons in the reduction half-reaction equal to the number of electrons in the oxidation half-reaction. The entire half-reaction must be multiplied by the integer to maintain charge balance. Example illustrates this procedure. 

C19-0014. The mercury battery, use of which is being discontinued because of the toxicity of mercury, contains HgO and Zn in contact with basic aqueous solution. The redox products are Hg and ZnO. Determine the oxidation and reduction half-reactions and the overall reaction for these batteries. 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

The description of redox reactions may afresh be carried out by introducing oxidation and reduction as an electron-transfer process. For this purpose, a process involving burning of magnesium in oxygen is considered, the reaction being written chemically as ... 

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