Chemical Reaction Engineering Laboratory Kinetics Of Saponification Of Ethyl Acetate (Batch Reactor)

INTRODUCTION: The design of a chemical reactor calls for prior information regarding: (a) the reaction rate equation and (b) the value of the reaction rate constant. This information is to be obtained invariably through laboratory-scale experiments conducted at constant temperature.
The equipment used is generally of batch type in case of homogenous reactions and of flow type in case of heterogeneous/catalatic reactions. The present experiment is very typical of the kinetic method that is normally used for simple homogenous reactions.

Aim of experiment: The aim of the experiment is to verify that the saponification of ethyl acetate in dilute aqueous solution :
is a second order reaction and to report the value of the reaction rate constant.

Materials required:
Ethyl acetate, pure -------2ml
NaOH (aqueous, 0.05M)------1 litre HCl (aqueous, 0.05M) ------ 500 ml Oxalic acid (standard 0.1M)----- 100 ml
Phenolphthalein indicator
Conical flask (1 litre), a dozen conical flasks (250ml), measuring pipette (5ml), burettes, 10ml and 25ml, pipettes, stop-clock.

1. With the help of standard oxalic acid (or preferably, standard potassium Hydrogen phthalate) solution, determine the exact normality of the NaOH solution. This must be 0.05+0.002. If not, re-adjust the concentration of NaOH solution to obtain it in this range. Using the NaOH, now determine the exact normality of the HCl solution.
2. Place 400ml of the alkali solution in a clean 1 litre conical flask, which serves as the reactor.
3. Transfer 25ml of the HCl solution into each of the twelve 250ml conical flasks, which are serially labeled.
4. Place the reactor on a magnetic stirrer and start stirring vigorously. Keep a clean and dry 25ml pipette handy.
5. By means of a clean and dry measuring pipette, transfer exactly 1.96ml of ethyl acetate into the reactor, and start the stop clock simultaneously. The ethyl acetate forms a 0.05-M solution in the reactor itself and the saponification reaction is now on.
6. At reaction times of 1,2,3,4,5,6,7,8,9,10,11,12 minutes, pipette out 25ml of the reaction mixture into each one the labeled conical flasks. (Do not stop the stopclock until all the twelve samples are transferred).
7. Back-titrate the contents of the twelve conical flasks with aqueous Na0H using phenolphthalein as indicator.
8. Note the room temperature.

Explanation: Note that when a sample of reaction mixture is transferred into the conical flask containing HCl, the alkali in the reaction mixture gets neutralized and so the saponification reaction stops. Back-titration of the contents of this flask therefore helps in determining the amount of unreacted NaOH present in the reaction mixture at the time of its removal from the reactor. In other words, the experiment helps to find the alkali concentration (CA) in the reactor as a function of time (t).

1. Bauman, "Experiments in Unit Processes and Reaction Engineering".
2. Walas, "Reaction Kinetics for Chemical Engineers".
3. McCabe and Smith, "Unit Operations of Chemical Engineering".

Readings and Calculations Preliminary Titrations:
(1) Standardization of NaOH: - Volume of Oxalic acid taken = 10 ml
Normality of Oxalic acid = ---------(given)
Volume of NaOH rundown = -----------ml
Normality of NaOH (N3) = ----------

(2) Standardization of HCl: -
Volume of HCl taken = 25 ml
Volume of NaOH rundown = ---------ml
Normality of HCl (N1) = ----------

Evaluation of NaOH concentrations: -

Reaction time, 1 2 3 4 5 6 7 8 9 10
t min.

Volume of HCl, V1 ml

Vol. of sample, V2 ml

NaOH rundown in back-
titration, V3 ml

Gm-moles of NaOH run-down =
of excess HCl, V3N3

gm-moles of NaOH
used up in back titration
= gm-moles of NaOH in
sample, V1N1-V3N3

Normality of NaOH in
sample, CA=(V1N1-V3N3)/V2

Differential method of Analysis:-
Prepare a plot CA versus t, and measure the slopes at the following values of CA:-
CA gm-moles /litre 0.025 0.03 0.035 0.04 0.045 0.05

-dCA/dt =rA

CA2 …

Prepare a plot of rA versus CA2. Check for a straight-line fit Report the value of the slope.

k=------litre/(gm-mole)(min) at ----oC

Integral Method of Analysis:-

1/CA - 1/CAo = kt
For the measured values of reaction time, tabulate 1/CA litre/gm-mole and t, min.

Plot 1/CA versus t. Check for a straight-line fit. Report the value of the slope:
k = --------litre/(gm-mole)(min) at ----oC

Selected Questions:
1. In the evaluation of NaOH concentrations the gm-moles of NaOH rundown are shown as V3N3 .The correct formula should have been V3N3 /1000.How does this affect the calculated values of CA?
2. A worker performing the present experiment reported the value of -dCA /dt at t=0 as 0.0175. What is the value of the reaction rate constant?
3. Plot rA vs CA on log-log paper and show that the reaction is of second order.
4. Derive the equation used for the integral method of analysis. What are the assumptions made in this derivation?
5. We have a vessel 1M in dia and 4M in height. The vessel is to be fed with a solution containing 0.5M NaOH and 0.5M ethylacetate. The required conversion is 80%. What must be the volumetric flow rate through the vessel? Report also the corresponding velocity and pressure drop through the vessel comment on your answers. Use the value of reaction rate constant as determined by the integral method.