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QUESTION:

This experiment studies the variation of [CH3COOH] with time in the ester hydrolysis reaction using colorimetry and titration. The reaction is monitored by measuring absorbance and determining the remaining [CH3COOH] in the reaction mixture.

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QUESTION:

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  1. O O H+ + + C2H5OH H2O OH CH3 C OC2H5 CH3 C QUESTION: ANS.1 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. O O H+ + + C2H5OH H2O OH CH3 C OC2H5 CH3 C QUESTION: ANS.2 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. p.01 Physical Method for Studying the Variation of [A] with Time < COLORIMETRY > < COLORIMETRY > C. Y. Yeung (CHW, 2009)

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

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

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

  7. p.05 Procedures: 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. Absorbance [Br2(aq)] p.06 Calibration of Colorimeter (continue) Step5: Plot a Calibration Curve of “Absorbance” versus “[Br2(aq)]”.

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

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

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

  12. rate = d(volume of H2) dt Example: p.10 Mg + 2HCl  MgCl2 + H2 (no colour change!)

  13. p.11 Next …. 5 Factorsaffecting the Rate of Rxn (p.15-21) Rate Eqns&Order of Rxn(p.25-36) Rate Eqns&Order of Rxn(p.37-43)

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

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