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Reduction Reaction

There are many reactions which occur in our daily life like rusting of iron, fading of color from the clothes, burning of combustible substances such as domestic use gas, wood, coal, etc. All such processes fall under the specific type of reaction called as oxidation – reduction reaction.

A large number of industrial processes like electroplating, extraction of metals like aluminum and sodium, bleaching of wood pulp, manufacture of caustic soda, etc., are also based on the redox reactions. Reduction – oxidation reactions also form the basis of electrochemical and electrolytic cells.

Carboxylic acids are reduced to primary alcohols on treatment with lithium aluminium hydride or better with diborane. Here -COOH group is reduced to -CH2OH

Reduction Reaction

Sodium borohydride does not reduce the carboxyl group.

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Reduction Reaction Definition

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Reduction reactions can be defined by two concepts :
  1. Classical or old concept.
  2. Electronic concept or new concept.

Classical Concept of Reduction Reactions

According to the classical concept, reduction reaction is defined as the addition of hydrogen or removal of oxygen.

To depict the classical concept of reduction, let us look at the following example :

Cl2 + H2

In the above reaction, Hydrogen is added to chlorine to form hydrogen chloride. Thus, chlorine gas is getting reduced to form hydrochloric acid.
H2S + Cl2 2HCl + S

Same as the first example, Hydrogen is added to chlorine by hydrogen sulfide, thereby reducing chlorine.

Reducing Agent

A substance which provides or supplies hydrogen or removes oxygen is said to be a reducing agent according to the classical concept. So, in the above example, since hydrogen sulfide is providing hydrogen to chlorine, Hydrogen sulfide is the ‘reducing agent’.

ZnO + C
Zn + CO

In the above reaction, carbon removes oxygen from zinc oxide, thereby reducing the zinc oxide.
This, carbon here is the reducing agent.

Fe2O3 + 3CO
2Fe + 3CO2

Carbon monoxide acts as a reducing agent and takes away oxygen from Fe2O3.

Oxidation Reduction Reactions

With respect to the classical concept, oxidation involves addition of oxygen and removal of hydrogen. It is the complete opposite of reduction.
C + O2 CO2

In the above reaction, Oxygen is added to carbon, thereby oxidizing it.

2Mg + O2

Oxygen is added to magnesium to convert it into its oxide, thereby oxidizing it.
Example of an oxidation reaction proceeding with removal of hydrogen is,

H2S + Cl2
2HCl + S

Hydrogen is removed from sulfur. So, sulfur is getting oxidized.

An oxidizing agent is one, which supplies oxygen or removes hydrogen from another reactant.Since chlorine is removing hydrogen from hydrogen sulfide, chlorine is the oxidizing agent.

MnO2 + 4HCl
MnCl2 + Cl2 + 2H2O

MnO2 supplies oxygen in the above example. So, manganese dioxide is an oxidizing agent.

Electronic Concept of Reduction

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A new concept, the electronic concept explains reduction reaction more clearly. These are defined in terms of electron transfer between the reactants, known as electronic concept of oxidation and reduction.

According to this concept, reduction is a process in which an atom or group of atoms taking part in chemical reaction gains one or more number of electrons. The eventual gain of electrons results in the decrease in positive charge or increase of negative charge of the species.

Examples of Reduction Reactions

Gain of electrons resulting in decrease in positive charge

Fe3+ + e- Fe2+

Iron (III) ion gains one electron and changes to Iron (II) ion, and hence gets reduced.

Sn4+ + 2e- Sn2+

Tin (IV) gets reduced to tin (ii) ion, with the addition of electrons.

Gain of electrons resulting in increase in negative charge

MnO4- + e- MnO42-

With the gain of electrons, MnO4- is reduced to MnO42-.

With the increase in negative charge, the oxidation number is decreased.

Reduction Reaction Example

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Na + Ag+ Na+ + Ag

In this reaction, sodium reduces silver ion to silver. So, sodium is the reducing agent.

Silver ion, on the other hand, has oxidized sodium to sodium ion (increase in oxidation number). Silver, therefore, is the oxidizing agent. This is a redox reaction since reduction and oxidation take place together.

Occurrence of Redox Reactions

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We know that oxidation involves loss of electrons and reduction involves gain of electrons. So, if a species is losing electron, some other species should gain these electrons, and vice versa. Therefore, to make a reaction effective, oxidation and reduction should always go hand in hand. Both the processes are complementary and proceed simultaneously.

These reactions, where reduction and oxidation go hand in hand are called as ‘REDOX’ reactions, or reduction- oxidation reactions. In a REDOX reaction, there is transference of electron from the reducing agent to the oxidizing agent as shown.
Redox reaction

The above reaction depicts the fact that an ion/compound acting as a reducing agent gets oxidized, and vice versa.

Summarizing the Whole Concept

  1. Oxidation : Loss of electrons.
  2. Reduction : Gain of electrons.
  3. Oxidizing agent : Species which gains electrons.
  4. Reducing agent : Species which loses electrons.

Electronic Concept of Oxidation

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It is a process in which an atom or group of atoms taking part in the chemical reaction loses one or more than one electrons. The eventual loss of electrons results in the increase of positive charge or decrease of negative charge of the species.

Oxidation due to loss of electrons resulting in increase in positive charge

Example: 1
Na Na+ + e-

Sodium has lost an electron and the oxidation number has increased from zero to 1.

Example: 2
Sn2+ Sn4+ + 2e-

Tin loses two electrons and gains 2+ charges, thereby increasing its oxidation state from +2 to +4.

Oxidation due to loss of electrons resulting in decrease of negative charge
2Cl$\to $ Cl2 + 2e-

Chlorine ion had a charge of negative one. This is increased to zero, thereby oxidizing chlorine ion to chlorine.

From the above examples, it is clear that,

Reducing agent is a species which loses electrons during a chemical reaction, thereby getting oxidized itself. Reducing agents are also called as Reductants.

An oxidizing agent on the other hand, is a species which gains electrons during a reaction and gets reduced. These are also called as Oxidants.
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