Anode reductions, electrochemical cells

In electrogravimetry the analyte is deposited as a solid film on one electrode in an electrochemical cell. The oxidation of Pb +, and its deposition as Pb02 on a Pt anode is one example of electrogravimetry. Reduction also may be used in electrogravimetry. The electrodeposition of Cu on a Pt cathode, for example, provides a direct analysis for Cu +. 

Identify the anode and cathode for the following electrochemical cells, and write the oxidation or reduction reaction occurring at each electrode. 

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

The cathode is defined as the electrode at which reduction occurs, i.e., where electrons are consumed, regardless of whether the electrochemical cell is an electrolytic or voltaic cell. In both electrolytic and voltaic cells, the electrons flow through the wire from the anode, where electrons are produced, to the cathode, where electrons are consumed. In an electrolytic cell, the dc source forces the electrons to travel nonspontaneously through the wire. Thus, the electrons flow from the positive electrode (the anode) to the negative electrode (the cathode). However, in a voltaic cell, the electrons flow spontaneously, away from the negative electrode (the anode) and toward the positive electrode (the cathode). 

The correct statement is (d). Electrons are produced at the anode and move toward the cathode, regardless of the electrode material. The electrons do not move through the salt bridge ions do. Electrons do not leave the cell they provide current within the circuitry. Reduction occurs at the cathode in both galvanic and electrolytic cells—in all types of electrochemical cells, in fact. 

FIGURE 14.2 A representation of an electrochemical cell as described in the text. One electrode is the anode, the other the cathode, and electrons generated by the oxidation process at the anode flow through the external circuit to the cathode, where reduction takes place. This flow of electrons constitutes electrical current in the external circuit. 

In an electrochemical cell, oxidation occurs on the anode and reduction on the cathode. The measured current density j (current per unit area of the electrode) is proportional to the difference in the rate of the two reactions... 

Fuel cells, like batteries, convert the chemical energy residing in a fuel into electrical energy on demand. As in batteries and other electrochemical cells, fuel cells consist of an anode, where oxidation occurs, a cathode, where reduction occurs, and an electrolyte, where ions carry the current between the electrodes. Fuel cells differ from batteries in that the fuel and oxidant are not contained within the fuel... 

The separator is often the weakest component in any electrochemical cell. There are also difficulties in employing ion-exchange diaphragms in aprotic media. Particularly with large industrial cells, it is advantageous to devise reaction conditions that allow the use of an undivided cell. One solution to these problems for an electrochemical reduction process employs a sacrificial anode of magnesium, alumin-... 

Most effort over the electrochemical reduction of benzene hydrocarbons has centred on finding a reaction medium, which is also a better solvent for the substrate than liquid ammonia. Aliphatic amines have proved useful solvents and they may be used in an undivided electrochemical cell. Base is generated at the cathode while an equivalent of acid is generated in the anode reaction so that mixing of the cel contents maintains a neutral solution. An alcohol is usually added as a proton donor to prevent the build-up of a localised highly basic environment. The simultaneous anode reaction is oxidation of the amine. Electrodes of platinum, aluminium or graphite have been used. Under these conditions, benzene [38] is converted... 

The Daniell cell illustrates the basic features of an electrochemical cell. Electrochemical cells always involve a redox reaction. Oxidation occurs at the cathode of the cell and reduction takes place at the anode. Electrons always flow from the anode to the cathode. Electrochemical cells come in many arrangements. To gain an appreciation for the variety of electrochemical cells, consider all the types of batteries available. 

Electrochemical cells produce electrical energy from a spontaneous chemical reaction. In electrolysis, the process is reversed so that electrical energy is used to carry out a nonspontaneous chemical change. A cell arranged to do this is called an electrolytic cell. An electrolytic cell is similar to an electrochemical cell except that an electrolytic cell s circuit includes a power source, for example, a battery. The same electrochemical cell terminology applies to electrolytic cells. Reduction occurs at the cathode and oxidation at the anode. 

Exchange Current Density. Let us now return to our electrochemical cell shown in Figure 3.8. This cell is a combination of two half-cells, with the oxidation reaction occurring at the anode and the reduction reaction occurring at the cathode resulting in a net flow of electrons from the anode to the cathode. Equilibrium conditions dictate that the rate of oxidation and reduction, roxid and rred, be equal, where both rates can be obtained from Faraday s Law ... 

Moreover, in the divided cell the exo.endo ratio of bromosilanes was 91 9 in the anode compartment bnt only 52 48 in the cathode compartment. Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the anodic reduction However, the increase in the current density dnring electrolysis cansed a decrease in the apparent current efficiency. This observation indicates a chemical natnre of the anodic process. Of course, the ultrasonic irradiation facihtates the formation of the organomagnesium intermediate at the sacrificial anode and the anthors reported a similar ultrasonic effect for the nonelectrochemical but purely sonochemical... 

Reduction of Ag" to Ag occurs in the beaker on the right, and the oxidation of copper occurs in the beaker on the left. The site at which oxidation occurs in an electrochemical cell is called the anode, and the site at which reduction occurs is called the cathode. 

