Speed up the colour change that occurs when an ammonium molybdate catalyst is added to sulfuric acid – that has been created by oxidising sodium thiosulfate with hydrogen peroxide.

Demonstration

Hydrogen peroxide oxidises sodium thiosulfate to sulfuric acid. Starting from an alkaline solution, the resulting pH change can be followed using Universal indicator which changes from blue to green to yellow to orange-red. Adding an ammonium molybdate catalyst speeds up the colour change.

Lesson organisation

This introductory demonstration is useful as a starting point for a discussion on catalysts. A white background will help the students to see the colour changes clearly.

Apparatus Chemicals

Eye protection

The teacher will require:

Flasks (1 dm3), 4

Measuring cylinder (100 cm3)

Beaker (100 cm3)

Sodium thiosulfate-5-water, 8.7 g.

Sodium ethanoate-3-water (sodium acetate tri-hydrate), 3.8 g or anhydrous sodium ethanoate, 2.3 g

Sodium hydroxide (CORROSIVE), 0.5 g

Ammonium molybate(VI) (HARMFUL), 0.08 g

Hydrogen peroxide, 100 ‘volume’ (HARMFUL), 14 cm3

Universal indicator solution (HIGHLY FLAMMABLE), a few cm3

Deionised or distilled water, 1.1 dm3

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection and consider wearing gloves when handling sodium hydroxide and 100 ‘volume’ hydrogen peroxide.

Sodium thiosulfate-5-water, Na2S2O3.5H2O(s) - see CLEAPSS Hazcard.

Sodium ethanoate-3-water, CH3CO2Na.3H2O(s) - see CLEAPSS Hazcard.

Sodium hydroxide, NaOH(s), (CORROSIVE) - see CLEAPSS Hazcard. 

Ammonium molybdate(VI),  (HARMFUL) - see CLEAPSS Hazcard.

Hydrogen peroxide, H2O2(aq), (HARMFUL) - see CLEAPSS Hazcard and Recipe Book 

Procedure

Before the demonstration

a Dissolve the sodium thiosulfate, sodium ethanoate and sodium hydroxide together in deionised or distilled water and make up to 1 dm3. Add sufficient Universal indicator solution to give an easily visible blue colour. Pour 225 cm3 of this solution into each of three 1 dm3 flasks labelled ‘Catalyst’. ‘No catalyst’ and ‘Control’, respectively.

b Make a solution of hydrogen peroxide from 14 cm3 of 100 volume hydrogen peroxide made up to 40 cm3 with deionised or distilled water. Divide this into two 20 cm3 portions.

c Weigh out 0.08 g of ammonium molybdate(VI).

For the demonstration itself

d Place the three flasks containing the blue solution on a bench. Add the weighed ammonium molybdate to the one labelled ‘Catalyst’ and swirl.

e Then add 20 cm3 portions of hydrogen peroxide solution to the flasks marked ‘Catalyst’ and ‘No catalyst’, leaving the third flask as a control for colour comparison.

f Over three or four minutes, the solution with the catalyst changes from blue through green, yellow and orange to orange-red. The solution without the catalyst follows the same sequence, but more slowly. It will not have reached the same red-orange colour of the first solution after an hour.

Teaching notes

The reaction is:

Na2S2O3(aq) + 4H2O2(aq) → Na2SO4(aq) + H2SO4(aq) + 3H2O(l)

The sulfuric acid produced by the reaction neutralises the sodium hydroxide (buffered by the sodium ethanoate) and gives the observed colour changes.

If the reaction is done with 20 volume hydrogen peroxide, the reaction is slower than with the diluted 100 volume hydrogen peroxide.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015