corrosion, rusting question...

[albertlee]

I know that when iron crodes, it also means it rusts...

 

however,

 

I thought, as redundant time has gone by, iron will react will oxygen apparantly, but in my textbook, it says it only rusts with oxygen+water...why????

[Primarygun]

Rusting of iron is the corrosion of iron by both water and oxygen so that it becomes hydrated iron(III) oxide.

Oxide layer of iron is not impermeable to water

[mezarashi]

Rust is hydrated iron(III) oxide. Water is a vital component to rusting. The water also helps remove the rust from the iron and thus you can have an iron bar "disappear" if exposed long enough. This is quite different from aluminum which has instead reacts to produce a "protective" aluminum oxide layer.

[YT2095]

IIRC, it has something to do with collisions of ions for the exchange to take place, Iron oxide can be made without water quite easily as it burns quite well in pure oxygen, BUT the iron has to be in a Liquid state for this to occur.

I`m sure there was alot more to it than just that, but that`s all I remember at the moment

[albertlee]

Iron oxide can be made without water quite easily as it burns quite well in pure oxygen, BUT the iron has to be in a Liquid state for this[/b'] to occur.

 

for what to occur???

[YT2095]

the reaction (oxidation).

[rthmjohn]

If you think of it this way, it is easier for iron to reduce (for oxidation to take place, reduction must also take place to make a redox reaction) the hydrogen in H2O than it is to reduce O2 gas (check out a reduction-half reaction table). For iron to burn, it has to be heated in pure O2. This highly exothermic process yields magnetite (Fe3O4).

[LucidDreamer]

Fe(s) --> Fe2+(aq) + 2e-

 

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

 

2H+(aq) + 2e- --> H2(g)

 

Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s)

 

4Fe2+(aq) + 4H+(aq) + O2(aq) --> 4Fe3+(aq) + 2H2O(l)

 

Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s)

 

Fe(OH)3(s) --> Fe2O3.H2O (not balanced)

 

So the water is needed to facilitate the loss of electrons in the first step and provide the hydroxide ions.

[albertlee]

If you think of it this way, it is easier for iron to reduce (for oxidation to take place, reduction must also take place to make a redox reaction) the hydrogen in H2O than it is to reduce O2 gas (check out a reduction-half reaction table). For iron to burn, it has to be heated in pure O2. This highly exothermic process yields magnetite (Fe3O4).

 

so, how does the hydrated iron oxide come from??

 

ps, sorry, I cant understand Lucid's steps of chemical equations....

[rthmjohn]

Upon contact with water, Fe is OXIDIZED to Fe2+ ions (Fe gives up its electrons so to speak). The electrons simultaneously REDUCE the dissolved O2 gas (yes oxygen dissolves in water) and H2O to make OH- (hydroxide) ions. The OH- then reacts with the Fe2+ to form iron(II)hydroxide (Fe(OH)2). Iron(II)hydroxide then loses its hydrogens and reduces more dissolved oxygen gas which makes a hydrated iron(III)oxide.

 

The equation for the last step is as follows: 4Fe(OH)2 + O2 --> 2Fe2O3 . xH20 + 2H2O (the degree hydration of the rust may vary)

 

I think Lucid's equations might have a small error.

[albertlee]

The electrons simultaneously REDUCE the dissolved O2 gas (yes oxygen dissolves in water) and H2O to make OH- (hydroxide) ions.

 

How do electrons reduce the dissolved O2 gas? since O2 gas is covalently bonded....

 

when you say REDUCE, do you mean you gain O2 gases by gaining its electrons??

 

a bit confused here....

 

plz help

[albertlee]

please help according to my previous message...

 

thx

[albertlee]

any one?????

[rthmjohn]

When I say reduce I mean that the O2 GAINS electrons, which breaks the covalent bond and forms oxide (O2-) ions. Remember that oxidation simply entails the loss of electrons; and reduction entails the gaining of electrons. Metals are good reducing agents because they give away their electrons. Nonmetals are good oxidizing agents because they tend to gain electrons from a reducing agent. Whenever there is reduction, there is always oxidation and vice versa. These are called redox reactions.

[albertlee]

wait...

 

do oxygen gas loose electrons to when dissolved in water??

 

how.. ?

 

I am not familar with the idea of a covalent bonded molecule in a liquid....

 

thx

[albertlee]

4Fe(OH)2 + O2 --> 2Fe2O3 . xH20 + 2H2O

 

how is the above step possible??

 

how does iron oxide bond with the water molecule??

[albertlee]

ok... this question has nothing to do with my last question here...

 

First we know that iron must be oxidized,

 

Fe + H2O -> FeOH + H2 (not balanced)

 

However, is it OH- ion that cause Fe to loose its electrons??

 

OH- already has excess electron, therefore it must be the H+ ion which causes Fe to loose electron

 

so, the oxidizing agent is actually H+ ions, right??

like this: Fe + 2H+ --> Fe2- + H2

 

 

2nd question: If there is no oxygen in water, does Iron still form iron hydroxide?? because in reality, I never see what does iron hydroxide look like....

 

 

any help??

 

thx alot

[qrasy]

No, there's no hidrogen released when the rust forms. I think it should be oxygen, electron+oxygen+water->hydroxide ion.

[albertlee]

so what element/molecule/compound attracts the electrons of iron in the 1st place during rusting??

 

I mean, the OH- ions should not be the factor, because it does not need more electrons...

[albertlee]

oh... please help...

 

Am I asking something nonsense??

 

please help...

 

thanks

[albertlee]

from wikipedia

When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidised to iron(II):

 

Fe → Fe2+ + 2e-

 

 

Can any one just explain how the above happens in rusting?? ie, what is the electrochemical process here? What does it have to do with electrochemistry??

 

Can any one please help here.....

 

 

thanks

[jdurg]

Every chemical reaction is electrochemistry. Chemical reactions are the movement of electrons from one atom/molecule to another. So no matter what it is, whether it's the burning of methane or the rusting of iron, it's an electrochemical equation.

 

For the rusting of iron, the oxygen gas that is dissolved in the water pulls electrons away from Fe forming Fe(2+) and O(2-). (So one oxygen molecule needs two iron atoms to form the two oxide anions). Water naturally dissociates into H+ and OH- ions, so those free oxygen atoms will become attracted to the H+ ions and form water. This leaves OH- ions in solution which bind to the Fe(2+) ions forming iron hydroxide. In solution, this leaves you with iron hydroxide. However, if the water is evaporated away, the iron hydroxide rearranges itself and forms iron-oxide with molecules of water trapped inside the crystal structure. This is what we call rust.

[albertlee]

thanks Jdurg

[sanjaygeorge]

well..........the fe2o3 and water have a type of molecular bonding

[albertlee]

However, if the water is evaporated away, the iron hydroxide rearranges itself and forms iron-oxide with molecules of water trapped inside the crystal structure. This is what we call rust.

 

oh yeah..

 

btw, I have seen rust formed by iron in water, without evaporating the water..

 

 

what's the chemistry here apart from the above quote??

 

 

thanks alot