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7.1 Introduction

Practically, all chemical reactions are carried out for the purpose of generating energy. Combusion of fuels, generation of electricity by batteries and even metabolism of food are redox (oxidation-reduction) reactions.

Oxidation : It is a process in which a substance (an ion or an atom) loses one or more electrons.

Reduction : It is a process in which a substance (an ion or an atom) gains one or more electrons.

Redox reaction : Electrons are not transferred to the surroundings in ordinary chemical reactions but electrons lost by one species, must be gained by another. Such chemical reactions involving loss of electrons by one species and the simultaneous gain by another constitute (oxidation - reduction) redox reaction.

Any redox reaction comprises two half reactions: one corresponding to oxidation and the other to reduction.


1) 2Ca + O2 ® 2CaO   Half reaction

2Ca - 4e- ® 2Ca+ 2     (Oxidation)

O2 + 4e- ® 20- 2         (Reduction)

2) Na + 1 Cl2 ® NaCl

Na - e- ® Na+           (Oxidation)

1 Cl2 + e- ® Cl-        (Reduction)

A Redox reaction can be represented as

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7.1 Introduction
7.2 Oxidation Numbers
7.3 Balancing Reactions

Chapter 8

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