Page 146 - Instant notes

Physical chemistry 132

Related topics Free energy (B6) Electrochemical cells (E3)
Fundamentals of equilibria Electrochemistry and ion

Notation

To avoid repetitive drawing of complicated cell diagrams, a common notation has been
adopted for cells. All electrical contacts between half-cells are assumed, changes of phase
(see Topic D4) are denoted by |, a salt bridge is shown as || and a junction between two
different solutions (a liquid junction in a glass frit) by . If there are multiple species in
the same phase these are separated by commas. The cell is shown starting at the left-hand
electrode and moving to the right-hand electrode through the solutions. Examples of cells
under standard conditions are:
−
+
Pt|H 2(g, p=1 atm)|H (aq, a=1)||Cl (aq, a=1)|AgCl(s)|Ag(s)
3+
2+
2+
Pt|Fe (aq, a=1), Fe (aq, a=1)||Zn (aq, a=1)|Zn(s)
Activity, a, or concentration, c, and pressure, p, or fugacity values are not necessary and
generally not quoted for standard cells (as they are defined), but are important away from
the standard state (see Topic E5). Ions in the solution that take no part in the redox
reaction are generally not included. In these cells, the half-cell on the left both
experimentally and as written is called the left-hand (LH) electrode, while that on the
right is called the right-hand (RH) electrode. Measured values of E cell or are
reported by convention as a positive or negative value, denoting that the right-hand (RH)
electrode has a more positive or negative potential than the left-hand (LH) electrode:

For those cells with half-cells separated by a glass frit or porous ceramic, there is a small
extra voltage associated with the liquid junction, which forms an extra component of
. This complication is avoided by using a salt bridge, which has a negligible voltage as it
contains two liquid junctions whose potential differences cancel.

Formal cell reaction

The overall formal cell reaction is obtained by writing both half-cell reactions as
reductions, e.g.
Pt|Fe 3+ (aq), Fe 2+ (aq)||Zn 2+ (aq)|Zn(s)
−
RH Zn 2+ (aq)+2e →Zn (s)
−
LH Fe 3+ (aq)+e →Fe 2+ (aq)
The number of electrons in each equation is then made equal (if necessary):
−
RH Zn 2+ (aq)+2e → Zn (s)
−
2LH 2Fe 3+ (aq)+2e →2Fe 2+ (aq)