Half-Reaction Method

Tags

Chem/Reaction

Cegep/2

Word count

327 words

Reading time

3 minutes

Method for balancing redox reaction in aqueous solution

For acidic reaction:

1. Write separate equations for the oxidation and reduction half-reactions.
2. For each half-reaction:
   1. Balance all elements except hydrogen and oxygen.
   2. Balance oxygen using $\mathrm{H}{\phantom{A}}_2\mathrm{O}$.
   3. Balance hydrogen using $\mathrm{H}{\phantom{A}}^{+}$.
   4. Balance the charge using electrons.
3. Balance electrons in the half-reactions.
4. Add and simplify the half-reactions.

For basic reaction:

1. Write separate equations for the oxidation and reduction half-reactions.
2. For each half-reaction:
   1. Balance all elements except hydrogen and oxygen.
   2. Balance oxygen using double $\mathrm{OH}{\phantom{A}}^{-}$.
   3. Balance hydrogen using $\mathrm{H}{\phantom{A}}_2\mathrm{O}$.
   4. Balance the charge using electrons.
3. Balance electrons in the half-reactions.
4. Add and simplify the half-reactions.

Examples

Balance the following reactions.

$\mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{-}+\mathrm{Fe}{\phantom{A}}^{2+}\to \mathrm{Fe}{\phantom{A}}^{3+}+\mathrm{Mn}{\phantom{A}}^{2+}$ (acidic solution)

$$\begin{array}{rl}\mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{-}& \to \mathrm{Mn}{\phantom{A}}^{2+}\\ \mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{-}+8\mathrm{H}{\phantom{A}}^{+}+5\mathrm{e}{\phantom{A}}^{-}& \to \mathrm{Mn}{\phantom{A}}^{2+}+4\mathrm{H}{\phantom{A}}_2\mathrm{O}\\ \\ \mathrm{Fe}{\phantom{A}}^{2+}& \to \mathrm{Fe}{\phantom{A}}^{3+}\\ \mathrm{Fe}{\phantom{A}}^{2+}& \to \mathrm{Fe}{\phantom{A}}^{3+}+\mathrm{e}{\phantom{A}}^{-}\\ \\ 5\mathrm{Fe}{\phantom{A}}^{2+}+\mathrm{MnO}{\phantom{A}}_4{\phantom{A}}^{-}+8\mathrm{H}{\phantom{A}}^{+}& \to 5\mathrm{Fe}{\phantom{A}}^{3+}+\mathrm{Mn}{\phantom{A}}^{2+}+4\mathrm{H}{\phantom{A}}_2\mathrm{O}\end{array}$$

$\mathrm{Ag}+\mathrm{CN}{\phantom{A}}^{-}+\mathrm{O}{\phantom{A}}_2\to \mathrm{Ag}(\mathrm{CN}){\phantom{A}}_2{\phantom{A}}^{-}$ (basic solution)

$$\begin{array}{rl}\mathrm{O}{\phantom{A}}_2& \to \\ \mathrm{O}{\phantom{A}}_2+2\mathrm{H}{\phantom{A}}_2\mathrm{O}+4\mathrm{e}{\phantom{A}}^{-}& \to 4\mathrm{OH}{\phantom{A}}^{-}\\ \\ \mathrm{Ag}+\mathrm{CN}{\phantom{A}}^{-}& \to \mathrm{Ag}(\mathrm{CN}){\phantom{A}}_2{\phantom{A}}^{-}\\ \mathrm{Ag}+2\mathrm{CN}{\phantom{A}}^{-}& \to \mathrm{Ag}(\mathrm{CN}){\phantom{A}}_2{\phantom{A}}^{-}+\mathrm{e}{\phantom{A}}^{-}\\ \\ 4\mathrm{Ag}+8\mathrm{CN}{\phantom{A}}^{-}+\mathrm{O}{\phantom{A}}_2+2\mathrm{H}{\phantom{A}}_2\mathrm{O}& \to 4\mathrm{Ag}(\mathrm{CN}){\phantom{A}}_2{\phantom{A}}^{-}+4\mathrm{OH}{\phantom{A}}^{-}\end{array}$$

$\mathrm{ClO}{\phantom{A}}^{-}+\mathrm{I}{\phantom{A}}^{-}\to \mathrm{Cl}{\phantom{A}}^{-}+\mathrm{IO}{\phantom{A}}_3{\phantom{A}}^{-}$ (basic solution)

$$\begin{array}{rl}\mathrm{I}{\phantom{A}}^{-}& \to \mathrm{IO}{\phantom{A}}_3{\phantom{A}}^{-}\\ \mathrm{I}{\phantom{A}}^{-}+6\mathrm{OH}{\phantom{A}}^{-}& \to \mathrm{IO}{\phantom{A}}_3{\phantom{A}}^{-}+3\mathrm{H}{\phantom{A}}_2\mathrm{O}+6\mathrm{e}{\phantom{A}}^{-}\\ \\ \mathrm{ClO}{\phantom{A}}^{-}& \to \mathrm{Cl}{\phantom{A}}^{-}\\ \mathrm{ClO}{\phantom{A}}^{-}+\mathrm{H}{\phantom{A}}_2\mathrm{O}+2\mathrm{e}{\phantom{A}}^{-}& \to \mathrm{Cl}{\phantom{A}}^{-}+2\mathrm{OH}{\phantom{A}}^{-}\\ \\ \mathrm{I}{\phantom{A}}^{-}+3\mathrm{ClO}{\phantom{A}}^{-}& \to \mathrm{IO}{\phantom{A}}_3{\phantom{A}}^{-}+3\mathrm{Cl}{\phantom{A}}^{-}\end{array}$$

Contributors

Ajitani Hifumi

Changelog

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