Chemical Method for Studying the Variation of [A] with Time

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Chemical Method for Studying the Variation of A
with Time
C. Y. Yeung (CHW, 2009)
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Chemical Method
Monitoring the change of conc. along with time by
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Example 1
O
O


ROH
OH-
R
C
O-
R
C
OR
(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.
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Example 1 (continued)
O
O


ROH
OH-
R
C
O-
R
C
OR
(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.
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p.05
Example 2
O
O
H catalyst


HI
I2
R
C
CH2I
R
C
CH3
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.
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Example 2 (continued)
O
O
H catalyst


HI
I2
R
C
CH2I
R
C
CH3
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.
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Example 3
MnO2(s)
O2(g)
2H2O2(aq)

2H2O(l)
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.
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Example 3 (continued)
MnO2(s)
O2(g)
2H2O2(aq)

2H2O(l)
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.
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QUESTION
p.09
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Next .
Physical Method for Studying the Variation of A
with Time