Oxidising And Reducing Agents
Category : JEE Main & Advanced
(1) Definition : The substance (atom, ion or molecule) that gains electrons and is thereby reduced to a low valency state is called an oxidising agent, while the substance that loses electrons and is thereby oxidised to a higher valency state is called a reducing agent.
Or
An oxidising agent is a substance, the oxidation number of whose atom or atoms decreases while a reducing agent is a substance the oxidation number of whose atom increases.
(2) Important oxidising agents
(i) Molecules made up of electronegative elements.
Example : O2, O3 and X2 (halogens).
(ii) Compounds containing an element which is in the highest oxidation state.
Example : ,
etc.
(iii) Oxides of elements, etc.
(iv) Fluorine is the strongest oxidising agent.
(3) Important reducing agents
(i) All metals e.g. Na, Zn, Fe, Al, etc.
(ii) A few non-metals e.g. C, H2, S etc.
(iii) Hydracids : HCl, HBr, HI, H2S etc.
(iv) A few compounds containing anelement in the lower oxidation state (ous).
Example : etc.
(v) Metallic hydrides e.g. NaH, LiH etc.
(vi) Organic compounds like HCOOH and (COOH)2 and their salts, aldehydes, alkanes etc.
(vii) Lithium is the strongest reducing agent in solution.
(viii) Cesium is the strongest reducing agent in absence of water. Other reducing agents are and KI.
(ix) Hypo prefix indicates that central atom of compound has the minimum oxidation state so it will act as a reducing agent.
Example : (hypophosphorous acid).
(4) Substances which act as oxidising as well as reducing agents
Examples : H2O2,SO2,H2SO3,HNO2,NaNO2,Na2SO3,O3 etc.
(5) Tips for the identification of oxidising and reducing agents
(i) If an element is in its highest possible oxidation state in a compound, the compound can function as an oxidising agent.
Example : etc.
(ii) If an element is in its lowest possible oxidation state in a compound, the compound can function only as a reducing agent.
Example : etc.
(iii) If an element is in its intermediate oxidation state in a compound, the compound can function both as an oxidising agent as well as reducing agent.
Example : etc.
(iv) If a highly electronegative element is in its highest oxidation state in a compound, that compound can function as a powerful oxidising agent.
Example : etc.
(v) If an electronegative element is in its lowest possible oxidation state in a compound or in free state, it can function as a powerful reducing agent.
Example : etc.
(6) Equivalent weight of oxidising and reducing agents
Equivalent weight of a substance (oxidant or reductant) is equal to molecular weight divided by number of electrons lost or gained by one molecule of the substance in a redox reaction.
Eq. wt. of O. A. \[=\frac{Molecular\,\,weight}{No.\,\,of\,\,electrons\,\,gained\,\,by\,\,one\,\,molecule}\]
\[=\frac{Molecular\,\,weight}{Change\,\,O.N.\,\,per\,\,mole}\]
Eq. wt. of R. A. \[=\frac{Molecular\,\,weight}{No.\,\,of\,\,electrons\,\,lost\,\,by\,\,one\,\,molecule}\]
\[=\frac{Molecular\,\,weight}{Change\,\,in\,\,O.N.\,\,per\,\,mole}\]
Equivalent weight of few oxidising/reducing agents
|
Agents |
O. N. |
Product |
O. N. |
Change in |
Total |
Eq. wt. |
| \[C{{r}_{2}}{{O}_{7}}^{2-}\] |
+ 6 |
\[C{{r}^{3+}}\] |
+ 3 |
3 |
3 × 2 = 6 |
Mol. wt./6 |
| \[{{C}_{2}}{{O}_{4}}^{2-}\] |
+ 3 |
\[C{{O}_{2}}\] |
+ 4 |
1 |
1 × 2 = 2 |
Mol. wt./2 |
| \[{{S}_{2}}{{O}_{3}}^{2-}\] |
+ 2 |
\[{{S}_{4}}{{O}_{6}}^{2-}\] |
+ 2.5 |
0.5 |
0.5 × 2 = 1 |
Mol. wt./1 |
| \[{{H}_{2}}{{O}_{2}}\] |
– 1 |
\[{{H}_{2}}O\] |
– 2 |
1 |
1 × 2 = 2 |
Mol. wt./2 |
| \[MnO_{4}^{-}\] |
– 1 |
\[{{O}_{2}}\] |
0 |
1 |
1 × 2 = 2 |
Mol. wt./2 |
|
|
+ 7 |
\[M{{n}^{2+}}\] |
+ 2 |
5 |
5 × 1 = 5 |
Mol. wt./5 |
|
|
+ 7 |
\[Mn{{O}_{2}}\] |
+ 4 |
3 |
3 × 1 = 3 |
Mol. wt./3 |
|
|
+ 7 |
\[MnO_{4}^{2-}\] |
+ 6 |
1 |
1 × 1 = 1 |
Mol. wt./1 |
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