23.1. Electrochemical Cells http://www.ck12.org
FIGURE 23.1
A galvanic (or voltaic) cell.There are many situations where an electric current is needed to make a device work, but it is inconvenient (or
impossible) to plug the device into a wall circuit. A cell phone or digital watch would not be very useful if we had
to plug the device into a power outlet in order for it to function.
Batteries provide us with a portable source of electricity that can be used when a power outlet is not available. The
first battery was developed by Alessandro Volta (1745-1827) in 1800. He constructed a “voltaic pile” consisting of
alternate discs of silver and zinc that were separated by paper soaked in salt water. According to some sources, Volta
tested the battery by measuring the strength of the electric current. His “measurement” involved seeing how intense
the shock produced by the battery was on a person (never a good idea).
A modernbatteryemploys oxidation-reduction reactions to generate a flow of electrons. The material being
oxidized will release electrons that can flow through an electrical circuit and produce work of some sort. The
electrons then flow into the material that is being reduced. This process will be illustrated through specific examples
as we look at a few common types of batteries that are in use today.
Lead-Acid Batteries
One common type of battery is the type used to start a car. When the circuit is open (the poles are not connected),
no electrons are flowing. However, when the circuit is closed by turning the ignition key to “on," the poles are
connected and electrons can flow from one pole to the other. This flow of electrons will create a spark in the engine
cylinders that ignites the gasoline. The ignition causes the piston to move up and down, powering the car.
A typical car battery involves two poles, one made of lead, and the other made of lead oxide. Each pole is submerged
in sulfuric acid, and the two compartments (one containing each pole) are separated by a barrier that is permeable
to ions and water. Based on its components, this type of battery is referred to as alead-acid battery. The overall
reaction that takes place is the following: