# How can I do redox reactions step by step?

Jul 1, 2018

A rather open-ended question...

#### Explanation:

And the first step is assignment of oxidation state...for elemental ions, i.e. $N {a}^{+}$, or ${K}^{+}$ or $C {r}^{3 +}$, the oxidation state of the metal is the CHARGE on the ion, so that we gots $N a \left(+ I\right) , K \left(+ I\right) , C r \left(+ I I I\right)$ respectively...

For compound ions, we have to divide up the anion/cation according to predetermined rules

Now $\text{permanganate}$, $M n {O}_{4}^{-}$, i.e. $M n \left(+ V I I\right)$ is a brightly coloured anion, that commonly undergoes reduction to give COLOURLESS $M {n}^{2 +}$..and since $+ V I I - I I = V$... WE CONCEIVE that manganese undergoes a FIVE ELECTRON REDUCTION....

${\underbrace{M n {O}_{4}^{-}}}_{\text{deep purple}} + 8 {H}^{+} + 5 {e}^{-} \rightarrow M {n}^{2 +} + 4 {H}_{2} O$ $\left(i\right)$

And note for this half equation both mass and charge are balanced, as is absolutely required. Now the electrons are conceived to COME from somewhere...to cause a corresponding oxidation...let us assume that it OXIDIZES a primary alcohol as shown...i.e. here we gots the oxidation $C \left(- I\right) \rightarrow C \left(+ I I I\right)$, a four electron oxidation....

$R \stackrel{- I}{C} {H}_{2} O H + {H}_{2} O \left(l\right) \rightarrow R \stackrel{+ I I I}{C} {O}_{2} H + 4 {H}^{+} + 4 {e}^{-}$ $\left(i i\right)$

We add $\left(i\right)$ and $\left(i i\right)$ in such a way that the electrons, virtual particles of convenience, are RETIRED....and thus $4 \times \left(i\right) + 5 \times \left(i i\right)$ gives....

$4 M n {O}_{4}^{-} + 32 {H}^{+} + 5 R C {H}_{2} O H + 5 {H}_{2} O \left(l\right) + 20 {e}^{-} \rightarrow 4 M {n}^{2 +} + 16 {H}_{2} O + 5 R C {O}_{2} H + 20 {H}^{+} + 20 {e}^{-}$

To give after cancellation...

$4 M n {O}_{4}^{-} + 5 R C {H}_{2} O H + 12 {H}^{+} \rightarrow 4 M {n}^{2 +} + 5 R C {O}_{2} H + 11 {H}_{2} O$

The which is balanced with respect to mass and charge as is absolutely required...

Confused yet? But all I have done is assign oxidation numbers according to predetermined rules...and based electron transfer on the basis of the change in oxidation number....