Experiment 9

Experiment 9

Topic :         Reaction Kinetics

 Purpose :       To investigate the hydrolysis of methyl ethanoate.

 Apparatus :  (a) One 10.0cm³ pipette and pipette filler.

                     (b) One 50.0cm³ burette.

                        (c) One 50.0cm³ measuring cylinder and one 10.0cm³ measuring cylinder.

                        (d) One 250cm³ conical flask.

                    (e) One 250cm³ titration flask.

                        (f) Three beakers.

                     (g) Stopwacth.

                        (h) A retort stands with clamp.          

            Materials :    KA 1  is 0.5 mol dm־³ of hydrochloric acid.

                                    KA 2  is aqueous sodium hydroxide of concentration 4.0 g mol־¹.

                              KA 3  is methyl ethanoate.

                                    Distilled water.

                                    Phenolphthalein.

 Procedure :   (a) Using a measuring cylinder, place 100cm³ of  KA 1  into a 250cm³

  conical flask. Using a 10.0cm³ measuring cylinder, add 5cm³ of  KA 3 

                             into the 250cm³ conical flask. When about half of the  KA 3  have been

                             added, start the stopwatch and shake the conical flask carefully for a

 while.

 Then pipette immediately 5.0cm³ of solution from this 250cm³ conical

  flask and transfer the sample to a 250cm³ titration flask containing 100

   cm³ of ice water. Note the time when half of the sample in the pipette

   has been transferred. When all of the 5.0cm³ is transferred, add one or

   two drops of phenolphthalein and titrate this solution immediately with

   KA 2 . Record your results in the table below.

                             (b) Repeat the procedure in the second paragraph above for sample 2, 3,

      and 4 withdrawn from the 250cm³ conical flask in the specific time

      intervals of 10, 20, and 30 minutes respectively. Record all your

      results in the table below.

(c) Repeat procedure (a) in the first paragraph but substitute solution

     KA 1  with distilled water to prepare sample 5. Repeat the procedure

     in the second paragraph for sample 5 after 30 minutes have elapsed.

     You are advised to continued writing your report while waiting for the

     30 minutes to end.

Results :        (d) Record and complete your readings in the table below.

                       

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Sample	1	2	3	4	5
Time of transferring sample/minute	0.5	10.0	20.0	30.0	30.0
Final reading/cm³	2.8	21.0	26.0	27.5	7.2
Initial reading/cm³	0.0	0.0	0.0	0.0	0.0
Volume of  KA 2 /cm³	2.8	21.0	26.0	27.5	7.2

 Question :

e)  Write a balanced equation for the hydrolysis of methyl ethanoate.

                        CH3COOCH + H2O      CH3COOH + CH3OH 

(f)  What is the purpose of titrating the sample of reaction mixture with  KA 2 ?

       To neutralize the hydrochloric acid, HCl and ethanoic acid, CH3COOH in the

       sample.

 g)  Plot a graph showing how the volume of  KA 2  used in the titration varies with the

      time.

 (h)  Why was the sample of reaction mixture added to 100cm³ of ice water before

       titration?

        To slow down or stop the reaction between  KA 1  and  KA 3 .

 i)  Using the same axes as (g), sketch an expected graph for hydrolysis of the ester

      Conducted at the same temperature but with 0.1 mol dm־³ ethanoic acid. Explain

      your answer.

       The ethanoic acid, CH3COOH is a weak acid, with ionize partially in water. On

       The contrary, the hydrochloric acid, HCl is a strong acid, which ionize

       completely when dissolved in water. Thus,, the concentration of hydrogen ions,

       H+ of the ethanoic acid is less than that of hydrochloric acid. Hence, the time

       taken by the ethanoic acid to complete the reaction is longer than that of

       hydrochloric acid.

 (j)  Based on the results of the experiments for sample 4 and 5, state the role of

      hydrochloric acid in the experiment.

       The hydrochloric acid, HCl acts as a catalyst for the hydrochloric of methyl

       ethanoate, CH3COOCH3 and hence increases the rate of hydrolysis of methyl

       ethanoate.

 Comments :   The reading is slightly different with the theory because there has

                        parallax error in the titration technique when the experiment is carried on.

\Conclution :   Hydrolysis of methyl ethanoate, CH3COOCH3 produces ethanoic acid,

                        CH3COOH  and methanol, CH3OH. Hydrochloric acid can act as a catalyst

                        to increase the rate of hydrolysis of methyl ethanoate.