REDOX REACTIONS

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Redox Reactions – Topper’s Notes
Date: 26/01/26
Page No: 01

REDOX REACTIONS (रेडॉक्स अभिक्रियाएं)

Chemistry is essentially the study of redox systems.
Redox का मतलब है ऐसी reactions जहां Oxidation & Reduction occur simultaneously (एक साथ होते हैं)।

1. Classical Concept (पुरानी अवधारणा)

  • Oxidation: Addition of Oxygen ($O_2$) OR removal of Hydrogen ($H_2$).
    Eg: $2Mg + O_2 \rightarrow 2MgO$ (Add of O)
  • Reduction: Removal of Oxygen OR addition of Hydrogen.
    Eg: $2HgO \xrightarrow{\Delta} 2Hg + O_2$ (Removal of O)
ZnSO₄ Solⁿ Anode (-) (Oxidation) CuSO₄ Solⁿ Cathode (+) (Reduction) Salt Bridge V e⁻ flow
Fig: Daniell Cell (Electrochemical Cell)

2. Electronic Concept (इलेक्ट्रॉनिक अवधारणा) IMP!

Simply put: Electron transfer is key.

  • Oxidation: Loss of electrons ($e^-$). (De-electronation)
    $Na \rightarrow Na^+ + e^-$ (Na has lost $e^-$)
  • Reduction: Gain of electrons ($e^-$). (Electronation)
    $Cl_2 + 2e^- \rightarrow 2Cl^-$
TRICK: OIL RIG
Oxidation Is Loss
Reduction Is Gain

3. Oxidation Number (O.N.)

It is the residual charge on an atom in a molecule.

★ Golden Rules for O.N. ★

  1. Free Elements ($H_2, O_2, Na, Fe$): Always 0.
  2. Monoatomic Ions: Equal to charge ($Na^+ = +1, Mg^{2+} = +2$).
  3. Oxygen: usually -2.
    • *Exception: Peroxides ($H_2O_2$) = -1, $OF_2$ = +2.
  4. Hydrogen: usually +1.
    • *Exception: Metal Hydrides ($NaH$) = -1.
  5. Halogens ($F, Cl, Br$): usually -1.
    (But if bonded to Oxygen, Cl/Br/I can be positive!)
  6. Algebraic Sum: Sum of O.N. in a neutral molecule is 0.

Stock Notation: Representing O.N. in Roman numerals.
Eg: $AuCl_3 \rightarrow Au(III)Cl_3$, $FeO \rightarrow Fe(II)O$.

Paradox of Fractional O.N.:
Sometimes we get fractions (like $4/3$ for C in $C_3O_2$).
Reality mein, atoms have different integer states. Average fraction hoti hai.

4. Types of Redox Reactions (महत्वपूर्ण)

  • a) Combination (संयोजन)

    $A + B \rightarrow C$ (One must be elemental).
    Eg: $C(s) + O_2(g) \xrightarrow{\Delta} CO_2(g)$

  • b) Decomposition (अपघटन)

    Breakdown of a compound.
    Eg: $2KClO_3(s) \xrightarrow{\Delta} 2KCl(s) + 3O_2(g)$
    (KClO3 mein O is -2, becomes 0 in O2)

  • c) Displacement (विस्थापन)

    Metal Displacement: A better reducing metal displaces another.
    $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$
    Non-metal Displacement: Metals displacing H from acid/water.
    $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

  • d) Disproportionation (असमानुपातन)

    Special reaction where the SAME element is Oxidised AND Reduced.
    Eg: $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$
    Oxygen starts at -1. Goes to -2 (water) and 0 (oxygen gas).

5. Balancing Redox Reactions (संतुलन)

Two methods exist. Exam mein koi bhi use kar sakte ho.

Method 1: Oxidation Number

  1. Write skeletal equation.
  2. Identify elements undergoing change in O.N.
  3. Calculate increase/decrease per atom.
  4. Equalize changes by multiplying.
  5. Balance ionic charges by adding $H^+$ (acidic) or $OH^-$ (basic).
  6. Balance H atoms by adding $H_2O$.

Method 2: Half Reaction

  1. Separate into Oxidation Half & Reduction Half.
  2. Balance atoms (except O and H).
  3. Balance O by adding $H_2O$.
  4. Balance H by adding $H^+$ (acidic).
  5. Balance Charge by adding electrons ($e^-$).
  6. Add halves so $e^-$ cancel out.

6. Electrode Potential ($E^\ominus$)

The tendency of an electrode to lose or gain electrons.
Standard Electrode Potential ($E^\ominus$): measured at 298K, 1M conc.

  • By convention, Hydrogen Electrode (SHE) is 0.00 V.
  • Negative $E^\ominus$ means stronger Reducing Agent than $H_2$ (Eg: Zn = -0.76V).
  • Positive $E^\ominus$ means weaker Reducing Agent (Eg: Cu = +0.34V).

Activity Series: Zn > Cu > Ag

By Alfaz
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