Galvanic cell actions

Galvanic cell action Chemical reaction causes electric current, as in a battery. 

In this chapter, we deal with two aspects of the connection between chemistry and electricity. The first is electrolysis, the splitting (lysis) of compounds when electricity passes through the solutions involved. The second topic is galvanic cell action, which is the generation of electricity (a flow of electrons) during chemical reaction. 

Oxidation reduction reactions occur at two electrodes. The electrode at which oxidation occurs is called the anode the one at which reduction takes place is called the cathode. Electricity passes through a circuit under the influence of a potential or voltage, the driving force of the movement of charge. There are two different types of interaction of electricity and matter. Electrolysis is when an electric current causes a chemical reaction. Galvanic cell action is when a chemical reaction causes an electric current, as in the use of a battery. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Although the law of mass action is equally valid for oxidation-reduction processes, and therefore conclusions as to the direction of reactions may be drawn from the knowledge of equilibrium constants, traditionally a different approach is used for such processes. This has both historical and practical reasons. As pointed out in the previous sections, in oxidation-reduction processes electrons are transferred from one species to another. This transfer may occur directly, i.e. one ion collides with another and during this the electron is passed on from one ion to the other. It is possible, however, to pass these electrons through electrodes and leads from one ion to the other. A suitable device in which this can be achieved is a galvanic cell, one of which is shown in Fig. 1.14. A galvanic cell consists of two half-cells, each made up of an electrode and an electrolyte. The two electrolytes are connected with a salt bridge and, if... 

A concentration cell is a limited form of a galvanic cell with a reduction half re-action taking place in one half cell and the exact reverse of that half reaction taking place in the other half cell. 

Volta s greatest contribution was, however, the discovery, in 1796. of the voltaic pile, which consisted of a series of units, each made from sheets of dissimilar metals such as zinc and silver separated by wet doth. Volta showed that metals could be arranged in au "electromotive series so that each became positive when placed in contact with the one next below it in the series. Although, as has already been mentioned, Volta considered that the source of the electric energy was at the surface of contact of, the metals, this theory was thrown in doubt when it was discovered that chemical action accompanied the operation of the pile. It is of interest that the question of the seat of the potential of the galvanic cell is not, even today, finally settled. Many improvements of the voltaic pile were made. It is, of course, the precursor of the modern galvanic cell. 

Transference numbers will also be found useful in obtaining precise values of the activities of ion constituents. It was another of Arrhenius tacit assumptions that ion concentrations may be used without error in the law of mass action. To investigate the limits of validity of that assumption, and to lay a foundation for the modern interionic attraction theory of solutions, it is necessary to consider the thermodynamics of solutions, and of the galvanic cell, subjects which are discussed in Chapters 5 and 6. 

Two necessary conceptions are those of the thermodynamic system and its surroundings. A thermodynamic system may be any arbitrarily selected portion of matter or space. It may be, for instance, a volume of gas, a heat engine, or a galvanic cell. The surroundings are the immediate environment of the system, with which it may exchange energy. Important types of interaction between a thermodynamic system and its surroundings are the flow of heat from one to the other, or the action of work of one on the other, or both. 

The galvanic potential differences in the electrodes of galvanic cells with oxoanionic solid electrolytes are determined by gas components which are not in reaction equilibrium with the cations, whose mobility determines the conductivity. Galvanic potential differences result according to the law of mass action for electrode reactions as follows ... 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

For electrochemical systems, electrochemical potentials jliip, T) (Section II) are used instead of chemical potentials. Under the action of driving forces, both chemical reactions (e.g., reaction in a galvanic cell) and charge transport (e.g., electron transport outside the cell from the anode to the cathode) may take place. The scheme... 

Just a few months after the appearance of the Volta pile it was found that the electric current can exert a chemical action. As early as May of 1800, Nicholson and Carlisle carried out water electrolysis. In 1803 the processes of metal electrodeposition were discovered. In 1807 Davy for the first time isolated alkali metals by electrolysis of salt melts. Thus almost simultaneously with the creation of the first electrochemical power source - the "galvanic cell" or "galvanic battery" - many electrochemical processes were discovered and the foundations were laid of the science which to-day we call electrochemistry. 

Examples include any item that runs on batteries, such as watches, calculators, etc. 3. Yes. A galvanic cell causes current to flow through an external circuit by electrochemical action. An electrolytic cell is a cell through which a current driven by an external source passes. 5. See the discussion in Section 19.2. 7. Oxidation a, c, d. Reduction b. 9. Reduction 11. Oxidation... 

Galvanic Cell An electrochemical cell that converts chemical energy into electrical energy by electrochemical action. 

Galvanic Cell n ) Cell consisting of two or more dissimilar metals or alloys in contact with the same body of an electrolytic solution such as seawater. Upon electrically connecting the dissimilar metals, a current flows as the result of accelerated corrosion of the more active of the dissimilar metals or alloys. (2) Electrolytic cell capable of producing electrical energy by electrochemical action. (Corrosion engineer s handbook, 3rd edn. Baboian R (ed) NACE International - The Corrosion Society, Houston, TX, 2002). 

Corrosion due to the action of local cells, that is, galvanic cells resulting from inhomogeneities between adjacent areas on a metal surface exposed to an electrolyte. 

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