EXPERIMENT 18
AN INVESTIGATION OF THE ORDER OF THE REACTION OF IODINE WITH PROPANONE
(TITRATION METHOD)
Chemical required
Iodine dissolved in potassium iodide solution (0.02 M)*, propanone in water (1 M), sulphuric acid (1 M), sodium
hydrogencarbonate solution (0.5 M), very dilute sodium thiosulphate(VI) solution (approx. 0.1 M) and starch solution
*5 g I2 + 33 g KI in 1dm3 solution
Apparatus required
Conical flasks, 10 cm3 pipette, titration apparatus, measuring cylinder (100 cm3).
Procedure
(a) Dilute 25.0 cm3 of sodium thiosulphate solution to 250 cm3 in a volumetric flask.
(b) Using a measuring cylinder, measure 25 cm3 of sulphuric acid, V1 cm3 of propanone solution and V2 cm3 of distilled water into
a conical flask as follows
Group/student pair
A
B
C
D
Volume of propanone solution (V1/cm3)
25.0
20.0
15.0
10.0
Volume of distilled water (V2/cm3 )
0.0
5.0
10.0
15.0
(c) By means of a measuring cylinder add 50 cm3 of iodine solution to the propanone mixture as quickly as possible and at the same
time start the stop-watch. Mix the contents.
(d) After 5 minutes, pipette 10 cm3 of the reaction mixture into another conical flask containing 10 cm3 of the sodium
hydrogencarbonate solution (measure this during the 5 minutes interval) noting the time at which this is done. Mix the solution
thoroughly and then titrate with the diluted sodium thiosulphate(VI) solution using starch as indicator.
(e) After 10, 15, 20, 25 and 30 minutes withdraw 10 cm3 portions of the reaction mixture from the flask and
carry out the above procedure each time.
(f) Record your results in the Result table.
Results
Time (min) when reaction mixture was added to NaHCO3
5
Final burette reading
Initial burette reading
Volume of Na2S2O3 used/cm3
Plot a graph of the volume of sodium thiosulphate(VI) needed to react
with the remaining iodine (y-axis) against the time at which the 10.0 cm3
samples of the reaction mixture were added to the sodium
hydrogencarbonate solution (x-axis).
10
15
20
Review questions
Answer the following questions
1. The reaction between propanone and iodine is catalysed by acid. What
is the function of the sodium hydrogencarbonate?
2. Did the iodine concentration in the reaction fall throughout the
experiment?
3. How can the rate of reaction be determined from the graph?
4. Did the iodine concentration change at a uniform rate throughout the
experiment?
5. Is the rate of change of iodine concentration dependent on the iodine
concentration? Give your reasons.
6. What is the order of reaction with respect to iodine? i.e. what is the value of n in the equation?
Rate of reaction = constant[I2]n
7. Does iodine take part in the rate determining step of this reaction?
8. What is the significance of the intercept of the graph with the y-axis?
9. What must have been the concentration of the sodium thiosulphate(VI)
solution?
10. Give the appropriate equations in this experiment.
Further work
Record the gradients of the graphs obtained by the four groups:
Group/Pair No.
A
B
C
D
Gradient of graph/cm3 min1
Plot the gradients of graphs above against the initial volume of propanone
solution added.
(a) How did the rate of reaction depend on the propanone concentration?
(b) What is the order of the reaction with respect to propanone?
25
30