Redox Reactions: A Quick Revision

Redox reactions are chemical reactions where both oxidation and reduction occur simultaneously. The term "redox" is a combination of "reduction" and "oxidation."

Key Concepts

• Oxidation: The loss of electrons by a species.
• Reduction: The gain of electrons by a species.
• Oxidizing agent: A substance that accepts electrons from another substance.
• Reducing agent: A substance that donates electrons to another substance.
• Oxidation number: A numerical value assigned to an atom in a compound to represent its apparent charge.

Rules for Assigning Oxidation Numbers

1. Free elements: Oxidation number of a free element is 0.
2. Monatomic ions: Oxidation number of a monatomic ion is equal to its charge.
3. Hydrogen: Usually +1, except in metal hydrides where it is -1.
4. Oxygen: Usually -2, except in peroxides (-1) and superoxides (-1/2).
5. Fluorine: Always -1.
6. Other halogens: Usually -1, except when combined with oxygen or a more electronegative halogen.
7. Sum of oxidation numbers: In a neutral compound, the sum of oxidation numbers of all atoms is 0. In a polyatomic ion, the sum is equal to the charge of the ion.

Balancing Redox Reactions

1. Half-reactions: Divide the overall equation into two half-reactions, one for oxidation and one for reduction.
2. Balance atoms: Balance the atoms other than hydrogen and oxygen in each half-reaction.
3. Balance oxygen: Add H₂O molecules to balance oxygen atoms.
4. Balance hydrogen: Add H⁺ ions to balance hydrogen atoms.
5. Balance charge: Add electrons to balance the charge in each half-reaction.
6. Equalize electrons: Multiply the half-reactions by appropriate coefficients to ensure that the number of electrons lost in one half-reaction equals the number of electrons gained in the other.  
7. Combine half-reactions: Add the balanced half-reactions together.
8. Simplify: If necessary, simplify the final equation by canceling out any common terms.

Examples of Redox Reactions

• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O
• Corrosion: 4Fe + 3O₂ → 2Fe₂O₃
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Electrolysis: 2H₂O → 2H₂ + O₂

Remember: Redox reactions are fundamental to many chemical processes and are essential for understanding various phenomena in nature and industry.