Question

${{H}_{2}}{{O}_{2}}$(aq) reduces $Cl{{O}_{4}}^{-}$(aq) in acidic medium. If 1 mole of ${{H}_{2}}{{O}_{2}}$ reduces 0.5 mole of $Cl{{O}_{4}}^{-}$(aq). What is the oxidation number of reduced species? (Answer is the modulus of the oxidation number).

Answer 1

Hint: This question is asked from the topic of redox reaction. To solve this kind of problem, you need to know the concept of oxidation state and redox reactions. As we know, oxidation state is also known as oxidation number which describes the loss of electron of an atom in a compound to form such a compound.

Complete step by step answer:
${{H}_{2}}{{O}_{2}}$ reduces the $Cl{{O}_{4}}^{-}$ and form a reduced species. Let us assume the reduced species as R and its oxidation number be x.
The redox reaction is as follows:
\[\begin{align}
& {{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\text{ + Cl}{{\text{O}}_{\text{4}}}^{-}\text{ }\to \text{ }{{\text{O}}_{\text{2}}}\text{ + reduced species (R)} \\
 & \text{ -2 +7 x} \\
\end{align}\]

Now, we can balance the oxidation numbers:
$\left( 7-x \right){{H}_{2}}{{O}_{2}}+2Cl{{O}_{4}}\to \left( 7-x \right){{O}_{2}}+2R$
From this, it is clear that (7-x) moles of ${{H}_{2}}{{O}_{2}}$ will reduce 2 moles of $Cl{{O}_{4}}^{-}$.
1 mole of ${{H}_{2}}{{O}_{2}}$ will reduce $\dfrac{2}{7-x}$ moles of $Cl{{O}_{4}}^{-}$.
Hence, $\dfrac{2}{7-x}=0.5\text{ mol }Cl{{O}_{4}}^{-}$ (given)
$4=7-x$
$x=3$
Thus, the oxidation number of Cl in R is equal to +3 and also R is $Cl{{O}_{2}}$.
So, answer is $\left| +3 \right|=3$

Additional Information:
Redox reaction is a reaction in which the reduction of one reactant and oxidation of another reactant takes place. When a molecule, atom or ion is simultaneously oxidized and reduced, it is a disproportionation reaction. This happens because the same entity gains and losses electrons in that reaction.

Note: You should not confuse the oxidation state and the valency. Valency is a number which indicates the number of electrons which an atom can gain or lose to become a stable species while oxidation number is the number which describes the degree of oxidation in a chemical compound. Oxidation number can be negative, zero and positive while valency is always positive.