Hydronium ion reduction

Although the NHE is fundamental to electrochemistry, it does not represent the primary electron-transfer step for hydronium ion reduction at an inert (glassy-carbon) electrode 2... 

Likewise, the electron-transfer reduction of H20 (with uncharged hydrogen and oxygen atoms), which must overcome the stabilization of the strong O—H bonds, results in a — 1 charge for oxygen rather than hydronium ion reduction ... 

The loss of electrons by magnesium atoms to form Mg cations indicates that this reaction between magnesium metal and hydronium ions involves oxidation and reduction. An atom of magnesium is oxidized, losing two electrons to form a Mg + cation. Because electrons must be conserved in every chemical process, electrons lost by Mg must be gained by some other species. In this example the electrons lost by Mg are gained by H3 O , which is reduced to form H2 and H2 O. 

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 ... 

Separating oxidation from reduction makes it possible to verify that electrons are conserved in a redox reaction. Note that the electrons produced in the oxidation of magnesium are consumed in the reduction of hydronium ions. The electrons required for a reduction must come from an oxidation. 

A starting material loses electrons in an oxidation, so electrons appear among the products of the oxidation half-reaction. A starting material gains electrons in a reduction, so electrons appear among the reactants of the reduction half-reaction. The reaction of magnesium metal with hydronium ions to produce hydrogen gas provides an example Mg(.y) -I- 2H3 0 ((2 q) q) H2(g) + 2H2 0(/) Here are the half-reactions for this... 

Chemists have chosen the reduction of hydronium ions to hydrogen gas as the reference half-reaction ... 

Every standard reduction potential has a specific sign. When a substance is easier to reduce than hydronium ions under standard conditions, its E ° is positive. When a substance is more difficult to reduce than hydronium ions under standard conditions, its E ° is negative. 

The Zn /Zn reduction potential is more negative than the H3 O /H2 reduction potential (-0.76 V vs. 0 V), so zinc is the anode in this cell. Zinc is oxidized and hydronium ions are reduced, causing electrons to flow from the more negative zinc electrode to the less negative SHE. Again, we reverse the direction of the half-reaction with the more negative potential and find E by subtracting the half-cell potentials ... 

Atmospheric O2 has a partial pressure of 0.20 bar, and atmospheric water vapor is saturated with carbon dioxide. This dissolved CO2 forms carbonic acid, which generates a hydronium ion concentration of about 2.0 X 10 M. The Nemst equation allows calculation of the half-cell potential for the reduction of 02(g) under these... 

In aqueous solutions two half-reactions are of special importance (a) the reduction of hydrogen in water or hydronium ions ... 

A similar reactivity toward hydrogen has been found with the carbonyl-platinates [Pt3(CO)6]n2- when n is greater than 3, although only formation of hydronium ions corresponding to a net reduction process can be observed for example (95),... 

Cathodic limits on mercury. In aqueous or other protic solvents the reduction of hydronium ion or solvent generally will limit the negative potential range. The nature of some electrode reactions at highly negative potentials on mercury has been examined.63 For example, K(OH2) and Na(OH2)4 ions are reduced reversibly in aqueous solutions, but the process is accompanied by a parallel irreversible reaction due to an amalgam dissolution reaction of the alkali metal with water that produces hydrogen. 

In dipolar aptotic solvents, the availability of hydronium ions is much lower and consequently the cathodic limit is extended. Reversible or nearly reversible waves can be readily observed for the reduction of Group I and some Group II metal ions.64,65... 

The role of hydronium ion and Br0nsted acids in electrode reactions has been described.51 53 For a reversible reduction [such as that for quinone that is represented in Eq. (7.10)] of the general form... 

The most fundamental redox process in electrochemistry is the reductive transformation of hydronium ion [H30(tq)] at a platinum electrode to molecular hydrogen [H2(g>]1... 

Another factor that affects the reduction of hydronium ion is its solvent sheath (solvation energy). By convention the pXa of H30(tq) in water is defined... 

Molecular Oxygen. The reduction of molecular oxygen is influenced by the solution matrix and its acidity. Thus, the redox thermodynamics of 02 are directly dependent upon hydronium ion activity ... 

An important point in these electron-transfer reductions is that the primary electron acceptor is the hydronium ion (H30+), which is transformed to a hydrogen atom (H-) that reacts with HO- (either free or bound via a covalent bond to the metal center). 

Some of the considerations for electron-transfer processes that have been discussed in previous chapters are fundamental to the electrochemistry of these examples. Thus, reductive processes always involve the most electrophilic (acidic, positive-charge density) center (substrate or substrate-matrix combination) that produces the least basic (nucleophilic) product. Under acidic conditions the primary reactant often is the hydronium ion (H30+) to give a hydrogen atom that couples with the substrate via covalent bond formation for instance... 

In both cases the overall process is an irreversible two-electron reduction via either (1) an EE path or (2) an ECEC path the first electron transfer is the most difficult and depends on the substrates electrophilicity. In the presence of hydronium ion the primary electron transfer will be to the most electrophilic center, for instance... 

Thus, the reduction of n-BuI is the equivalent of the addition of two hydrogen atoms [H ] (generated via the electrochemical reduction of the two hydronium ions). 

In the presence of hydronium ions (H30+), the reduction of the quinones is an irreversible two-electron process (ECEC), in which the first step is the more difficult (requiring the more negative potential). 

Whereas hydroquinone (p-HOPhOH) in acetonitrile is oxidized via an irreversible two-electron process at +1.18 V versus SCE (Eq. 12.34 and Table 12.2), its dimethyl ether (p-MeOPhOMe) is significantly more resistant with a reversible one-electron oxidation at +1.30 V versus SCE (Figure 12.5).16 The initial oxidation of the latter is followed by a second irreversible one-electron oxidation ( + 1.81 V vs. SCE) that yields a product that is reduced at +0.59 V versus SCE [consistent with the reduction of benzoquinone in the presence of hydronium ions (Table 12.2)] ... 

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