Experiment: Batch Reactor
Unit Operations Lab I (CHEGR3787L)
Fall 2004
August 27, 2004
Objectives:
(1) Using method of excess reactants confirm that the saponification of ethyl acetate is first
order with respect to NaOH.
(2) Confirm that the reaction is first order with respect to NaOH and ethyl acetate
a. Run experiments with different starting concentration of NaOH and ethyl acetate.
Using the rate of zero time and conduct a least squares analysis (page 209 of
Fogler)
b. Compare the concentration versus time data obtained from the experiment to the
curve predicted by the supplied excel sheet
(3) Determine the activation energy of the reaction and compare to the literature value
Background
Researchers typically use a batch reactor to study reaction kinetics under ideal conditions.
A batch reactor can be used to find the reaction rate constant, activation energy and order of the
reaction. The use of a batch reactor for the most part eliminates the effects due to fluid flow on
the resulting reaction rates. Consequently, the data reflect the intrinsic kinetics for the reaction
being investigated.
Your goal for this experiment is to find the kinetics for a liquid-phase (mildly
exothermic) irreversible reaction. The reaction chosen for the experiment is given below
NaOH (A)  CH 3COOC 2 H 5 (B)  CH 3COONa  C 2 H 5OH
 rA  kCA CB
(1)
This reaction is first order with respect to NaOH and ethyl acetate. You will determine if
your experimental data confirms the order of the reaction and the activation energy. You will use
two probes to measure the extent of reaction. Procedure to use the two probes described later.
Unit Operations Lab I
1 of 5
Fall 2004
Detectors
Conductivity and pH probes would be used during the experiment to determine the extent
of reaction.
Conductivity probe measure the conductivity of the solution. In the present experiment,
sodium hydroxide and sodium acetate would contribute to the conductivity of the solution. Let k0
be conductivity of the solution at the start of the reaction. At the start, only sodium hydroxide
would contribute to the conductivity. As reaction proceeds, sodium acetate starts forming after
consumption of sodium hydroxide resulting in different conductivity results. The consumption of
NaOH can be found using the following relationship
C (t ) k (t )  k 

C A0
k0  k
Where,
C(t) = Concentration of NaOH at time t
CA0= Initial concentration of NaOH
k (t ) = Conductivity of solution at time t
k 0 = Initial conductivity of solution
k  =Conductivity at the end of reaction
The conductivity contribution due to sodium hydroxide is different from the contribution
due to sodium acetate. So one has to find for k  each reaction run.
The pH probe measures the pH of the solution. The pH of the solution is directly related to the
amount of OH ions present in the solution. . The formulas needed to calculate the concentration
of [OH]- from pH is given below:
pH
 pOH  14
 pOH  14  pH
pOH
  log[OH ]
[OH ]  10 pOH
For example, if pH = 12 means [OH]- = 0.01 M
Unit Operations Lab I
2 of 5
Fall 2004
Note: the value of 14 is valid only for 25C temperature. This value changes with change in
the reaction temperature. Use the appropriate value for the experimental temperature.
Tasks and Questions for each week
Answers to the question should be submitted at that start of the class for that week. For example,
answers to week 1 questions should be submitted at the start of week 1 experiments. Failure to
submit answers in a timely fashion would affect the 50 points for answers and sample calculation
part of the grade for the report.
Week 1
(1) Describe how do you find order of a reactant using the method of excess reactants? Give
the graph that would be used to prove that the reaction is first order with respect to
NaOH. Use differential method of analysis
(2) Find a relation of Kw as a function of temperature. Kw gives the value for pH + pOH for a
given temperature.
(3) What is the weight of NaOH to be added to 1.5 liters solution to obtain, 0.2, 0.1 and 0.05
M solution? How much of NaOH would you add, if it were supplied as 50 weight percent
solution to get the above concentration?
Experiment
-
-
Please read the reactor manual for general operation and safety
Ensure that pH meter is working by using buffer.
You get that data by starting the data acquisition system and measuring the readings.
Add enough NaOH to 1.5 liter solution to obtain 0.05 M solution
Switch “MAIN” and “STIRRER” on front panel. Stir it until all NaOH dissolves in water
Add enough ethyl acetate to the solution to obtain 0.5 M concentration
Measure the change in NaOH concentration with time using pH meter and conductivity
meter. Also not the temperature reading from the pH meter. The data acquisition system
should automatically record the data.
You click on “reactor” icon on the desktop to start the data acquisition
Ensure that the time interval is set to obtain 10 data points per minute
Carry out the experiment until the data reaches a steady state
Stop the data acquisition and save file as text file.
You can open this file under excel and use “delimited” to get the data.
Repeat the batch reaction using 0.1M NaOH and 0.1M ethyl acetate
All reactions done at room temperature
Make sure you take the last conductivity data, as it is required to calculate the
NaOH concentration as a function of time.
Unit Operations Lab I
3 of 5
Fall 2004
Week 2
(1) Please show the graph obtained using differential analysis method for finding order of
NaOH reaction using week 1 data from run 1.
(2) Derive the expression for NaOH concentration as a function of time for a bimolecular
second order reaction (which is the case for the present one). Please drive it for the case
where initial concentration of NaOH is different from the one for ethyl acetate. Also,
derive the equation for the case where starting concentration is same for both reactants.
One uses integral method from the rate equation to obtain these expressions.
Experiment
-
-
Conduct bath reaction at room temperature with the following concentration
o 0.1 M ethyl acetate and 0.05 M NaOH
o 0.05 M ethyl acetate and 0.1 M NaOH
o 0.15 M ethyl acetate and 0.1 M NaOH
Reaction rate is calculated as change in NaOH concentration over time
Note: Ensure you have the excel file that gives the theoretical data for this reaction as a function
of time and starting concentration
Week 3
(1) Show graphs comparing the experimental data with theoretical values for the different
batch reactor runs.
(2) How do you find the order of reaction using the least square analysis (see page 209
Fogler). Can you use data generated till now to predict the order of the reaction with
respect to ethyl acetate and NaOH? If not, how many more runs need to be done and what
are they?
Hint: You can take two data points from each run for doing multiple regression. You have to
multiple regression with rate as dependent variable and concentration of ethyl acetate and NaOH
as independent variable. See example show in the other file.
Experiment
If you team answered that you need more runs in question (2), please conduct those runs this
week. If not, carry out the experiment given below to find the activation energy of the reaction.
Unit Operations Lab I
4 of 5
Fall 2004
-
Conduct the bath reaction using 0.1M ethyl acetate and 0.1M NaOH at four different
temperatures
Temperatures should be between 10-30C.
Lower temperature runs would be conducted by chilling 0.1 M NaOH
The data would be extrapolated to calculate the initial reaction rate
This initial reaction rate would be used to calculate the rate constant at different
temperatures
Rate constant can be plotted in an Arrhenius plot to obtain the activation energy and
frequency factor.
Note: Please prepare 0.1M NaOH the day before the lab and keep it in a refrigerator overnight
for the low temperature reaction.
Unit Operations Lab I
5 of 5
Fall 2004