Redox reaction - calculate number of electrons - Chemistry (10.18)

Question

How many electrons are exchanged when the next redox reaction is balanced?

O2 + H2O + Fe → Fe(OH)3

the answer is 12.

I came up with

3OH^- + Fe → Fe(OH)3 + 3e^-

so 3 electrons at this point, I do not know, what else to do after this. (left O2 and H2O)

Answer 1

First, let's consider the reduction half reactions.

O2 + 4 e- + 2H+ =====> 2OH^1- (the oxidation state of O in OH^1- is 2-, so we have both charge and mass balance at this point.

Oxidation half reaction.

Fe⁰ =====> Fe³⁺ + 3 e-

To balance the electrons, we must multiply the reduction half reaction by 3 and the oxidation half reaction by 4 and add the equations.

3O2 +12 e- + 6H+ =====> 6OH^1- plus

4Fe⁰ =====> 4Fe³⁺ + 12 e-

Adding

3O2 +4Fe⁰ + 6H+ =====> 6OH^1- + 4Fe³⁺ We still need 6 more OH^1- to make 4Fe(OH)3, so we note that this must come from H2O on the left side of the equation. In place of 6H+ let's use 6H2O and get

3O2 + 6H2O + 4Fe ======> 4Fe(OH)3

Answer 2

I would balance the equation first:

3O₂ + 6H₂O + 4Fe → 4Fe(OH)₃

Now you can see that 6 oxygen atoms will "take" 2 electrons each and 4 Fe atoms will give up 3 electrons each.

I would write the half reactions like this:

4Fe → 4Fe³⁺ + 12e^-

3O₂ + 6H₂O + 12e^- → 12OH^-