CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 396
11.8 Thermodynamics of galvanic cells
11.8.1 Basic terms
Agalvanic cellis an electrochemical system, see11.1.7, in which a potential difference occurs
between the anode and the cathode due to electrochemical reactions. This potential difference
is called the(equilibrium) cell potential, in older literature also theelectromotive force
of the cell.
The anode, i.e. the electrode at which oxidation takes place, is charged negatively. The
cathode, i.e. the electrode at which reduction takes place, is charged positively, compare with
the beginning of section11.2.
In the simplest case, both electrodes share a single electrolyte. If it is not so, the galvanic
cell is made up of two or more half-cells. Thehalf-cell is formed by an electrode and an
electrolyte that surrounds it.
Note:In electrochemistry the termelectrodeis commonly used as a synonym to the term
half-cell, e.g. “calomel electrode” or “glass electrode”.The interface between two half-cells is usually a frit which prevents the electrolyte surround-
ing the anode from mixing with that surrounding the cathode. A potential difference occurring
at the interface between the two electrolytes is called thediffusionorliquid junction po-
tential.
We often seek to suppress this liquid potential. This can be achieved by connecting the
electrolytes by asalt bridge,which is usually a U-shape tube filled with a saturated solution
of potassium chloride or potassium sulfate.
If no electric current passes through a galvanic cell, we say that the cell is in azero-current
state. The zero-current state may be attained in two ways:
a) the cell is disconnected,
b) the cell is connected with an external source compensating for the equilibrium cell potential.
From the thermodynamic point of view, we classify galvanic cells as reversible and irre-
versible. Areversible cellis such in which
a) no reaction proceeds in the zero-current state,
b) if the voltage of the external source exceeds the cell potential, the same reactions take place
in it as in a cell without any external source, but in the opposite direction.