Question

Permanganate ion is reduced to `MnO_2`, and ammonia is oxidized to nitrate ion. Can you write a balanced chemical equation to represent the redox process?

Answer 1

This is a good one...............

Explanation:

You really have to make a meal out of these redox equations.

Permanganate ion (STRONGLY COLOURED PURPLE) could be reduced to almost colourless `Mn^(2+)`....

`stackrel(+VII)[MnO_4]^(-) +8H^(+) + 5e^(-) rarr Mn^(2+) + 4H_2O` `(i)`

But here it is reduced to `MnO_2`.

`stackrel(+VII)[MnO_4]^(-) +4H^(+) + 3e^(-) rarr stackrel(+IV)"MnO"_2 + 2H_2O` `(ii)`

Ammonia is OXIDIZED to nitrate anion.......(there is no associated colour change; ammonia pen and inks very badly, whereas ammonium ion/salt is odourless, but I do not suggest you use this method to differentiate them!)

`stackrel(-III)"NH"_3 +3H_2O rarr (stackrel(+V)"NO"_3)^(-) +9H^(+)+ 8e^(-)` `(iii)`

Now if I have done my sums right; `(i)`, `(ii)`, and `(iii)` ARE STOICHIOMETRICALLY BALANCED WITH RESPECT TO MASS AND CHARGE. If they are not, then they cannot be accepted as a representation of chemical reality. I think they are balanced, and so the final redox equation eliminates the electrons, and we take the sum...........`8xx(i)+5xx(iii)`:

`8MnO_4^(-) +5NH_3 + cancel(15H_2O) + cancel(64)19H^(+) rarr 8Mn^(2+) + 5NO_3^(-) +cancel(45H^(+))+ cancel(32)17H_2O`

And we cancel out common reagents to give (finally!):

`8MnO_4^(-) +5NH_3 +19H^(+) rarr 8Mn^(2+) + 5NO_3^(-) + 17H_2O`

Even despite the whack coefficients this is stoichiometrically balanced AS IS ABSOLUTELY REQUIRED...........

But you specified the reduction of permanganate to `MnO_2`, so this is ...........`3xx(i)+5xx(iii)`:

`8MnO_4^(-) +3NH_3 +5H^(+)rarr 8MnO_2(s) + 3NO_3^(-) +7H_2O`

But I note (well eventually I did!) that you specified BASIC conditions; all I have to do is add `5xxHO^-` to each side of the equation............and eliminate the waters..........

`8MnO_4^(-) +3NH_3 rarr 8MnO_2(s) + 3NO_3^(-) +5HO^(-) + 2H_2O`

Manganese metal has a particularly rich redox chemistry. For another example of this redox manifold, see here. See here for redox reactions in general.