Introduction to Redox reaction (oxidation and reduction)

 

Oxidation-reduction (Redox reaction) reactions involve the transfer of electrons between substances. They take place simultaneously because if one substance loses electrons, another must gain them. 

  Redox reactions involve two opposing yet complementary processes: 

OXIDATION AND REDUCTION.

  For example, all single-replacement reactions are redox reactions.

We often come across processes like;

Rusting of iron particles, 

Fading of the colour of the clothes, 

Burning of combustible substances such as cooking gas, wood, coal, etc. 

  All these processes fall in the category of redox reactions. A large number of industrial processes such as;

Electroplating, 

Extraction of metals like aluminium and sodium,

Bleaching of wood pulp, 

Manufacture of caustic soda, etc. are also based on the redox reactions.

Redox reactions also form the basis of electrochemical and electrolytic cells. 

Electrochemistry is the study of the interchange of chemical and electrical energy.

DEFINITION OF OXIDATION & REDUCTION

Oxidation and reduction can be defined in the following ways:

IN TERMS OF ELECTRON TRANSFER

Oxidation is the loss of electrons and reduction is the gain of electrons. Since electrons are negative, the loss of electrons will appear as an increase in charge. For example, when zinc loses two electrons, its charge increases from 0 to +2. When an element gains electrons, the charge on the element appears to decrease, hence, there is a reduction of the charge. For example, chlorine gains one electron and changes from an oxidation number of 0 to -1. Metals are oxidized while non-metals are reduced.

IN TERMS OF OXYGEN TRANSFER

Oxidation is the gain or addition of oxygen and reduction is a loss or removal of oxygen.

IN TERMS OF HYDROGEN TRANSFER

Oxidation is the loss or removal of hydrogen while reduction is the addition or gain of hydrogen.

IN TERMS OF OXIDATION NUMBER

Oxidation is an increase in the oxidation number or oxidation state of an atom in a reaction while reduction is a decrease in the oxidation number or oxidation state of an atom in a reaction.

IN TERMS OF ELECTRONEGATIVE AND ELECTROPOSITIVE ELEMENTS 

Oxidation is the addition of electronegative elements and removal of electropositive elements while reduction is the addition of electropositive elements and removal of electronegative elements.

EXAMPLES OF OXIDATION

1) Oxidation Of Carbon: Carbon is oxidized when it reacts with oxygen.

C + O2 -> CO2

2) Oxidation Of Hydrogen Sulphide: When hydrogen sulphide reacts with bromine, it loses hydrogen and becomes oxidized while bromine is reduced to hydrogen bromide.

H2S + Br2 -> 2HBr + S

3) Oxidation Of Magnesium: Magnesium is oxidized when it loses two electrons.

Mg -> Mg2+ + 2e-

When magnesium reacts with fluorine, it is oxidized to magnesium fluoride while fluorine is reduced due to the addition of magnesium which is an electropositive element.

Mg + F2 -> MgF2

4) Oxidation Of Iodine: When potassium iodide reacts with hydrogen peroxide, iodine becomes oxidized due to the removal of potassium which is an electropositive element.

2KI + H2O2 -> I2 + 2KOH

5) Oxidation Of Iron: When iron reacts with sulphur, it is oxidized due to the addition of sulphur which is an electronegative element.

Fe + S -> FeS

EXAMPLES OF REDUCTION

1) Reduction Of Ferric Chloride: When ferric chloride reacts with hydrogen, it is reduced to ferrous chloride due to the removal of chlorine which is an electronegative element.

2F3Cl3 + H2 -> 2F3Cl2 + 2HCl

2) Reduction Of Lead Oxide: When lead oxide reacts with carbon, it is reduced to lead while carbon is oxidized to carbon monoxide.

PbO + C -> Pb + CO

3) Reduction Of Mercuric Chloride: When mercuric chloride reacts with tin (ii) chloride (stannous chloride), it is reduced to mercury (i) chloride (mercurous chloride) due to the addition of an electropositive element.

SnCl2 + 2HgCl2 -> SnCl4 + Hg2Cl2

4) Reduction Of Copper (Ii) Oxide: Copper (ii) oxide can be reduced to copper when it reacts with hydrogen.

CuO + H2 -> Cu + H2O

5) Reduction Of Nitrogen: When nitrogen reacts with hydrogen, it is reduced to ammonia by gaining hydrogen.

N2(g) + 3H2(g) -> 2NH3(g)

6) Reduction Of Zinc Oxide: Zinc oxide is reduced to zinc by the removal of oxygen when it reacts with carbon.

ZnO + C -> Zn + CO

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