Catalysis of the reaction between sodium thiosulfate and hydrogen peroxide
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.
Read our standard health & safety guidance
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 and chemicals
Eye protection
The teacher will require:
Flasks (1 dm3), 4
Measuring cylinder (100 cm3)
Beaker (100 cm3)
Quantities for one demonstration:
Sodium thiosulfate-5-water, 8.7 g.
Sodium ethanoate acetate-3-water (sodium acetate tri-hydrate), 3.8 g
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, a few cm3
Deionised water, 1.1 dm3
Technical notes
Sodium thiosulfate (Low hazard) Refer to CLEAPSS Hazcard 95C and Recipe card 67
Sodium ethanoate (Low hazard) Refer to CLEAPSS Hazcard 38A
Sodium hydroxide (Corrosive) Refer to CLEAPSS Hazcard 91 and Recipe card 65
Ammonium molybate(VI) (Harmful) Refer to CLEAPSS Hazcard 9A
Hydrogen peroxide (Harmful) Refer to CLEAPSS Hazcard 50 and Recipe card 32
Before the demonstration
SAFETY: Wear eye protection and consider wearing gloves when handling sodium hydroxide and 100 volume hydrogen peroxide.
Dissolve the sodium thiosulfate, sodium ethanoate and sodium hydroxide together in deionised 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.
Make a solution of hydrogen peroxide from 14 cm3 of 100 volume hydrogen peroxide made up to 40 cm3 with deionised water. Divide this into two 20 cm3 portions.
Weigh out 0.08 g of ammonium molybdate.
Procedure
HEALTH & SAFETY: Wear eye protection
a Place the three flasks containing the blue solution on a bench. Add the weighed ammonium molybdate to the one labelled ‘Catalyst’ and swirl.
b 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.
c 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 + 4H2O2(aq) → Na2SO4(aq) + H2SO4(aq) + 3H2O(I)
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 and Safety checked, October 2007
Updated 29 Oct 2008
4 out of 5
Used in BTEC Applied Science coursework with the effect of a catalyst on the rate of reaction
Submitted by: jo cross on 20 January 2010