After oxygen atoms are balanced with and hydrogen atoms with

CHEMISTRY · Single correct · A Level · Chemistry · Oxidation, reduction and redox reactions · Oxidation, reduction and redox reactions

Problem

After oxygen atoms are balanced with

\text{H}_2\text{O}

and hydrogen atoms with

\text{H}^+

, the reduction half-reaction

\text{MnO}_4^- \rightarrow \text{Mn}^{2+}

still shows a charge of

+7

on the left and

+2

on the right. Which statement best justifies why 5 electrons must be added to the left side to complete the balancing? (a) Five electrons are added to cancel the

+5

charge introduced when

8\,\text{H}^+

ions are placed on the left during balancing. (b) Manganese gains 5 electrons as its oxidation state decreases from

+7

\text{MnO}_4^-

+2

\text{Mn}^{2+}

, making the charges equal. (c) Because there are five oxygen atoms in

\text{MnO}_4^-

, five electrons must be added to balance the half-reaction. (d) Since the

\text{Mn}^{2+}

ion must lose 5 electrons to reach the

+7

state of

\text{MnO}_4^-

, the 5 electrons are written on the right side.

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