Question

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Acids and bases are referred to as being 'strong' or 'weak' as distinct from being 'concentrated' or 'diluted'. Explain how these two variables, electrolyte strength and solution concentration, affect the conductivity of aqueous solutions.

Answer 1

The conductivity of an aqueous solution of an acid or a base is directly related to the strength and concentration of the species dissolved.

Explanation:

It takes free-moving charged particles to conduct electricity. Pure water barely disassociates to produce charged ions that conduct electricity across the liquid. Solvents that ionizes when dissolved in water supplies charged particles rendering the solution a conductor of electricity.

The `"strength"` of an electrolyte describes its capability to form ions through disassociation under aqueous conditions. [1] Under constant concentration and quantity, a relatively "strong" electrolyte produces more ions than a "weak" one does. With all these electricity-conducting particles, solutions of strong electrolyte demonstrate higher conductivity than those of weak electrolyte does. The strength of an electrolyte is innate to this species and is independent of the concentration of the electrolyte.

For example, given the fact that `"HCl"` is stronger than `"CH"_3 "COOH"` as an acid ("electrolyte") one would expect `"HCl"` to disassociate to a further extent and yields more ions than `"CH"_3 "COOH"` does. The `"HCl"` solution would thus be more conductive than the `"CH"_3 "COOH"` despite that the two solutions have the same molar concentration. So is the case for strong and weak bases.

What's special about acids and bases as electrolytes is that in addition to conductivity, their ionic concentrations also reflect the chemical reactivity. For example, an acidic solution with a high `["H"^(+)]` tend to erode reactive metals vigorously, as seen in the rate at which `"H"_2 (g)` bubbles off. These solutions thus demonstrate "strong" acidic traits.

The molar electrolyte concentration is directly related to the conductivity of the solution. Consider the Le Chatlier's Principle for the acid disassociation equilibrium of `"CH"_3 "COOH"`:

`"CH"_3"COOH"(aq) rightleftharpoons "CH"_3 "COO"^(-)(aq) + "H"^(+)(aq)`

Increasing the concentration of `"CH"_3 "COOH"` the electrolyte as a reactant causes the equilibrium to shift to the right, increasing the concentration of ions in the solution. The conductivity and acidity of the solution thus increase.

To wrap up:

• Switching to a stronger electrolyte of identical concentration increases the conductivity of the solution
• Increasing the molar electrolyte concentration of the solution without changing the electrolyte increases the conductivity of the solution.

Note that at high concentrations, however, the conductivity of solutions start to decrease as one keep adding to the electrolyte. For example, `"H"_2 "SO"_4` return to its partially disassociated form `"HSO"_4^(-)` as the portion of the solute in the solution outweighs that of the solvent.

Reference
[1] "Strong Electrolytes and Weak Electrolytes Tutorial", AUS-e-TUTE, https://www.ausetute.com.au/electrolytes.html