E q u i l i b r i u m : D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t P u r p o s e To determine the equilibrium constant of a reaction. L e a r n i n g O b j e c t i v e s Take a reaction to equilibrium by setting up and monitoring a reaction in a reflux apparatus. Measure the amount of acid at equilibrium by carrying out an acid-base titration. Apply the information from a balanced chemical equation and data obtained in the laboratory to de- termine the concentrations of reactants and products at equilibrium. Calculate the value of the equilibrium constant using data obtained in the laboratory. L a b o r a t o r y S k i l l s To set up and monitor a reflux apparatus. To carry out an acid-base titration. E q u i p m e n t Two 50-mL graduated cylinders Two 125-mL Erlenmeyer flasks 1-mL pipet 25-mL buret Equipment necessary to assemble the reflux apparatus shown in Figure 1. C h e m i c a l s Anhydrous ethanol (ethyl alcohol) Anhydrous acetic acid Concentrated sulfuric acid I n t r o d u c t i o n From the beginning of this course, we have generally assumed that chemical reactions go to completion, that is, the reaction proceeds in the forward direction until one of the reactants is completely used up. However, many reactions do not go to completion and are able to move both in the forward and reverse directions simultaneously. Such a reaction is called a reversible reaction. A double arrow in the chemical equation designates a reversible reaction, as shown in Reaction 1: aA + bB −−−⇀↽−−− cC + dD (Reaction 1) 1 D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t A reversible reaction has two reaction rates: a forward reaction rate, where the reactants A and B are consumed andtheproductsCandDareproduced,andareversereactionrate,wheretheproductsCandDareconsumedand thereactantsAandBareproduced. Allreversiblereactionseventuallyreachapointatwhichtheforwardreaction rate equals the reverse reaction rate. This point is called equilibrium. At equilibrium, the concentration of reactants and products do not change with time. It is important to remember that even though the concentration of reactants and products do not change with time, the reaction has not stopped. Equilibrium is a dynamic state. The state will persist as long as the reaction conditions remain constant. A reaction at equilibrium follows the law of mass action which gives the relationship between concentrations of the reactants and products at equilibrium. According to the law of mass action, the relationship between concentrations of reactants and products at equilibrium for the above reaction is given in Equation 1: 𝐾eq = [C]𝑐[D]𝑑 [A]𝑎[B]𝑏 (Equation 1) Thisrelationshipiscalledtheequilibrium-constantexpression. Theconstant, 𝐾eq, isapositivenumberwhose value depends on the reaction and temperature. In today’s experiment, students will be determining the equilibrium constant for the reac

1. E q u i l i b r i u m :
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
P u r p o s e
To determine the equilibrium constant of a reaction.
L e a r n i n g O b j e c t i v e s
Take a reaction to equilibrium by setting up and monitoring a
reaction in a reflux apparatus.
Measure the amount of acid at equilibrium by carrying out an
acid-base titration.
Apply the information from a balanced chemical equation and
data obtained in the laboratory to de-
termine the concentrations of reactants and products at
equilibrium.
Calculate the value of the equilibrium constant using data
obtained in the laboratory.
L a b o r a t o r y S k i l l s
To set up and monitor a reflux apparatus.
To carry out an acid-base titration.
E q u i p m e n t

2. Two 50-mL
graduated cylinders
Two 125-mL
Erlenmeyer flasks
1-mL pipet
25-mL buret
Equipment necessary
to assemble the
reflux apparatus
shown in Figure 1.
C h e m i c a l s
Anhydrous ethanol
(ethyl alcohol)
Anhydrous acetic
acid
Concentrated sulfuric
acid
I n t r o d u c t i o n

3. From the beginning of this course, we have generally assumed
that chemical reactions go to completion, that is,
the reaction proceeds in the forward direction until one of the
reactants is completely used up. However, many
reactions do not go to completion and are able to move both in
the forward and reverse directions simultaneously.
Such a reaction is called a reversible reaction. A double arrow
in the chemical equation designates a reversible
reaction, as shown in Reaction 1:
aA + bB −−−⇀↽−−− cC + dD (Reaction 1)
1
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
A reversible reaction has two reaction rates: a forward reaction
rate, where the reactants A and B are consumed
andtheproductsCandDareproduced,andareversereactionrate,wher
etheproductsCandDareconsumedand
thereactantsAandBareproduced.
Allreversiblereactionseventuallyreachapointatwhichtheforwardr
eaction
rate equals the reverse reaction rate. This point is called
equilibrium. At equilibrium, the concentration of

