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Electrolysis : Arrhenius's Theory, Strong And Weak Electrolytes


The current through a solution, or the charge transported per second, is proportional to the charges on the ions, the numbers of ions, and the speeds with which they move. The charges on the ions remain constant, hence the increase in equivalent conductivity with dilution may be attributed to the increasing number of ions, i.e. increased dissociation, and the limiting value at infinite dilution to complete dissociation. The degree of ionisation at any dilution will then be α = Λ/Λ∞. This theory, due to Arrhenius, assumes that the speeds of the ions do not change on dilution.

The tables below give the values of α for 0.1N solutions at 18°, the ions produced being in brackets. Electrolytes largely ionised at this dilution are called strong electrolytes (e.g., HCI), those slightly ionised are called weak electrolytes (e.g., acetic acid).

acidsbases
Hydrochloric (H˙,Cl')0.92Sodium hydroxide (Na˙,OH')0.91
Nitric (H˙,NO3')0.92Potassium hydroxide (K˙,OH')0.91
Sulphuric (H˙,HSO4')0.61Barium hydroxide (Ba˙˙,2OH')0.81
Phosphoric (H˙,H2PO4')0.28Ammonium hydroxide (NH4˙,OH')0.013
Hydrofluoric (H˙,F')0.085
Acetic (H˙,CH3COO')0.013
Carbonic (H˙,HCO3')0.0017
Hydrosulphuric (H˙,HS')0.0007
Boric (H˙,H2BO3')0.0001
Hydrocyanic (H˙,CN')0.0001


Whereas acids and bases differ considerably in strength, most salts are strong electrolytes, as the following table shows. Sodium acetate is largely ionised, although acetic acid is weak; ammonium chloride is a strong electrolyte, although ammonia is a weak base as measured by conductivity.

salts

Potassium chloride (K˙,Cl') 0.86
Sodium chloride (Na˙,Cl') 0.85
Potassium nitrate (K˙,NO3') 0.83
Silver nitrate (Ag˙,NO3') 0.82
Sodium acetate (Na˙,CH3COO') 0.80
Barium chloride (Ba˙˙,2Cl') 0.75
Potassium sulphate (2K˙,SO4'') 0.73
Sodium carbonate (2Na˙,CO3'') 0.70
Zinc sulphate (Zn˙˙,SO4'') 0.40
Copper sulphate (Cu˙˙,SO4'') 0.39
Mercuric chloride (Hg˙˙,2Cl') < 0.01
Mercuric cyanide (Hg˙˙,2CN') very small
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