Question

Write balanced half-reactions in which $Mn{O}_4^{-}$acts as an oxidant at

$$\begin{gathered} \left(\mathrm{a}\right)\mathrm{pH}=0; \\ \left(\mathrm{b}\right)\mathrm{pH}=10; \\ \left(\mathrm{c}\right)\mathrm{pH}=15. \end{gathered}$$

Short answer

The balanced half-reactions are,

$$\begin{gathered} \left(\mathrm{a}\right){{\mathrm{MNO}}_4}^{-}+8{\mathrm{H}}^{+}+5{\mathrm{e}}^{-}\Leftarrow {\mathrm{mn}}^2+4{\mathrm{H}}_2\mathrm{O} \\ \left(\mathrm{b}\right){\mathrm{MnO}}_4^{-}+4{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O} \\ \left(\mathrm{c}\right){\mathrm{MnO}}_4^{-}+{\mathrm{e}}^{-}\rightleftharpoons {{\mathrm{MnO}}_4}^{2-} \end{gathered}$$

Step-by-step solution

Definition of redox titration

• Redox reactions are the oxidation-reduction chemical reactions in which the oxidation states of the reactants change. The term redox refers to the reduction-oxidation process.
• All redox reactions can be divided down into two types of reactions: reduction and oxidation.
• In a redox reaction, or Oxidation-Reduction process, the oxidation and reduction reactions always happen at the same time.

Balanced Half-reactions inMnO4-

a)

At pH = 10, in strongly acidic conditions, ${\mathrm{KMnO}}_4$is reduced to colorless ${\mathrm{Mn}}^{2+}$:

role="math" localid="1654846415242" $MN{O_4}^{-}+8H^{+}+5e^{-}\Leftarrow M{n}^{2+}+4H_{2}O$

b)

At , pH = 10 in slightly alkaline conditions, is reduced to solid brown ${\mathrm{MnO}}_2$:

role="math" localid="1654846745489" ${\mathrm{MnO}}_4^{-}+4{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}\rightleftharpoons {\mathrm{MnO}}_2\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}$

c)

At , in strongly alkaline conditions, ${\mathrm{KMnO}}_4$is reduced to green Manganate anion

${\mathrm{MnO}}_4^{-}+{\mathrm{e}}^{-}\rightleftharpoons {{\mathrm{MnO}}_4}^{2-}$

is reduced to green Manganate anion