Oxidation and Reduction

Classical Concept of Oxidation

The term oxidation was first used to describe chemical reactions in which oxygen was added to an element of a compound. The phenomenon of combustion was the earliest example of oxidation. Later on the term oxidation was extended to describe many more reactions which occurred without the use of even oxygen. Lets try to find out what is answer of the question, What is oxidation ! according to classical concept.

• Addition of Oxygen:  Reduction is a chemical reaction in which oxygen is removed from any chemical species (atom, ion or molecule).
  2Mg + O₂ → 2MgO 
  S + O₂ →  SO₂
  2CO + O₂ →  2CO₂
  Na₂SO₃ + H₂O₂ →  Na₂SO₄ + H₂O

• Removal of Hydrogen : Oxidation is a chemical reaction in which hydrogen is removed from any chemical species (atom, ion or molecule).  
  H₂S + Cl₂ → 2HCl + S
  4HI + O₂ →  2H₂O + 2I₂
  4HI + MnO₂ →  MnCl₂ + 2H₂O + Cl₂

• Addition of an Electronegative Element: Oxidation is a chemical reaction in which an electronegative element is added into any chemical species (atom, ion or molecule).
  Fe+ S →  FeS (Oxidation of iron)
  SnCl₂ + Cl₂ →  SnCl₄ (Oxidation of stannous chloride)
  2Fe + 3F₂ →  2FeF₃ (Oxidation of iron)

• Removal of an Electropositive Element: Oxidation is a chemical reaction in which an electropositive element is removed from any chemical species (atom, ion or molecule).  
  2KI + H₂O₂ →  2KOH + I₂  (Oxidation of potassium iodide)
  2K₂MnO₄ + Cl₂ →  2KCl + 2KMnO₄ (Oxidation of potassium manganate)
  2KI + Cl₂ →  2KCl + I₂ (Oxidation of potassium iodide)

• A substance which brings oxidation is known as oxidizing agent. 

Classical Concept of Reduction

• Removal of Oxygen:  Reduction  is a chemical reaction in which oxygen is removed from into any chemical species (atom, ion or molecule).
  CuO + C → Cu + CO    
  H₂O + C →  CO + H₂ 
  2CO + O₂ →  2CO₂ 
  Fe₃O₄ + 4H₂ →  3F₂ + 4H₂O

• Addition of Hydrogen : Reduction is a chemical reaction in which hydrogen is added to any chemical species (atom, ion or molecule).  
  Cl₂ + H₂ →  2HCl
  S + H₂ →  H₂S
  C₂H₄ + H₂ →  C₂H₆

• Removal of an Electronegative Element: Reduction is a chemical reaction in which an electronegative element is removed from any chemical species (atom, ion or molecule). 2HgCl₂ + SnCl₂ →  Hg₂Cl₂ + SnCl₄ (Reduction of mercuric chlorine)
  2FeCl₃ + H₂ →  2FeCl₂ + 2HCl (Reduction of ferric chloride)
  2FeCl₃ + H₂S →  2FeCl₂ + 2HCl + S (Reduction of ferric chloride)

• Addition of an Electropositive Element: Reduction is a chemical reaction in which an electropositive element is added any chemical species (atom, ion or molecule).  
  HgCl₂ + Hg →  Hg₂Cl₂ (Reduction of mercuric chloride)
  CuCl₂ + Cu →  Cu₂Cl₂ (Reduction of cupric chloride)

• The substance which brings reduction is known as reducing agent. 

• A substance, which undergoes oxidation, acts as a reducing agent while a substance, which undergoes reduction, acts as an oxidizing agent.

  Mg, S, Cu, Na₂SO₃, H₂S, HI, H₂, C, KI are reducing agents, while O2, Cl₂, F₂, H₂O₂, MnO₂, FeCl₃, CuCl₂, Fe₃O₄, CuO, etc., are oxidizing agents in the above examples.

• All oxidation and reduction reactions are complimentary of one another and occur simultaneously, one can not take place without the other.

• No single oxidation and no single reduction process is known. The simultaneously oxidation and reduction reactions are generally termed as redox reactions.  e.g., 2FeCl³ + SnCl² → 2FeCl² + SnCl⁴  or  2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺

  In above example iron undergoes reduction from +3 to +2 and tin undergoes oxidation from +2 to +4.

Modern Concept of Oxidation & Reduction

According to the modern concept, loss of electrons is oxidation whereas gain of electrons is reduction.

• Examples of oxidation reactions are:
  Na → Na⁺ + e
  Zn → Zn²⁺ + 2e
  Fe²⁺ → Fe³⁺ + e
  Sn²⁺ → Sn⁴⁺ + 2e
  H₂O₂ → O₂ + 2H⁺ + 2e
  2S₂O₃^2– → S₄O₆^2– + 2e
  [Fe(CN)₆]^4– → [Fe(CN)₆]^3– + e
  MnO₄^2– → MnO₄^– + e

• Examples of reduction reactions are:-
  Cl₂ + 2e → 2Cl
  S + 2e → S^2–
  Cu²⁺ + 2e → Cu
  MnO₄– + 8H⁺ + 5e → Mn²⁺ + 4H₂O
  Cr₂O₇2– + 14H+ + 6e → 2Cr3+ + 7H₂O
  H₂O₂ + 2H⁺ + 2e → 2H₂O

• Oxidation and reduction can be represented in a general way as shown below:

  M4- → M3- → M2- → M- → M0 → M+ → M2+ → M3+ → M4+

• In a redox process the valency of the involved species changes. The valency of a reducing agent increases while the valency of an oxidizing agent decreases in a redox reaction. The valency of a free element is taken as zero.

• Decrease in valency When there is no change in valency it means there is no oxidation or reduction, e.g., 
  BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl    or BaSO₄ → Ba²⁺ + SO₄^2–    (No change in valency)

Watch this Video for more reference
 

Conclusion

• Oxidation is a process in which one or more electrons are lost or valency of the element increases.

• Reduction is a process in which one or more electrons are gained or valency of the element decreases.

• Oxidizing agent is a material which can gain one or more electrons, i.e., valency decreases.

• Reducing agent is a material which can lose one or more electrons, i.e., valency increases.

• Redox reaction involves two half reactions, one involving loss of electron or electrons (oxidation) and the other involving gain of electrons or electrons (reduction).

Question 1: Which of the following chemical reactions represent oxidation process?

a. Cl₂ + 2e → 2Cl
b. S + 2e → S^2–
c. 2S₂O₃^2– → S₄O₆^2– + 2e
d. MnO₄– + 8H⁺ + 5e → Mn²⁺ + 4H₂O

Question 2: According to the modern concept, oxidation is the process of 

a. gain of electrons

b. lose of electrons

c. addition of hydrogen

d. removal of hydrogen

Question 3: Which of the following conditions does not represent oxidation?

a. Removal of hydrogen.

b. Addition of oxygen.

c. Removal of electropositive element.

d. Addition of electrons.

Question 4: Which of the following statements is incorrect ?

a. Oxidation is a process in which one or more electrons are lost or valency of the element increases.

b. Reduction is a process in which one or more electrons are gained or valency of the element decreases.

c. Oxidizing agent is a material which can gain one or more electrons,

d. Reducing agent is a material whose valency decreases during reaction.

Related Resources

• Click here to know the syllabus of chemistry for  IIT JEE

• Have a look at past year papers of IIT JEE

• You can refer to redox reactions. 

To read more, Buy study materials of Redox Reactions comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.