ELECTRO-CHEMICAL EFACTIONS. A system in which chemical energy is transformed into elec trical is termed a galvanic or voltaic cell, and a combination of two or more cells is termed a galvanic or voltaic battery. One of the first questions that la:Mir:illy suggest themselves to the student of elect ro-chemistry is, What are the characteristics of those chemical changes whose energy may assume the electrical form? In other words. What chemical reactions can he utilized for the construction of cells? The an swer is, All changes, and only suelt changes, in irhieli acids, bases, or sails, take part, or bury he caused to take part, ran Le employld eleelro chemirally. Sub-tanees of these are termed 'electrolytes,' and they are usually cm ploycd. in cells, in the form of aqueous solutions, which their molecules are assumed to be disso ciated into fragments. termed 'ions' (see pn,sco CIA1140,), that are charged with either positive or negative electricity. In most cells the chemi cal action eonsists, on the one hand, in the solution of one of the metallic electrodes, its rartieles entering the solution in the form, of ions; and on the other hand. in the deposition upon the other electrode of another metal, whose particles formed the ions of the solution. For ex ample. the Daniell cell consists, on the one hand, of a zinc electrode immersed in a solution of zinc sulphate, and on the other hand, of a copper electrode immersed in a solution of copper sul phate, the two solutions being in contact, and the two metals being connected by a metal wire; the chemical action of the cell consists, on the one hand, in the entrance of zinc into the solu tions in the form of zinc inns, and on the other hand, in the deposition, on the copper electrode, of copper ions in the state of metallic copper. The two simuLanvous changes evidently ensue in the system at two separate points connected with one another by a suitable •ondnetor of elec This 'chemical action at a distance' is an essential characteristic of eleetro-chemival action. If in the Daniell cell the two chemical changes took place at point, in immediate contact with each other (e.g. if zinc wore caused to replace copper by being -imply placed in a solution of copper sulphate), a certain amount of heat. but no electricity. w011141 be produced.
An excellent example of changes in which elec trolytes may and may not take part is presented by the mere flow of hydrogen or oxygen gas, at constant temperature, from of higher to places of lower pressure. lf, say, hydrogen gas is contained in two separate vessels under un equal pressures, a flow of gas will naturally take place from one vessel into the other, if com munication is established between them. But the equalization of pressures can also be caused to take place as follows: let two bars of platinum coated with 'platinum black' (finely divided platinum, which has a great capacity for occlud ing gases) be half immersed in dilute solution of sulphuric acid, while the other halves are ex posed to hydrogen gas coo*ained in two vessels. under two different pressures: n» eonnecting the platinum bars outside the solution by a metal wire, it will be found that a current of electricity passes through the latter, while hydrogen passes through the solution until the pressures in the two have been equalized. The action thus
taking place is in dilute aqueous solution, the molecules of sulphuric acid arc broken up into eleetro-positive hydrogen ions II and electro-raTative ions SO,: neutral hydro gen molecules from the vessel of greater pres sure enter the solution in the form of hyd ,rogen ions• and, replacing the free hydrogen of the acid. drive them from the solution in th- form of neutral molecules into the yesel of lower pressure. Evidently here, too, in the ease of any other voltaic cell, the electric (»went is caused by ions taking part in the change.
Et i:c.rao.AlonvE FORCE. We can now discuss the question as to what determines the electro motive force of voltaic yell, i.e. the difference of potential existing between its two electrodes, It was once believed that a considerable differ enee Of potential is established whenever two different metals are placed in contact with each other; but this is now positiNely known to be wrung. If differences of potenthil arise from the communication of the electrodes outside the solution, those differences are certainly very small and need not be taken into account. In the case of cells with two different solutions (like the Daniell cell). a difference of potential is known to exist at the surface of contact of the two solutions: but this, too, is very slight and need not be considered here. The only possible seats of the main difference of potential in a cell are. therefore, the surfaces of contact between the metallic electrodes and the electrolyte solu tions—i.e. at the places where ions form and disappear. But how are differences of potential established there? A clear answer to this ques tion is furnished by modern electro-chemical theory. If a liquid is placed in a closed vessel, its vapor gradually attains a maximum pres sure which balances the vapor-tension of the liquid at the given temperature. Again, if an excess of sugar is placed in some water, the sugar dissolves, or, so to speak, vaporizes into the water, and the portion dissolved gradually at tains a maximum 'osmotic pressure' which bal ances the 'solution-tension' of sugar at the given temperature. Similarly, if a metal is brought in contact with water, its ultimate particles tend to rush into the volume of the water until a maxi mum osmotic pressure has been attained. Since, according to the theory of electrolytic dissocia tion, the free particles of a metal in solution are invariably charged with positive electricity, the osmotic pressure of a metal in solution can evi dently be exerted only by its electropositive ions. Supposing, now, that a bar of some metal is placed in a solution of one of its salts, and remembering that the salt is dissociated into metallic and acidic ions, we readily distinguish three possible cases, viz. the osmotic pressure of the metallic ions may happen to be greater than. less than, or equal to the maximum osmotic pressure that would balance the 'electrolytic solu tion-tension' of the metal.