page 1 of 3
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.
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- .
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:
Take a Study Break!