Oxidation and reduction

Classical concept of oxidation and reduction

Oxidation

A chemical process that involves the addition of oxygen or any other electronegative species or removal of hydrogen or any other electropositive species.

i. Addition of oxygen:
2Mg + O₂ → 2MgO (Mg is oxidized to MgO).

ii. Addition of electronegative species:
Ca + Cl₂ → CaCl₂ (Ca is oxidized to CaCl₂).

iii. Removal of hydrogen:
H₂S + Cl₂ → 2HCl + S (H₂S is oxidized to S).

iv. Removal of electropositive species:
2KI + Cl₂ → 2KCl + I₂ (KI is oxidized to I₂)

Reduction

A chemical process that involves the addition of hydrogen or any other electropositive species or removal of oxygen or any other electronegative species.

i. Addition of hydrogen:
H₂ + Cl₂ → 2HCl (Cl₂ is reduced to HCl)

ii. Addition of electropositive species:
2KI + Cl₂ → 2KCl + I₂ (Cl₂ is reduced to KCl)

iii. Removal of oxygen
ZnO + C → Zn + CO (ZnO is reduced to Zn)

iv. Removal of electronegative species
2FeCl₃ + SnCl₂ → 2FeCl₂ + SnCl₄ (FeCl₃ is reduced to FeCl₂)

Modern or electronic concept of oxidation and reduction

Oxidation is the process in which there is loss of electron.

i. Loss of electron
Mg → Mg⁺⁺ + 2e^–

ii. Increase in positive charge
Fe⁺⁺ → Fe⁺⁺⁺ + e^–

Reduction is the process in which there is a gain of an electron.

i. Gain of electron
Cu⁺⁺ + 2e^– → Cu

ii. Decrease in positive charge
Fe⁺⁺⁺ + e^– → Fe⁺⁺

Oxidation number

It is the charge which appears to be associated with the atom on removing all the other atoms as ion from the compound.

Rules for assignment of oxidation number:

1. The oxidation number of an element in an elementary or free state is zero.
2. The algebraic sum of the oxidation number of all the elements in the compound is zero.
3. The algebraic sum of oxidation numbers of all the elements in an ion is equal to the charge on the ion.
4. The oxidation of alkali metal is +1 and alkaline earth metal is +2 in all the compounds.
5. The oxidation number of hydrogen in compounds is +1 except in metallic hydrides (-1).
6. The oxidation number of oxygen in compounds is -2 except in peroxide (-1) and in superoxides (-1/2).

Calculate the oxidation number of underlined elements

i. HNO₃
Let oxidation number of N = x
1 + x + (-2).3 = 0
x = +5

ii. K₂Cr₂O₇
Let ON of Cr = x
+2 + 2x -14 = 0
x = +6

Difference between oxidation number and valency

Oxidation number	Valency
It is the charge which appears to be associated with the atom on removing all the other atoms as ion from the compound.	It is the combining capacity of an atom of element.
It can be a whole number or fractional value.	It is a whole number.

Oxidation and reduction in terms of oxidation number
Oxidation = Increase in oxidation number
Reduction = Decrease in oxidation number

Redox reaction

The reaction in which oxidation and reduction occur simultaneously is called the redox reaction.

Oxidation half-reaction: Sn⁺² → Sn⁺⁴ + 2e^–
Reduction half-reaction: Fe⁺³ + e^– → Fe⁺²

Oxidizing and reducing agents

The agent which oxidizes others but itself get reduced is called an oxidizing agent (oxidant).
The agent which reduces other but itself get oxidized is called the reducing agent ( reductant).

Oxidizing agent: FeCl₃
Reducing agent: SnCl₂

Oxidizing agent: H₂S
Reducing agent: KMnO₄

Auto-redox reaction

A chemical reaction in which the same species undergoes oxidation as well as reduction is called a disproportional reaction or auto-redox reaction.

Steps for balancing redox reaction by oxidation number method

1. Write the skeleton redox reaction and write the oxidation number of those elements whose oxidation number is changed.
2. Balance those elements whose oxidation number is changed.
3. Calculate the change in oxidation number per mole and multiply by the criss-cross method.
4. Balance all other element and balance hydrogen and oxygen in the end.

Balance the following reaction by oxidation number method

1. Cu + HNO₃ → Cu(NO₃)₂ + NO + H₂O

2. P₄ + NaOH + H₂O → PH₃ + NaH₂PO₂

Balancing redox reaction by ion electron method.

1. Fe⁺⁺ + Cr₂O₇^— → Fe⁺⁺⁺ + Cr⁺⁺⁺
solution:

i. Splitting the reaction into oxidation half and reduction half

Oxidation half: Fe⁺⁺ → Fe⁺⁺⁺
Reduction half: Cr₂O₇^— → Cr⁺⁺⁺

ii. (a) Balancing oxidation half:

Fe⁺⁺ → Fe⁺⁺⁺ + e^–

(b)Balancing reduction half:

iii. Adding oxidation half and reduction half reactions

This reaction is in an acidic medium. To balance in an alkaline medium, OH should be introduced.

Some Important Questions

1. Define oxidation and reduction in terms of classical and electronic concept.
2. What is a redox reaction? Oxidation and reduction occur simultaneously. Justify.
3. Define oxidant and reductant in terms of classical, electronic and oxidation number.
4. What is a disproportional reaction? Write any two examples.
5. Balance the following reactions by oxidation number method: