Following the rate of the iodine-propanone reaction by a titrimetric method
Equation:
I2(aq) + CH3COCH3(aq) + H+(aq) → CH3COCH2I(aq) + 2H+(aq) + I−(aq)
Sulfuric acid acts as a catalyst in this process.
A large excess of both propanone and sulphuric acid are used so that their concentrations
remain effectively constant during the reaction. This allows us to measure the influence of
iodine alone on the rate of reaction.
Method
1. Add 25 cm3 of sulfuric acid to 25 cm3 of propanone in a beaker and mix well.
2. Add 50 cm3 of 0.02 mol dm-3 iodine solution to the beaker and immediately start the
timer.
3. Using a pipette, remove a 10 cm3 sample of the mixture to a conical flask. Add a
spatula of sodium hydrogencarbonate immediately after removing it from the flask to
quench the reaction. Note the time the sodium hydrogencarbonate was added.
4. Withdraw samples every three minutes and repeat this process.
5. Titrate these samples against sodium thiosulphate (Na2S2O3), using starch indicator.
The indicator will turn from blue-black to colourless as iodine reacts with thiosulphate.
6. Repeat for all samples to find their concentrations.
7. Use these results to plot a graph of concentration against time.
Safety
- Propanone is an irritant and highly flammable. Keep away from naked flames.
- Sodium thiosulphate releases sulphur dioxide when it reacts. Keep the room
well-ventilated.