Lecture 4 – Oxidation, Reduction & Oxidation state

Oxidation and Reduction:

Table 3:

In terms of:	Oxidation	Reduction
Oxygen	Gain	Loss
Hydrogen	Loss	Gain
Electrons	Loss	Gain
Oxidation State	Increase	Decrease

Oxidation state:

Oxidation state is the number assigned to an element in a chemical combination of a compound. This shows the number of electron involved in bonding, to form compounds. It is usually equal to the valency of the element.

i.                    The oxidation state of an element in uncombined form is always 0.

ii.                  The oxidation state of a compound is always 0.

Examples:

Calculate the oxidation state of:

i.                 C in CO₂

Let the oxidation state of C be x.

The oxidation state of O is generally -2

The oxidation state of a compound is always 0

Therefore;

x + 2(-2) = 0

x - 4 = 0

x = 4

Oxidation state of C in CO₂ is +4.

ii.                  H in H₂O

Let the oxidation state of H be x.

The oxidation state of O is generally -2

The oxidation state of a compound is always 0

Therefore;

2x + (-2) = 0

2x - 2 = 0

x = 2/2 = +1

Oxidation state of H in H₂O is +1.

iii.                S in SO₃

Let the oxidation state of S be x.

The oxidation state of O is generally -2

The oxidation state of a compound is always 0

Therefore;

x + 3(-2) = 0

x - 6 = 0

x = 6

Oxidation state of S in SO₃ is +6.

iv.               Fe in Fe₂O₃

Let the oxidation state of Fe be x.

The oxidation state of O is generally -2

The oxidation state of a compound is always 0

Therefore;

2x + 3(-2) = 0

2x - 6 = 0

x = 6/2 = 3

Oxidation state of Fe in Fe₂O₃ is +3.

v.                 Fe in FeO

Let the oxidation state of Fe be x.

The oxidation state of O is generally -2

The oxidation state of a compound is always 0

Therefore;

x + (-2) = 0

x - 2 = 0

x = 2

Oxidation state of Fe in FeO is +2.