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

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Presentation transcript:

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)

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

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.

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.

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.

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.

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.

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.

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

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