Question

Why is the oxidation of `Mn^(2+)` in ACIDIC media represented differently to the oxidation of `Mn^(2+)` in an ALKALINE medium? What do we mean by `H^+` or `H_3O^+`?

Answer 1

Because chemistry follows experiment...........

Explanation:

`H^+` is very much a conceptual particle. When we write `H^+` in a reaction we mean the hydronium ion, `H_3O^+`, which AGAIN is a conceptual species. The actual species is (probably) a cluster of 3-4 water molecules, with an extra proton to give `H_7O_3^+`, or `H_9O_4^+`. The clusters exchange the proton between themselves. We write `H^+` or `H_3O^+` for convenience.

When we write a redox equation, of course we balance mass and charge, and the use of `H^+` or `H_3O^+` is a means to this end. It happens that in ACIDIC medium, `Mn^(2+)`, may be oxidized to `MnO_4^(-)`:

`Mn^(2+) +4H_2O rarr MnO_4^(-) + 8H^(+) + 5e^-`

In alkaline media, `Mn^(2+)` may be oxidized to `"manganate ion"`, `MnO_4^(2-)`, which is `Mn(VI+)`:

`Mn^(2+) +8HO^(-) rarr MnO_4^(2-) + 4H_2O + 4e^-`

`"Manganates"` give green salts.

Mass and charge are balanced (I think) as required. Note that this representation follows the actual experiment, not vice versa.

If there is a further issue or query, fire away.