1. Chemical Method for Studying the Variation of [A] with Time
p.01
Chemical Method for Studying the Variation of [A] with Time
C. Y. Yeung (CHW, 2009)

2. Monitoring the change of conc. along with time by:
p.02
Chemical Method = …
Monitoring the change of conc. along with time by:
sampling,
quenching, and
titration

3. O O R C OR’ + OH- R C O- + R’OH
Example 1:
p.03 O O R C
OR’ +
OH- R C O- +
R’OH
(aq)
(aq)
(aq)
(aq)
Step1:
Known amounts of ester and NaOH in beakers are placed in a thermostatic bath at known temperature.
Step2:
Start the reaction by mixing the reactants rapidly. The time is noted.
Step3:
At regular time intervals, a fixed volume of the mixture is pipetted into about 5 volumes of ice-water to quench the reaction.

4. O O R C OR’ + OH- R C O- + R’OH
Example 1 (continued):
p.04 O O R C
OR’ +
OH- R C O- +
R’OH
(aq)
(aq)
(aq)
(aq)
Step4:
Remaining [OH-] in the reaction mixture is determined by titration against standard HCl using phenolphthalein.
Step5:
Repeat steps 3 and 4 to obtain variation of [OH-] against t.

5. Example 2: O O R C CH3 + I2 R C CH2I + HI Step1:
H+ catalyst R C
CH3 + I2 R C
CH2I + HI
Step1:
Known amounts of ketone, I2 and HCl in beakers are placed in a thermostatic bath at known temperature.
Step2:
Start the reaction by mixing the reactants rapidly. The time is noted.
Step3:
At regular time intervals, a fixed volume of the mixture is pipetted into NaHCO3(aq) to quench the reaction by removing the H+ catalyst.

6. Example 2 (continued): O O R C CH3 + I2 R C CH2I + HI Step4:
H+ catalyst R C
CH3 + I2 R C
CH2I + HI
Step4:
Remaining [I2] in the reaction mixture is determined by titration against standard Na2S2O3(aq) using starch indicator.
Step5:
Repeat steps 3 and 4 to obtain variation of [I2] against t.

7. Example 3: 2H2O2(aq) 2H2O(l) + O2(g) Step1:
MnO2(s)
2H2O2(aq)
2H2O(l) +
O2(g)
Step1:
Known amounts of H2O2 is mixed with a small amount of MnO2(s) catalyst in a beaker placed in a thermostatic bath at known temperature.
Step2:
At regular time intervals, a fixed volume of the mixture is pipetted into excess dilute H2SO4(aq) to quench the reaction by removing the MnO2(s) catalyst.

8. Example 3 (continued): 2H2O2(aq) 2H2O(l) + O2(g) Step4:
MnO2(s)
2H2O2(aq)
2H2O(l) +
O2(g)
Step4:
Remaining [H2O2] in the reaction mixture is determined by titration against standard KMnO4(aq).
5H2O2 + 2MnO4- + 6H+  2Mn2+ + 8H2O + 5O2
Step5:
Repeat steps 3 and 4 to obtain variation of [H2O2] against t.

9. QUESTION: CH3 C O OC2H5 + H2O OH C2H5OH
p.09
CH3 C O
OC2H5 +
H2O OH
C2H5OH H+

10. Physical Method for Studying the Variation of [A] with Time
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Physical Method for Studying the Variation of [A] with Time