126
Microscale Chemistry
THE ROYAL
SOCIETY OF
CHEMISTRY
16. The reaction between hydrogen
peroxide and dichromate ions
Topic
Transition elements – colours of ions, redox reactions, variable oxidation states.
Level
Post-16.
Timing
5 min.
Description
In this experiment dichromate(VI) ions are reduced to chromate(III) ions by hydrogen
peroxide which is itself oxidised to oxygen gas. The experiment provides several
points for student observation and illustrates an interesting redox reaction.
Apparatus (per group)
▼
One student worksheet
▼
One clear plastic sheet (eg ohp sheet).
Chemicals (per group)
Solutions contained in plastic pipettes, see p. 2
▼
Potassium dichromate
0.2 mol dm–3
▼
Hydrogen peroxide
5% 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:
Cr2O72–(aq) + H2O2(aq) + 8H+(aq) → 2Cr3+(aq) + 5H2O(I) + 2O2(g)
Safety
Students must wear eye protection.
It is the responsibility of the teacher to carry out a risk assessment.
40
Microscale Chemistry
THE ROYAL
SOCIETY OF
CHEMISTRY
16. The reaction between hydrogen
peroxide and dichromate ions
In this experiment you will be looking at the reaction between two substances
containing oxygen.
Instructions
1.
Cover the worksheet with a clear plastic sheet.
2.
Put one drop of potassium dichromate solution in the circle below.
3.
Add one drop of 5% hydrogen peroxide to the potassium dichromate solution.
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. What type of reaction is occurring?
Questions