Explaining The Corrosion Of Iron In NaOH With Equilibrium

[LoganJohnston]

The question asks:

 

Recall that you observed very little corrosion occurring on the iron nail immersed in NaOH(aq) solution. This observation is difficult to explain from an electrochemistry perspective since electrochemistry principles predict a spontaneous reaction that should cause corrosion. Explain why there was no corrosion on this nail from an equilibrium perspective using the half-reaction shown below:

O2(g) + 2 H2O(l) + 4 e- --> 4 OH-(aq)

 

I understand that the reaction is spontaneous between NaOH(aq) and Fe(s), yet no visible precipitate forms. From an equilibrium perspective though, I am truly stumped.

 

It obviously has something to do with Le Châtelier's Principle, but since the solution is a condensed state I'm not sure how to apply the principle.

 

Any thoughts?

[John Cuthber]

 

 

I understand that the reaction is spontaneous between NaOH(aq) and Fe(s),

I don't

[studiot]

Iron stops rusting at high enough pH, which is why reinforcing bars don't rust in concrete.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&biw=&bih=&q=ph+to+stop+iron+rusting&gbv=2&oq=ph+to+stop+iron+rusting&gs_l=heirloom-hp.3...1953.9172.0.9984.23.2.0.21.21.0.141.282.0j2.2.0....0...1ac.1.34.heirloom-hp..21.2.282.cfqLBSQ60cA

Edited December 30, 2015 by studiot

[LoganJohnston]

I don't

using standard E values

 

O2(g) + 2 H2O(l) + 4 e– ⇌ 4 OH– (aq) ... E°+0.40

Fe(s) ⇌ Fe2+(aq) + 2 e– ... E°+ 0.45

 

And this sentence from the question : "This observation is difficult to explain from an electrochemistry perspective since electrochemistry principles predict a spontaneous reaction that should cause corrosion."

[John Cuthber]

OK where did the oxygen (O₂) come from?

Neither Fe nor NaOH is O₂

The reaction of iron with water might be spontaneous and the reaction with oxygen certainly is.

But that doesn't say anything about the reaction of NaOH and Fe.

[LoganJohnston]

OK where did the oxygen (O₂) come from?

Neither Fe nor NaOH is O₂

The reaction of iron with water might be spontaneous and the reaction with oxygen certainly is.

But that doesn't say anything about the reaction of NaOH and Fe.

 

I agree, I worked that out on NYE.

 

So to summarize: the reaction of iron in the presence of water and O₂ is spontaneous, yes.

 

In the presence of hydroxide, there is no O₂ to react with so no reaction occurs, which explains the lack of corrosion seen in the earlier lab the question references.

 

I understand all of that just fine now, but I will have to email my teacher as I don't understand why the question states the reaction is spontaneous.

 

In answer to my original question of explaining the non-reaction with the provided half-reaction, could it be that the concentration of OH^-_(aq) is much higher than the concentration of O_2(g), thus no O_2(g) is present to react with the iron nail?

[John Cuthber]

It may be significant that, in the presence of hydroxide:

there are fewer hydrogen ions to react thus

Fe +2H⁺ --> Fe⁺⁺ + H₂

And that Fe(OH)₂ and Fe(OH)₃ are insoluble so they get in the way of the reaction of Fe with O₂

 

Rusting is complicated.

[LoganJohnston]

Yes it is

 

I have since been explained the answer thusly:

 

"Corrosion is the result of a redox reaction. Shown below is the reduction half-reaction. Now if we add more hydroxide ions, the reaction will shift towards the left, correct? This will impede the reduction half-reaction – it will not occur to any great extent. Hence there will be very little corrosion."

 

We were on the right track, although something seems wrong with the way this question was worded. Or perhaps something was wrong with my interpretation.

 

Oh well, something to think about I suppose. Thank you all for your input.