Energetics of oxidation-reduction (redox) reactions in solution are conveniently studied by arranging the system in an electrochemical cell. Charge transfer from the excited molecule to a solid is equivalent to an electrode reaction, namely a redox reaction of an excited molecule. Therefore, it should be possible to study them by electrochemical techniques. A redox reaction can proceed either by electron transfer from the excited molecule in solution to the solid, an anodic process, or by electron transfer from the solid to the excited molecule, a cathodic process. Such electrode reactions of the electronically excited system are difficult to observe with metal electrodes for two reasons firstly, energy transfer to metal may act as a quenching mechanism, and secondly, electron transfer in one direction is immediately compensated by a reverse transfer. By usihg semiconductors or insulators as electrodes, both these processes can be avoided. 

FIGURE 12.1 In an electrochemical cell, a reaction takes place in two separate regions. Oxidation occurs at one electrode, and the electrons released travel through the external circuit to the other electrode, where they cause reduction. The site of oxidation is called the anode, and the site of reduction is called the cathode. 

The specific value of the crossover depends on both the emulsion used and the conditions of exposure. For exposures made in air, the crossover occurs at a reduction potential that is less negative the lower the pAg.of the emulsion (248). Honda and associates (264) attribute this dependence to "anodic and cathodic shifts of the band edge of the silver halide, respectively." They support this explanation by experiments on the effects of silver ions and halide ions on the sensitized photocurrent in an electrochemical cell having an AgBr or AgCl sheet crystal window. 

Electrochemical cells are made of two conducting electrodes, called the anode and the cathode. The oxidation reaction takes place at the anode, where electrons are released to flow through a wire to the cathode. At the cathode, reduction takes place. For the oxidation and reduction reactions to occur, the electrodes must be in a conducting solution called an electrolyte. The electrochemical cell voltage depends on the types of materials, usually conducting metals, used as electrodes, and the concentration of the electrolyte solution. (See Figure 6.5.)... 

When E° values are established for a number of substances, these values are placed in a table that shows an equation for the half reaction associated with each E° value. Either the oxidation half reactions at the anodes or the reduction half reactions at the cathodes may be listed. For the zinc/hydrogen electrochemical cell described above, the half reactions would be as follows ... 

To calculate the E°, the voltage, of an electrochemical cell, the voltage for the oxidation half reaction at the anode is added to the voltage for the reduction half reaction at the cathode E°a ll = E°oxid react + E°red react. For an electrochemical cell with zinc and copper electrodes, E° = 0.76 + 0.34 = 1.10 V, voltage equals 0.76 + 0.34, which equals 1.10 volts. The sign for the zinc reduction potential half reaction is changed because the half reaction is reversed to show that oxidation occurs at the zinc electrode. The two half reactions can also be added to show the overall reaction in the electrochemical cell ... 

Except for its source of outside current, the electrolytic cell has the same elements as the electrochemical cell an anode and a cathode placed in an electrolyte in which cations (positive ions) move toward the cathode, and anions (negative ions) move toward the anode. The oxidation half reaction at the anode and the reduction half reaction at the cathode can be added together to find the overall redox reaction for the cell. The process is called electrolysis. If a coating of silver metal is desired on a piece of silver jewelry, electrolysis can be performed to coat or plate the silver jewelry in an electrolytic cell. The electrolyte silver nitrate (AgN03) solution supplies a source of silver ions (Ag+). The cathode is the silver jewelry, from which silver ions are reduced to silver metal. The anode... 

The reasons to perform electrochemistry, in particular, electrosynthesis, in a microfluidic system are the following (Rode et al., 2009) (1) reduction of ohmic resistance in the electrochemical cell, by decreasing the distance between anode and cathode, (2) enhancement of mass transport by increase of electrode surface to cell volume ratio, also realized by small interelectrode gaps, (3) performing flow chemistry to establish single-pass conversion, and (4) coupling of cathode and anode processes, permitting simultaneous formation of products at both electrodes. The latter... 

In 2002, Burghard and coworkers described an elegant method for the electrochemical modification of individual SWCNTs [177]. To address electrically individual SWCNTs and small bundles, the purified tubes were deposited on surface-modified Si/Si02 substrates and subsequently contacted with electrodes, shaped by electron-beam lithography. The electrochemical functionalization was carried out in a miniaturized electrochemical cell. The electrochemical reduction was achieved by reduction of 4-N02C6H4N2+BF4 in DMF with NBu4+BF4 as the electrolyte (Scheme 1.27a), anodic oxidation was accomplished with aromatic amines in dry ethanol with LiC104 as the electrolyte salt (Scheme 1.27b) [177]. 

Electron Transfer in Electrochemistry. In electrochemical cells electron transfer occurs within the electrode-solution interface, with electron removal (oxidation) at the anode, and with electron introduction (reduction) at the cathode. The current through the solution is carried by the ions of the electrolyte, and the voltage limits are those for electron removal from and electron insertion into the solvent-electrolyte [e.g., H20/(H30+)(C10j ) (Na )(-OH) ... 

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