On the basis of trends in the properties of the `3d`-series elements , suggested possible `M^(2+)` aqua ions for use as reducing agents , and write a balanced chemical equation for the reaction of one of those ions with `O_(2)` in acidic solution.

Explanation

Because oxidation state `+2` is most stable for the later elements of `3d`-series element, strongs reducing agents include ions of the metals on the left of the series: such ions include `V^(2+)` (aq) and `Cr^(2+)` (aq) the `Fe^(2+)` (aq) ion is only weakly reducing. The `Co^(2+)` (aq), `Ni^(2+)` (aq), and `Cu^(2+)` (aq) ions are not oxidized in water.
`Fe^(3+)overset(+0.77)rarrFe^(2+)overset(-0.44)rarrFe`
The chemical equation for the oxidation is then
`4Fe^(2+) (aq) + O_(2)(g) + 4H^(+) (aq) rarr 4Fe^(3+) (aq) +2H_(2)O(I)`