Half cell reactions

Because electrons are transferred, not created or destroyed, in redox reactions, any such reaction may be considered to be a combination of two half reactions, each involving an oxidation-reduction couple. For example, in the reaction of Fe with Sn ,  

Half-cell reactions bear a striking resemblance to the Bronsted acid-base equilibria. In the latter, a proton shuttles back and forth between the acid and base which are called a conjugate acid-base pair. In a redox halfcell, electrons are the particles transferring between the oxidized and reduced forms of a redox couple. 

In the generalized half-cell equation, n represents the number of electrons necessary to transform a species to the next stable, lower oxidation state. In contrast to proton transfer reactions which occur in steps of one proton at a time, many redox reactions involve the simultaneous transfer of several electrons. 

---

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

This is the isocitrate dehydrogenase reaction of the TCA cycle. Writing the two half-cell reactions, we have... 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

The half-cell reactions supported by this structure are... 

Unlike burning hydrogen in air, in a fuel cell the electrolyte partitions the overall reaction into half-cell reactions on either side of the cell. 

Although it is attractive to directly convert chemical energy to electricity, PEM fuel cells face significant practical obstacles. Expensive heavy metals like platinum are typically used as catalysts to reduce energy barriers associated with the half-cell reactions. PEM fuel cells also cannot use practical hydrocarbon fuels like diesel without complicated preprocessing steps. Those significantly increase the complexity of the overall system. At this time, it appears likely that PEM fuel cells will be confined to niche applications where high cost and special fuel requirements are tolerable. 

The overall reaction describes what goes on in the entire electrochemical cell. In half of the cell, the right beaker, reaction (7) occurs. In the other half of the cell, the left beaker, reaction (2) occurs. Hence, reactions (7) and (2) are called half-cell reactions or half-reactions. 

H+], calculation of, 192, see also Hydrogen ion Haber, Fritz, 151 Haber process, 140, 150 Hafnium, oxidation number, 414 Haldane, J. B. S., 436 Half-cell potentials effect of concentration, 213 measuring, 210 standard, 210 table of, 211, 452 Half-cell reactions, 201 Half-life, 416 Half-reaction, 201 balancing, 218 potentials, 452 Halides... 

The convention is adopted of writing all half-cell reactions as reductions M"+ + ne - M... 

The term 4H20 is omitted, since the reaction is carried out in dilute solution, and the water concentration may be assumed constant. The hydrogen ion concentration is taken as molar. The complete reaction may be divided into two half-cell reactions corresponding to the partial equations ... 

The standard potential e of reaction (1) is 0.56 volt and of reaction (2) 0.60 volt. By suitably controlling the experimental conditions (e.g. by the addition of barium ions, which form the sparingly soluble barium manganate as a fine, granular precipitate), reaction (1) occurs almost exclusively. In moderately alkaline solutions permanganate is reduced quantitatively to manganese dioxide. The half-cell reaction is ... 

The overall half-cell reaction may therefore be written as ... 

The reaction of hydrogen in acidic solution is a half-cell reaction and can therefore be handled like the metal/metal salt solution system ... 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

The half-cell reactions taking place in a MH v / Ni battery may be written as follows ... 

Figure 9.3 The lead storage battery. The key to obtaining electrical energy from a redox chemical reaction is to physically separate the two half-cell reactions so that electrons are transferred from the anode through an external circuit to the cathode. In the process, electrical work is accomplished.

Adding these two half-cell reactions gives reaction (9.88).s... 

The half-cell reactions in the cell can be written as before... 

In assembling cells for making thermodynamic measurements, one should try not to combine half-cells in a manner that results in a junction potential. Figure 9.7 is a schematic representation of the Daniell cell, which is one with a junction potential. The half-cell reactions are... 

These pe values actually represent electron activities for a new set of half-cell reactions derived from reactions 1, 2, and 3 ... 

The permanganate ion is a powerftil oxidizing agent that oxidizes water to oxygen under standard conditions. Here are the half-cell reactions ... 

C19-0029. Mercury batteries have the following half-cell reactions ... 

Ab initio atomic simulations are computationally demanding present day computers and theoretical methods allow simulations at the quantum mechanical level of hundreds of atoms. Since an electrochemical cell contains an astronomical number of atoms, however, simplifications are essential. It is therefore obvious that it is necessary to study the half-cell reactions one by one. This, in turn, implies that a reference electrode with a known fixed potential is needed. For this purpose, a theoretical counterpart to the standard hydrogen electrode (SHE) has been established [Nprskov et al., 2004]. We will describe this model in some detail below. 

Thus, if Cu2+ and H+ are existing at unit activity, the reduction of Cu2+ is easier than the reduction of H+, and Cu2+ ions must be discharged in preference to H+, until the Cu2+ concentration has dropped to such a point that the E for its half-cell reaction is less then that for the H+ H2 half-cell. 

Depending on the processes occurring during the cell reaction at the individual electrodes, the cell reaction can be separated into two half-cell reactions formulated as reduction by electrons. For the cell reaction described by Eq. (3.1.42), these reactions are... 

---