• 25 cm3 of sulphuric acid, V1 cm3 of propanone solution and V2 cm3 of distilled water was measured using a measuring cylinder, and was then added into a conical flask as follows :
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
• 50 cm3 of iodine solution was added to the propanone mixture as quickly as possible and at the same time the stop-watch was started. The contents were mixed.
• After 5 minutes, 10 cm3 of the reaction mixture was pipetted into another conical flask containing 10 cm3 of the sodium hydrogencarbonate solution (it was measured during the 5 minutes interval), the time was noted at which this was done. The solution was mixed thoroughly
• The solution was titrated with the diluted sodium thiosulphate solution (in burette). When the mixture was nearly colourless, two or three drops of starch solution was added and then titration was continued until the blue starch-iodine complex colour was discharged by one drop of sodium thiosulphate solution.
• After 10, 15, 20, 25 and 30 minutes, 10 cm3 portions of the reaction mixture was withdrawn further from the flask and the above procedure was carried out each time.
Results
Time when the reation mixture was added to NaHCO3 / min
5
10
15
20
25
30
Final burette reading / cm3
20.12
18.00
16.40
14.74
13.11
11.80
Initial burette reading / cm3
00.02
00.00
00.00
00.04
00.01
00.00
Volume of Na2S2O3 used / cm3
20.10
18.00
16.40
14.70
13.10
11.80
Pair No.
A
B
C
D
Gradient of Graph 1 / cm3 min-1
0.282
0.257
0.189
0.123
Conclusion
The gradient of graph 1 is -0.33 and that of graph 2 is 0.010.
As the reaction rate in does not change with the decreasing concentration of iodine,
the order of reaction with iodine is 0.
As the reaction rate increases with the concentration of propanone,
the order of reaction with propanone is 1.
Questions
• Explain why the volume of sodium thiosulphate solution needed to react with the remaining iodine in a 10 cm3 portion of the reaction mixture can be taken as a measure of [I2(aq)]t (concentration of iodine at time t) ?
It is because the number of moles of I2 is proportional to the number of moles of Na2S2O3.
• What is the function of sodium hydrogencarbonate solution in this experiment? Explain how it works.
Sodium hydrogencarbonate solution acts as a quenching spench in this experiment. When some reaction mixture is transferred to sodium hydrogencarbonate solution, it will neutralized the sulphuric acid in the reaction mixture which acts as a catalyst in order to stop the reaction.
• Deduce the concentration of the sodium thiosulphate solution you used from the intercept of Graph 1.
I2 (aq) + 2S2O32- (aq) S4O62- (aq) + 2I- (aq)
No. of moles of I2 = 0.02 x 50/1000 x 1/10
= 0.0001 mol
No. of moles of S2O32- = 2 x 0.0001
= 0.0002 mol
Volume of sodium thiosulphate = 19.7 cm3
Concentration of S2O32- = ( 0.0002 x 10 ) / ( 19.7 / 1000 )
= 0.102 M
Discussion
The temperature during the experiment is not constant, it affects the rate of reaction and thus the result.
To improve the experiment, some known amounts of reactants are placed in a thermostatic bath for several minutes to allow the temperature of the reactants to reach the temperature of the thermostatic bath.
Also the experiment was not conducted accurately, as pipette was not used as often as it should have been. To improve the experiment, pipette should be used to deliver the reaction mixture into the conical flask
A colorimeter could be used to provide the most accurate result. Moreover, repeat the titration several times can also eliminate the effects bought by error.
For further investigation, we can investigate the effect of temperature on the order of the reaction of iodine with propanone by allowing iodine react with propanone at different temperature.
Reference
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