Redox chemistry with dichromate ions

Reduce dichromate ions by using hydrogen peroxide, and observe the colour changes that occur. What does this colour change mean? What kind of reaction does it suggest? 

This experiment should take five minutes. You can also use this reaction in a series of microscale experiments to investigate the chemistry of the transition elements and observe and compare their colour changes.

Equipment

Apparatus

• Eye protection
• Student worksheet
• Clear plastic sheet (eg OHP sheet)

Chemicals

• Potassium dichromate 0.2 mol dm^–3
• Hydrogen peroxide 5% solution

Health, safety and technical notes

• Read our standard health and safety guidance.
• Students must wear suitable eye protection (Splash resistant goggles to BS EN166 3).
• Potassium dichromate, K₂Cr₂O₇, 0.2 mol dm^–3 is a carcinogen, mutagen, reproductive toxin, skin and respiratory sensitiser. It is also toxic if inhaled, corrosive to skin and eyes and toxic to aquatic life. Wear splash-proof eye-protection if transferring large amounts. Avoid skin contact. See CLEAPSS Hazcard HC078c.
• Hydrogen peroxide, 5% solution H₂O₂ (aq) is of low hazard. See CLEAPPS Hazcard HC050.

Procedure

1. Cover the worksheet with a clear plastic sheet.
2. Put one drop of potassium dichromate solution in the circle provided.
3. Add one drop of 5% hydrogen peroxide to the potassium dichromate solution.

Observations

On adding the hydrogen peroxide solution, the reaction mixture immediately turns a deep blue colour.

After a while bubbles are seen and the colour gradually fades to a pale blue-green due to hexa-aqua chromium(III) ions.

The reaction is: Cr₂O₇^2–(aq) + H₂O₂(aq) + 8H⁺(aq) → 2Cr³⁺(aq) + 5H₂O(I) + 2O₂(g)

Questions

1. Observe carefully. Are there any changes over the next few minutes?
2. Give an explanation for your observations and try to write an equation for the reaction.
3. What type of reaction is occurring?