432 H. C. BIDDLE 



THEORETICAL 



It can be shown that the deposition of copper by solutions 

 of ferrous salts is a reversible reaction governed by the ordinary 

 laws of chemical equilibrium. It is to Arrhenius largely that we 

 owe the view that certain substances in solution are more, or less, 

 dissociated into electrically charged parts, or ions. This theory 

 has proved of the highest value in affording an insight into 

 the principles of chemical reactions. Different substances differ 

 much in their tendency to pass into the ionic form and this 

 tendency is greatly influenced by external conditions, particu- 

 larly by the nature of the solvent. 



The chief source of ions is the dissociation of electrically 

 neutral molecules, such as occurs in the aqueous solutions of 

 salts, acids, and bases. They may further be formed from elec- 

 trically neutral substances which enter the ionic condition by 

 partially, or wholly, appropriating the electric charge of ions 

 already present.^ As an example of this mode of formation 

 may be mentioned the reduction of ferric salts by the action of 

 metallic iron. 2FeCl3 + Fe = 3FeCl2. The solution of metallic 

 copper in ferric chloride is an action of the same nature. 

 Cu + 2FeCl3 = CuCl, + 2FeCl,. 



As is seen the deposition of copper by a ferrous salt would 

 be the reverse of this last reaction. 



The conditions under which such reduction should occur 

 may be readily determined. In a system which contains a solu- 

 tion of iron and cupric salts in contact with metallic copper and 

 in which the several constituents have attained a constant value, 

 a condition of equilibrium subsists on the one hand between fer- 

 ric, cuprous, ferrous, and cupric ions (Fe + Cu .^ ^Fe + Cu ), 



and on the other hand between ferric, ferrous, and cuprous ions 

 and the active mass of the metallic copper (Cu -f Fe ^ > Fe" 

 + Cu ). If a, b, c, d, respectively, represent the active masses 

 of the ions in the first instance, an equation of equilibrium may 



be thus formulated, — = A' . ^ The active mass of the copper 



'^ F. W. KiJSTER : Zeit. f. Elec. Chem., Vol. IV, p. 105. 

 = W. Nernst : Theoretical Chemistry, p. 358. 



