Electron transfer plays a fundamental role in governing the pathway of chemical reactions. Yet
the speed and size of the electron mean that tracing its movement is difficult using tradition
methods such as spectroscopy and synthetic chemistry. Consequently our knowledge of the
driving force for many reactions remains elusive. Electrochemical methods offer the potential
to investigate these processes directly by the detection of the electrons involved. This course will
highlight the main principles behind the kinetic aspects of electrochemistry and concentrate on
the application of electrochemical techniques to the investigation of charge transfer driven
processes. In the following sections a general overview of electrolysis
reactions is covered.
Electrode Reactions
A typical electrode reaction involves the transfer of charge between an electrode and a species
in solution. The electrode reaction usually referred to as electrolysis, typically involves a series
of steps:
| * |
Reactant (O) moves to the interface: this is termed mass
transport |
| * | Electron transfer can then occur via quantum
mechanical tunnelling between the electrode and reactant close to the electrode (typical
tunnelling distances are less than 2 nm) |
| * | The product (R) moves
away from the electrode to allow fresh reactant to the surface |
Figure 1 Animation of a simple electrode reaction
The above electrode steps can also be complicated by:
| * | The applied voltage on the
electrode |
| * | The reactivity of the species |
| * | The nature of the electrode surface |
| * | The structure of the interfacial region over which
the electron transfer occurs |
Before we begin to look at the above processes in detail, let us consider a few examples of
electrode reactions. Perhaps the 'simplest' is a single electron transfer
Figure 2 A single electron
transfer reaction
Here the reactant Fe3+ moves to the interface where it undergoes a one electron
reduction to form Fe2+. The electron is supplied via the electrode which is part of
a more elaborate electrical circuit. For every Fe3+ reduced a single electron must
flow. By keeping track of the number of electrons flowing (ie the current) it is possible to say
exactly how many Fe3+ molecules have been reduced.
A further example of electrochemistry in action is metal deposition
Figure 3 Copper deposition at
a Cu electrode
In this case the electrode reaction results in the fomation of a thin film on the orginal surface.
It
is possible to build up multiple layers of thin metal films simply by passing current through
appropriate reactant solutions.
A final example of electrode reactions is shown in figure 4
Figure 4 Electron transfer
followed by chemical reaction
In this case an organic molecule is reduced at the electrode forming the radical anion. This
species however is unstable and undergoes further electrode and chemical reactions. We will see
later how electrochemistry can be employed to study and initiate a whole range of chemical
processes.
Equilibrium electrochemistry
The above are all examples of electrolysis reactions, where an electron is forced in or out of the
electrode. In this situation a current flows for as long as the reaction continues. This differs
significantly from the electrochemical topics you have encountered in earlier courses. Before we
begin to look at electrolysis in detail it is perhaps useful to briefly recall some points from the
year I course on equilibrium electrochemistry. Then typically we were using a two compartment
cell
with separate reactants in each side. When electrodes were placed within each compartment and
a circuit made between the two halves, a voltage was obtained. It is important to note that this
measurement is performed so that no current is allowed to flow between the compartments.
Hence we obtain an equilbrium. The voltage measured then predicts which way electrons would
like to flow (if they could), this is purely thermodynamic measurement. Like data from chemical
equilibrium measurements the infomation tells us whether it is thermodynamically favourable
or
not for a reaction to proceed.
Such experiments allow the measurement of the following quantities
Enthalpies of reaction
Entropies of reaction
Free energies of reaction
Equilibrium constants
Solution pH
However these measurements reveal nothing regarding the kinetics of the process.
To gain kinetic information we must watch the establishment of the equilibrium - this is
essentially the area of electrode dynamics, the topic of the lecture course. We will examine a
model of the kinetic in the next document, before we do this however, we need to gain a
microscopic view of why electrode reactions occurr, and what this odd term voltage has to do
with 'chemistry'.
Electron Transfer and Energy levels
The key to driving an electrode reaction is the application of a voltage (V). If we consider the units of volts
V = Joule/Coulomb
we can see that a volt is simply the energy (J) required to move charge
(c). Application of a voltage to an electrode therefore supplies
electrical energy. Since electrons possess charge an applied voltage can alter the 'energy' of the
electrons within a metal electrode. The behaviour of electrons in a metal can be partly understood
by considering the Fermi-level (EF). Metals are comprised
of closely packed atoms which have strong overlap between one another. A piece of metal
therefore does not possess individual well defined electron energy levels that would be found in
a single atom of the same material. Instead a continum of levels are created with the available
electrons filling the states from the bottom upwards. The Fermi-level corresponds to the energy
at which the 'top' electrons sit.
Figure 5 Representation of
the Fermi-Level in a metal
at three different applied voltages
This level is not fixed and can be moved by supplying electrical energy. Electrochemists are
therefore able to alter the energy of the Fermi-level by applying a voltage to an electrode.
Figure 6 Schematic
representation of the reduction of
a species (O) in solution
Figure 6 shows the Fermi-level within a metal along with the orbital energies (HOMO and
LUMO) of a molecule (O) in solution. On the left hand side the Fermi-level has a lower value
than
the LUMO of (O). It is therefore thermodynamically unfavourable for an electron to
jump from the electrode to the molecule. However on the right hand side, the Fermi-level is
above
the LUMO of (O), now it is thermodynamically favourable for the electron transfer to
occur, ie the reduction of O.
Figure 7 Animation of the reduction of
a species (O) in
solution
Whether the process occurs depends upon the rate (kinetics)
of the electron transfer reaction and the next document describes a model which explains this
behaviour
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