Question

Complete the following chemical equation:
\[8MnO_{4}^{-}+3{{S}_{2}}O_{3}^{2-}+{{H}_{2}}O\to \]

Answer 1

Hint: To complete this equation, we first need to determine what kind of reaction is this. This is an oxidation-reduction or a redox reaction. A reaction in which electrons are transferred between two species is known as a redox reaction or an oxidation-reduction reaction.

Complete answer:
When permanganate ($MnO_{4}^{-}$) and thiosulfate react in an aqueous solution (${{H}_{2}}O$), manganese dioxide ($Mn{{O}_{2}}$) is formed along with sulfate ($SO_{4}^{2-}$) and hydroxide ($O{{H}^{-}}$).
The unbalanced chemical equation is written as follows:
\[8MnO_{4}^{-}+3{{S}_{2}}O_{3}^{2-}+{{H}_{2}}O\to Mn{{O}_{2}}+SO_{4}^{2-}+O{{H}^{-}}\]
Let us now balance this equation.
We know that the usual oxidation state of Oxygen (O) is -2 i.e., ${{O}^{2-}}$.
And the usual oxidation state of hydrogen (H) is +1 i.e., ${{H}^{+}}$
So, for manganese (Mn), the oxidation state will be +7 i.e., $M{{n}^{7+}}$ on the reactant side, and +4 on the product side i.e., $M{{n}^{4+}}$. Hence it is reduced.
\[\begin{align}
  & M{{n}^{+7}}\xrightarrow{+3{{e}^{-}}}M{{n}^{+4}} \\
 & 8M{{n}^{+7}}+24{{e}^{-}}\to 8M{{n}^{+4}} \\
\end{align}\]
Similarly, for sulfur (S), the oxidation state will be +2, i.e., ${{S}^{2+}}$ on the reactant side, and
+4 on the product side i.e., ${{S}^{4+}}$. Hence it is oxidized.
\[\begin{align}
  & {{S}^{+2}}\xrightarrow{-4{{e}^{-}}}{{S}^{+4}} \\
 & 6{{S}^{+2}}-24{{e}^{-}}\to 6{{S}^{+4}} \\
\end{align}\]
Upon substituting these coefficients, we get the equation as follows
\[8MnO_{4}^{-}+3{{S}_{2}}O_{3}^{2-}+{{H}_{2}}O\to 8Mn{{O}_{2}}+6SO_{4}^{2-}+O{{H}^{-}}\]
Upon balancing the remaining charge and equating the number of oxygen (O) and hydrogen (H) atoms on both the reactants and the products side we get the final balanced equation as follows:
\[8MnO_{4}^{-}+3{{S}_{2}}O_{3}^{2-}+{{H}_{2}}O\to 8Mn{{O}_{2}}+6SO_{4}^{2-}+2O{{H}^{-}}\]
So, the above equation is the completed chemical equation.

Note:
It should be noted that a balanced redox equation is an equation in which the coefficients of atoms for each element of the products are equal to the coefficients of atoms for each element of the reactants.
There are two ways to balance a redox equation
1. Half-reaction method
2. Oxidation number method