Core practical 13a - Iodine quench titrations

Question 1

Summary of 13a Steps (Iodine propanone)

Answer 1

1. To a beaker add 25 cm3 propanone (1M) to 25 cm3 of Sulfuric acid (1M)
2. Add 50 cm3 of (0.02M) iodine solution. Immediately start the clock swirl the beaker well to mix.
3. Using a 10cm3 pipette, withdraw a sample of the mixture and transfer it to a conical flask.
4. Quench by adding a spatula of sodium hydrogen carbonate to the conical flask
5. TITRATE the iodine present in the conical flask with (0.01M )sodium thiosulfate solution. When the colour
   turns a pale yellow add the starch indicator. The end point is then when the mixture goes from blue to colourless.
6. Every 3 minutes withdraw another 10 cm3 sample and repeat steps 4 and 5

Question 2

What is the rate equation for this reaction?

Answer 2

rate = k[CH3COCH3)][H+]

-1st order with respect to both the acid catalyst and the propanone

Question 3

If there is a zero order reactant what does it mean for the mechanism

Answer 3

• There must be more than one step and the zero-order reactant cant be in the rate-determining step

Question 4

What does the method allow to be calulcated?

Answer 4

• The order of iodine in the reaction

Question 5

Why do we add starch indicator later in the titration?

Answer 5

• If we add the starch complex early it will react with the high concentrations of Iodine
• forming an INSOLUBLE IODINE STARCH COMPLEX