1. (DO NOT QUENCH by Using NaOH!)
QUESTION:
ANS.1
CH3 C O
OC2H5 +
H2O OH
C2H5OH H+
Step1:
Known amounts of ester and dilute HCl(aq) 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 large volume of ice-water to quench the reaction.
(DO NOT QUENCH by Using NaOH!)

2. QUESTION: CH3 C O OC2H5 + H2O OH C2H5OH
ANS.2
CH3 C O
OC2H5 +
H2O OH
C2H5OH H+
Step4:
Remaining [CH3COOH] in the reaction mixture is determined by titration against standard NaOH using phenolphthalein.
Step5:
Repeat steps 3 and 4 to obtain variation of [CH3COOH] against t.

3. < COLORIMETRY > < COLORIMETRY >
p.01
Physical Method for Studying the Variation of [A] with Time
< COLORIMETRY >
< COLORIMETRY >
C. Y. Yeung (CHW, 2009)

4. Monitoring the change of conc. along with time by:
p.02
Colorimetry = …
Monitoring the change of conc. along with time by:
measuring the variation of absorbance by colorimeter
absorbance  [coloured substance]
[colour intensity]

5. (for coloured reactant)
p.03
rate 
- d(aborbance) dt
(for coloured reactant)

6. I2 + CH3COCH3  CH3COCH2I + H+ + I-
p.04
Examples:
Br2 + HCOOH  2Br- + 2H+ + CO2
I2 + CH3COCH3  CH3COCH2I + H+ + I-
BrO3- + 5Br- + 6H+  3Br2 + 3H2O
H2O2 + 2I- + 2H+  I2 + 2H2O

7. Calibration of Colorimeter + Measurement
Procedures:
p.05
Calibration of Colorimeter + Measurement
Calibration of Colorimeter
Step1:
Use distilled water as sample, the absorbance is set to zero.
Step2:
A standard solution of Br2(aq) [e.g. 1.0M] is put into the sample cell in the colorimeter.
Step3:
Record the absorbance of 1.0M Br2(aq).
Step4:
Repeat steps 2 and 3 with different [Br2(aq)].

8. Calibration of Colorimeter (continue)
p.06
Calibration of Colorimeter (continue)
Step5:
Plot a Calibration Curve of “Absorbance” versus “[Br2(aq)]”.
[Br2(aq)]
Absorbance

9. Br2 + HCOOH  2Br- + 2H+ + CO2 Measurement
p.07
Measurement
Br2 + HCOOH  2Br- + 2H+ + CO2
Step1:
Put the Br2 containing reaction mixture in the sample cell and put into the colorimeter.
Step2:
Start the stop watch to monitor the variation of absorbance with time.
time
Absorbance

10. Monitoring the change of conc. along with time …
p.08
Monitoring the change of conc.
along with time …
time
Absorbance
[Br2(aq)]
Absorbance
Measurement
Calibration A1 A1 t1
[Br2]t1
Time
[Br2] t1
[Br2]t1

11. Monitoring the change of conc. along with time by:
p.09
Another Physical Method …
Monitoring the change of conc. along with time by:
measuring the volume of gas formed by syringe

12. Mg + 2HCl  MgCl2 + H2 d(volume of H2) rate = dt Example:
(no colour change!)
rate =
d(volume of H2) dt

13. Next …. 5 Factors affecting the Rate of Rxn (p.15-21)
Rate Eqns & Order of Rxn (p.25-36)
Rate Eqns & Order of Rxn (p.37-43)

14. Assignment p.24 Ex. Q.1-5 [due date: 11/2(Wed)]
Pre-lab [due date: 12/2(Thur)