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The Chemistry of Chromium (Demo)

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    222005

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    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 A, C, and D, and HCl is added to them.
    • HCl and granulated Zn are added to dish B.
    • BaCl2 is 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.
    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.

    Contributors

    The Chemistry of Chromium (Demo) is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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