[Schrödinger]

Hey guys.

I reduced FeCl₃ to FeCl₂ using steel wool (which can be treated as Fe)
Trying to write the oxidation and reduction half reactions, I got stuck.

Suppose I have these possible reactions :

a:Fe³⁺  + e^- → Fe²⁺                  E₁
b:Fe → Fe²⁺  + 2e^-                    E₂
c:Fe → Fe³⁺  + 3e^-                    E₃

E₂ and E₃ are oxidation potentials and E₁ is reduction potential. I've not written all of them as reduction reactions (as is the convention) because that's not what I'm bothered about. It's just more convenient for this situation.

E1, E2 and E3 are all positive.

Suppose E₁+E₂ > E₁+E₃ >0, can I say that a + b will happen and not a + c?

Please don't bother yourself with balancing the equations to make the number of electrons transferred equal.

I hope you've understood what I wanted to convey.

[renge ishyo]

First, notice that equation a is just a combination of equations b and c. If you combine b and c you get a:

(equation b)  Fe   Fe²⁺ + 2e^-     E₂
(equation c)  Fe³⁺ + 3e^-   Fe    -(E₃)
--------------------------------------------------------------------------------------------------------
(equation a) Fe³⁺ + e^-   Fe²⁺    E₁

If you combine a and b you get c:

(equation a) Fe³⁺ + e^-   Fe²⁺    E₁
(equation b)  Fe²⁺ + 2e^-    Fe    -(E₂)
--------------------------------------------------------------------------------------------------------
(equation c)  Fe³⁺ + 3e^-   Fe    -(E₃)

If you combine a and c you get b:

(equation a) Fe³⁺ + e^-   Fe²⁺    E₁
(equation c)  Fe     Fe³⁺ + 3e^-  E₃
--------------------------------------------------------------------------------------------------------
(equation b)  Fe    Fe²⁺ + 2e^-    E₂

These relationships all yield the general relationship that E1 = E2 - (E3). Since E1 is positive E2 must be larger than the positive value of E3. This is consistent with your inequality after simplification (E1 + E2 > E1 + E3 > 0 can also be written as simply E2 > E3 > 0 since E1 is added to both E2 and E3).

Now, if we try to add equation a directly to equation b we would get E1 + E2 = E3 which is inconsistent with the relationship E1 = E2 - E3 that appears above. So a + b shouldn't happen directly as written.

However, if we directly add equation a with equation c we would get E1 + E3 = E2 which is consistent with the relationship E1 = E2 - E3 that appears above. Therefore, a + c should be able to happen directly as written (in fact, this is observed if you look at the equations written out in the third example, none of them are "flipped").

I hope this was what you were curious about.

[Schrödinger]

Thanks renge ishyo.
I was actually referring to a general case. It turns out that the given example reactions can be got by adding the other two (with or without flipping). The examples I gave were probably not very useful.

Anyway, what I wanted to ask was, if there are three unrelated equations A,B and C with E1, E2 and E3 as the respective E^o values all of which are positive, can I jump to the conclusion that the one with the highest E^o value will occur and not the other two (because its is more negative than that of the other two reactions)?

Or will there be exceptions? (as usual )

[Borek]

Are you comparing formal or standard potentials?

[Schrödinger]

Sorry for my limited knowledge, but what exactly is formal potential?

[Borek]

Standard potential is a potential as read from the tables - it assumes activities of all substances involed are equal to 1. That (approximately) means pH 0, all concentrations equal 1M and all pressures (if there are gases involved) 1 atm.

Formal potential is a potential as observed in the real solution, taking into account all real concentrations (and using Nernst equation).

It may happen that reaction that seems to be obvious just from comparing standard potentials in reality is replaced by other, or even goes in the opposite direction, just because formal potential substantially differs from the standard one. That's for example what happens to the gold. Gold can't be oxidized by atmospheric oxygen - unless it is put in the solution af cyanides. They create very stable complex which lowers formal potential of the gold oxidation enough to make the reaction possible.

[Schrödinger]

Standard potential is a potential as read from the tables - it assumes activities of all substances involed are equal to 1. That (approximately) means pH 0, all concentrations equal 1M and all pressures (if there are gases involved) 1 atm.

Formal potential is a potential as observed in the real solution, taking into account all real concentrations (and using Nernst equation).

It may happen that reaction that seems to be obvious just from comparing standard potentials in reality is replaced by other, or even goes in the opposite direction, just because formal potential substantially differs from the standard one.

I was referring to standard potentials. Suppose that formal potential(real values) are the same as the standard electrode potentials (something like an ideal experiment), then would it be right to jump to the conclusion? (referring to my earlier post)

[renge ishyo]

Anyway, what I wanted to ask was, if there are three unrelated equations A,B and C with E1, E2 and E3 as the respective Eo values all of which are positive, can I jump to the conclusion that the one with the highest Eo value will occur and not the other two (because its  is more negative than that of the other two reactions)?

In the case of three unrelated reactions can you generalize the result? I would say the answer is no.

An easy way to see how this can be is if you have a reaction with the largest positive E/ :delta: G that reacts much much slower than the other reactions with lower positive E values (since the reactions are unrelated, the kinetics can differ substantially depending on the species involved). If the other two reactions were kinetically much faster you would observe initially that only they would be taking place despite their lower E values.

Also, I wouldn't generalize by saying that after you reach equilibrium that "only" the reaction with the largest E value is taking place. All that the standard E and G values tell you are the relative amounts of each species you will have left at equilibrium if you started with 1M concentrations under strictly specified conditions. ALL reactions are still taking place back and forth at equilibrium, the standard E just gives you an overall idea of the relative amounts of each species you will have in solution at equilibrium. The E is more like a census of a city than it is a description of the processes that take place within that city.

[Schrödinger]

Thanks renge.
I didn't think of kinetics. That made a lot of things clear.

Besides, I forgot the famous example of NaCl electrolysis.... Cl₂ evolves and not O₂ due to the over potential of O₂