Question

What is an oxidation number or oxidation state? How is it useful to identify redox reactions, oxidant and reductant?

Answer 1

Hint: A category of reactions involving transfer of electrons from one species to another is called oxidation reduction or redox reaction. In redox reaction one species gain electrons and other species lose electron

Complete step by step answer:
A redox reaction is a chemical reaction which involves simultaneous oxidation and reduction. In free radical and ionic reactions oxidation or reduction are defined as processes by which an element undergoes a net loss or gain of electrons, respectively. In reference to organic molecules oxidation is a process by which carbon atoms gain bonds to more electronegative elements, most commonly oxygen. The process by which a carbon atom gains bonds to less electronegative elements, most commonly hydrogen, is called reduction. Oxidation reactions are those in which the central carbon of the functional group is transformed into a more highly oxidised form, and reduction reactions are those in which central carbon are transformed into a more highly reduced form.
The charge carried by the central atom in a complex if all the ligands are removed along with the electron pairs that are shared with the central atom is called an oxidation number. Oxidation number is generally represented by roman numbers
Eg: In ${\left[ {{\text{Cu}}{{\left( {{\text{CN}}} \right)}_{\text{4}}}} \right]^{{\text{3 - }}}}$ complex the oxidation number of Copper is ${\text{ + 1}}$ and it is written as ${\text{Cu}}\left( {\text{{\rm I}}} \right)$$$
In general oxidation is termed as addition of oxygen to the substrate, removal of hydrogen or removal of one electron. In oxidation reactions the oxidising agent accepts electrons from organic molecules being oxidised.

Additional information:
Most commonly used oxidising agents are ${{\text{O}}_{\text{2}}}{\text{, }}{{\text{O}}_{\text{3}}}{\text{, Mn}}{{\text{O}}_{\text{2}}}{\text{, Mn}}{{\text{O}}_{\text{4}}}^{\text{ - }}{\text{, Cr}}{{\text{O}}_{\text{3}}}{\text{, C}}{{\text{r}}_{\text{2}}}{{\text{O}}_{\text{7}}}^{{\text{2 - }}}{\text{, Cr}}{{\text{O}}_{\text{2}}}{\text{C}}{{\text{l}}_{\text{2}}}{\text{, A}}{{\text{r}}_{\text{2}}}{\text{O, Os}}{{\text{O}}_{\text{4}}}{\text{, HI}}{{\text{O}}_{\text{4}}}{\text{, Pb}}{\left( {{\text{OAc}}} \right)_{\text{4}}}$
The oxidation reactions are classified into various types according to the type of bond cleavage involved during the process they are,
Substitution of hydrogen by oxygen, Addition of oxygen, Elimination of hydrogen or dehydrogenation, oxidative cleavage, oxidative coupling, addition of other electronegative atoms and substitution with other electronegative atoms.
Reduction reaction are classified as three types
Catalytic reduction, dissolving metal reduction and complex metal hydride reduction

Note: An example for redox reaction involving inorganic species or molecules is
${\text{C}}{{\text{u}}^{\text{ + }}}\left( {{\text{CuCl}}} \right){\text{ + F}}{{\text{e}}^{{\text{ + 3}}}}\left( {{\text{FeC}}{{\text{l}}_{\text{3}}}} \right)\xrightarrow{{}}{\text{C}}{{\text{u}}^{{\text{ + 2}}}}\left( {{\text{CuC}}{{\text{l}}_{\text{2}}}} \right){\text{ + F}}{{\text{e}}^{{\text{ + 3}}}}\left( {{\text{FeC}}{{\text{l}}_{\text{2}}}} \right)$
Here ${\text{C}}{{\text{u}}^{\text{ + }}}$ loses an electron which means that it is oxidised and the other species ${\text{F}}{{\text{e}}^{{\text{3 + }}}}$ is the oxidizing agent and it gains two electrons which means it is reduced