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In the iron oxidation that results in rust, the first step is to steal 2 electrons from $\ce{Fe}$. The main thief is said to be the oxygen dissolved in water, witch uses the stolen electrons to form $\ce{OH-}$ according to the reaction $\ce{O2 + 2H2O + 4e- -> 4OH-}$.

What is really going on in this reaction? I cannot believe that this 3 reactants bump together at the same time and magically $\ce{4 OH-}$ comes out of the collision. I imagine that there must be some steps between the reactants and $\ce{4OH-}$. Is it that, first, $\ce{O2}$ steals the two electrons from the $\ce{Fe}$ lattice becoming $\ce{2O-}$ and then, later, each $\ce{O-}$ takes an $\ce{H}$ from $\ce{H2O}$ to form $\ce{2OH-}$? That does not seem right to me, because this means that what is happening is:

$$\ce{O2 + 4e- -> 2O-}$$

$$\ce{2O- + 2H2O -> 4OH-}$$

But the reaction "$\ce{O2 + 4e- -> 2O-}$" implies that $\ce{2O-}$ is more stable than $\ce{O2}$. Is that right?

Henrique Rigitano
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  • Welcome to ChemSE. Please take a moment to familiarize yourself with the tools this web site offers in terms of formatting. In addition to other sites, mathjax and mhchem (related to the LaTeX package of same name) offer useful assistance for mathematical and chemical expressions; a brief starter is provided here: https://chemistry.meta.stackexchange.com/questions/86/how-can-i-format-math-chemistry-expressions-here/88 – Buttonwood Jul 09 '17 at 22:53
  • The equations you mention are book keeping starting materials, on one side, and products, on the other. They are not about the mechanism how the interconversion happens. At least partially related is this https://chemistry.stackexchange.com/questions/76520/why-is-the-notation-for-reactions-not-explicit/76537#76537 And no, while it is already rare two molecules collide with each other such that there is a chemical reaction, Oxygen + Hydrogen + 4 electrons colliding at once to form four $\ce{OH-}$ is even less probable. – Buttonwood Jul 09 '17 at 22:57

2 Answers2

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Bringing up rust seems to indicate confusion between the concept of corrosion and the concept of oxidation. Practically, iron corrosion is the breakdown of iron as a result of oxidation reactions.

To explain corrosion, understand water can self-ionize producing hydronium, $H_3O^+$, and hydroxide, $OH^-$ as well as ionize small amounts of iron causing it to dissolve. The dissolved iron ions collide with the hydroxide ions to form iron hydroxide. Iron hydroxide may fall out of the solution as it is not very soluble, or if the concentration is low enough the surplus of hydronium donates a proton to the iron hydroxide converting it to iron oxide.

Oxidation, which is a more abstract concept, involves any reaction which results in the loss of an electron. Instead of considering corrosion, consider other reactions. In photosynthesis water is oxidized because it loses an electron. In combustion, fuel is oxidized because it looses an electron.

Chemical reactions can get extremely complex, so it may be prudent to master the language of chemistry before delving into the complexities of specific reactions. Many reactions are described with the language of redox (Reduction and Oxidation) chemistry.

Agriculturist
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  • Ok, I understand the reduction and oxidation nomenclature and also what happens with iron once it is dissolved in water. What I don't understand is what, exactly, takes the iron electrons form iron so that it ends up positive and get expelled from the lattice to end up in the solution. every explanation says that the iron electrons are taken by oxygen diluted in water. those explanations present the partial equation in my question ( diluted oxygen gas + water + electrons = hydroxide). I still don't understand the mechanism of that partial reaction. – Henrique Rigitano Jul 10 '17 at 20:00
  • Try rewriting your question omitting the use of the word "I", "me", "my", or anything in the first person. – Agriculturist Jul 10 '17 at 22:07
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As Argriculturist has said water can self ionise into $\ce{H+}$ and $\ce{OH-}$, but this takes a long time. So, 4 water molecules will self ionise into 4 hydrogen ions and 4 hydroxide ions

$\ce{4H2O -> 4H+ + 4OH-}$

The 4 hydrogen ions will soon react with the dissolve oxygen to form water molecules.

$\ce{O2 + 4H+ + 4e- -> 2H2O}$

So when we combine the equation, it will form this equation

$\ce{4H2O + O2 + 4H+ + 4e- -> 4H+ + 4OH- + 2H2O}$

And by cancelling the reapeating molecules in both sides, it become like this

$\ce{2H2O + O2 +4e- -> 4OH-}$

Simon-Nail-It
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  • This is what I thought, if there're mistakes, please tell – Simon-Nail-It Jul 10 '17 at 23:32
  • interesting! now, going deeper, for the reaction $\ce{O2 +4H+ + 4e- -> 2H2O}$ to happen, how the $\ce{4e-}$ got there? in other words, what is the species that pulled the electrons out of the iron lattice? $\ce{O2}$? – Henrique Rigitano Jul 11 '17 at 18:17
  • Iron is conductive... Here is a link to how the corrosion process works http://www.rustbullet.com.au/technical/how-it-works/introduction-to-corrosion-and-process/ – Agriculturist Jul 12 '17 at 14:53
  • This site tells it is $\ce{H+}$ that pulls the electrons out of iron ($\ce{2 H+ + 2 e- –> H2}$). But wikipedia (https://en.wikipedia.org/wiki/Rust) tells that it is oxygen ($\ce{O2 + 4  e− + 2  H2O → 4  OH−}$). Who is right? – Henrique Rigitano Jul 13 '17 at 18:01