Question 16.7: Balancing Redox Reactions Balance the following redox reacti......

1. Follow the half-reaction method for balancing redox reactions. Begin by assigning oxidation states.

2. Separate the overall reaction into two half-reactions.

OXIDATION      \mathrm{I}^{-}(a q)~\longrightarrow~\mathrm{I}_{2}(s)

REDUCTION   \mathrm{Cr}_{2}\mathrm{O}_{7}^{~2-}(a q) \longrightarrow \mathrm{C r}^{3^{+}}(a q)

3. Balance each half-reaction with respect to mass.

• Balance all elements other than H and O.

• Balance O by adding \mathrm{H}_{2}\mathrm{O}.

• Balance H by adding \mathrm{H}^{+}.

2~\mathrm{I}^{-}(a q)\longrightarrow\mathrm{I}_{2}(a q)

\mathrm{Cr}_{2}\mathrm{O}_{7}^{~2-}(a q)\longrightarrow\mathrm{2}\:\mathrm{Cr}^{\mathrm{3}^{+}}(s)

2~\mathrm{I}^{-}(a q)\longrightarrow\mathrm{I}_{2}(s)

\mathrm{Cr}_{2}\mathrm{O}_{7}^{~2-}(a q)\longrightarrow2\,\mathrm{Cr}^{~3+}(a q)+7\,\mathrm{H}_{2}\mathrm{O}(l)

2~\mathrm{I}^{-}(a q)\longrightarrow\mathrm{I}_{2}(s)

\mathrm{14~H^{+}(a q)+\mathrm{C r_{2}O_{7}^{~2-}(a q)}\longrightarrow2\,C r^{3+}(a q)+7~\mathrm{H_{2}O(l)}}

4. Balance each half-reaction with respect to charge.

2~\mathrm{I}^{-}(a q)\longrightarrow\mathrm{I}_{2}(s)+2~\mathrm{e}^{-}

6~\mathrm{e}^{-}+14{\mathrm{~H}}^{+}(a q)+{\mathrm{Cr}}_{2}{\mathrm{O}}_{7}^{~2-}(a q)\longrightarrow2\,\mathrm{Cr}^{~3+}(a q)+7\,\mathrm{H_{2}O(l)}

5. Make the number of electrons in both half-reactions equal.

3 \times[2\,\mathrm{I}^{-}(a q) \longrightarrow\,\mathrm{I}_{2}(s) +2~\mathrm{e}^{-}]

6~\mathrm{e}^{-}+14\,\mathrm{H}^{+}(a q)+\mathrm{Cr}_{2}\mathrm{O}_{7}^{~2-}(a q)\longrightarrow2\,\mathrm{Cr}^{3+}(a q)+7\,\mathrm{H}_{2}\mathrm{O}(l)

6. Add the half-reactions together.

6~\mathrm{I}^{-}(a q) \longrightarrow 3\ \mathrm{I}_{2}(s)+ \cancel{6~\mathrm{e}^{-}}

\underline{\cancel{6~\mathrm{e}^{-}}~+14\,{\mathrm{H}}^{+}(a q)+\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-}(a q)\longrightarrow2\,\mathrm{Cr}^{3+}(a q)+7\,\mathrm{H}_{2}\mathrm{O}(l)~~~~~}

6\,\mathrm{I}^{-}(a q)+14\,\mathrm{H}^{+}(a q)+\mathrm{Cr}_{2}\mathrm{O}_{7}^{~2-}(a q)\longrightarrow 3\,\mathrm{I}_{2}(s)+2\,\mathrm{Cr}^{~3+}(a q)+7\,\mathrm{H}_{2}\mathrm{O(l)}

7. Verify that the reaction is balanced.

Reactants              Products
6 I                            6 I
14 H                         14 H
2 Cr                         2 Cr
7 O                          7 O
+6 charge             +6 charge