Page 160 - Instant notes

Physical chemistry 146

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and under these conditions, λ values are constant for any ion and do not vary when the
counterion is varied. They have been tabulated for many ions, which allows values of
to be calculated for different salts.

Weak electrolytes

For a weak electrolyte, the salt does not completely dissolve into its constituentions and
for example the equilibrium:

is present. Dilute concentrations favor the formation of ions and as
because complete dissociation occurs. Increasing the concentration decreases the fraction
of dissociated ions (the degree of dissociation, α) in accordance with Le Chatelier’s
principle (see Topic C1). Undissociated electrolyte is uncharged and cannot contribute to
the conductivity and so the molar conductivity falls much more rapidly with
concentration than for a strong electrolyte (Fig. 2b). As this fall is much larger than that
due to the electrostatic attraction of ions, the fraction of salt present as ions, α, at any
electrolyte concentration c is given by .
Λ m is measured at c, and in the example if is measured or calculated, α can be used
to determine the concentration of cation and anion (each αc) and undissociated electrolyte
(c− c) and obtain a value for the equilibrium constant, K, for the ion dissociation
reaction (see Topic C5). In the example, at low ionic strength (see Topic E2),

Transport numbers

The transport number, t, of an ion is the fraction of the total charge carried by the ion in
an electrolyte. For the cation, and the anion, these are given by
t +=n +λ +/Λ m and t −=n −λ −/Λ m
respectively, and the sum of the transport numbers for the cations and the anions must
add up to 1. Since λ and Λm vary with concentration and electrolyte, so does t. The
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limiting transport number, t , is the value measured when c→0.