(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!)

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.

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

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]

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

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

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)].

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

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

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

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

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

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)

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