4. reactants and products do not change with time. It is important
to remember that even though the concentration
of reactants and products do not change with time, the reaction
has not stopped. Equilibrium is a dynamic state.
The state will persist as long as the reaction conditions remain
constant.
A reaction at equilibrium follows the law of mass action which
gives the relationship between concentrations
of the reactants and products at equilibrium. According to the
law of mass action, the relationship between
concentrations of reactants and products at equilibrium for the
above reaction is given in Equation 1:
�eq =
[C]�[D]�
[A]�[B]�
(Equation 1)
Thisrelationshipiscalledtheequilibrium-constantexpression.
Theconstant, �eq, isapositivenumberwhose
value depends on the reaction and temperature.
In today’s experiment, students will be determining the
equilibrium constant for the reaction of ethyl alcohol
(C2H5OH) with acetic acid (HC2H3O2) to produce ethyl acetate
(CH3COOC2H5) and water according to Reac-
tion 2:

5. C2H5OH(aq) + HC2H3O2(aq) −−−⇀↽−−− CH3COOC2H5 +
H2O (Reaction 2)
The equilibrium expression for this reaction is given in
Equation 2:
�eq =
[CH3COOC2H5][H2O]
[C2H5OH][HC2H3O2]
(Equation 2)
This reaction is a bit unusual for general chemistry students
because it does not occur in dilute aqueous solution.
The reaction begins by mixing anhydrous ethyl alcohol with
anhydrous acetic acid (called glacial acetic acid).
Note that this means the there is no (or very little) water present
in the reactants but, because water is a product,
the concentration of water changes during the reaction. Some
sulfuric acid is added to act as a catalyst to allow
the reaction reach equilibrium faster. The reaction mixture is
heated to boiling and then maintained at boiling for
1-1.5 hours. This gives the reaction sufficient time to reach
equilibrium. The reaction mixture is then analyzed
to determine the equilibrium concentrations from which the
equilibrium constant may be determined.
Studentswill determinetheconcentrationof aceticacid by
titrationagainst0.25 MNaOHsolution. The acid-base

6. 2
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
neutralization reaction is shown in Reaction 3:
HC2H3O2(aq) + NaOH(aq) −−−→ NaC2H3O2(aq + H2O(l)
(Reaction 3)
At the endpoint, the number of moles of NaOH added will be
equal to the number of moles of acetic acid con-
tained in your sample. The number of moles of NaOH added can
be calculated from the the volume of NaOH
solution and the molarity of NaOH solution. The number of
moles of acetic acid contained in your sample is
equal to the volume of your solution used in the titration times
the molarity of acetic acid. Because this neutral-
ization reaction has a 1:1 stoichiometric relationship between
the acid and the base, you can use Equation 3 to
determine the molarity of the acetic acid in your sample:
Vacid×Macid = VNaOH×MNaOH (Equation 3)
Figure 1: R e fl u x a p p a r a t u s
It is important to remember that this formula only works for
acid-

7. base titrations in which one mole of acid neutralizes one mole
of
base. Forexample, itwouldnotworkfortitrationsof sulfuricacid
(H2SO4) with sodium hydroxide.
When setting up the reflux apparatus (Figure 1), be sure to
place
the clamps in the positions shown to stabilize the assembly.
Suf-
ficient distance should be allowed between the wire gauze and
the Bunsen burner to allow for adjustment of flame height. The
water inlet on the condenser should be connected to a water
sup-
ply using a rubber hose. The water outlet should have a rubber
hose leading to a sink or trough. Be certain that the rubber
hoses
are firmly attached so that no water leaks into the reaction
flask.
The water supply should then be adjusted so that there is a
steady
flow through the cooling jacket of the condenser in the
indicated
direction.

8. 3
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
P r o c e d u r e
You will be working in pairs on this experiment. Each student
should hand in a separate data sheet.
A . D e t e r m i n a t i o n o f i n i t i a l c o n c e n t r a t i o n s
1. Measure 31.5 mL (0.5 mol) of glacial acetic acid and 29.1 mL
(0.5 mol) of ethyl alcohol in separate clean, dry
50-mL graduated cylinders.
2. Pour the two reactants simultaneously into the round-bottom
flask. Mix thoroughly.
3. Immediately remove 1 mL of the reaction mixture using a 1-
mL pipet.
4. Place the 1 mL sample in a 125-mL Erlenmeyer flask
containing 30 mL of deionized water.
5. Add three drops of phenolphthalein indicator and titrate the
sample with the standard 0.25 M NaOH fur-
nished.
6. Record the volume of NaOH required.
7. Calculate the initial concentration of acetic acid using
Equation 3.

