Datalogging Experiment (2)
38
Kinetics of Iodination of Propanone by Colorimetry
Student Handout
Purpose
To determine the rate equation of the reaction between acidified propanone solution and
iodine.
Introduction
As the following reaction proceeds,
CH
3COCH3(aq) + I
2(aq) ⎯⎯→ CH3COCH2I(aq) + H+(aq) + I
-(aq)
colourless (brown/yellow) colourless colourless
the concentration of iodine decreases and the brown/yellow colour intensity of the
reacting solution also decreases. The change in colour intensity allows the use of
colorimetry to follow the reaction kinetics.
For dilute solutions, absorbance is proportional to concentration. Decrease in
concentration of species can be measured by the change in absorbance. Relative rate of
disappearance of iodine, in terms of decrease in absorbance and not in terms of actual
decrease in concentration of iodine, is measured from the absorbance vs time plot. This
avoids the time-consuming conversion to concentration units using a calibration curve.
Order of reaction with respect to each reactant is determined by separately doubling the
concentrations of each of CH3COCH3(aq), H+(aq) and I2(aq). A first order reaction with
respect to CH3COCH3(aq) is confirmed when, while keeping the concentrations of I2(aq)
and H+(aq) constant, the initial rate of decrease in concentration of I2(aq) (or the initial
rate of decrease in absorbance) doubles as the concentration of CH3COCH3(aq) is
doubled.
Safety
Avoid skin contact with chemicals.
Materials and Apparatus
1 M sulphuric acid, 1 M propanone solution
0.02 M iodine solution, deionised water, a datalogger with colorimeter accessories,
computer, micro-tip plastic pipette, 5 cm3 graduated pipette.
IRRITANT
EYE PROTECTION
MUST BE WORN
