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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: telle_star on March 10, 2006, 11:50:42 PM

Title: crocoite + hydrochloric acid = chromium oxide + ???
Post by: telle_star on March 10, 2006, 11:50:42 PM

Hey all,
I was just wondering if anyone would know the complete balanced equation between crocoite and HCl to make chromium oxide? A reference would be helpful to since i can find the reaction anywhere.
I got an idea...
PbCr04 + HCl ---> CrO3 + ??
Please *delete me*
Thanks ???

Title: Re:crocoite + hydrochloric acid = chromium oxide + ???
Post by: madscientist on March 11, 2006, 01:08:41 AM

Quote

In the mid 18th century analysis of Siberian "red lead" (PbCrO4, crocoite) from Siberia showed that it contained quite a lot of lead, but also a further material. This was eventually identified as chromium oxide. Chromium oxide was discovered in 1797 by Louis-Nicholas Vauquelin, who prepared the metal itself in the following year. Starting from crocoite the procedure was to powder the mineral and to precipitate the lead out through its reaction with hydrochloric acid (HCl in water). The residue was chromium oxide, CrO3. Heating this oxide in an oven in the presence of charcoal as a reducing agent gave the metal itself.

http://www.webelements.com/webelements/elements/text/Cr/hist.html

Have a good read of the above quote, it should tell you how you need to write out your reaction, think about what happens, what species are/is produced in the precipitation reaction with lead and HCl.

cheers,

madscientist :albert:

Title: Re:crocoite + hydrochloric acid = chromium oxide + ???
Post by: jen on March 13, 2006, 07:05:57 AM

I asked my teacher and this's an answer question
PbCrO4 + HCl ---> CrO3 + Cl2  + PbCl2 + H2O   ::)

Title: Re:crocoite + hydrochloric acid = chromium oxide + ???
Post by: Borek on March 13, 2006, 07:26:25 AM

Quote from: jen on March 13, 2006, 07:05:57 AM

I asked my teacher and this's an answer question
PbCrO4 + HCl ---> CrO3 + Cl2  + PbCl2 + H2O   ::)

No way this can be balanced. Cl^- gets oxidized to Cl₂, but there is no reducing agent.

Title: Re:crocoite + hydrochloric acid = chromium oxide + ???
Post by: Alberto_Kravina on March 13, 2006, 01:29:04 PM

I think that a possible reation could be:

PbCrO₄ + 2 HCl ----> PbCl_2(s) + "H₂CrO₄"

And maybe this instable chromic acid could decompose to CrO₃ and H₂O

I'm not sure if this reaction happens, though.
I'm not sure....maybe if Excess HCl is added dichromate could form... ???

Title: Re:crocoite + hydrochloric acid = chromium oxide + ???
Post by: Negahrebas on March 14, 2006, 09:24:47 PM

2CrO₄^2- + 2H¹⁺ = Cr₂O₇^2- + H₂O ???

==>

2PbCrO₄ + 2HCl = PbCr₂O₇ + PbCl₂ + H₂O ???

 ;D