Simple Voltaic Cells

Outline

• Electrochemical Cells
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

Electrochemical Cells

Voltaic cells are simple chemical "wet" batteries in which chemical reactions, usually in an acid or base solution, allow ions or free electrons to move from one location to another, creating electric current that can be used to do work. A complete voltaic cell is made of two half cells. In one, a metal is oxidized and gives up electrons. In the other, ions of a different metal are reduced by taking on electrons.

A complete voltaic cell is made of two half cells, represented here by beakers. The reaction in each half cell will produce or absorb electrons carrying electric charge that can do work as it travels along an external circuit. This assembly mimics the operation of "wet" cells like a car battery, and is named for Alessandro Volta, who first devised this method of storing electricity in the eighteenth century.

In our example, the half-cell on the left contains a solution of nitritate, which will dissolve solid copper into copper ions as its reaction proceeds. The second half cell contains a solution of silver nitrate, where silver ions are dissolved with always-soluble nitrate ions.

In order for electrons to travel from one half reaction to the other, we'll need an external wire circuit. We'll use a voltmeter to measure the current generated during the reaction.

For the anode, we'll use a copper electrode, a piece of pure copper metal, attached to one end of our wire and partly submerged in the nitrate solution. The nitric acid begins dissolving the copper, leaving electrons to gather on the anode.

For the cathode, we'll use a silver electrode, a piece of pure silver, attached to the other wire and partly submerged in the silver nitrate solution, which already contains silver ions and soluble nitrate ions.

To complete the circuit, we'll use a salt bridge, a tube fill with sodium nitrate gel, which allows ions to travel the length of the tube but does not allow the two solutions to mix.

As soon as we add the salt bridge and complete the circult, electrons from the dissolving copper oxidation reaction begin traveling the external wire. When they reach the silver electron, they attract silver ions in the solution and bond with the ions, forming solid silver that is deposited on the silver electrode.

The current flow of electrons leaves excess positive charge as copper ions in the copper nitrate solution and removes positive charges in the silver nitrate solution. To balance this, nitrate ions from the silver nitrate solution travel through the salt bridge to the copper nitrate solution and return the solution concentration net charge to zero. Sodium ions from the salt bridge travel to the silver nitrate solution and raise the positive charge there.

This reaction will continue as long as there are ions to carry charge in each solution.

Check out the YouTube video of A CuAg Voltaic Cell for a narrated explanation of how this works.

Designing Batteries

Designing voltaic cells for particular situations may require the use of different combinations of solutions and electrode substances. Not all metals or material sare appropriate, since they may react in ways that affect current flow or the oxidation or reduction reaction, may be too costly, or dangerous to handle. For measurement purposes, a hydrogen gas electrode is used as the reference for assigning cell voltages.

Commercial batteries come in a number of forms. Wet-cell batteries that can be recharged are used in cars to run electrical systems and aid ignition. Dry cell and alkaline batteries are used in most flashlight and portable systems because they are long-lasting. Nickel-cadmium batteries and lithium batteries are expensive to make, but are lightweight and supply a steady low-level current for electronics such as cell phones and laptop computers, as well as airplane and car systems.

Practice with the Concepts

In a salt bridge, positive ions move to which half-cell, the anode or cathode?

Oxidation always occurs at which electrode?

Discussion Questions

• Why does nitric acid make a good battery acid?

Optional Readings

• Battery University has a lot of information about batteries, from basic principles to descriptions oof exotic batteries designed for specific situations.

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