Experiment 10 : Investigation of the Order of the Reaction of Iodine with
Propanone (Titration Method) [Worksheet Report]
Aim :
The purpose of this experiment is to determine the order of reaction of the iodination of propanone, and to
establish a rate equation for this reaction.
Introduction :
The rate of a chemical reaction is affected by the concentration of reactants according to the rate equation,
which has to be determined experimentally.
In the iodination of propanone,
CH3COCH3(aq) + I2(aq)  CH3COCH2I(aq) + HI(aq)
the reaction is acid catalysed. The rate equation could be represented by
d[I2]
- dt = k[CH3COCH3]a[I2]b
By titrating the unreacted iodine remianing in the reaction mixture against standard thiosulphate solution at
various times, the rate of reaction can be determined. Since the propanone is used in large excess, its
concentration remains more or less constant. So the rate equation reduces to approximately
d[I2]
- dt = constant  [I2]b
and hence the order of reaction (i.e., value of b) with respect to iodine can be determined.
By further varying the initial concentration of propanone while keeping the initial concentration of iodine
constant, the relationship between rate of reaction and concentration of propanone can also be established, as
the rate equation would then become
d[I2]
- dt = constant  [CH3COCH3]
Requirements :
0.01 M Na2S2O3 (A)
1 M H2SO4 (B)
0.02 M I2 in KI solution (5g I2 and 33g KI in 1
dm3 solution) (C)
3 M aqueous propanone (D)
0.5 M NaHCO3 (E)
starch solution
titration apparatus (burette, conical flasks, etc.)
10.0 cm3 pipette
beakers
stop watch
100 cm3 measuring cylinder
Procedures
2. Pipette 25.0 cm3 of 1M sulphuric(VI) acid (B), and deliver from burettes, V1 cm3 of 1M propanone (D)and V2
cm3 of distilled water into a conical flask as follows :
[Hazard Warning : 1 M sulphuric(VI) acid is corrosive, and propanone is flammable]
(Note : Each group should prepare one of the combinations of solutions indicated below.)
Group
3.
4.
5.
6.
7.
8.
9.
1
2
3
4
5
Volume of propanone solution (V1 / cm3) (D)
25.0
20.0
15.0
10.0
5.0
Volume of de-ionized water (V2 / cm3)
0.0
5.0
10.0
15.0
20.0
Run 50 cm3 of the 0.02M iodine solution (C) from a burette into a beaker. Transfer this iodine solution to the
propanone mixture as quickly as possible and at the same time start the stop watch. Mix the contents.
Pipette 10.0 cm3 of the reaction mixture into another conical flask. Exactly after 5 minutes, add 10 cm3 of the
0.5M sodium hydrogencarbonate solution (E). Mix the solution thoroughly and then titrate with the diluted
sodium thiosulphate solution until the solution becomes a straw colour. Then add starch indicator and continue
titration until the solution becomes colourless.
After 10, 15, 20, 25 and 30 minutes, withdraw 10.0 cm3 portions of the reaction mixture from the flask and
carry out step (4) for each portion.
Record your results in the Results Table (input data into the computer).
Share data with other groups (your teacher will help you by using the network in the laboratory).
Input data on gradients into the second table in the spreadsheet in the computer.
Send the spreadsheet to the printer. Attach this with your report.
Experiment 10, Page 1
Results (Input these into the appropriate cells in the spreadsheet in the computer):
Time when reaction mixture was added to NaHCO3
/ min
5
10
15
20
25
30
Final burette reading / cm3
Initial burette reading / cm3
Volume of Na2S2O3 used / cm3
Analysis of results :
1. Plot a graph (automatically done by the computer)of the volume of sodium thiosulphate needed to react with the
remaining iodine against the time at which the 10.0 cm3 sample of the reaction mixture was added to the sodium
hydrogencarbonate solution.
Read the y-intercept of your graph and hence find the concentration of the given sodium thiosulphate solution.
2. Acid is used as a catalyst for the reaction between propanone and iodine. What is the purpose of adding sodium
hydrogencarbonate in step (4) ?
3. How does the concentration of iodine change throughout the experiment ?
4. Find the gradient of the graph.
5. Does the rate of the reaction, which in this case is directly proportional to the gradient of the graph, vary with
different concentrations of iodine ?
6. What is the order of the reaction with respect to iodine ?
7. Is iodine involved in the rate determining step of this reaction ?
8. Record the gradients of the graphs obtained by the five groups.
Group
1
2
3
4
5
Gradient of graphs / cm 3 min-1
9.
Plot the gradients of graphs against the initial volumes of propanone solution added. Deduce from the graph the
relationship between the rate of the reaction and the concentration of propanone. Hence determine the order of
the reaction with respect to propanone.
Give the rate equation for this reaction.
Conclusions:
Draw conclusions concerning the order of reaction of the iodination of propanone. What are the orders of
reaction with respect to the various reactants involved? What is the rate law of this reaction? What is the
initial concentration of the given sodium thiosulphate solution?
Discussions:
What are the main difficulties encountered? What are the main sources of error? Do these errors cause serious
difficulty in drawing a conclusion for the experiment? Does being able to see the results of more groups help in
drawing the conclusion?
Experiment 10, Page 2