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CHS AP Chemistry Lab: Chemical Equilibrium
Many chemical reactions, especially those of organic substances, do not go to completion. Rather, they come to a point
of chemical equilibrium before the reactants are fully converted to products. At the point of equilibrium, the
concentrations of all reactants remain constant with time. The position of equilibrium is described by a function called
the equilibrium constant, Kc, which is the ratio of the amount of product present to the amount of reactant remaining
once the point of equilibrium has been reached. You will determine the equilibrium constant for an esterification
reaction.
Early in the study of chemical reactions, it was noted that many chemical reactions do not produce as
much product as might be expected, based on the amounts of reactants taken originally. Somewhat like
why the batch of cookie dough never yields the number of cookies the recipe promises?
Some reactions appear to stop because the products produced by the original reaction themselves begin
to react, in the reverse direction to the original process. As the concentration of products begins to build
up, product molecules will react more and more frequently. Eventually, as the speed of the forward
reaction decreases while the speed of the reverse reaction increases, the forward and reverse processes
will be going on at exactly the same rate. Once the forward and reverse rates of reaction are identical,
there can be no further net change in the concentrations of any of the species in the system. At this point, a dynamic
state of equilibrium has been reached. The original reaction is still taking place but is opposed by the reverse of the
original reaction also taking place.
In this experiment you will determine the equilibrium constant for the esterification reaction between n-propyl alcohol
and acetic acid which forms n-propyl acetate (an ester) and water:
CH3CHOOH +
CH3CH2CH2OH

CH3COOCH2CH2CH3
+
H2O
You will set up the reaction is such a way that the initial concentrations of the reactants are known. The reaction will
then be allowed to stand for one week to come to equilibrium. As the reactants react the acidity of the mixture will
decrease, reaching a minimum once the system reaches equilibrium. You will be able to qualitatively be able to measure
this with your sense of smell. Week one the acetic acid is strong (vinegar) and week two it will smell like the ester npropyl acetate which is a common component of nail polish remover.
The quantity of acid present in the system will be determined by titration with standard sodium hydroxide solution.
Esterification reactions are typically catalyzed by the addition of a strong mineral acid. You will be using a small amount
of 6M sulfuric acid as a catalyst. The concentration of acetic acid in the mixture is determined by the technique of
titration. Acetic acid reacts with sodium hydroxide on a 1:1 stoichiometric basis.
Materials: 50 mL buret, plastic wrap, standard 0.109 M NaOH solution, phenolphthalein
indicator solution, 1 mL pipet and pump, 1-propanol, glacial acetic acid, 6 M sulfuric acid
Procedure:
A] Week one: four total titrations need to be complete before it sits for one week!
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CHS AP Chemistry Lab: Chemical Equilibrium
Obtain approximately 100 mL of standard 0.10 M NaOH solution in a clean, dry beaker. Rinse the buret several times
with small portions of NaOH solution (discard the rinsings); then fill the buret with NaOH solution. Keep the remainder
of the NaOH solution in the beaker covered until it is needed.
Clean two 250 mL Erlenmeyer flasks for use as titration vessels. Rinse the flasks with tap water; follow with small
portions of distilled water. Place approximately 25 mL of distilled water in each flask, and set aside until needed.
Clean and dry a 125 mL Erlenmeyer flask. Label the flask as reaction mixture. Cover a rubber stopper with plastic wrap
or baggie. This will contain the mixture for the entire lab and will need to be used next week. Rubber stoppers will be
decomposed by the vapors of the reaction mixture unless there is plastic protection.
Clean and dry a small graduated cylinder. Using the graduate, obtain 14 +/- 0.2 mL of glacial acetic acid (0.25 mol) and
transfer the acetic acid to the reaction mixture flask.
Rinse the graduated cylinder with water and redry. Obtain 19 +/- 0.2 mL of 1-propanol (0.25 mol) and add to the acetic
acid in the reaction mixture flask. Stopper the flask and swirl the flask for several minutes to mix the reagents.
Using the 1 mL pipet, transfer 1.00 mL of the reaction mixture to each of the two 250 mL Erlenmeyer flasks. Restopper
the flask containing the reaction mixture to prevent evaporation.
Add 3-4 drops of phenolphthalein indicator to each of the two samples to be titrated.
Titrate each mixture until the pale light permanent pink color appears. Be sure to pay attention to the level of NaOH in
the buret and do not let it run below the 50 mL mark. Refill as necessary keeping a record of total volume added during
the titration. Record the amount of NaOH added for each trial. Discard the two trials.
