Question

Which of the following is a redox reaction?
A.$2{\text{Na}}\left[ {{\text{Ag}}{{\left( {{\text{CN}}} \right)}_2}} \right] + {\text{Zn}} \to {\text{N}}{{\text{a}}_2}\left[ {{\text{Zn}}{{\left( {{\text{CN}}} \right)}_4}} \right] + 2{\text{Ag}}$
B.${\text{Ba}}{{\text{O}}_2} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{BaS}}{{\text{O}}_4} + {{\text{H}}_2}{{\text{O}}_2}$
C.${{\text{N}}_2}{{\text{O}}_5} + {{\text{H}}_2}{\text{O}} \to 2{\text{HN}}{{\text{O}}_3}$
D.${\text{AgN}}{{\text{O}}_3} + {\text{KI}} \to {\text{AgI}} + {\text{KN}}{{\text{O}}_3}$

Answer 1

Both oxidation and reduction should take place in the reaction. The decrease in oxidation number will be termed as reduction and increase in oxidation number will be termed as oxidation.

Complete step by step answer:
There are several reactions in which transfer of electrons from one chemical substance to another takes place and such reactions are known as redox reaction or oxidation reduction reaction. In redox reaction oxidation and reduction takes place simultaneously. The chemical substance whose oxidation number increase or addition of oxygen occurs is said to be oxidised and whose oxidation number decrease is said to be reduced.
Oxidation state of an element in a compound represents the number of electrons lost or gained during its change from one state into another. Let us check the change in oxidation state in each of the given option:
$2{\text{Na}}\left[ {\mathop {{\text{Ag}}}\limits^{ + 1} {{\left( {{\text{CN}}} \right)}_2}} \right] + \mathop {{\text{Zn}}}\limits^0 \to {\text{N}}{{\text{a}}_2}\left[ {\mathop {{\text{Zn}}}\limits^{ + 2} {{\left( {{\text{CN}}} \right)}_4}} \right] + 2\mathop {{\text{Ag}}}\limits^0$
As we can see that the oxidation number of silver on the reactant side is + 1 and on the product side is zero. So there is a decrease in oxidation state of silver hence the reduction of silver is taking place. On the other hand oxidation number of zinc on the reactant side is zero and on the product side is plus 2. So an increase in oxidation state of zinc is taking place hence the above reaction is a redox reaction.
$\mathop {{\text{Ba}}}\limits^{ + 2} {\mathop {\text{O}}\limits^{ - 1} _2} + {\mathop {\text{H}}\limits^{ + 1} _2}\mathop {{\text{S}}{{\text{O}}_4}}\limits^{ - 2} \to \mathop {{\text{Ba}}}\limits^{ + 2} {\text{S}}{{\text{O}}_4} + {{\text{H}}_2}{\mathop {\text{O}}\limits^{ - 1} _2}$
In the above reaction the oxidation state of all the elements are the same on both sides of the reaction hence it is not a redox reaction. Due to peroxide linkage the oxidation state of oxygen is minus 1.
${{\text{N}}_2}{{\text{O}}_5} + {{\text{H}}_2}{\text{O}} \to 2{\text{HN}}{{\text{O}}_3}$
In the above reaction also the oxidation of nitrogen remains the same that is plus 5 on both sides of the reaction. It is also not a redox reaction.
${\text{AgN}}{{\text{O}}_3} + {\text{KI}} \to {\text{AgI}} + {\text{KN}}{{\text{O}}_3}$
Oxidation state of silver remains the same and that of iodine and nitrogen also remains the same. It is also not a redox reaction.

Thus, the correct option is A.

Note:
Oxidation can also be defined as the addition of oxygen or loss of hydrogen from a molecule. Reduction can be defined as the removal of oxygen and addition of hydrogen.