CHAPTER 20: ELECTROCHEMISTRY 20-1 Voltaic Cells Oxidation is the loss of electrons. Reduction is the gain of electrons. There are 2 types of electrochemical cells we will study in this chapter …. Voltaic cells use chemical changes to produce electricity. Electrolytic cells use electricity to produce chemical change. We will look at voltaic cells first….

Plating Without Power Look at the metals involved in a plating question and write the equation like a single replacement reaction. Look at the reduction/oxidation chart to determine whether replacement will occur. If replacement occurs, there will be plating, if no replacement, then no plating because there is no reaction.

Voltaic Cell anode/oxidized /- cathode/reduced/+

Voltaic Cell (cont’d) The half-cell that loses electrons is the – electrode or anode. The half-cell that gains electrons is the + electrode or cathode. You can determine which metal is oxidized/reduced using your reduction/oxidation potential chart. You will draw the flow of electrons from oxidized half-cell to reduced half-cell. If you are making your own half-cells, the metals will be in solutions of sulfate, nitrate, ….etc, where both metals have the same – ion.

Voltaic Cell With a Salt-bridge A salt-bridge is a pathway used to maintain solution neutrality by allowing passage of ions from one half-cell to the other. The half-reactions can be determined by looking at which metal is higher on the reduction or oxidation chart…you will have one on your PUI page. You should balance the half-reactions to have the same electrons lost and gained.

Voltaic Cells (cont’d) So lets draw a voltaic cell from scratch using copper and zinc…

Voltaic Cell With Porous Cup anode/oxidized /- cathode/reduced/+

Standard Notation for a Voltaic Cell anode/oxidized /- cathode/reduced/+ Zn  Zn 2+ + 2e- Cu2+ + 2e-  Cu

Voltaic Cell Potentials Once you have drawn a voltaic cell, and labeled all of the parts and written the half-reactions, you will be asked to solve for the cell potential. You will use the equation ECell = ERed - EOx . Look up the reduction potentials on your chart and plug in. A + value means voltage is produced by the cell.

How are Cell Potentials Established? A metal is connected, with its 1 M solution, to a 1 M solution of HCl with a standard hydrogen electrode with platinum wire. (often called a platinum-black electrode). This standard is the zero –point and the value that shows on the voltage meter is the reduction potential for the chart.