R.P 7a- initial rate method (iodination of propanone)

catalyst used for the iodination of propanone

dilute sulfuric acid

what causes the colour change in the reaction and why

The iodine decolourises during the reaction as it turns into iodopropanone and hydrogen iodide

reactants

• hydrogen peroxide

• iodide ions

• thiosulfate ions

equations

H₂O₂ (aq) + 2H⁺ (aq) + 2I^- (aq) → I₂ (aq) + 2H₂O(l)

2S₂O₃^2-(aq) + I₂ (aq) → 2I^- (aq) + S₄O₆^2- (aq)

what are the sodium thiosulfate ions (2S₂O₃^2-) used for

remove iodine as it forms

step 1

fill 50cm³ burette with potassium iodide

rinse with KI before doing so

step 2

Transfer 10.0 cm³ of hydrogen peroxide solution from a burette to a 100 cm³ beaker

beaker should be clean and dry

step 3 and 4

Use a 50 cm3 measuring cylinder to add 25 cm3 of sulfuric acid to a 250 cm3 beaker

Use a 25 cm3 measuring cylinder to add 20 cm3 of distilled (deionised) water into the 250 cm3 beaker

step 5, 6, 7

Use a plastic dropping pipette to add about 1 cm3 of starch solution to this beaker (the 250 one)

Use the burette to add 5.0 cm3 of potassium iodide solution to the mixture in the 250 cm3 beaker

Add 5.0 cm3 of sodium thiosulfate solution from a burette to the mixture in the 250 cm3 beaker

step 8 and 9

Stir the mixture in the 250 cm3 beaker. add 10 cm3 hydrogen peroxide solution into the 250 cm3 beaker and immediately start the timer

Stop timer when mixture turns blue-black, record time

step 10,11

Rinse the 250 cm3 beaker with distilled (deionised) water and dry it with a paper towel

Repeat steps in four further experiments changing the concentration of potassium iodide

step 12

Plot a graph of initial rate (y) versus concentration (x) to determine the order

sources of error

• human error in timing- too slow to record time

• temperature

how to minimise human error in timing

use a colorimeter to measure colour absorbance

colour absorbance is proportional to concentration of coloured species