Oxidation and Reduction Explained

Oxidation and Reduction Explained

Oxidation (Old Concept)

Oxidation is a process which involves either of the following:

1. Addition of Oxygen

2. Removal of Hydrogen

3. Addition of Electronegative Element or Group

4. Removal of Electropositive Element or Group

Examples:

• 2Mg + O₂ → 2MgO (oxidation of Mg)

• H₂S + Cl₂ → 2HCl + S (oxidation of H₂S)

• Fe + S → FeS (oxidation of Fe)

• 2KI + H₂O₂ → 2KOH + I₂ (oxidation of KI)

Reduction (Old Concept)

Reduction is a process which involves either of the following:

1. Addition of Hydrogen

2. Removal of Oxygen

3. Addition of Electropositive Element or Group

4. Removal of Electronegative Element or Group

Examples:

• H₂ + Cl₂ → 2HCl (reduction of Cl₂)

• CuO + C → Cu + CO (reduction of CuO)

• HgCl₂ + Hg → Hg₂Cl₂ (reduction of HgCl₂)

• 2FeCl₃ + H₂ → 2FeCl₂ + 2HCl (reduction of FeCl₃)

Modern Concept of Oxidation and Reduction

According to the modern concept:

• Oxidation is the loss of electrons.

• Reduction is the gain of electrons.

Examples:

• Na → Na⁺ + e⁻ (oxidation of Na)

• Zn → Zn²⁺ + 2e⁻ (oxidation of Zn)

• Cl₂ + 2e⁻ → 2Cl⁻ (reduction of Cl₂)

• S + 2e⁻ → S²⁻ (reduction of S)

Oxidising Agent (O.A.)

A substance which undergoes reduction is called an oxidising agent.

Example:

• CuO + C → Cu + CO

Oxidation:

• C

Reduction:

• CuO

Common Oxidising Agents:

• O₂, O₃, H₂O₂, KMnO₄, K₂Cr₂O₇

Reducing Agent (R.A.)

A substance which undergoes oxidation is called a reducing agent.

Example:

• H₂O + C → CO + H₂

Oxidation:

• C

Reduction:

• H₂O

Reducing Agent:

C

Common Reducing Agents:

• H₂, CO, H₂S, SO₂, C, SnCl₂

Redox Reaction

A reaction in which both oxidation and reduction take place simultaneously is called a redox reaction.

Example:

• CuO + C → Cu + CO

Oxidation:

• C

Reduction:

• CuO

Oxidation Number (O.N.)

The charge present on an atom in a molecule or ion is called the oxidation number. It may be zero, positive, or negative.

Rules for Determining Oxidation Number:

1. The oxidation number of an atom in a free state is zero.

2. The oxidation number of alkali metals (Li, Na, K, Rb, Cs) in a molecule is always +1.

3. The oxidation number of alkaline earth metals (Be, Mg, Ca, Sr, Ba) in a molecule is always +2.

4. The oxidation number of hydrogen is (+1) when in a hydrogen ion and (–1) when in a hydride ion.

5. The oxidation number of oxygen is (–2) in oxide, (–1) in peroxide, and (–1/2) in superoxide.

6. The sum of oxidation numbers of atoms in a molecule is equal to zero.

7. The sum of oxidation numbers of atoms in an ion is equal to the magnitude of the charge, with the sign.

Examples of Oxidation Number Calculations:

• Oxidation Number of Mn in KMnO₄:

  • Let O.N. of Mn = X
    1 + X + (–2) × 4 = 0
    1 + X – 8 = 0
    X = +7

• Oxidation Number of Cr in K₂Cr₂O₇:

  • Let O.N. of Cr = X
    1 × 2 + X × 2 + (–2) × 7 = 0
    2 + 2X – 14 = 0
    X = +6

• Oxidation Number of C in C₁₂H₂₂O₁₁:

  • X × 12 + 1 × 22 + (–2) × 11 = 0
    12X + 22 – 22 = 0
    X = 0

  • ZnO + C → Zn + CO

  • Oxidation - C

  • Reduction - ZnO

  • Reduction - agent - C

  • The oxidation number of sulphur is 4.^2-

    • Let O.N. of S = x

      • x + (-2) x 4 = -2

      • x - 8 = -2

      • x = +6

  • Oxidation number of iron in Fe(CO)₅.

    • Let O.N. of N = x

      • x + 0 x 5 = 0

      • x = 0

  • Oxidation number of nitrogen in NH₄+

    • Let O.N. of N = x

      • x + 1 x 4 = +1

      • x = +1 - 4

      • x = -3

  • Cations are formed by the loss of electrons.

    • Na (g) - e → Na⁺ (g)

    • Mg (g) - 2e → Mg ⁺⁺ (g)

  • Anions are formed by the gain of an electron. 

    • Cl (g) + e → Cl^- (g)

    • O (g) + 2e → O^2- (g).

Types of Reactions

1. Decomposition Reactions

In these reactions, a compound decomposes into two or more components, with at least one component being in the elemental state.

Example:

• 2NaH (s) → ∆ Na (s) + H₂ (g)

• 2H₂O (l) → ∆ 2H₂ (g) + O₂ (g)

2. Combination Reactions

In combination reactions, compounds are formed as a result of the chemical combination of two or more elements.

Examples:

• H₂ (g) + 1/2 O₂ (g) → H₂O₂ (l)

• C (s) + O₂ (g) → CO₂ (g)

• 3Mg (s) + N₂ (g) → Mg₃N₂ (s)

3. Displacement Reactions

In these reactions, an atom or ion in a compound is replaced by an atom or ion of another element.

Examples:

• FeSO₄ (aq) + Zn (s) → ZnSO₄ (aq) + Fe (s)

• MgO (aq) + 2Na (s) → Na₂O (aq) + Mg (s)

4. Double Displacement Reactions

Such reactions, in which there is an exchange of ions between the reactants, are called double displacement reactions.

Example:

• _Na₂SO₄ (aq) BaCl₂ (aq) → BaSO₄ (s) + 2naCl (aq)

When sodium sulphate (Na₂SO₄) solution is treated with barium chloride solution, a white precipitate of barium sulphate (BaSO₄) is formed. This is the test of the sulphate ion, and it is an example of a double displacement reaction.

Final Thoughts

Oxidation and reduction are key chemical processes that occur together in redox reactions.

Oxidation involves the addition of oxygen, removal of hydrogen, or loss of electrons, while reduction involves the gain of electrons, addition of hydrogen, or removal of oxygen. Substances undergoing reduction act as oxidising agents, and those undergoing oxidation act as reducing agents. The oxidation number indicates the charge on an atom in a compound or ion.

Reactions are classified as decomposition, combination, displacement, or double displacement based on the exchange or formation of elements. Understanding these concepts helps explain how substances interact and change during chemical reactions.