If the oxidation and reduction steps of an electrode reaction
are rapid (high exchange current densities) then the passage of
charge across the electrode-solution interface will barely displace
the reaction equilibrium. Such an electrode is said to be
non-polarisable in the sense that its potential, for small
currents, is stable and equal to the equilibrium electrode
potential. If, on the other hand, reaction equilibrium is
established only slowly due to the kinetic inhibition of a step
involved in the electrode reaction, then the electrode is said to
be polarisable. To induce the reaction to proceed in a given
direction the kinetic inhhibition of the reaction must be overcome
by applying a high overpotential. Electrode polarisation and the
presence of overpotentials are important concepts in understanding
electrode processes. They underlie the fact that galvanic cells
always deliver current at less than the equilibrium e.m.f. and that
an applied potential greater than the equilibrium e.m.f. is
required in order to drive a reaction in an electrolytic cell.
Futhermore, a number of important electochemical devices (e.g. the
lead-acid accumulator) and electroanalytical techniques (e.g.
polarography) make use of the inhibition (high overpotential) of
certain electrode reactions.
- Simultaneous measurement of current and voltage
- Simplified implementation: all pre-settings already prepared
- Record the current-potential curve for the electrolysis of a 1
M hydrochloric acid solution using graphite rod electrodes and
determine the decomposition voltage.
- Discuss the physical processes determining the form of this
- By replacing the graphite rod cathode with a series of
different metal rod electrodes, compare the overpotentials for
hydrogen evolution at these metals.
What you can learn about
- Electrode kinetics
- Irreversible processes
- The electrode-electrolyte interface
- Voltammetry and current-potential curves
- Relevance to electrolysis
- Fuel cells
Software included. Computer not provided.