Galvanic cells harness the electrical energy available from the
electron transfer in a
redox reaction to perform useful electrical work. The key to
gathering the
electron flow is to separate the oxidation and reduction
half-reactions,
connecting them by a wire, so that the electrons must flow through that
wire. That electron
flow, called a current, can be sent through a circuit which could be
part of any number
of electrical devices such as radios, televisions, watches, etc.
The figure below shows two typical setups for galvanic cells. The
left hand cell
diagram shows and oxidation and a reduction half-reaction joined by both a
wire and a
porous disk, while the right hand cell diagram shows the same cell
substituting a
salt bridge for the porous disk.
Figure %: Diagram of a Galvanic Cell
The salt bridge or porous disk is necessary to maintain the charge
neutrality of each half-cell by allowing the flow of ions with minimal mixing of the half-cell
solutions. As
electrons are transferred from the oxidation half-cell to the reduction
half-cell, a negative
charge builds in the reduction half-cell and a positive charge in the
oxidation half-cell. That
charge buildup would serve to oppose the current from anode to
cathode--
effectively stopping the electron flow--if the cell lacked a path for ions
to flow between the
two solutions.
The above figure points out that the electrode in the oxidation
half-cell is
called the anode and the electrode in the reduction half-cell is called the
cathode. A good
mnemonic to help remember that is "The Red Cat ate An Ox" meaning
reduction
takes place at the cathode and oxidation takes place at the anode.
The anode, as the source of the negatively charged electrons is usually
marked with a
minus sign (-) and the cathode is marked with a plus sign (+). Physicists
define the
direction of current flow as the flow of positive charge based on an 18th
century
understanding of electricity. As we now know, negatively charged electrons
flow in a
wire. Therefore, chemists indicate the direction of electron flow on cell
diagrams and not
the direction of current. To make that point clear, the direction of
electron flow is indicated
on with a arrow and the symbol for an electron, e-
.
Figure %: Diagram of a Galvanic Cell Showing Direction of Electron Flow
Line Notation for Galvanic Cells
Instead of drawing a cell diagram such as or chemists have devised a shorthand way of completely
describing a cell
called line notation. This notation scheme places the constituents of
the cathode on the
right and the anode components on the left. The phases of all reactive
species are listed and
their concentrations or pressures are given if those species are not in
their standard
states (i.e. 1 atm. for gasses and 1M for solutions). All phase
interfaces are noted with a
single line ( | ) and multiple species in a single phase are separated by
commas. For
example, a half-cell containing 1M solutions of CuO and HCl and a Pt
electrode for the
reduction of Cu2+ would be written as:
