Page 151 - Instant notes

E5
ELECTROCHEMISTRY AND ION
CONCENTRATION

Key Notes
The cell voltage and half-cell potentials depend on the activity of
the ions in the cell reaction and the half-cell reaction. The
relationships linking cell voltage and activity and half-cell
potential and activity are the Nernst equations for cell and half-
cell respectively.
An electrochemical cell at equilibrium has no cell voltage. This
means that the equilibrium constant for the cell reaction can be
obtained from the standard cell voltage, .
Measurements of cell voltages as a function of ionic
concentration can be used to determine the mean activity
coefficient of the ions in the cell reaction at each concentration.
The standard cell voltage can also be found, as the ionic
concentrations tend to zero.
Electrolyte concentration cells have a cell reaction that involves
the movement of an ion from a solution of one concentration to
another. The Nernst equation for this cell allows the free energy
change for this process to be calculated. The cell voltage
corresponds to the voltage set up between these two solutions
across a membrane which allows only this ion to pass through it.
Cells that contain the hydrogen electrode half-cell show a
dependence of the cell voltage on pH given by the Nernst
equation. This equation can be used to convert between the
standard state and the biological standard state. A cell consisting
of the hydrogen electrode and a reference electrode could in
principle be used to measure changes in the pH of a solution.
Practical difficulties mean that a cell consisting of the glass
electrode and a reference electrode is preferred.

Related topics Thermochemistry (B3) Thermodynamics of ions in
solution (E2)
Free energy (B6)
Fundamentals of equilibria Electrochemical thermodynamics
(C1) (E4)