Oxidation-Reduction Reactions

Introduction

• Definition: Oxidation-reduction (redox) reactions involve the transfer of electrons between two species.
• Importance: Vital for biological functions like photosynthesis and respiration, and for processes like combustion and rusting.

Oxidation States

• Oxidation State (OS): Indicates the number of electrons an atom gains, loses, or seems to use when bonding with other atoms.
• Guidelines for Assigning OS:
  1. Individual atom: OS = 0
  2. Sum in neutral species: OS = 0; in ions: OS = ion charge.
  3. Group 1 metals: OS = +1; Group 2: OS = +2.
  4. Fluorine in compounds: OS = -1.
  5. Hydrogen in compounds: OS = +1.
  6. Oxygen in compounds: OS = -2.
  7. In binary metal compounds: Group 17 elements = -1, Group 16 = -2, Group 15 = -3.

Examples of Assigning Oxidation States

• Example 11:
  • Fe(s) + O2(g) → Fe2O3(g): Fe = +3, O = -2.
  • Fe2+(aq): Fe = +2.
  • Ag(s) + H2S → Ag2S(g) + H2(g): Ag = +1, H = +1, S = -2.
• Example 22:
  • Na3PO3: P = +3.
  • H2PO4: P = +5.

Identifying Oxidized and Reduced Elements

• Example 33:
  • Zn + 2H⁺ → Zn²⁺ + H₂: Zn oxidized, H⁺ reduced.
  • 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu: Al oxidized, Cu²⁺ reduced.
  • CO₃²⁻ + 2H⁺ → CO₂ + H₂O: Not a redox reaction.

Redox Reaction Concept

• Half-Reactions: Always occur together; one is reduced (gains electrons) while the other is oxidized (loses electrons).
• Mnemonics:
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain.
• Agents:
  • Oxidizing agent: Accepts electrons, oxidizes other species.
  • Reducing agent: Donates electrons, reduces other species.

Types of Reactions

Combination Reactions

• General Form: A + B → AB
• Example: H₂ + O₂ → H₂O
  • H₂ and O₂: OS = 0; H in H₂O: OS = +1, O: OS = -2.

Decomposition Reactions

• General Form: AB → A + B
• Example: H₂O → H₂ + O₂
  • H₂O: OS of H = +1, O = -2; H₂ and O₂: OS = 0.

Single Replacement Reactions

• General Form: A + BC → AB + C
• Example: Cl₂ + NaBr → NaCl + Br₂
  • Cl reduced, Br oxidized.

Double Replacement Reactions

• General Form: AB + CD → AD + CB
• Example: Reaction of MgSO₄ with Na₂C₂O₄.

Combustion Reactions

• Characteristic: Involves burning, often exothermic.
• Example: CₓHᵧ + O₂ → CO₂ + H₂O
  • Can involve other oxidizing agents like Cl₂ or F₂.

Disproportionation Reactions

• Definition: A substance is both oxidized and reduced in a reaction.
• Example: 2H₂O₂ → 2H₂O + O₂
  • O in H₂O₂ reduced to H₂O, oxidized to O₂.

References

• Petrucci et al., “General Chemistry: Principles & Modern Applications.”
• Sadava et al., “Life: The Science of Biology.”

Contributors

• Authors: Christopher Spohrer, Christina Breitenbuecher, Luvleen Brar

This document is licensed under CC BY-NC-SA 4.0.