PRELIMS

Oxidation, Reduction, and Chemical Reactions

Chemical reactions and redox processes are fundamental to chemistry.

Oxidation and Reduction (Redox)

1. Oxidation (Classical): Addition of Oxygen; removal of Hydrogen; addition of electronegative element; removal of electropositive element.
2. Reduction (Classical): Addition of Hydrogen; removal of Oxygen; addition of electropositive element; removal of electronegative element.
3. Modern Concept (Electron Transfer):
   • Oxidation: Loss of electrons ( ).
   • Reduction: Gain of electrons ( ).
4. Agents:
   • Oxidising Agent (O.A.): Substance that undergoes reduction (e.g., ).
   • Reducing Agent (R.A.): Substance that undergoes oxidation (e.g., ).
5. Redox Reaction: Reaction where both oxidation and reduction occur simultaneously.

Oxidation Number (O.N.)

The charge present on an atom in a molecule or ion.

• Rules for Determination:
  1. Free state atom = 0.
  2. Alkali Metals (Group 1) = +1.
  3. Alkaline Earth Metals (Group 2) = +2.
  4. Hydrogen = +1 (ion) or -1 (hydride).
  5. Oxygen = -2 (oxide) or -1 (peroxide).
  6. Sum of O.N. in a neutral molecule = 0.
  7. Sum of O.N. in an ion = Magnitude of charge.
• Example Calculation:
  • Mn in : (+1) + x + (-2 * 4) = 0 .
  • Cr in : (+1 * 2) + 2x + (-2 * 7) = 0 .
  • C in : .

Balancing Redox Reactions (Basics)

• Oxidation Number Method: Identify atoms whose oxidation number changes; equalize the total increase and decrease in oxidation number by multiplying with suitable coefficients; balance remaining atoms and charges.
• Half-Reaction (Ion-Electron) Method: Split the reaction into separate oxidation and reduction half-equations; balance atoms and charges in each half (using and as needed); equalize electrons lost/gained; add the two halves.

Classification of Chemical Reactions

1. Decomposition: One compound breaks into two or more components (e.g., ).
2. Combination: Compounds formed by combination of two or more elements (e.g., ).
3. Displacement: One element replaces another in a compound (e.g., ).
4. Disproportionation: Only one substance is both oxidised and reduced simultaneously (e.g., ).
5. Substitution (Organic): One or more atoms/groups are replaced in an organic molecule (e.g., Ethyl chloride to Ethyl alcohol).
6. Neutralisation: Acid + Base Salt + Water ( ).
7. Reversible: Reactants combine to form products and products recombine to reactants (e.g., Haber's process for ).
8. Irreversible: Proceeds in only one direction (e.g., calcination of ).

UPSC Relevance

Redox reactions are core to battery technologies and environmental chemistry.

• Oxidation Number: Essential for balancing redox reactions and determining the oxidation state in coordination compounds.
• Reaction Types: Understanding the difference between decomposition and disproportionation is key for chemistry MCQs.
• Industrial Links: Haber's process (reversible) and Neutralisation in wastewater treatment are common application-based questions.