Theoretical chemistry from the standpoint of Avogadro's rule & thermodynamics / [Walther Nernst].
- Walther Nernst
- Date:
- 1911
Licence: In copyright
Credit: Theoretical chemistry from the standpoint of Avogadro's rule & thermodynamics / [Walther Nernst]. Source: Wellcome Collection.
791/840 page 767
![For the theory of excess-potential see the interesting study by G. Moller (Ann. d. Phys. [4*.] 27, p. 566, 1908). Chemical Application of the Osmotic Theory.—It has long been customary to draw chemical deductions from the galvanic potential series of the metals, which is given in the table (p. 764), and which is now seen to find quantitative expression in the solution pressures or the decomposition pressure so determined; it must not, however, be assumed that copper is always precipitated by zinc. On the contrary, the ionic concentration is a second important factor, and the above (p. 761) experiment with the Daniell cell shows that under suitable conditions zinc can be reduced by copper. Similar conclusions may be drawn as to the decomposition potential of the anions; thus it is known that bromine throws out iodine from solution of iodide, and the chlorine similarly precipitates bromine quickly and very completely (how complete the precipitation is may be easily calculated from the solution pressure according to principles already given). We have, in fact, the well-known simple reactions— Br2 + 2 J = J9 + 2Br ] Cl2 + 2Br = Br2 + 2C1. We see further, that chlorine must be capable of evolving oxygen from acid solution, but not so bromine or iodine. It is known, how- ever, that the evolution of oxygen by chlorine takes place very slowly, unlike the rapid deposition of bromine by chlorine. This need not surprise us in view of what has gone before; for the chlorine, in order to pass to the ionic state, must replace the 0 ions that are present in excessively small quantities, since the OH ions, which are more abundant, do not give up their negative charge except by means of a potential 0-3 volt higher than that of chlorine. One of the most interesting reactions is the decomposition of water by metals with formation of hydrogen; the conditions for this process can easily be deduced from the foregoing considerations (p. 754). The electrical forces in question act not only on the ions of the metals in question, but also on all the positive ions present; for example, on the hydrogen ions which always exist in aqueous solution. The separation of hydrogen ions must occur as soon as the osmotic pressure of the hydrogen ions and the electrostatic attraction are sufficient to overcome the electrolytic solution pressure of hydrogen at atmospheric pressure, that is, we must have e1>e2 or V Cj > C, where the index 1 refers to the metal and 2 to the hydrogen, and nx is the chemical valency of the metal in question. We see, therefore, that the favourable conditions for decomposition of water are :—](https://iiif.wellcomecollection.org/image/b28047850_0791.jp2/full/800%2C/0/default.jpg)
