ELECTROCHEMISTRY Voltaic and Electrolytic Cells EDHS Fall 2010 IB CHEM

Review… Redox reaction are those involving the oxidation and reduction of species. LEO says GER… Losing electrons  Oxidation Gaining Electrons  Reduction NOTE: Oxidation and reduction must occur together. They cannot exist alone.

Let’s Look at the REDOX Reaction between Zinc and Copper Oxidation Half-Reaction: Zn(s)  Zn2+(aq) + 2 e–. Reduction Cu2+(aq) + 2 e–  Cu(s)

Some Vocabulary… Electrodes: are usually metal strips/wires connected by an electrically conducting wire. Salt Bridge: is a U-shaped tube that contains a gel permeated with a solution of an inert electrolyte. Anode: is the electrode where oxidation takes place. Cathode: is the electrode where reduction takes place.

Voltaic Cell

Voltaic Cells In a spontaneous redox reaction, electrons flow from the oxidizing reactant (reducing agent) to the reducing reactant (oxidizing agent). If the two half–reactions can be separated, this flow of electrons, instead of occurring at the surface of the metal, occurs through an external wire and an electric current is generated.

Voltaic Cells Continued… A battery, like the ones found in a flashlight or calculator, contains oxidizing and reducing substances. As the electrons are transferred they are “tapped” in order to provide the voltage necessary to power the flashlight or calculator.

Analogy for Voltaic Cells…. A good analogy for the flow of electrons is the flow of water. Water flows spontaneously downhill. Dams and waterwheels are examples of ways that the energy of flowing water is tapped to generate power. Sometimes we want water to flow uphill. In this case we need to supply energy, in the form of a pump, to make this happen.

Redox Reactions as a Source of Power In order for a redox reaction to serve as a source of power, the reaction must be spontaneous. What is the reaction is not spontaneous? In this case, we can use electricity to make the reaction “go”.

Electrolytic Cells An electrolytic cell is a device which uses electricity to drive a non–spontaneous redox reaction. For example, water can be separated into hydrogen and oxygen gas (a non–spontaneous reaction) using electricity.

Negative electrode (cathode):  Na+ + e-  Na Positive electrode (anode):  2 Cl-  Cl2 + 2 e-