The Chemistry of Chromium (Demo)

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Page ID: 3058

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Chemical Concepts Demonstrated

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

Demonstration

A Cr³⁺ solution is prepared. Some is added to crystallizing dishes A and B.
NaOH is added to the Cr³⁺ solution. Some is poured into dish C.
More NaOH is added to the solution. Some of this is poured into dish D.
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 A, C, and D, and HCl is added to them.
HCl and granulated Zn are added to dish B.
BaCl₂is added to the remaining solution. HCl is then added. Finally, Pb(NO₃)₂ is added.

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.
A, C, & 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.