Page:Text-book of Electrochemistry.djvu/181

 1 66 DEGREE OF DISSOCIATION chap.

that the deviation from the law of mass action is only apparent.

Divalent Acids. — The above formulsB apply to electro- lytes formed from two monovalent ions. When the con- centration is great, a strong divalent acid, such as sulphuric acid, appears to dissociate according to —

U^Oi = H + HSO4.

As the solution is diluted, the HSO4 ions suffer further dissociation —

• HSO4 = H + SO4.

Each of these dissociations is regulated by a particular equation, and the equilibrium is so masked thereby that it cannot be determined. The same applies to salts consisting of polyvalent ions.

In the case .of most of the di- and poly-valent adds (sulphuric acid is almost the only exception) only the first phase of the dissociation takes place at the dilutions at which we commonly work, i.e. only the first hydrogen ion is split off. The other possible dissociation can therefore be neglected, and with a fair degree of exactitude we can apply Ostwald's formula, although this is only rigidly applicable to electrolytes consisting of two monovalent ions.

Influence of Substitution on the Dissociation of Acids. — It has been known for a very long time that an acid, such as acetic acid, becomes stronger by replacement (substitution) of one hydrogen atom by a clilorine atom; mono- chloracetic acid (CH2CICOOH) is considerably stronger than acetic acid (CHaCOOH); dichloracetic acid (CHClgCOOH) is stronger than monochloracetic acid; and trichloracetic acid (CCI3COOH) is the strongest of the substitution products. The series of strengths can be recognised from the dissociation constants, IT, because the greater this constant is the greater is the quantity of substance dissociated at a particular dilution, r, or the degi-ee of dissociation of the acid, and it

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