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Chromate Dichromate Equilibrium

Hazards

  • Chromium salts are considered carcinogenic. Use care.
  • 6M HCl and 6M NaOH are corrosive and toxic. Avoid contact.

Chemicals and Solutions

  • Potassium chromate (KCrO) solution 0.1M
  • Potassium dichromate (KCrO) solution 0.1M
  • 6M hydrochloric acid
  • 6M sodium hydroxide

Materials

2 ungraduated cylinders

Procedure

  1. Add some 0.1M KCrO to one cylinder and then add 6M HCl. The solution will shift from yellow to orange.
  2. To the second cylinder add some 0.1M KCrO and then add 6M NaOH. The solution will shift from orange to yellow.

Hint: This demonstration can also be done on an overhead projector using petri dishes instead of cylinder.

Discussion

The reaction:

\( \ce{$\underset{\text{yellow}}{\ce{2CrO4^2-_{(aq)} + 2H+_{(aq)}}}$  ->  $\underset{\text{orange}}{\ce{Cr2O7^2-_{(aq)} + H2O}}$} \)

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