Types of Redox Reaction

1. Combination reactions:
   

A + B → C

For such a reaction to be redox reaction, either A or B or both should be in elemental form.

Ex:

   2 C⁺²O⁻²₂ (g) + O0₂ (g) → 2 C⁺⁴ O⁻²₂ (g)

   Fe0 (s) + S0 (s) → Fe⁺²S⁻² (s)

2. Decomposition reactions:
   

Breakdown of a compound to form 2 or more products

For a decomposition to be redox reaction, one or more of the products have to be in elemental form

Ex:

3. Displacement Reactions: An ion (or atom) in a compound is replaced by an ion (or atom) of another element.

X + YZ → XZ + Y

1. Metal displacement:
   A metal in a compound is displaced by another metal in the uncombined state.
   Ex: Cu⁺² S⁺⁴ O⁻²₄ + Zn0 (s) →Cu0 (S) + Zn⁺² S⁺⁴ O⁻²₄ (aq)
   
   Note:
   ZnSO₄ (aq) + Cu (s) → Zn (s) + Cu SO₄ (aq)
   the above reaction does not take place as Zn is more electropositive than Cu.
   Only a more electropositive element can displace a less electropositive element from its compound
   i.e., the displacing metal should be a better reducing agent than the metal that is displaced
   
2. Non – metal displacement:
   Includes
   

1. Hydrogen displacement:
   

All alkali metals displace Hydrogen from cold water

2K (s) + 2H₂O (l) →2KOH (aq) + H₂ (g)

Alkaline earth metals Ca, Sr and Ba displace hydrogen from cold water

Ca (s) + 2H₂O (l) → Ca (OH)₂ (aq) + H₂ (g)

Less active metals like Mg & Fe react with steam to product dihydrogen gas.

Many metals displace hydrogen from acids

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

Zn (s) + H₂SO₄ (aq) → ZnSO₄ + H₂ (g)

2. Displacement of oxygen (rare):
   

2H₂O (l) + 2F₂ (g) → 4HF (aq) + O₂ (g)

3. Displacement of halogen:
   

Oxidizing power:

F > Cl > Br > I

Thus, Fluorine can replace Cl, Br & I

Chlorine can replace Br & I

Cl₂ (g) + 2KBr (aq) → 2KCl (aq) + Br₂ (l)

Bromine can replace Iodine

Br₂ (l) + 2KCl (aq) → 2KBr (aq) + I₂ (s)

4. Dispropartionation reactions:
   

• An element undergoes oxidation and reduction simultaneously.
• The reacting element undergoing reduction and oxidation simultaneously exists in the intermediate oxidation state so that both increase and decrease in oxidation is possible. Ex: Cu⁺¹₂ O (s) + H₂ SO₄ (aq) → Cu0 (s) + Cu⁺²SO₄ (aq) + H₂O (l) Here Cu is in +1 oxidation state in Cu₂O. After reacting with H₂SO₄ it forms Cu metal which has 0 Oxidation state and CuSO₄ where, its oxidation state is +2.
  So Cu undergoes oxidation to form CuSO₄ and reduction to form Cu metal.