In which of the following reactions the metal is reduced ?

Concept:

• The oxidation number is also called the oxidation state, the total number of electrons that an atom either gains or loses to form a chemical bond with another atom.
• The oxidation number of a free element is always 0.
• The oxidation number of a monatomic ion equals the charge of the ion.

Oxidation: Oxidation is a process which occurs when there is:

• Loss of hydrogen or electropositive atom
• Loss of more than one electron
• Gain of an oxygen atom or electronegative atom
• Increase in oxidation number

Reduction: Reduction is a process which occurs when there is:

• Gain of hydrogen or electropositive atom
• Gain of more than one electron
• Loss of an oxygen atom or electronegative atom
• Decrease in oxidation number

Explanation:

In equation:

There is a decrease in oxidation number

So, MnO-4 → MnO2  is going for reduction.