Complex ion-electron equations
The data booklet has some examples of complex ion-electron equations. As well as ions and electrons, they involve hydrogen ions (H+) and water (H2O).
For example, consider permanganate ions (a strong oxidising agent).
\(MnO{_{4}}^{-}(aq) + 8{H^ + }(aq) + 5{e^ - } \to M{n^{2 + }}(aq) + 4{H_2}O\)
Complex ion-electron equations can be written if they don't appear in the data booklet. Several steps must be followed.
Example
What is the ion-electron equation for the reduction of dichromate ions to chromium(III) ions?
The basic change involved from the question can be written.
\(C{r_2}O{_{7}}^{2-}(aq) \to C{r^{3 + }}(aq)\)
Firstly, we can balance the chromium ions on both sides.
\(C{r_2}O{_{7}}^{2-}(aq) \to 2C{r^{3 + }}(aq)\)
As there is oxygen present on the left hand side, we can balance this by adding water molecules to the right hand side.
\(C{r_2}O{_{7}}^{2-}(aq) \to 2C{r^{3 + }}(aq) + 7{H_2}O(l)\)
The addition of water has added hydrogen into the equation, which must also be balanced.
This can be achieved by adding hydrogen ions to the left hand side.
\(C{r_2}O{_{7}}^{2-}(aq) + 14{H^ + }(aq) \to 2C{r^{3 + }}(aq) + 7{H_2}O(l)\)
Finally, the electric charge must be balanced on both sides. Totalling the charge on the left hand side gives an overall charge of 12+ (the total of the charges on the negative dichromate ion and positive hydrogen ions), while the right hand side is 6+ (from the two chromium ions).
The charge can be balanced by adding electrons to the left hand side to give a balanced complex ion-electron equation:
\(C{r_2}O{_{7}}^{2-}(aq) + 14{H^ + }(aq) + 6{e^ - } \to 2C{r^{3 + }}(aq) + 7{H_2}O(l)\)