Oxidation-Reduction Reactions 2-1 (2)

Chemical Reactions: Oxidation-Reduction Reactions

Overview

• Chemical reactions include oxidation-reduction (redox) reactions which are crucial for energy transfer in biological systems and batteries.

  • Reactions can include processes such as rusting of iron.

General Equation for Rusting

• Example: (4Fe + 3O_2 \rightarrow 2Fe_2O_3)

Oxidation and Reduction

Definitions

• Oxidation: Loss of electrons.

  • Example: (Zn \rightarrow Zn^{2+} + 2e^{-})

• Reduction: Gain of electrons.

  • Example: (Cu^{2+} + 2e^{-} \rightarrow Cu)

Key Concept

• Oxidation and reduction occur simultaneously in redox reactions.

Oxidation Numbers

Purpose

• Used to determine if an oxidation-reduction reaction has occurred.

Assigning Oxidation Numbers

• Elements in elemental form: Oxidation number = 0.

• Monatomic ions: Oxidation number = charge.

Specific Rules

• Nonmetals: Generally have negative oxidation numbers, but can vary.

  • Oxygen: Usually -2, but -1 in peroxides.

  • Hydrogen: -1 when bonded to metals, +1 with nonmetals.

• Fluorine: Always -1.

• Other halogens: -1 when negative, can be positive in oxoanions (e.g., (ClO_3^{-})).

Sum of Oxidation Numbers

• In a normal compound: Sum = 0.

• In a polyatomic ion: Sum = charge of the ion.

Half-Reactions for Oxidation-Reduction

Example of a Redox Reaction

• In zinc and copper(II) sulfate:

  • Oxidation: (Zn \rightarrow Zn^{2+} + 2e^{-})

  • Reduction: (Cu^{2+} + 2e^{-} \rightarrow Cu)

Oxidizing and Reducing Agents

Definitions

• Oxidizing agent: Gains electrons and is reduced; oxidizes other substances.

• Reducing agent: Loses electrons and is oxidized; reduces other substances.

Balanced Red-Ox Equations

Principle

• Balance loss of electrons with gain of electrons.

• Example: (Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu)

Balancing Oxidation-Reduction Reactions

Method

1. Write separate equations for oxidation and reduction half-reactions.

2. Balance elements except H and O.

3. Use (H_2O) to balance O and (H^+) to balance H.

4. Balance charge with electrons.

Example

• For the reaction between (ClO_3^{-}) and (SO_2):

  1. Split into half-reactions:

     • (ClO_3^{-} \rightarrow Cl^{-})

     • (SO_2 \rightarrow SO_4^{2-})

  2. Balance separately.

  3. Equalize electrons and add.

Oxidation with Oxygen

Historical Definition

• Addition of oxygen to a reactant indicates oxidation.

  • Example reactions:

    • (4K + O_2 \rightarrow 2K_2O)

    • Combustion reactions like (C + O_2 \rightarrow CO_2)

Gain and Loss of Hydrogen

Organic Reactions

• Oxidation: Loss of hydrogen atoms.

• Reduction: Gain of hydrogen atoms.

  • Example: (CH_3OH \rightarrow H_2CO + 2H)

Summary Table of Oxidation and Reduction Characteristics

Oxidation

• Involves:

  • Loss of electrons

  • Addition of oxygen

  • Loss of hydrogen

Reduction

• Involves:

  • Gain of electrons

  • Loss of oxygen

  • Gain of hydrogen

Learning Checks

Identifying Oxidation and Reduction in Reactions

• Identify substances oxidized and reduced:

  • A. (4Fe + 3O_2 \rightarrow 2Fe_2O_3)

  • B. (6Na + N_2 \rightarrow 2Na_3N)

  • C. (2K + I_2 \rightarrow 2KI)