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

Becaue oxidation state =2 is most stable for the later elements of 3d-series elements, strong 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)` ions is only weakly reducing. The `Co^(2+)(aq),Ni^(2+)(aq), and Cu^(2+)(aq)` ions are not oxidized in water. ltBrgt `Fe^(3+)(+0.77)toFe^(2+)overset(-0.44)toFe`
The chemical equation for the oxidation is then
`4Fe^(2+)(aq)+O_(2)(g)+4H^(+)(aq)to4Fe^(3+)(aq)+2H_(2)O(l)`.