Electromotive Force and Chemical Equilibrium

The electromotive force of a galvanic cell is a measure of the electrical work which can be obtained from the reaction in the cell. The total or maximum work which can be obtained from the cell reaction includes also the work which is done against the external forces owing to the changes in volume (formation of gas, etc.) of the reacting substances. From the definition of affinity (p. 318) it follows, therefore, that the electromotive 

The relationship between chemical equilibrium and the electromotive force of galvanic cells was first recognised by van t Hoff in 1886. It was not until much later, however, that a cell in chemical equilibrium was investigated experimentally, as it is not easy to find cells in which the equilibrium is not entirely to the one side or the other. In most cells the reaction and the production of current proceed until one of the reacting substances has disappeared almost entirely (e.g. the precipitation of copper by zinc in the Daniel cell). At the instigation of Bredig, Kniipffer investigated a cell made up as follows  

When a positive current is sent through the cell in the direction from left to right, thallium goes into solution at the chloride electrode, and a corresponding amount of thallous chloride is precipitated, while thallous thiocyanate is decomposed and a corresponding amount of metallic thallium is deposited at the other electrode. The reaction in the cell may therefore be represented by the equation 

If the concentrations are so chosen that this reaction would take place spontaneously, the cell will produce a current in the direction stated above. On the other hand, if the concentrations are taken so that the affinity is in the opposite direction, the cell will give a current in the reverse direction. 

By Chapter IX. the affinity of the reaction for dilute solutions is given by [KCl] 

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The changes, however, are both numerous and significant. First of all, there is a change in the organization of the subject matter. For example, material formerly contained in the section entitled Analytical Chemistry is now grouped by operational categories spectroscopy electrolytes, electromotive force, and chemical equilibrium and practical laboratory information. Polymers, rubbers, fats, oils, and waxes constitute a large independent section. 

Section 8 now combines all the material on electrolytes, electromotive force, and chemical equilibrium, some of which had formerly been included in the old Analytical Chemistry section of earlier editions. Material on the half-wave potentials of inorganic and organic materials has been thoroughly revised. The tabulation of the potentials of the elements and their compounds reflects recent lUPAC (1985) recommendations. 

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