Adding and analyzing catalyst:
**Do not add catalyst unless you have time to do the last two titrations. Be sure to
complete them today so the reaction mixture can sit for a complete week until the next lab
session.
Refill buret (if needed) with standard solution NaOH. Clean out the titration flasks, rinse,
and fill with approximately 25 mL of distilled water. Add, with swirling, 10 drops of 6 M sulfuric acid catalyst to the
reaction mixture. Immediately pipet a 1.00 mL sample of the catalyzed reaction mixture into each of the titration flasks.
Do not delay pipetting, or the concentration of acetic acid will begin to change as the reaction occurs.
Titrate these two samples with the NaOH, record volume used for trials 3 & 4.
Stopper the 125 mL flask containing the acetic acid/1-propanol mixture. Place the reaction mixture in your locker until
next week.
B] Week two: only two titration trials needed…easy peasy lemon squeezy 
After standing for a week, the reaction system of 1-propanol and acetic acid will have come to equilibrium.
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CHS AP Chemistry Lab: Chemical Equilibrium
Rinse a buret and 1 mL pipet with distilled water. Rinse and fill buret with the standard 0.10 M NaOH solution.
Clean and rinse two 250 mL Erlenmeyer flasks. Place approximately 25 mL of distilled water in each of the
flasks.Uncover your reaction mixture. Pipet 1.00 mL samples into each of the two flasks. You should notice the odor of
the reaction mixture has changed markedly from the sharp vinegar odor of acetic acid that was present last week.
Add 3-4 drops of phenolphthalein to each sample, and titrate the samples to the pale pink endpoint with the standard
NaOH solution; record the total volume added in each flask.
Complete the data table and calculations.
WEEK ONE
Initial buret volume
Trial 1 uncatalyzed
Trial 2 uncatalyzed
Trial 5
Trial 6
Final buret volume
Total volume NaOH
used
WEEK TWO
Initial buret volume
Final buret volume
Total volume NaOH
used
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Trial 3 catalyzed
Trial 4 catalyzed
CHS AP Chemistry Lab: Chemical Equilibrium
Concentration of standard NaOH solution________
a. Volume of NaOH to titrate 1 mL
initial uncatalyzed mixture (1st week)
Trial 1
Trial 2
Mean volume
b. Concentration of acetic acid, M, in
original mixture
c. Volume of NaOH to titrate 1 mL
catalyzed reaction mixture (1st week)
Trial 3
Trial 4
Mean volume
d. Volume correction for sulfuric acid
e. Volume of NaOH to titrate 1 mL of
equilibrium mixture (2nd week)
Trial 5
Trial 6
Mean volume
f. Volume (corrected) of NaOH to
titrate acetic acid in equilibrium
mixture
g. Concentration of acetic acid, M, in
equilibrium mixture
h. Change in concentration of acetic
acid in reaching equilibrium
Complete ICE box using the information above:
CH3CHOOH +
CH3CH2CH2OH

CH3COOCH2CH2CH3
+
H2O
I
C
E
Calculate the equilibrium constant for the reaction. Using the theoretical value for this reaction, calculate percent error:
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CHS AP Chemistry Lab: Chemical Equilibrium
QUESTIONS:
1. Sulfuric acid was used as a catalyst in this reaction. Would the presence of a catalyst affect the position of the
equilibrium (the relative amounts of substances present once equilibrium was reached)? Why?
2. Could some other acid have been used as the catalyst? Why?
3. The 6 M sulfuric acid used as the catalyst for this reaction quite naturally contains some water, and water is one of the
products of the esterification reaction. How will the presence of a small amount of water in the catalyst affect the
position of the acetic acid/1-propanol equilibrium? How will this water affect the value determined for the equilibrium
constant?
4. A common misconception among students is that you leave liquids and solids out of equilibrium constants. All the
species involved in this reaction were liquids, and all of their concentrations were included in the calculation of the
equilibrium constant. Explain.
5. How would a change in temperature affect the equilibrium reaction studied? (Assume it is exothermic) Would the
position of equilibrium change? Would the value of the equilibrium constant change? Explain.
6. Draw a structural depiction of the esterification process of this lab.
a. Highlight and label the functional groups of the carboxylic acid, alcohol, and ester.
b. Label the hybridization of each carbon and oxygen atom in the structural diagram.
c. Label 1 bond angle in each reactant and product (4 total).
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