Question

Which of these half-reactions represents reduction?

I. Fe2+ → Fe3+
II. Cr2O72- → Cr3+
III. MnO4- → Mn2+

Answer 1

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

• 
  `Cr_(2)O^(2-)_(7) \rightarrow Cr^(3+)`
• 
  `MnO^(-)_(4) \rightarrow Mn^(2+)`

Step-by-step 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+)`