Chemical reactions which involve both oxidation as well as reduction process simultaneously, are known as redox reactions (‘red’) from reduction and ‘ox’ from oxidation). All these reactions are always accompanied by energy change in the form of heat, light or electricity.
Types of Redox Reactions:
(i) Intermolecular redox reactions: In such reactions, oxidation and reduction take place separately in two compounds. e.g.,
(ii) Intramolecular redox reactions: In these reactions, oxidation and reduction take place in a single compound. e.g.,
(iii) Disproportionation reactions: These reactions involve reduction and oxidation of same element of a compound. e.g.,
This reaction is also known as auto-redox reaction.
Classification of Redox Reactions:
1. Direct Redox Reactions-
Chemical reaction in which oxidation, as well as reduction, is carried out simultaneously in the same container is known as direct redox reaction. In such reactions, energy is generally liberated in the form of heat energy.
2. Indirect Redox Reactions-
A reaction in which oxidation and reduction are carried out separately in two separate half-cells, is known as indirect redox reaction. In such reactions, energy is generally liberated in the form of electrical energy.
Oxidation and Reduction
Addition of oxygen to an element of compound, of the removal of hydrogen from a compound, eg.,
2Mg + 02 -------- > 2MgO
2H2S + 02 ---------- > 2H20 + 2S
Addition of hydrogen to an element or compound, or the removal of oxygen from a compound, eg.,
H2S + Cl2 ---------> 2HCI + S
Fe2O3 - 3C0 -----------> 2Fe + 3C02
Addition of electronegative element or removal of any other electropositive element.
Zn+ S —> ZnS
2KI + Cl2 —> 2KCI + l2
Addition of electropositive element or removal of any other electronegative element.
2HgCl2+ SnCI4---> Hg2Cl2 + SnCI4
SiCI4 + 4Na ---------> Si+ 4NaCi
Oxidation is the loss of electrons by an atom, ion or molecule, it is also known as de-eiedrociatron,
Zp —-> Zn2 + + 2e-
Reduction is the gain of electrons by an atom, ion or molecule. This process is known as etectronation.
Cu2+ + 2e- -----------> Cu
Oxidation involves increase in oxidation number.
Reduction involves decrease in oxidation number.
Oxidation is caused by an oxidizing agent.
Reduction is caused by a reducing agent.
Reductants and Oxidants:
Oxidant or oxidizing agent is a chemical substance which can accept one or more electrons and causes oxidation of some other species. In other words, the oxidation number of oxidant decreases in a redox reaction.
Molecules of most electronegative elements such as O2, O3, halogens.
Compounds having an element in its highest oxidation state e.g.,
K2Cr2O7, KMnO4, HCIO4, H2SO4, KCIO3, Ce(SO4)2,
Oxides of metals and non-metals such as MgO, CrO3, CO2, etc.
Reductant or reducing agent is a chemical substance which can give one or more electrons and causes reduction of some other species. In other words, the oxidation number of reductant increases in a redox reaction.
All metals such as Na, Al, Zn, etc., and some non – metals, e.g., C, S, P, H2, etc.
Metallic hydrides like NaH, LiH, KH, CaH2 etc.
The compounds having an element in its lowest oxidation state such as-
H2C2O4, FeSO4, Hg2Cl2, SnCl2, SnCl2,H2S,SO2,Na2S2O3 etc.
SO2, HNO2 and H2O2 can act both as oxidant as well as reductant.
Eq. wt. of oxidant/reductant = Molar Mass/Change in oxidation number
For disproportionation reaction:
Eq. wt. of oxidant / reductant = sum of eq wt. of two half-reactions eg.