Page 145 - Instant notes

E4
ELECTROCHEMICAL
THERMODYNAMICS

Key Notes
A common notation is used for electrochemical cells. The
symbols |, || and are used to denote a phase boundary, salt bridge
and liquid junction respectively. All outside electrical
connections are assumed to be present and activities (or
concentrations) and fugacities (or pressures) are specified where
necessary. The left hand (LH) half-cell is shown on the left-hand
side of the diagram with the right-hand (RH) half-cell on the
right-hand side. The cell voltage is the potential difference
between RH and LH half-cells when a salt bridge is used.
The formal cell reaction is obtained by taking the difference
between the RH and LH half-cell reduction reactions, whilst
ensuring that the same number of electrons is present in each
half-cell reaction.
Changes in half-cell potential can be measured by measuring the
voltage of a cell with the half-cell as the RH electrode and a
reference electrode as the LH electrode. This is because the
reference electrode maintains a constant half-cell potential even
with small changes in reagent activity.
The standard reduction potential of a half-cell is the standard cell
potential of a cell with the half-cell as the RH electrode and the
standard hydrogen electrode as the LH electrode. values,
when tabulated, give an electrochemical series that shows an
increase in the oxidizing power of the oxidant and a decrease in
the reducing power of the reductant in the redox reaction as
increases. This can be used to predict whether a redox reaction
between an oxidant and a reductant is spontaneous under standard
conditions.
The cell voltage and changes in the cell voltage with a small
change in temperature at constant pressure can be used to
determine changes in Gibbs free energy, entropy and enthalpy
and the equilibrium constant for the formal cell reaction and the
half-cell reduction reactions. Changes in the cell voltage with a
small change in pressure at constant temperature can be used to
calculate changes in volume for the cell reaction and half-cell
reaction respectively.