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The Chemistry of Chromium

Chemical Concepts Demonstrated

  • Cr2+, Cr 3+, and Cr (IV) oxidation states of chromium

Demonstration

  1. A Cr3+ solution is prepared.  Some is added to crystallizing dishes A and B.
  2. NaOH is added to the Cr3+ solution. Some is poured into dish C.
  3. More NaOH is added to the solution. Some of this is poured into dish D.
  4. 3% H2O2 is added to the rest of the solution. Some of this is pour into dish E.  HCl is added to this dish.
  • Solution E is compared with AC, and D, and HCl is added to them.
  • HCl and granulated Zn are added to dish B.
  • BaClis added to the remaining solution. HCl is then added.  Finally, Pb(NO3)2 is added.
chrom.gif

Observations and Explanations

 

Dish Observation / Explanation
A + B

Violet solution of Cr(H2O)6 3+.

C

The color of the solution changes from violet to an "acid green".

D

Green Cr(OH)3 precipitates from the green solution.  More base will cause the solid to redissolve to give a green chromite Cr(OH)4 -solution. 

E

The green solution changes to orange as the CrO42-/Cr2O72- ions are formed.

E + HCl

After the HCl is added a series of erratic color changes are observed.  When the reaction is complete the solution is green.

AC, & D with HCl

Solution A remains violet. Solution C changes from green back to violet. Solution D produces another green solution.

B with HCl & Zn

When the solution becomes acidic, several pieces of Zn are added to the dish.  The bright blue color of Cr2+ (aq) will be visible momentarily, but air oxidation rapidly converts this to a green solution.

Original solution + BaCl2 + Pb(NO3)2

A yellow precipitate of BaCrO4 will form.  Adding HCl will redissolve the precipitate and produce an orange-yellow solution.  Pb(NO3)2 produces another yellow precipitate, PbCrO4.

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