10.01 Combustion, Oxidation, and Reduction

1. Combustion Reactions

Definition:

• Combustion is a chemical reaction where a substance reacts with oxygen, releasing energy in the form of heat and light. It is an exothermic reaction.

Characteristics of Combustion:

• Exothermic: Releases energy.
• Requires Oxygen: Oxygen is a key reactant.
• Produces Oxides: Typically carbon dioxide and water when hydrocarbons burn completely.

Common Combustion Reactions:

1. Complete Combustion of Methane (Natural Gas):CH₄(g)+2O₂(g)→CO₂(g)+2H₂O(g)
   • Methane (CH₄): Main component of natural gas.
   • Products: Carbon dioxide (CO₂) and water vapor (H₂O).
2. Combustion of Glucose in Respiration:C₆H₁₂O₆(s)+6O₂(g)→6CO₂(g)+6H₂O(l)+heat
   • Glucose (C₆H₁₂O₆): Derived from carbohydrates in food.
   • Process: Respiration in biological cells.
   • Products: Carbon dioxide, water, and energy (heat).

Key Terms:

• Fuel: A substance like methane that combusts readily, releasing significant energy.

2. Oxidation and Reduction (Redox) Reactions

Definitions:

• Oxidation: Gain of oxygen or loss of electrons by a substance.
• Reduction: Loss of oxygen or gain of electrons by a substance.
• Redox Reaction: A reaction involving both oxidation and reduction.

Important Concepts:

• Oxidizing Agents: Substances that oxidize others and are themselves reduced (e.g., oxygen, hydrogen peroxide).
• Reducing Agents: Substances that reduce others and are themselves oxidized (e.g., hydrogen, carbon, carbon monoxide).

General Principles:

• Oxidation Never Occurs Alone: It is always paired with reduction.
• Electron Transfer: Oxidation involves loss of electrons; reduction involves gain of electrons.

Key Definitions:

• Redox Reaction: Reaction involving the transfer of electrons between two species, resulting in oxidation of one and reduction of the other.

3. Examples of Redox Reactions

Example A: Oxidation of Copper

• Reaction: 2Cu(s)+O₂(g)→2CuO(s)
• Process:
  • Oxidation: Copper (Cu) loses oxygen to form copper(II) oxide (CuO).
  • Reduction: Oxygen gains electrons to form oxide ions.

Reverse Reaction: Reduction of Copper(II) Oxide

• Reaction: CuO(s)+H₂(g)→Cu(s)+H₂O(g)
• Process:
  • Reduction: Copper(II) oxide loses oxygen to regenerate copper.
  • Oxidation: Hydrogen gains oxygen to form water.

Example B: Reduction of Zinc Oxide by Carbon

• Reaction: ZnO(s)+C(s)→Zn(s)+CO(g)
• Process:
  • Reduction: Zinc oxide loses oxygen to form zinc metal.
  • Oxidation: Carbon gains oxygen to form carbon monoxide.

4. Industrial Importance of Redox Reactions

Extraction of Metals:

• Reduction of Metal Oxides:
  • Example: Extraction of Iron from Hematite (Fe₂O₃) in a Blast Furnace:
    1. Reduction of Carbon Dioxide: CO₂(g)+C(s)→2CO(g)
    2. Reduction of Iron(III) Oxide: Fe₂O₃(s)+3CO(g)→2Fe(s)+3CO₂(g)
  • Reducing Agents Used: Carbon, carbon monoxide.
• Extraction of Other Metals:
  • Metals like Zinc, Lead, Copper: Extracted by reducing their oxides with carbon.

5. Everyday Oxidation Reactions

a. Corrosion:

• Definition: The chemical attack on metals by substances like oxygen and water.
• Example: Rusting of Iron:
  4Fe(s) + 3O₂(g) + 6H₂O(l) → 4Fe(OH)₃(s)
  • Product: Rust (Iron(III) oxide-hydroxide).
  • Impact: Weakens structures like cars, railings, and bridges.

b. Rancidity:

• Definition: Oxidation of fats and oils in food, leading to unpleasant taste and smell.
• Prevention Methods:
  • Adding antioxidants.
  • Storing in refrigerators or airtight containers to slow oxidation.

6. Key Vocabulary

• Combustion: Chemical reaction with oxygen, releasing energy.
• Respiration: Biological combustion of glucose in cells to release energy.
• Redox Reaction: Involving both oxidation and reduction.
• Oxidizing Agent: Substance that causes another to oxidize, itself gets reduced.
• Reducing Agent: Substance that causes another to reduce, itself gets oxidized.
• Corrosion: Chemical degradation of metals due to reaction with environment.
• Rancid: Oxidized organic material, particularly fats and oils.