Oxidation & Reduction

Oxidation & Reduction

Oxidation (old concept):

Oxidation is a process which involves either of the following –

(i)   addition of oxygen          

(ii)  removal of hydrogen

(iii) addition of electro negative element or group.

(iv) removal of electro positive element or group.

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 Kl)

Reduction (old concept):

Reduction is a process which involves either of the following –

(i)   addition of hydrogen

(ii)  removal oxygen

(iii) addition of electro positive element or group.

(iv) removal of electronegative element or group.

H₂ + Cl₂  →  2HCl         (reduction of Cl₂)

CuO + C  →  Cu + CP   (reduction of CuO)

HgCl₂ + Hg  →  Hg₂Cl₂ (reduction of HgCl₂)

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

Modern concept of oxidation and Reduction:

According to modern concept, loss of electrons is called oxidation whereas gain of electrons is called reduction. Example:

Na  →  Na⁺ + e     (oxidation of Na)

Zn  →  Zn²⁺ + 2e  (oxidation of Zn)

Cl₂ + 2e  →  2Cl^–  (reduction of Cl₂)

S + 2e  →  S^2–       (reduction of S)

Oxidising agent (O.A.):

A substance which undergoes reduction called oxisising agent

Cuo + C  →  Cu + CO

Oxidation – C, Reduction – CuO,

Example – O₂, O₃, H₂O₂, KMnO₄, K₂Cr₂O₇ etc.

Reduction agent (R.A.):

A substance which undergoes oxidation is called reducing agent.

H₂O + C  →  CO + H₂

Oxidation – C, Reduction – H₂O, Reduction agent - C

Example – H₂, CO, H₂S, SO₂, C, SnCl₂ etc.

Redox Reaction:

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

Example –

CuO + C  →  Cu + CO

Oxidation – C, Reduction – CuO

Oxidation Number (O.N.):

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

Rules for determination of oxidation number:

(i)  Oxidation number of an atom in free state is zero.

(ii) Oxidation number of alkali metals (li, Na, K, Rb, Cs) in molecule is always + 1.

(iii) Oxidation number of alkaline earth metals (Be, Mg, Ca, Sr, Ba) in a molecule is always +2

(iv) Oxidation number of hydrogen – (+1) hydrogen ion, (–1) hydride ion

(v) Oxidation number of oxygen – (–2) oxide, (–1) peroxide, – 1/2 superoxide

(vi) Sum of Oxidation number of atoms in a molecule is equal to zero.

(vii) Sum of oxidation number of atoms in a ion is equal to magnitude of charge with sign.

Oxidation Number of Mn in KMnO₄:

Let O.N. of Mn = X

1 + X + (–2) x 4 = 0

1 + X – 8 = 0

X = +7

Oxidation Number of Cr in K₂Cr₂O₇:

Let O.N. of Cr = X

1 x 2 + X x 2 + (–2) x 7 = 0

2 + 2X – 14 = 0

X = +6

Oxidation Number of C in C₁₂H₂₂O₁₁:

X x 12 + 1 x 22 + (–2) x 11 = 0

12X + 22 – 22 = 0

X = 0

Types of Reactions:

1.  Decomposition reactions:

In these reactions, compound either of its own or upon heating decomposes to give two or more components out of which at least one is in the elemental state.

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.

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 / ion present in a compound gets replaced by an atom / ion of another element.

FeSO₄ (aq) + Zn (s)  →  ZnSo₄ (aq) + Fe (s)

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

4. Disproportionate reaction:

The chemical reaction in which only one substance is oxidized as well as reduced simultaneously is called disproportionation reaction.

Cl₂ + 2NaOH  →  NaCl + NaOCl + H₂O

P₄ + NaOH + 2H₂O  →  2NaH₂PO₂ + 2PH₃

5. Substitution reaction:

In these reactions one or more atoms or groups present in organic molecule get substituted or replaced by suitable atoms or groups.

C₂H₂Cl (Ethyl chloride) + KOH (aq)  →  C₂H₅OH (Ethyl alcohol) + KCl

6. Neutralisation reaction:

When an acid reacts with a base, salt and water is formed. This reaction is called neutralization reaction.

acid + base  →  salt + water

HCL + NaOH  →  NaCl + H₂O

7. Reversible reaction:

A reaction in which rectants combine to form products  and again products recombine to reactants is called reversible reaction.

N₂ (g) + 3H₂ (g) →←  2NH₃ (g)

8. Irreversible reaction:

A reaction which proceeds in only one direction is called irreversible reaction.

CaCO₃ (s)  →∆  CaO (s) + CO₂ (g)