Question

Consider the following reaction:
$HCHO+2{{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}+3O{{H}^{-}}\to 2Ag+HCO{{O}^{-}}+4N{{H}_{3}}+2{{H}_{2}}O$
Which of the following statements regarding oxidation and reduction is correct?
A. HCHO is oxidised to$HCO{{O}^{-}}$ and ${{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}$is reduced to Ag
B. HCHO is reduced to $HCO{{O}^{-}}$ and is ${{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}$ oxidised to Ag
C. ${{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}$ is reduced to Ag while is $O{{H}^{-}}$oxidised to $HCO{{O}^{-}}$
D. ${{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}$ is oxidised to $N{{H}_{3}}$ while $HCHO$ is reduced to ${{H}_{2}}O$

Answer 1

To answer the correct option in the above question, we should first separate the oxidation and reduction part. We will then assign the oxidation states to each compound in the reaction.

Complete step by step solution :
First of all we will write the reaction that is present in question.
$HCHO+2{{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}+3O{{H}^{-}}\to 2Ag+HCO{{O}^{-}}+4N{{H}_{3}}+2{{H}_{2}}O$
Now, we should separate the oxidation and reduction reaction from the redox reaction. We will consider assigning the oxidation states to different molecules that are present in reaction.
So, we know that:
Oxidation number of H=+1
Oxidation number of O=-2
Oxidation number of $N{{H}_{3}}$= 0
Oxidation number of HCHO= 0
Oxidation number of Ag= 0
Now, we will assign the above oxidation number to above reaction.

& \,\,\,\,\,\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,+1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,+2\,\,\,\,\,\,\,\,\,\,\,\,\, \\\ & HCHO+2{{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}+3O{{H}^{-}}\to 2Ag+HCO{{O}^{-}}+4N{{H}_{3}}+2{{H}_{2}}O \\\ \end{aligned}$$ So, by observing the above reaction we came to know that the oxidation number of C changes from 0 to +2. It can be verified as: $$HCHO\to HCO{{O}^{-}}$$ And from this we can say that it is an oxidation reaction and we can say that the Carbon atom gets oxidised. Similarly, the oxidation number of Ag changes from +1 to 0. $$2{{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}\to Ag$$ The above reaction is a reduction reaction where Ag gets reduced. So, from the above reactions and discussion we can say that HCHO is oxidised to$$HCO{{O}^{-}}$$ and $${{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}} \right]}^{+}}$$is reduced to Ag. _So, option A is correct._ **Note** :We should know that oxidation and reduction can be defined in terms of the adding or removing oxygen to a compound. And we can say that oxidation is the gain of oxygen and reduction is the loss of oxygen. With respect to hydrogen we can say that oxidation is the loss of hydrogen and reduction is the gain of hydrogen.