Hazards
- Chromium salts are considered carcinogenic. Use care.
- 6M HCl and 6M NaOH are corrosive and toxic. Avoid contact.
Chemicals and Solutions
- Potassium chromate (K₂CrO₄) solution 0.1M
- Potassium dichromate (K₂Cr₂O₇) solution 0.1M
- 6M hydrochloric acid
- 6M sodium hydroxide
Materials
2 ungraduated cylinders
Procedure
- Add some 0.1M K₂CrO₄ to one cylinder and then add 6M HCl. The solution will shift from yellow to orange.
- To the second cylinder add some 0.1M K₂Cr₂O₇ 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:
\( \ce{$\underset{\text{yellow}}{\ce{2CrO4^2-_{(aq)} + 2H+_{(aq)}}}$ -> $\underset{\text{orange}}{\ce{Cr2O7^2-_{(aq)} + H2O}}$} \)