Page 345 - Physical chemistry understanding our chemical world

312 ELECTROCHEMISTRY

The activity of a solution It is unwise to speak in broad terms of ‘the activity of
a solution’ because so many different situations may be considered. For example,
consider the following two examples.
(1) The activity of a mixture of liquids. It is rarely a good idea to
Amalgams are liquid suggest that the activity of a liquid in a mixture is equal to its mole
when very dilute, but fraction x because of complications borne of intermolecular inter-
are solid if the mole actions (e.g. see Chapter 2 and Section 5.6 concerning Raoult’s
fraction of mercury law). Thankfully, it is generally rare that an electrochemist wants
drops below about 70 to study liquid mixtures of this sort (except amalgams diluted to a
per cent. maximum mole fraction of about 1 per cent metal in Hg), so we
will not consider such a situation any further.
(2) The activity of a solute in a liquid solvent. The activity a and concentration c
may be considered to be wholly identical if the concentration is tiny (to a maximum
−3
of about 10 −3 mol dm ), provided the solution contains no other solutes. Such a
concentration is so tiny, however, as to imply slightly polluted distilled water, and is
not particularly useful.
For all other situations, we employ the Debye–H¨ uckel laws (as below) to calculate
the activity coefficient γ . And, knowing the value of γ , we then say that a = (c ÷
c ) × γ (Equation (7.25)), remembering to remove the concentration units because
O
a is dimensionless.

Why does the cell emf alter after adding LiCl?

Ionic ‘screening’

Consider the Daniell cell Zn|Zn ||Cu |Cu. The cell emf is about 1.1 V when pre-
2+
2+
pared with clean, pure electrodes and both solutions at unit activity. The emf decreases
to about 1.05 V after adding lithium chloride to the copper half-cell. Adding more
LiCl, but this time to the zinc solution, increases the emf slightly, to about 1.08 V.
No redox chemistry occurs, so no copper ions are reduced to
2+
In fact, a similar result copper metal nor is zinc metal oxidized to form Zn . No com-
is obtained when adding plexes form in solution, so the changes in emf may be attributed
most ionic electrolytes. entirely to changing the composition of the solutions.
Lithium and chloride ions are not wholly passive, but interact
with the ions originally in solution. Let us look at the copper ions,
each of which can associate electrostatically with chloride ions,
causing it to resemble a dandelion ‘clock’ with the central copper
We need a slightly
ion looking as though it radiates chloride ions. All the ions are
different form of γ solvated with water. These interactions are coulombic in nature,
when working with
so negatively charged chloride anions interact attractively with the
electrolyte solutions:
we call it the mean positive charges of the copper cations. Copper and lithium cations
ionic activity coefficient repel. Conversely, the additional Li + ions attract the negatively
−
γ ± ,as below. charged sulphates from the original solution; again, Cl and SO 2−
4
anions repel.