Redox reactions in galvanic cells

139 REDOX REACTIONS IN GALVANIC CELLS When discussing oxidation-reduction reactions we have not mentioned ways in which the directions of such reactions can be predicted. In other words, discussions in the previous chapters were aimed at understanding how oxidation-reduction reactions proceed, but there was no mention of why they take place. In this and the next few sections the problem will be dealt with in some detail. 

The direction of chemical reactions can always be predicted from thermodynamical data. Thus, if the Gibbs free energy change of a reaction is calculated, 

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 

The direction of this electron flow in the cell is strongly associated with the direction of the chemical reaction(s) involved in the process. Electrically speaking, the direction of electron flow depends on the sign of the potential difference between the electrodes electrons will flow from the negative electrode through the lead towards the positive electrode. The magnitudes of electrode potentials 

If solutions of iron(III) chloride and tin(II) chloride are mixed, this reaction proceeds instantaneously. 

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