Phosphoric acid vs. Hydrochloric acid when removing rust and rerusting

[Crushgear64]

Homework Statement

Why does a 12mol/L solution of HCl used to clean rust off iron get a brown coating after being exposed to air and washed with water, while the same concentration solution of phosphoric acid takes a day to have an apparent brown coating.

Homework Equations

Explain the similarities and differences in the chemistry of each acid when used to remove rust.

The Attempt at a Solution

6HCl+ Fe2O3 →2FeCl3 + 3H2O

2H3PO4 + Fe2O3 → 2FePO4 + 3H2O

Phosphoric is a weak acid? Lower concentration of hydrogen ions.

 

[chemisttree]

Why does a 12mol/L solution of HCl used to clean rust off iron get a brown coating after being exposed to air and washed with water, while the same concentration solution of phosphoric acid takes a day to have an apparent brown coating.

Here is your hint...

 

[AGNuke]

Actually, FeCl₃ is a water soluble salt, which is yellow in anhydrous state, but forms brown, acidic aqueous solution. It is also a deliquescent.

So when FeCl₃ is formed, it absorbs water formed alongside itself, and from air and thus forming a brown coating.

Actually, the real chemistry behind this involves Co-ordination chemistry.
FeCl₃ absorbs water to form its hydrated salt, FeCl₃.6H₂O; which is actually [Fe(H₂O)₄Cl₂]Cl⋅2H₂O.

This is like CuSO₄.5H₂O; which is actually [Cu(H₂O)₄]SO₄.H₂ in aqueous form. The [Cu(H₂O)₄]²⁺ complex ion is the reason the Copper Sulphate gets its blue colour.

Same here with [Fe(H₂O)₄Cl₂]⁺ ion, which is brown.

The case of Ferric Phosphate is different. Firstly, it is also yellow salt, but is not a good deliquescent, which can form brown coating similarly to previous salt as quickly.

 

[chemisttree]

Actually, FeCl₃ is a water soluble salt, which is yellow in anhydrous state, but forms brown, acidic aqueous solution. It is also a deliquescent.

So when FeCl₃ is formed, it absorbs water formed alongside itself, and from air and thus forming a brown coating.

Actually, the real chemistry behind this involves Co-ordination chemistry.
FeCl₃ absorbs water to form its hydrated salt, FeCl₃.6H₂O; which is actually [Fe(H₂O)₄Cl₂]Cl⋅2H₂O.

This is like CuSO₄.5H₂O; which is actually [Cu(H₂O)₄]SO₄.H₂ in aqueous form. The [Cu(H₂O)₄]²⁺ complex ion is the reason the Copper Sulphate gets its blue colour.

Same here with [Fe(H₂O)₄Cl₂]⁺ ion, which is brown.

The case of Ferric Phosphate is different. Firstly, it is also yellow salt, but is not a good deliquescent, which can form brown coating similarly to previous salt as quickly.

While these are good points, I don't think this answers the OP's question. For example, why discuss the color of FeCl₃ if it has been removed by washing? And the question suggests that the brown coating forms after rinsing so it isn't there initially but develops over time.

 

[AGNuke]

Hydrated FeCl₃ is formed over time. See, the water from the reaction is not enough, so it tries to absorb it from the atmosphere, to a certain extent. However, to form full hydrated [Fe(H₂O)₄Cl₂]Cl⋅2H₂O, so that [Fe(H₂O)₄Cl₂]⁺ can show its colour, we have to form its aqueous solution, which is formed only when we wash it with water.

 

[chemisttree]

So you think the HCl is anhydrous? And it's being washed in a solvent other than water?
The title of the thread indicates what is going on. It is re-rusting.