Electrochemistry

Introduction

Electrochemistry is the study of chemical processes that cause electrons to move, which is the basis for redox (oxidation-reduction) reactions. These reactions are fundamental to understanding how chemical energy is converted to electrical energy and vice versa.

Redox Reactions

General Concepts

• Redox (oxidation-reduction) reaction: A chemical reaction in which electrons are transferred from one reactant to another.

• Oxidation: Loss of electrons by a substance. Mnemonic: OIL (Oxidation Is Loss)

• Reduction: Gain of electrons by a substance. Mnemonic: RIG (Reduction Is Gain)

• Reducing agent: The species that donates electrons and is itself oxidized.

• Oxidizing agent: The species that accepts electrons and is itself reduced.

Example:

• Zn loses 2 electrons (is oxidized); Zn is the reducing agent.

• Cu2+ gains 2 electrons (is reduced); Cu2+ is the oxidizing agent.

Identifying Agents in Redox Reactions

• In , Fe is the reducing agent.

• In , Ag+ is the oxidizing agent.

Half-Reactions

Every redox reaction can be split into two half-reactions: one for oxidation and one for reduction.

• Oxidation half-reaction: Shows the loss of electrons.

• Reduction half-reaction: Shows the gain of electrons.

Example:

• Overall:

• Oxidation:

• Reduction:

Oxidation Numbers

Definition and Assignment Rules

The oxidation number (or oxidation state) of an atom in a compound is the charge it would have if the compound were ionic. Assigning oxidation numbers helps identify which atoms are oxidized and which are reduced.

• Elemental form: Oxidation number is 0.

• Monatomic ion: Oxidation number equals the ion's charge.

• Common nonmetals:

  • Fluorine: -1

  • Hydrogen: +1 (with nonmetals), -1 (with metals)

  • Oxygen: -2 (most compounds), -1 (peroxides)

  • Group I metals: +1; Group II metals: +2

  • Cl, Br, I: -1 (except when combined with oxygen or other halogens)

• Sum of oxidation numbers equals the charge on the molecule or ion.

Example: Assign oxidation numbers in the following compounds:

Balancing Redox Reactions

Stepwise Procedure (Acidic and Basic Conditions)

Balancing redox reactions ensures both mass and charge are conserved. The process involves several steps:

1. Assign oxidation numbers to all elements to identify what is oxidized and what is reduced.

2. Separate the equation into two half-reactions (oxidation and reduction).

3. Balance all atoms except O and H in each half-reaction.

4. Add to balance oxygen atoms.

5. Add to balance hydrogen atoms (for acidic solution).

6. Add electrons () to balance charges in each half-reaction.

7. Multiply half-reactions by appropriate factors so the number of electrons lost equals the number gained.

8. Add the half-reactions together, canceling like terms.

9. For basic solutions, add ions to both sides to neutralize and form .

10. Verify that all atoms and charges are balanced.

Example:

• Balance: (acidic conditions)

• Balance: (acidic and basic conditions)

Additional info:

• Balancing in basic solution requires converting to using .

• Always check that both mass and charge are balanced in the final equation.