IB Demo 5.
2(1)
Studying the Kinetics of a reaction using Colorimetry
Theory
This experiment is used to find the orders with respect to the initial concentrations of reagents in a reaction between iodine and propanone and to therefore find out by experiment the rate equation for the reaction. Colorimeters are used for determining the concentration of coloured substances in solution. They are generally fitted with analogue outputs, which enable them to be used with dataloggers. Most colorimeters used in schools are fitted with 4ml plastic cuvettes. These cuvettes have to be inserted the correct way and the colorimeter first standardised against solutions of known concentration.
The Rate of Reaction between iodine and propanone
The reaction between iodine and propanone (acetone) in acid solution is represented by: CH3COCH3 + I2 + H+(catalyst) CH3COCH2I +2H+ + I- (all aqueous)
The progress of the reaction can be followed by measuring the absorbance (due to absorption of light by iodine molecules) and the rates of reaction determined at different concentrations. It is possible from rate and concentration measurements to determine orders of reaction for iodine, propanone and acid.
Method
1. Make up a 2 mol dm-3 propanone solution by transferring 14.7cm3 of propanone to a 100cm3 volumetric flask and make into a 100cm3 standard solution using distilled water only.
Calibration
a) Set up the colorimeter with a filter b) Make up the following samples, in boiling tubes using two burettes
Concentration of Iodine solution being made Volumes: 0.01M iodine solution distilled water Total volume / /10-3M 2 2 8 10 3 3 7 10 4 4 6 10 5 5 5 10 6 6 4 10
cm3 3 / cm
/
cm3
c) After the colorimeter has stabilised, set it to zero absorbance with water. d) Transfer the samples to the cuvette and try out different filters. The aim is to select a filter where all the concentrations give absorbance below 1. e) Record in a suitable table (for the filter you have chosen) values of absorbance and concentration. f) Plot a graph using Excel of absorbance against concentration for your chosen filter. Label the graph and show the R2 value. g) Record on your graph the wave number of the filter you have chosen.
IB Demo 5.2(1)
Experiment
1. Using burettes, put into two different boiling tubes the volumes shown in column a. cm3
Volumes
a 2 2 4 2 10
b 2 2 2 4 10
c 2 2 0 6 10
d 4 2 2 2 10
e 1 2 5 2 10
f 2 4 2 2 10
g 2 6 0 2 10
0.01M iodine solution In one test tube: 2M hydrochloric acid Distilled water In other test tube: Total volumes 2M propanone solution
2. Zero the colorimeter with distilled water, using the filter chosen for calibration and set up the colorimeter for kinetic measurements. 3. Adjust the timespan to 5 minutes and the voltage from 0-1.2V. 4. Add the mixture from one test tube to the propanone solution in the other, shake, and quickly pour into a cuvette, ensuring the cuvette in placed in the colorimeter the correct way, then start the datalogger recording. 5. When recording has finished save the data. 6. Repeat for column b resetting the colorimeter each time.
Calculations
1. Using the information above merge together the appropriate graphs that will allow the determination of the order of reaction with respect to [propanone] 2. Calculate or otherwise work out the gradients of the lines. 3. Determine the order of reaction with respect to [propanone] 4. Title the graph and label the gradient of each line using the Insight software. 5. Printout the graph. 6. Repeat to work out the order of reaction with respect to [iodine] and [H+] 7. Determine the overall order of reaction 8. Write down the experimentally determined rate equation. 9. Calculate the error in both the pipette and burette.
IB Demo 5.2(1)
Apparatus
4 Burettes and funnels Boiling tubes 1x10ml pipette Datameter 1000 1x50ml Beaker Distilled water
1x100ml volumetric flask pipette filler colorimeter
100ml 0.01 mol dm-3 iodine solution Propanone 2 mol dm-3 Hydrochloric acid
