Question

Consider the following reaction:
$HCHO+2\left[Ag\right(N{H}_3{)}_2{]}^{+}+3O{H}^{-}\to 2Ag+HCO{O}^{-}+4N{H}_3+2H_{2}O$
Which of the following statements regarding oxidation and reduction is correct?

• A. $HCHO$ is oxidised to $HCO{O}^{-}$ and $\left[Ag\right(N{H}_3{)}_2{]}^{+}$is reduced to $Ag$
• B. $HCHO$ is reduced to $HCO{O}^{-}$ and $\left[Ag\right(N{H}_3{)}_2{]}^{+}$ is oxidised to $Ag$
• C. $\left[Ag\right(N{H}_3{)}_2{]}^{+}$ is reduced to $Ag$ while $O{H}^{-}$ is oxidised to $HCO{O}^{-}$
• D. $\left[Ag\right(N{H}_3{)}_2{]}^{+}$ is oxidised to $N{H}_3$ while $HCHO$ is reduced to $H_{2}O$

Answer 1

The correct option is A $HCHO$ is oxidised to $HCO{O}^{-}$ and $\left[Ag\right(N{H}_3{)}_2{]}^{+}$is reduced to $Ag$

Separating the oxidation and reduction reaction from the redox reaction:

$HCHO+2\left[Ag\right(N{H}_3{)}_2{]}^{+}+3O{H}^{-}\to 2Ag+HCO{O}^{-}+4N{H}_3+2H_{2}O$

assigning the oxidation number on central atom ($C$ and $Ag$) in each molecules by considering oxidation number of $H=+1,O=-2,N{H}_3=0$ we get oxidation state as:

$H\stackrel{0}{C}HO+2\left[\stackrel{+1}{Ag}\right(N{H}_3{)}_2{]}^{+}+3O{H}^{-}\to 2\stackrel{0}{Ag}+H\stackrel{+2}{C}O{O}^{-}+4N{H}_3+2H_{2}O$

as the oxidation number of $C$ changes from 0 to +2 as:

$HCHO\to HCO{O}^{-}$

its a oxidation reaction where $C$ gets oxidized

Similarly, as the oxidation number of $Ag$ changes from +1 to 0 as:

$\left[Ag\right(N{H}_3{)}_2{]}^{+}\to Ag$

its a reduction reaction where $Ag$ gets reduced.