Electrochemical Cells for AP Chemistry
Practice problems for these concepts can be found at:
- Electrochemistry Multiple Choice Review Questions for AP Chemistry
- Electrochemistry Free-Response Questions for AP Chemistry
In the example above, the electron transfer was direct, that is, the electrons were exchanged directly from the zinc metal to the cupric ions. But such a direct electron transfer doesn't allow for any useful work to be done by the electrons. Therefore, in order to use these electrons, indirect electron transfer must be done. The two half-reactions are physically separated and connected by a wire. The electrons that are lost in the oxidation half-reaction are allowed to flow through the wire to get to the reduction half-reaction. While those electrons are flowing through the wire they can do useful work, like powering a calculator or a pacemaker. Electrochemical cells use indirect electron transfer to produce electricity by a redox reaction, or they use electricity to produce a desired redox reaction.
Galvanic (Voltaic) Cells
Galvanic (voltaic) cells produce electricity by using a redox reaction. Let's take that zinc/copper redox reaction that we studied before (the direct electron transfer reaction) and make it a galvanic cell by separating the oxidation and reduction half-reactions. (See Figure 16.1.)
Instead of one container, as before, two will be used. A piece of zinc metal will be placed in one, a piece of copper metal in another. A solution of aqueous zinc sulfate will be added to the beaker containing the zinc electrode and an aqueous solution of copper(II) sulfate will be added to the beaker containing the copper metal. The zinc and copper metals will form the electrodes of the cell, the solid portion of the cell that conducts the electrons involved in the redox reaction. The solutions in which the electrodes are immersed are called the electrode compartments. The electrodes are connected by a wire and … nothing happens. If the redox reactions were to proceed, the beaker containing the zinc metal would build up a positive charge due to the zinc cations being produced in the oxidation half-reaction. The beaker containing the copper would build up a negative charge due to the loss of the copper(II) ions. The solutions (compartments) must maintain electrical neutrality. To accomplish this, a salt bridge will be used. A salt bridge is often an inverted U-tube that holds a gel containing a concentrated electrolyte solution, such as KNO3 in this example. Any electrolyte could be used as long as it does not interfere with the redox reaction. The anions in the salt bridge will migrate through the gel into the beaker containing the zinc metal, and the salt-bridge cations will migrate in the opposite direction. In this way electrical neutrality is maintained. In electrical terms, the circuit has been completed and the redox reaction can occur. The zinc electrode is being oxidized in one beaker, and the copper(II) ions in the other beaker are being reduced to copper metal. The same redox reaction is happening in this indirect electron transfer as happened in the direct one:
The difference is that the electrons are now flowing through a wire from the oxidation halfreaction to the reduction half-reaction. And electrons flowing through a wire is electricity, which can do work. If a voltmeter was connected to the wire connecting the two electrodes, a current of 1.10 V would be measured. This galvanic cell shown in Figure 16.1 is commonly called a Daniell cell.
In the Daniell cell shown in Figure 16.1 note that the compartment with the oxidation half-reaction is on the left and the compartment undergoing reduction is on the right. This is a convention that you will have to follow. The AP graders look for this. The electrode at which oxidation is taking place is called the anode, and the electrolyte solution in which it is immersed is called the anode compartment. The electrode at which reduction takes place is called the cathode, and its solution is the cathode compartment. The anode is labeled with a negative sign (–), while the cathode has a positive sign (+). The electrons flow from the anode to the cathode.
Remember: Oxidation is an anode process.
Sometimes the half-reaction(s) involved in the cell lack a solid conductive part to act as the electrode, so an inert (inactive) electrode, a solid conducting electrode that does not take part in the redox reaction, is used. Graphite and platinum are commonly used as inert electrodes.
Note: The electrode must be a conductor on to which a wire may be attached. It can never be an ion in solution.