An equilibrium involving chromate(VI) and dichromate(VI) ions

Read our standard health & safety guidance

Lesson organisation

This experiment is most appropriate with A-level students, given the potential hazards with solutions containing chromate(VI) and dichromate(VI) ions. Otherwise it could be carried out as a teacher demonstration.

This experiment can be carried out individually by students, but the potassium chromate(VI) solution used should be prepared beforehand by the teacher or technician, given the hazards presented by the solid.

It should take no more than 5 min.

Apparatus and chemicals

For one demonstration:

Eye protection: goggles
Test-tube
Test-tube holder
Dropping pipette

Potassium chromate(VI) (K₂CrO₄) solution, 0.2 mol dm^-3 (Toxic, Oxidising, Dangerous for environment), about 1 cm³
Sodium hydroxide solution, 1.0 mol dm^-3 (Corrosive at this concentration), about 10 cm³
Dilute sulfuric acid, 1.0 mol dm^-3 (Irritant at this concentration), about 5 cm³

Technical notes

Potassium chromate(VI) (Toxic, Oxidising, Dangerous for environment) Refer to CLEAPSS Hazcard 78 and Recipe Card 53
Sodium hydroxide (Corrosive at concentration used) Refer to CLEAPSS Hazcard 91 and Recipe card 65
Dilute sulfuric acid (Irritant at concentration used) Refer to CLEAPSS Hazcard 98A and Recipe card 69

Procedure

HEALTH & SAFETY: wear goggles

a Put 10 drops of potassium chromate(VI) solution in a test-tube.

b Add about 5 cm³ of sulfuric acid and shake to mix thoroughly. What do you observe?

c Now add about 10 cm³ of sodium hydroxide solution and shake the mixture. Do you get back to where you started?

d Can you repeat the whole process by adding sulfuric acid and sodium hydroxide alternately all over again?

Teaching notes

The process occurring involves the equilibrium between chromate(VI), dichromate(VI) and hydrogen ions:

The addition of acid encourages the equilibrium towards the right, producing more orange-coloured dichromate(VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease, and this brings the equilibrium back to the left-hand side, regenerating yellow chromate(VI) ions.

It is important that students realize that both ions are always present and exist in equilibrium with one another, but that the yellow chromate(VI) ions predominate under alkaline conditions and the orange dichromate(VI) ions predominate in acidic solutions.

Students may be asked if this a redox reaction. Their answer is often ‘yes’, but examination of oxidation numbers will show that chromium remains in the +6 oxidation state throughout.

Health & Safety checked, August 2008

Web Links

A set of still pictures showing the colour changes described here can be found at:
http://jchemed.chem.wisc.edu/JCESOFT/CCA/CCA8/MAIN/8/08/32/thumbs.html

(Websites accessed August 2008)