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Phosphoric acid vs. Hydrochloric acid when removing rust and rerusting

  1. Jul 28, 2012 #1
    1. The problem statement, all variables and given/known data

    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.

    2. Relevant equations

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

    3. The attempt at a solution

    6HCl+ Fe2O3 →2FeCl3 + 3H2O

    2H3PO4 + Fe2O3 → 2FePO4 + 3H2O

    Phosphoric is a weak acid? Lower concentration of hydrogen ions.
     
  2. jcsd
  3. Aug 1, 2012 #2

    chemisttree

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    Here is your hint....
     
  4. Aug 2, 2012 #3

    AGNuke

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    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.
     
  5. Aug 3, 2012 #4

    chemisttree

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    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.
     
  6. Aug 3, 2012 #5

    AGNuke

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    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.
     
  7. Aug 4, 2012 #6

    chemisttree

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    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.
     
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