Page 141 - Instant notes

E3
ELECTROCHEMICAL CELLS

Key Notes
An electrochemical cell is constructed of two half-cells, each of
which is connected by an outside electrical circuit (as an
electrical connection) and a salt bridge (as an ionic connection).
Each half-cell consists of a metal (an electrode) at which a redox
reaction occurs which establishes a characteristic potential on the
metal surface. The simplest half-cell is a metal dipping into a
solution of its ions.
In a galvanic cell, current is allowed to pass in the spontaneous
direction (electrons move from low to high potential) by simply
connecting the two electrodes together. For thermodynamic
measurements, the potential difference (voltage) is measured
without current flow by connecting a high impedance voltmeter
between the electrodes. In the electrolytic cell a voltage is applied
to the electrodes to force reaction in the non-spontaneous
direction.
Other important half-cells are the metal-insoluble salt electrode,
the gas electrode and the redox electrode. These last two
electrode reactions do not involve a metal and an inert platinum
metal electrode is required to produce the electrode potential.
Cell voltages and half-cell potentials are often compared under
standard conditions called the standard state at a specified
temperature (usually 298 K). The standard state requires all
reagents to be pure with all gases at unit fugacity, all ions at unit
activity and all electrical connections between half-cells to be
made with platinum. The biological standard state is often used
for biological systems, as the standard state is inappropriate.
Related topics Enthalpy (B2) Electrochemical
thermodynamics (E4)
Thermochemistry (B3)
Entropy and change (B5) Electrochemistry and ion
concentration (E5)
Thermodynamics of ions in
solution (E2)

Cells