Chapter 20 Electrochemistry Electrochemistry is the branch of chemistry that focuses on the electricity related aspects of oxidation-reduction reactions. If you separate the oxidation reaction from the reduction reaction in an appropriate way, you can use the electrons being transferred to produce electricity. The oxidation reaction can be separated from the reduction reaction by using either a salt bridge or a porous disk. Both of the devices function in the same way-they allow ions to migrate from one solution to the other without letting the two solution mix. An electrode is a conductor that is used to establish contact with the non-metallic portion of a circuit (an electrolyte solution). The electrode where the oxidation reaction occurs is called the anode. The electrode where the reduction reaction occurs is called the cathode.

Diagram of an Electrochemical Cell (battery or galvanic cell) (You should be able to completely label a diagram similar to this.) Zn (s) + Cu+2 (aq) Cu (s) + Zn+2 (aq)

Oxidation is at the Anode Reduction is at the Cathode Line Notation is a short hand way of describing a redox reaction. Anode electrode Anode solution Cathode solution Cathode electrode An Ox and a Red Cat Oxidation is at the Anode Reduction is at the Cathode

Zn (s) + Cu+2 (aq) Cu (s) + Zn+2 (aq) Anode electrode Anode solution Cathode solution Cathode electrode Zn (s) + Cu+2 (aq) Cu (s) + Zn+2 (aq) Let’s write the line notation for the reaction above. First identify the oxidation reaction (that is where the anode will be identified). Second identify the reduction reaction (that is where the cathode will be identified).

Electrical Potential (voltage or how forcefully the electrons “want” to be transferred. The electrical potential for a battery must be positive in order to operate. We can use a table of standard reduction potential to determine the direction that a galvanic cell will operate (also the direction of the redox reaction). For example, if we wanted to use: Fe (s), Fe+3 (aq), Al (s), and Al+3 (aq) what would the redox reaction look like, what would be the cell potential, and what substance would the anode and cathode be made of? See page 626 to find the standard reduction potentials needed.

Balance the Redox reaction below: H2S (g) + MnO4-1 (aq) S (s) + Mn+2 (aq) Find xo: Draw a cell and label all parts of the cell: Write the line notation for the cell:

Classwork/Homework: Complete problems 3, 9, 16, 15, 18, 27 on pages 636-637.

Voltaic Cell (battery) versus Electrolytic Cell (electrolysis) If a redox reaction has a negative value for xo, then we can use electricity to force the reaction to occur. When we use electricity to make a redox reaction occur, we are using an Electrolytic Cell.