Question

Define redox reaction. Give one example.

Answer 1

A redox reaction is a type of chemical reaction in which reduction and oxidation takes simultaneously in a single step. The reaction is associated with change in oxidation states of the substances.

Complete answer:
A redox reaction is an oxidation-reduction reaction where transfer of electrons takes place between the two reactants which are participating in it. The transfer of electrons is identified or monitored by observing the change in the oxidation states of the substrates involved in the reaction.
A general redox reaction is written in following manner:
${A^0} + {B^0} \to {A^ + } + {B^ - }$
From the above reaction it is clear that the substrate $A$ is producing ${A^ + }$ ion and the substrate $B$ is producing ${B^ - }$ ion. $A$ is undergoing loss of electron generating ${A^ + }$ and $B$ is undergoing gain of electrons producing ${B^ - }$ ions.
The single reaction can be represented as a combination of two half reactions. The oxidation half reaction involves loss of electrons resulting in increase in the oxidation state of a reactant. The reduction half reaction involves gain of electrons results in decrease in the oxidation state of a reactant.
The species which has the tendency to undergo a reduction in redox reactions is denoted as electron-accepting species also called as oxidizing agents. The species which has the tendency to donate electrons is denoted as the electron-donating species also called as reducing agent.
Thus the two half reaction is written as:
Oxidation half reaction:
$A \to {A^ + } + {e^ - }$
Reduction half reaction:
$B + {e^ - } \to {B^ - }$
Each of the half reactions should be balanced in such a way so that all the number of electrons is equal for loss and gain by the reacting species.
For example the metal displacement reaction used in cells or metallurgical processes is a redox reaction. A solution of copper sulfate on treatment with zinc metal undergoes a redox reaction. The reaction is
$Zn(s) + CuS{O_4}(aq) \to ZnS{O_4}(aq) + Cu(s)$
The oxidation state of $Zn\left( s \right)$ is $0$ and that of copper in $CuS{O_4}$ is $+ 2$. Upon reaction the generated $ZnS{O_4}$ has the oxidation state of zinc $+ 2$ and that of copper is $0$. Thus change of oxidation state takes place between the reacting species.
The half reactions are as follows:
Oxidation half reaction: $Zn \to Z{n^{2 + }} + 2{e^ - }$
Reduction half reaction: $C{u^{2 + }} + 2{e^ - } \to Cu$

Note:
The zinc metal acts as the reducing agent which reduces copper from $+ 2$ to $0$ oxidation state and itself undergoes oxidation. The $C{u^{2 + }}$ ion acts as the oxidizing agent which oxidizes zinc from $0$ to $+ 2$ oxidation state and itself undergoes reduction.