3. 3
Oxidation Number
It is defined as the imaginary charge left on the atom when all other atoms of the
compound have been removed in their usual oxidation states that are assigned
according to set of rules.
A term that is often used interchangeably with oxidation number is oxidation state
1) The oxidation state of a free element (i.e. in its uncombined state) is zero.
Example : each atom in H2, Cl2, Na, S8 have the oxidation number of zero.
2) For a monatomic ion, the oxidation state is equal to the net charge on the ion.
Example : The oxidation number of sodium in Na+ is +1.
The oxidation number of chlorine in Cl – is –1.
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3) The algebraic sum of oxidation states of all atoms in a molecule is equal to zero,
while in ions, it is equal to the net charge on the ion.
Example:
In H2SO4,
2 × (oxidation number of hydrogen) + (oxidation number of S)+ 4 (oxidation number
of oxygen) = 0 .
In SO4
2–,
(oxidation number of S) + 4 (oxidation number of oxygen) = – 2.
4) Hydrogen has an oxidation number of +1 in all its compounds except in metal
hydrides where it has – 1 value.
Example:
Oxidation number of hydrogen in hydrogen chloride (HCl) is + 1.
Oxidation number of hydrogen in sodium hydride (NaH) is –1.
5) Fluorine has an oxidation state of – 1 in all its compounds.
5. 5
6) The oxidation state of oxygen in most compounds is –2. Exceptions are peroxides,
super oxides and compounds with fluorine.
6. 6
7) Alkali metals have an oxidation state of + 1 and alkaline earth metals have an oxidation state of + 2 in
all their compounds.
Calculation of oxidation number using the above rules.
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Decrease in valency When there is no change in valency it means
there is no oxidation or reduction, e.g.,
BaCl2 + H2SO4 → BaSO4 + 2HCl
BaSO4 → Ba2+ + SO4
2– (No change in valency)
Key Points
8. 8
How to Remember Oxidation and Reduction
You could just memorize oxidation: lose electrons-reduction: gain electrons,
but there are other ways. There are two mnemonics to remember which
reaction is oxidation and which reaction is reductions.
The First One is OIL RIG
•Oxidation Involves Loss of electrons
•Reduction Involves Gain of electrons.
The Second Is 'LEO the Lion Says GER’
•Lose Electrons in Oxidation
•Gain Electrons in Reduction.
9. 9
Problem -1.
For the reaction:
2 AgCl(s) + H2(g) → 2 H+(aq) + 2 Ag(s) + 2 Cl-
Identify the atoms that undergo oxidation or reduction and list
the oxidizing and reducing agents.
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Solution:
The first step is to assign oxidation states to each atom in the reaction.
AgCl:
Ag has a +1 oxidation state Cl has a -1 oxidation state
•H2 has an oxidation state of zero
•H+ has a +1 oxidation state
•Ag has an oxidation state of zero.
•Cl- has a -1 oxidation state.
The next step is to check what happened to each element in the reaction.
•Ag went from +1 in AgCl(s) to 0 in Ag(s). The silver atom gained an electron.
•H went from 0 in H2(g) to +1 in H+(aq). The hydrogen atom lost an electron.
•Cl kept its oxidation state constant at -1 throughout the reaction.
11. 11
Question 1: 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 2: 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.
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Question 3: 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.
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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).
