Cell-containing redox reactions

The electrical current needed to start an automobile engine is provided by a lead storage battery. This battery contains aqueous sulfuric acid in contact with two electrodes. One electrode is metallic lead, and the other is solid Pb02. Each electrode becomes coated with solid PbSOq as the battery operates. Determine the balanced half-reactions, the overall redox reaction, and the anode and cathode in this galvanic cell. 

The zinc anode and copper cathode of a Daniell cell are both metals, and can act as electrical conductors. However, some redox reactions involve substances that cannot act as electrodes, such as gases or dissolved electrolytes. Galvanic cells that involve such redox reactions use inert electrodes. An inert electrode is an electrode made from a material that is neither a reactant nor a product of the cell reaction. Figure 11.6 shows a cell that contains one inert electrode. The chemical equation, net ionic equation, and half-reactions for this cell are given below. 

Photocorrosion can be prevented by adding a redox couple to the electrolyte whose potential is more favourable than the decomposition potential such that the redox reaction occurs preferentially. When n-CdS is used as photoanode in aqueous electrolytes, the electrode is photocorroded since the reaction, CdS -1- 2h - S -1- Cd, occurs readily. By adding NaOH and sodium polysuphide to the electrolyte (Ellis et al, 1976), photocorrosion is prevented. The /S redox couple preferentially scavenges the photoholes. At the anode, sulphide is oxidized to polysulphide (free sulphur) and free sulphur is reduced back at the dark cathode. Similarly n-Si anodes have been stabilized by using a nonaqueous electrolyte containing a ferricinium/ferrocene redox couple (Legg et al, 1977 Chao et al, 1983). Unfortunately, a similar stabilization technique cannot be applied to photoelectrolysis cells. Some examples of electrode... 

Bard and co-workers have developed the technique of Scanning Electrochemical Microscopy (SECM) [3], to provide information about the redox activity of a wide variety of assemblies. In common with STM, SECM uses high-resolution piezoelectric elements to scan a microelectrode tip across the interface of interest. However, in SECM the microelectrode acts as a working electrode in an electrochemical cell that contains a redox-active species. A redox reaction occurs at the microelectrode, e.g. Ox + ne = Red, and by monitoring the current generated at the tip, the surface can be mapped in terms of its redox activity. 

Redox reactions can be studied using electrochemical cells. An electrochemical cell for the chemical reaction in Example 10.8 is shown in Fig. 10-2. The Cu and Zn electrodes dip into solutions of their respective ions and the salt bridge (containing concentrated KC1) maintains electrical contact between the two solutions. Electrons will flow from the Zn half-cell to the Cu half-cell if Zn is oxidized to Zn2+, with concomitant reduction of Cu2+ to Cu in the Cu half-cell. The value of E for this reaction may be determined by measuring the potential difference (in volts) that has to be applied to the cell to prevent the electron flow. 

Biological Roles of Zinc and Copper. Zinc and copper are essential cofactors at the active site of a number of enzymes. Zinc is a component of more than 200 proteins and enzymes (Table II). Copper, sim-lleT to iron, participates both in redox reactions and as a proton doner (Table III). The normal human adult body contains approximately 50-100 mg of copper and 2.0 g of zinc. The vast majority of tissue copper is found in the liver, kidney, heart and brain. In the blood, copper exists within the red blood cell as superoxide dlsmutase and in the serum as ceruloplasmin. Copper is a component of aerobic metabolism, bone synthesis, and erythrocyte development. Zinc is found primarily in the liver, kidney, bone and prostate. Zinc is essential for normal growth of tissues, wound repair, skin structure, reproduction, taste perception, and the prevention of dwarfism. 

Table 8.3 lists a few representative standard electrode potentials (or reduction potentials). Figure 8.6 exemplifies the principle of an electrochemical cell. The hydrogen electrode is made up of a B-electrode (which does not participate directly in the reaction), which is covered by H2(g), which acts as a redox partner [H2(g) = 2H +2e ]. Pt acts as a catalyst for the reaction between H and H2(g) and acquires a potential characteristic of this reaction. The salt bridge between the two cells contains a concentrated solution of salt (such as KCl) and allows ionic species to diffuse into and out of the half-cells this permits each half-cell to remain electrically neutral. 

Potentiometric transducers measure the potential under conditions of constant current. This device can be used to determine the analytical quantity of interest, generally the concentration of a certain analyte. The potential that develops in the electrochemical cell is the result of the free-energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition is satisfied. For electrochemical cells containing an anode and a cathode, the potential difference between the cathode electrode potential and the anode electrode potential is the potential of the electrochemical cell. If the reaction is conducted under standard-state conditions, then this equation allows the calculation of the standard cell potential. When the reaction conditions are not standard state, however, one must use the Nernst equation to determine the cell potential. Physical phenomena that do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential. An example of this would be ion-concentration gradients across a semi-permeable membrane this can also be a potentiometric phenomenon and is the basis of measurements that use ion-selective electrodes (ISEs). 

An electrochemical cell consists of two parts, called half-cells, in which the separate oxidation and reduction reactions take place. Each half-cell contains an electrode, which is the object that conducts electrons to or from another substance, usually a solution of ions. In Figure 21-1, the beaker with the zinc electrode is where the oxidation part of the redox reaction takes place. The beaker with the copper electrode is where the reduction part of the reaction takes place. The reaction that takes place in each half-cell is the half-reaction, sometimes called half-cell reaction, that you studied in Chapter 20. The electrode where oxidation takes place is called the anode of the cell. The electrode where reduction takes place is called the cathode of the cell. Which beaker in Figure 21-1 contains the anode and which contains the cathode ... 

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