Note on The Concept Of Oxidation & Reduction

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Oxidation & Reduction

Oxidation can be defined as loss of electron or gain of oxygen or removal of hydrogen. The term reduction indicates a gain of electron or gain of hydrogen or removal of oxygen. Oxidation and reduction can be simply understood by the term LEOGER which means Loss of Electron Oxidation Gain of Electron Reduction.

There are 2 concepts that define Oxidation and Reduction

  1. Classical Concept of Oxidation and Reduction
  2. Electronic Concept of Oxidation and Reduction

Classical Concept of Oxidation and Reduction

According to classical concept, oxidation is

  1. The addition of oxygen or electronegative radical/element.
  2. Removal of hydrogen or electropositive radical/element.

$${2MgO+O_2\longrightarrow2MgO} \space (addition\space of\space oxygen)$$

$${FeCl_2}+3/2Cl_2\longrightarrow{Fecl_2}\space (addition\space of\space electronegative\space element)$$

$${H_2S}+Cl_2\longrightarrow{HCl+S}\space (removal\space of\space hydrogen)$$

$${Hg_2Cl_2\longrightarrow{HgCl_2}+Hg}\space (removal\space of\space elecetropositive\space element)$$

According to classical concept, reduction is

  1. Addition of hydrogen or electropositive radical/element.
  2. Removal of oxygen or electronegative radical/element.

$${NH_3}+{HCl}\longrightarrow{NH_4Cl}\space (addition\space of\space hydrogen)$$

$${2Na+H_2O}\longrightarrow2NaOH\space (addition\space of\space electropositive\space element)$$

$${2KClO_3\longrightarrow2KCl+3Cl_2}\space (removal\space of\space oxygen)$$

$${H_2S\longrightarrow{H_2}+S}\space (removal\space of\space electropositive\space element)$$

Electronic Concept of Oxidation & Reduction

According to Electronic Concept Oxidation is defined as

  1. Loss of electron

a. Increase in positive charge.

b. Decrease in negative charge.

$$Mg\longrightarrow Mg^{++}+2e^{-} (loss\space of\space electron)$$

$$Fe^{++}\longrightarrow Fe^{+++}+e^{-} (increase\space in\space positive\space charge$$

$$2Cl^{-}\longrightarrow{Cl_2}+2e^{-} (decrease\space in\space negative\space charge)$$

According to Electronic Concept, reduction is defined as

  1. Gain of electron

a. Decrease in positive charge

b. Increase in negative charge

$$O+2e^{-}\longrightarrow{O}^{--} (gain\space of \space electron)$$

$$Fe^{+++}+e^{-}\longrightarrow{Fe}^{++} (decrease\space in\space positive\space charge) $$

$$S+2e^{-}\longrightarrow{S}^{--} (increase\space in\space negative\space charge$$

Classification

  1. Redox reaction
  2. Disproportionation reaction

A. Redox reaction

According to modern concept, Oxidation means loss of electron and Reduction means a gain of an electron. If one chemical compound loses electron then other must be gaining it which shows oxidation and reduction occur in the same reaction. In a chemical reaction oxidation and reduction occurs simultaneously.

For e.g

$$Zn+H_2SO_4\longrightarrow{ZnSO_4}+H_2\uparrow$$

$$Zn+2H^{+}+SO_4^{--}\longrightarrow{Zn}^{++}+SO_4^{--}+H_2$$

Oxidation Half Reduction half

$$Zn\longrightarrow{Zn}^{++}+2e^{-} (loss\space of\space electron)$$

$$2H^{+}+2e^{-}\longrightarrow{H_2} (increase\space in\space negative\space charge)$$

Oxidation & Reduction In Terms Of Oxidation Number (O.N)

According to this concept, the increase in oxidation number means Oxidation (loss of the electron) whereas the decrease in oxidation number means Reduction (gain of electron).

$${Zn+H_2SO_4\longrightarrow{ZnSO_4}+H_2}$$

The O.N of Zn increased from 0 to 2 i.e Zn got oxidized.

The O.N of H2 decreased from 2 to 0 i.e H2 got reduced.

Oxidation Number & Valency

  1. When all other atoms of a compound are removed, the remaining charge of an atom is known as oxidation number whereas the losing gaining or sharing capacity of an element during the chemical reaction is called valency.
  2. Oxidation number can be a positive, negative or fractional number but except for inert gases i.e 0, the valency for all element is positive.
  3. The oxidation number of the same element differs in the different compound like O.N of carbon in CO, CH4, CH2Cl2 are +2, +4, 0 respectively whereas valency of an element is usually same like valency of carbon in CO, CH4, CH2Cl2 is 4. However, many elements show multiple valencies like valency of copper is 2 or 3.

Oxidizing Agent (Oxidant) & Reducing Agent (Reductant)

1. Oxidizing Agent (Oxidant)

a. Classical concept

The chemical substance/species (atoms or compounds) which can oxidize other (increase in positive charge) & itself gets reduced (decrease in positive charge). Such substances are called oxidizing agent or oxidant.

b. Electronic Concept

The chemical substance/species (atoms or compounds) that accepts electron & decrease its own oxidation number is known as oxidizing agent or oxidant.

For e.g. KMnO4, K2Cr2O7, concentrated HNO3, concentrated H2SO4 etc.

2. Reducing Agent (Reductant)

a. Classical Concept

The chemical substance/species (atoms or compounds) which can reduce other (increase negative charge) & itself gets oxidized (decrease in negative charge). Such substances are called reducing agent or reductant.

b. Electronic Concept

The chemical substance/species (atoms or compound) that donate the electron and increase its own oxidation number is known as reducing agent or reductant.

For eg. Lithium aluminum hydride (LiAlH4), all metals etc.

This concept can be described by the following reaction;

$${HNO_3+H_2S}\longrightarrow{NO}+{H_2S+S}$$

Here, HNO3 acts as an oxidizing agent cause it oxidized H2S to S i.e it increased the oxidation number of sulphur from -2 to 0 whereas H2S acts as a reducing agent cause it reduces HNO3 to NO i.e it decreased the oxidation number from +5 to +2.

B. Disproportionate reaction

The redox reaction (i.e both oxidation and reduction happens in the same reaction) in which same chemical substance/species act as oxidizing as well as reducing agent. Such type of reaction is called disproportionate reaction.

This concept can be described by the following reaction

$$2NaOH+Cl_2\longrightarrow{NaCl}+NaClO+H_2O$$

In above reaction, Cl2 gets reduced into NaCl i.e oxidation number of Cl2 decreased from 0 to -1.

In the same reaction, Cl2 gets oxidized into NaClO i.e oxidation number of Cl2increased from 0 to1.

In the above reaction, Cl2 acts as both oxidizing & reducing agent. Hence, the reaction is a disproportionate reaction.

When the oxidation state of the element is intermediate i.e. intermediate between maximum and lowest then the element behaves as both oxidant and reductant.

Reference:

pulse, Tracy. Introduction to chemistry. u.s.a: flexbook, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.

  • Oxidation and reduction can be simply understood by the term LEOGER which means Loss of Electron Oxidation Gain of Electron Reduction.
  • The chemical substance/species (atoms or compounds) that accepts electron & decrease its own oxidation number is known as oxidizing agent or oxidant.
  • The chemical substance/species (atoms or compounds) which can reduce other (increase negative charge) & itself gets oxidized (decrease in negative charge). Such substances are called reducing agent or reductant.
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nirmal salami

In this Chapter Add have a Rules to calculate the oxidation number a More better.


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rules of balancing reaction through ionic form

Rules of balancing reaction through ionic form


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