Electrode 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,... 

Scheme 3 Electrode half-reaction scheme for five- and six-coordinate Mn(lll) porphyrins [56-58].

H2O. Moreover, the half-reactions in aqueous solution exhibit a pH dependence corresponding to the addition of one proton in conjunction with each electron. Thus, under the conditions of Fig. 12, the [Mn2 02] " center (1) is converted to an [Mn2 " " 0(0H)] + center (2) in the first reduction and to an [Mn2 "(OH)2] center (3) in the second. Electrode half-reactions, Nemst equations, and values for the... 

S.C.E., saturated calomel electrode A calomel electrode saturated with KCI. The electrode half-reaction is Hg2Cl2(.v) + 2e 2Hg(/) + 2C1-. 

Although the anode half-cell always appears on the left in the shorthand notation, its location in a cell drawing is arbitrary. This means that you can t infer which electrode is the anode and which is the cathode from the location of the electrodes in a cell drawing. You must identify the electrodes based on whether each electrode half-reaction is an oxidation or a reduction. 

The hydrogen electrode. The hydrogen electrode is discussed first because it is the primary reference electrode used to define an internationally accepted scale of standard potentials in aqueous solution. By convention, the potential of an electrode half-reaction that is measured with respect to the normal hydrogen electrode (NHE also written as SHE, standard hydrogen electrode) is defined as the electrode potential of the half reaction. This convention amounts to an arbitrary assignment for the standard potential of the hydrogen electrode as zero at all temperatures. Thus, there is in effect a separate scale of electrode potentials at each temperature level. 

In a system involving reagents and products at equilibrium, the rates of the reactions in each direction are equal. Equilibrium can thus be seen as a limiting case, and any kinetic model must give the correct equilibrium expression. For reactions at an electrode, half-reactions > the equilibrium expression is the Nernst equation. 

By convention, the electrode potential of any half-reaction is expressed relative to that of a standard hydrogen electrode (half-reaction 2H+ -p 2e -H2) and is called the standard electrode potential, E . Table 34.1 shows the values of E" for selected half-reactions. With any pair of half-reactions from this series, electrons will flow from that having the lowest electrode potential to that of the highest. " is determined at pH = 0. It is often more appropriate to express standard electrode potentials at pH 7 for biological systems, and the symbol is used in all circumstances, it is important that the pH is clearly stated. 

There are also batteries in which the sum of the two electrode half-reactions produces not a chemical reaction but the transfer of a species from one electrode to the other. Such cells are called concentration cells to distinguish them from those in which a chemical reaction takes place. 

A positive electrode potential indicates that the half-reaction in question is spontaneous with respect to the standard hydrogen electrode half-reaction. That... 

H2O. Moreover, the half-reactions in aqueous solution exhibit a pH dependence corresponding to the addition of one proton in conjunction with each electron. Thus, under the conditions of Fig. 12, the [Mn2 O2] center (1) is converted to an [Mn2 O(OH)]5+ center (2) in the first reduction and to an [Mn2 (OH)2] center (3) in the second. Electrode half-reactions, Nernst equations, and pA a values for the phen complex and for several related systems are summarized in Table 5. Precise data are somewhat difficult to obtain for these systems owing to the slow electron-transfer kinetics (fcgj, 10 cm s or less), the need to activate the working electrode surface to achieve a useful response, and the limited stability of some species in the presence of coordinating buffers. Nonetheless, pAa values for the bpy complex obtained from the Pourbaix diagram in Ref. 93 lead to pH-independent potentials of +1.1 and +0.5 V, respectively, for [Mn2 O(OH)]3+ and [Mn2 O2] + reduction, values that compare reasonably well to the results in Table 4, given the difference in solvent environments. 

For all electrode systems, an electrode half-reaction can be written from which the potential of the electrode is described. The electrode system can be represented by M/M"" ", in which the line represents an electrode-solution interface. For the silver electrode, we have... 

Problems based on Faraday s law often ask you to calculate current, mass of material, or time. The electrode half-reaction provides the key to solving these problems because it is related to the mass for a certain quantity of charge. 

On the absorption of a photon by a n-type semiconductor, one electron is excited from the valence band and promoted to the conduction band [see Figure 4.9(b)] to leave a net positive charge - a hole (h" ) - in the valence band. The electron in the conduction band is forced to the back contact and is transferred to the counter electrode. At the electrode I electrolyte interface of this latter electrode, the electrons react with protons to produce hydrogen, while the simultaneously created holes in the valence band of the semiconductor (the minority carriers) generate oxygen by oxidizing water. Thus, the electrode half-reactions in the PEC cell are as follows ... 

The solvent (water) can sometimes itself be the reactant in an electrode half reaction. This is the case in neutral or alkaline solutions where, instead of H+, H2O is the source of hydrogen (water is reduced to hydrogen). On the other hand, in neutral or acidic solutions, instead of OH , H2O is the source of oxygen (water is oxidised to oxygen). 

The negative electrode half-reaction (with charging being forward) is ... 

The PdH reference electrode is thus similar to the standard hydrogen electrode, which is based on the same half reaction between hydronium ions and molecular hydrogen. However, there are also some differences. For instance, palladium is not as good a catalyst for the electrode half reaction as platinum. As a consequence, under identical conditions, the reference potential of the palladium hydrogen electrode differs from that of the standard hydrogen electrode by a constant potentiaL In addition, because hydronium ions are involved in the reference half reaction, the potential of the PdH electrode is pH dependent If the PdH electrode were connected to another reference electrode (e.g., Ag/AgQ), the PdH electrode could serve as a pH indicator. However, when the PdH electrode functions as a reference electrode, it cannot indicate pH. In the same manner, a glass electrode cannot measure pH it must be used together with a suitable reference electrode. 

Enthalpy Changes (kj mol ) for Electrode Half-Reaction M (aq)—>M(s) for Alkali Metals... 

For a hydrogen electrode half-reaction, H2(g) + e —> H (aq), with E = 0, the same calculation gives... 

Write the cell reactions and electrode half-reactions for the following cells ... 

The standard electrode potentials of these electrodes (half-reactions) can be found in Table 4.1. Note that flH2o(i) in Equation 4.28 is very close to 1 in dilute solution. However, in high concentrated solutions, flH20(i) can significantly deviate from 1, and this should be taken into account in calculating the electrode potential. 

The three contributors in the electrochemical reactions in a lithium-ion battery are the anode, cathode, and electrolyte (Fig. 3.1). During insertion (or intercalation), ions move into the electrode, while during the reverse process, extraction (or deintercalation), ions move back out. To control the rate of electron transfer, the cathode must be physically and electrically isolated from the anode using an ionically conductive but electrically insulating medium, typically a liquid or polymeric electrolyte [5]. Following are the electrochemical reactions occurring in a lithium-ion battery (LiCo02 system). The positive electrode half reaction is... 

Water electrolysis is a method to generate hydrogen. The water electrolysis process can be treated as a superposition of concurrent or sequential electrochemical reaction occurring in the vicinity of electrodes (half reactions) whose overall effect is to split water molecule under the influence of a direct electric current and separate gaseous products (hydrogen and oxygen) [80]. 

EXAMPLE 19-12 Predicting Electrode Half-Reactions and Overall Reactions in Electrolysis... 

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