Question

Which of these half-reactions represents reduction?

Answer 1

Answer: The half-reactions represents reduction are as follows.

• $Cr_{2}O^{2-}_{7} \rightarrow Cr^{3+}$
• $MnO^{-}_{4} \rightarrow Mn^{2+}$

Explanation:

A half-reaction where addition of electrons take place or a reaction where decrease in oxidation state of an element  takes place is called reduction-half reaction.

For example, the oxidation state of Cr in $Cr_{2}O^{2-}_{7}$ is +6 which is getting converted into +3, that is, decrease in oxidation state is taking place as follows.

$Cr_{2}O^{2-}_{7} + 3 e^{-} \rightarrow Cr^{3+}$

Similarly, oxidation state of Mn in $MnO^{-}_{4}$ is +7 which is getting converted into +2, that is, decrease in oxidation state is taking place as follows.

$MnO^{2-}_{4} + 5 e^{-} \rightarrow Mn^{2+}$

Thus, we can conclude that half-reactions represents reduction are as follows.

• $Cr_{2}O^{2-}_{7} \rightarrow Cr^{3+}$
• $MnO^{-}_{4} \rightarrow Mn^{2+}$