Galvanic cells, also known as voltaic cells, are electrochemical cells in which spontaneous oxidation-reduction reactions produce electrical energy.
Electrochemical cells have two conductive electrodes, called the anode and the cathode. Electrodes can be made from any sufficiently conductive materials, such as metals, semiconductors, graphite, and even conductive polymers. In between these electrodes is the electrolyte, which contains ions that can freely move.
Electron flow is observed at both the anode and cathode: the anode will undergo oxidation (loss of electrons) while the cathode experiences reduction (gain of electrons).
– The metal of the anode will oxidize, going from an oxidation state of 0 (in the solid form) to a positive oxidation state, and it will become an ion.
– At the cathode, the metal ion in the solution will accept one or more electrons from the cathode, and the ion’s oxidation state will reduce to 0. This forms a solid metal that deposits on the cathode.
Direction of Electron Flow
The two electrodes must be electrically connected to each other, allowing for a flow of electrons that leave the metal of the anode and flow through this connection to the ions at the surface of the cathode.
This flow of electrons is an electrical current that can be used to do work, such as turn a motor or power a light. A battery is a set of voltaic cells that are connected in parallel. For instance, a lead–acid battery has cells with the anodes composed of lead and cathodes composed of lead dioxide.
Half Equations
The operating principle of the voltaic cell is a simultaneous oxidation and reduction reaction, called a redox reaction. This redox reaction consists of two half-reactions. In a typical voltaic cell, the redox pair is copper and zinc, represented in the following half-cell reactions:
– Zinc electrode (anode): Zn(s) → Zn2+(aq) + 2 e–
– Copper electrode (cathode): Cu2+(aq) + 2 e– → Cu(s)
Cell Potential
Standard Cell potentials (difference of the two electrodes) can be calculated from the potentials of the half-reactions:
Cell potential = Reduction potential + Oxidation potential
When solving for the standard cell potential, the species oxidized and the species reduced must be identified. This can be done using an activity series. A table of standard reduction potentials displays the reduction potentials in decreasing order. The species at the top have a greater likelihood of being reduced while the ones at the bottom have a greater likelihood of being oxidized. Therefore, when a species at the top is coupled with a species at the bottom, the one at the top will become reduced while the one at the bottom will become oxidized. A positive cell potential indicates that the reaction proceeds spontaneously in the direction in which the reaction is written. The cell potential for all galvanic/voltaic cells is positive because the voltaic cell generates potential.
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Key Points
• An electrochemical cell, also called galvanic or voltaic cell, is a device that produces an electric current from energy released by a spontaneous redox reaction in two half-cells.
• The anode will undergo oxidation (loss of electrons) and reduction (gain of election) will happen at cathode.
• Electrons leave the metal of the anode and flow through the connecting wire to the ions at the surface of the cathode.
• Cell potential = Reduction potential + Oxidation potential.
Key Terms
Electrodes: an electrical conductor that provides the physical interface between the electrical circuit providing the energy and the electrolyte.
Electrolyte: a substance containing free ions that carry electric current.
Active electrode: electrode that participates in the oxidation-reduction reaction of an electrochemical cell; the mass of an active electrode changes during the oxidation-reduction reaction
Anode: an electrode in an electrochemical cell at which oxidation occurs; information about the anode is recorded on the left side of the salt bridge in cell notation
Cathode: an electrode in an electrochemical cell at which reduction occurs; information about the cathode is recorded on the right side of the salt bridge in cell notation
Cell notation: shorthand way to represent the reactions in an electrochemical cell
Cell potential: difference in electrical potential that arises when dissimilar metals are connected; the driving force for the flow of charge (current) in oxidation-reduction reactions
Galvanic cell: electrochemical cell that involves a spontaneous oxidation-reduction reaction; electrochemical cells with positive cell potentials; also called a voltaic cell
Inert electrode: electrode that allows current to flow, but that does not otherwise participate in the oxidation-reduction reaction in an electrochemical cell; the mass of an inert electrode does not change during the oxidation-reduction reaction; inert electrodes are often made of platinum or gold because these metals are chemically unreactive.
Voltaic cell: another name for a galvanic cell