Electrochemistry: Redox Reactions and Cells

Learning Objectives

Define oxidation and reduction and identify oxidizing and reducing agents.

Assign oxidation states to atoms in compounds and ions.

Distinguish between a galvanic (voltaic) cell and an electrolytic cell.

Describe the basic components of an electrochemical cell, including the anode, cathode, and salt bridge.

The Chemistry of Electron Transfer

Electrochemistry deals with the relationship between chemical reactions and electrical energy. The reactions involved are oxidation-reduction (redox) reactions, where electrons are transferred from one species to another.

Oxidation and Reduction

Oxidation Is Loss of electrons.

Reduction Is Gain of electrons.

(A useful mnemonic is OIL RIG).

The species that is oxidized (loses electrons) is called the reducing agent.

The species that is reduced (gains electrons) is called the oxidizing agent.

Oxidation States: To track electron transfer, we assign an oxidation state (or oxidation number) to each atom.

An atom in its elemental form has an oxidation state of 0 (e.g., Fe, O₂).

The oxidation state of a monatomic ion is its charge (e.g., Na⁺ is +1, Cl⁻ is -1).

The sum of oxidation states in a neutral compound is 0; in a polyatomic ion, it's the ion's charge.

Oxidation corresponds to an increase in oxidation state; reduction corresponds to a decrease.

Electrochemical Cells

An electrochemical cell is a device that can generate electrical energy from a chemical reaction or use electrical energy to cause a chemical reaction.

There are two main types:

1.Galvanic (or Voltaic) Cell:

Uses a spontaneous redox reaction to generate an electric current.

This is the basis of all batteries.

Chemical energy is converted into electrical energy.

2.Electrolytic Cell:

Uses an external source of electrical energy to drive a non-spontaneous redox reaction.

This is used for processes like electrolysis (e.g., splitting water into H₂ and O₂) and electroplating.

Electrical energy is converted into chemical energy.

Components of a Galvanic Cell

Anode: The electrode where oxidation occurs. In a galvanic cell, it is the negative electrode. (Mnemonic: An Ox).

Cathode: The electrode where reduction occurs. In a galvanic cell, it is the positive electrode. (Mnemonic: Red Cat).

Salt Bridge: A device containing an inert electrolyte (like KNO₃) that connects the two half-cells. Its purpose is to maintain charge neutrality by allowing ions to flow between the half-cells, completing the electrical circuit. Without it, the cell would quickly stop working due to charge buildup.

Electrons always flow from the anode to the cathode through the external wire.

Key Terms

Redox Reaction: A type of chemical reaction that involves a transfer of electrons between two species. It comprises two half-reactions: oxidation (loss of electrons) and reduction (gain of electrons).
Oxidation State: A number assigned to an element in a chemical combination which represents the number of electrons lost (or gained, if the number is negative) by an atom of that element in the compound.
Galvanic Cell (Voltaic Cell): An electrochemical cell that derives electrical energy from a spontaneous redox reaction taking place within the cell.
Electrolytic Cell: An electrochemical cell that uses an external electric current to drive a non-spontaneous chemical reaction.
Anode: The electrode at which oxidation occurs. Electrons flow away from the anode.
Cathode: The electrode at which reduction occurs. Electrons flow towards the cathode.