What is the effect of acidity to the equilibrium of chromate and dichromate

Try this class practical to investigate an equilibrium between chromate(VI), dichromate(VI) and hydrogen ions

In this experiment, students add dilute sulfuric acid to an aqueous solution of potassium chromate(VI). They observe the resulting colour changes, before reversing the reaction using aqueous sodium hydroxide.

The 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. The 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 five minutes.

Equipment

Apparatus

• Eye protection (goggles)
• Test tube
• Test tube holder
• Dropping pipette

Chemicals

• Potassium chromate(VI) solution, 0.2 M (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT), about 1 cm3
• Sodium hydroxide solution, 1.0 M (CORROSIVE), about 10 cm3
• Dilute sulfuric acid, 1.0 M (IRRITANT), about 5 cm3

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection (goggles) throughout.
• Potassium chromate(VI) solution, K2CrO4(aq) (TOXIC, OXIDISING, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC078a and CLEAPSS Recipe Book RB069.
• Sodium hydroxide solution, NaOH(aq) (CORROSIVE) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
• Dilute sulfuric acid, H2SO4(aq), (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.

Procedure

1. Put 10 drops of potassium chromate(VI) solution in a test tube.
2. Add about 5 cm3 of sulfuric acid and shake to mix thoroughly. What do you observe?
3. Now add about 10 cm3 of sodium hydroxide solution and shake the mixture. Do you get back to where you started?
4. 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:

2CrO42–(aq) (yellow) + 2H+(aq) ⇌ Cr2O72–(aq) (orange) + H2O(l)

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

Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.

© Nuffield Foundation and the Royal Society of Chemistry

Health and safety checked, 2016

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Discussion

LeChatelier's Principle states that if a system is at equilibrium and something is changed so that it is no longer at equilibrium, the system will respond in an effort to counteract that change. If more reactant is added, the equilibrium will shift to the right in order to consume some of the extra reactant, resulting in more product. If some of the product is removed from the system, the equilibrium will shift to the right to produce more of that product. The equilibrium can be shifted to the left by either adding product to or removing reactant from the system.

Narration

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the right towards the dichromate ion. As nitric acid is added to the chromate solution, the yellow color turns to orange. When sodium hydroxide is added to the chromate solution, the orange color turns back to yellow. The sodium hydroxide reacts with hydrogen ions, removing them from the solution. When one reactant is removed from an equilibrium system, the equilibrium shifts from the right back toward the left, in this case forming yellow chromate ion again.

What happens to chromate ion in acidic pH?

How does increasing the concentration of H+ affect the chromate dichromate equilibrium?

What is the effect of pH on dichromate solution?

What is the effect of increasing pH on a solution of potassium dichromate?