Chemical Equilibrium, Law Of Mass-action : Strong Electrolytes

It has long been known that the Ostwald dilution formula does not apply to solutions of strong acid or bases, or solutions of largely ionised salts, i.e., to strong electrolytes. Since the law of mass-action fails for strong electrolytes the law of solubility product will not apply to them strictly, although it does so in a qualitative form, as experiments show.

In the modern theory, the strong electrolytes are practically completely ionised in dilute solutions, and since no un-ionised substance is present the law of mass-action cannot apply. Instead of the solubility product equation:

c_C x c_A = const = K

for the constancy of the product of the concentrations of the cation C and anion A, of a salt, we must now write:

a_C x a_A = const = K'

where a denotes the activity. We may replace a by f_c, where f is the activity coefficient, which depends on the concentration and is not a constant as would be required by the law of mass-action; hence:

a_C x a_A = (c_C x f_C) x (c_A x f_A = K'

The law of mass-action applies if activities are substituted for concentrations. In very dilute solutions the activity coefficients (which are proportional to 1 - k√c) become nearly unity, and when the solubility of the salt and hence the concentrations of its ions are small, the solubility product law is very approximately true. This is seen from the following results for the solubilities of silver nitrite in presence of silver nitrate or potassium nitrite. The concentrations are in mols per litre.

Conc of AgNO or KNO₂	Solubility of AgNO₂ in presence of		Solubility calc.
	AgNO₃	KNO₂
0	0.0269	0.0269	0.0269
0.00258	0.0260	0.0259	0.0259
0.00588	0.0244	0.0249	0.0247
0.02355	0.0192	0.0203	0.0187

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Chemical Equilibrium, Law Of Mass-action
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