Batch Reactor (Saponification) Expt

Batch Reactor Objective To study the reaction kinetics and to determine the rate constant and the overall order of the saponification reaction (Hydrolysis of Ethyl acetate using a base (aq. NaOH))

Theory The Batch reactor is the generic term for a type of vessel widely used in the process industries. In a batch reactor the reactants and the catalyst are placed in the reactor and the reaction is allowed to proceed for a given time whereupon the mixture of unreacted material together with the products is withdrawn. Provision for mixing may be required. In an ideal batch reactor, the concentration and temperature are assumed to be spatially uniform. In practice, the condition can be approximately realized by vigorous agitation or stirring. All the elements of the fluid spend the same amount of time in the reactor, and hence have the same residence time. Saponification is a process that produces soap, usually from fats and lye. In technical terms, saponification involves base (usually caustic soda (NaOH)) hydrolysis of triglycerides, which are esters of fatty acids, to form the sodium salt of a carboxylate.

The saponification of ethyl acetate:              The mechanism by which esters are cleaved by base involves a series of equilibria. The hydroxide anion adds to (or "attacks") "attacks") the carbonyl group of the ester. The immediate product is called an orthoester:

Expulsion of the alkoxide generates a carboxylic acid:

The alkoxide is more basic than the conjugate base of the carboxylic acid, and hence proton transfer is rapid:

Even though the first two reactions are reversible, the final step is irreversible thus making the entire reaction irreversible. The overall reaction kinetics depends on each and every step. Rate of reaction =  [   ] [  ] Here, since concentrations of ethyl acetate and base are the same (taken initially to be the same; so remain same throughout the reaction, based on stoichiometry), the equivalent rate expression is –

  [    ] Total order of the reaction is (x + y). ‘k’ is the reaction rate constant.

The differential method is a well-known technique of finding the rate law from batch experiments. Refer to [1,2] for details.

Procedure 4.1 Preparation of solutions Hydro Chloric Acid (HCl):

 M HCl is required to stop/freeze the saponification reaction.

Sodium Hydroxide (NaOH):   M of NaOH was taken as reactant. Ethyl Acetate (CH3COOC2H5):

 M ethyl acetate solution.

The value   will be specified during the lab session.

4.2 Calibration of pH meter The Calibration of the pH meter has to be don e before the start of the reaction [3]. The procedure will be demonstrated during the lab session.

4.3 Experimental Procedure 1. Fill up the test tubes with 6 ml of 0.1 M solution and set it aside in a test tube stand. 2. Fill the burette with 0.1 M NaOH solution for titration. 3. Place the 1000 mL beaker on the magnetic stirrer and insert the pH meter probe in it. 4. Pour 250 mL of 0.1 M NaOH and 250 mL of 0.1 M Ethyl acetate into the beaker and start the timer at the same instant. 5. Pipette out 6 ml of the reactant mixture every 30 seconds and add to the test tubes containing HCl solution in order to freeze the reaction. The value on pH meter should also be recorded for every 30 seconds. 6. Two drops of phenolphthalein should be added to the solution (reactant mixture + HCl) and has to be titrated against 0.1 M NaOH solution in order to e stimate the unreacted HCl in the solution. 7. The experiment is stopped when there is no change in pH meter readings and the successive readings of the consumption of NaOH during titration.

Precautions 

 





Once calibration is done, set the pH meter in reading mode and do not switch back to calibration mode during the experiment. Freeze the reacting mixture using HCl as soon as a sample is collected. Ethyl acetate is a volatile compound. So, cover the beaker containing Ethyl acetate solution in order to avoid vaporization of ethyl acetate from the solution. Both HCl and NaOH are corrosive in nature. So, these chemicals must be handled carefully. HCl solution must be prepared by slowly adding HCl to distilled water and not vice versa

References [1] http://www.umich.edu/~elements/fogler&gurmen/html/course/lectures/five/index.htm [2] Fogler, H. Scott. "Elements of chemical reaction engineering." (1999) [3] http://en.wikipedia.org/wiki/PH_meter#Calibration_and_use