9. Showyourcalculationsonthereportsheet.
Rememberthattheendpointof thetitrationisthefirst pinkcolor
that persists for more than 30 seconds. Do not continue the
titration until the solution becomes darker pink
or purple. Since equal number of moles of acetic acid and ethyl
alcohol were used to prepare the reaction
mixture, the initial concentration of ethyl alcohol will be equal
to the initial concentration of acetic acid that
is calculated.
8. Place two or three boiling chips in the reaction mixture in the
round bottom flask to ensure smooth boiling.
9. Carefully add 20 drops of concentrated sulfuric acid.
10. Reconnect the condenser to the flask and begin heating the
mixture. Be certain that the condenser is snugly
4
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
fitted to the flask and that water is flowing through the
condenser. Done correctly, no fumes can escape
from either the top or the bottom of the condenser. As the
mixture boils, you should note fumes rising
a few inches into the condenser and liquid condensing at that

10. point and dropping back into the reaction
flask. This condition is known as reflux and will insure a
constant temperature of the mixture. The reaction
mixture should boil gently for one to one and one-half hours to
allow the reaction sufficient time to reach
equilibrium.
B . D e t e r m i n a t i o n o f t h e b l a n k
While the reflux process is taking place, perform the following
titration to determine the amount of NaOH
solution required to neutralize the sulfuric acid added to the
reaction mixture.
11. Prepare a blank solution by adding the same amount of
H2SO4 (20 drops) that was added to the reaction
mixture to 60.6 mL of RO water (the same volume as the
reaction mixture) in a 125-mL Erlenmeyer flask
12. Mix thoroughly.
13. Pipet 1 mL of this blank solution into a second 125-mL
Erlenmeyer flask.
14. Add 30 mL of RO water, three drops of phenolphthalein,
and titrate as before with 0.25 M NaOH solution.
15. Record the volume of NaOH required to reach the endpoint.
C . D e t e r m i n a t i o n o f fi n a l c o n c e n t r a t i o n s

11. When the reaction has reached equilibrium, turn off the heat and
allow the mixture to cool to room temper-
ature. Then disconnect the condenser.
16. Pipet 1 mL of the reaction mixture into a 125-mL
Erlenmeyer flask.
17. Add 30 mL of RO water, three drops of phenolphthalein,
and titrate as before with 0.25 M NaOH.
18. Record the volume of NaOH required to reach the endpoint.
This volume represents the amount of NaOH
solutionneededtoneutralizetheaceticacidandthesulfuricacidcontai
nedinthereactionmixture. Subtract
the volume required to neutralize the sulfuric acid, determined
by the blank, from this volume to obtain the
5
D e t e r m i n a t i o n o f a n E q u i l i b r i u m C o n s t a n t
volumeof NaOHsolutionrequiredtoneutralizetheaceticacid.
Usethecorrectedvolumeand?? tocalculate
the equilibrium concentration of the acetic acid.
D . C a l c u l a t i o n o f t h e e q u i l i b r i u m c o n s t a n t
The reaction started with the initial concentrations of acetic
acid and ethyl alcohol begin equal and the two

12. reactants react in a 1:1 ratio.
• Thus, the equilibrium concentration of ethyl alcohol is equal
to the equilibrium concentration of acetic
acid.
• One mole of ethyl acetate is formed for every mole of acetic
acid reacted, so the equilibrium concentra-
tionof ethylacetatewillbeequaltothechangeinconcentrationof
aceticacid. Thechangeinconcentra-
tionof aceticacidisequaltotheinitialconcentrationof
aceticacidminustheequilibriumconcentration
of acetic acid.
• Onemoleof waterisformedforeverymoleof
ethylacetateformedandtheinitialconcentrationof water
is very small. Thus, the equilibrium concentration of water is
equal to the equilibrium concentration of
ethyl acetate.
From these equilibrium concentrations, use Equation 2 to
calculate the value of Keq for this reaction. Show your
calculations on the data sheet.
6
Determination of an Equilibrium ConstantPurposeLearning
ObjectivesLaboratory SkillsEquipment-1emChemicals-
1emIntroductionProcedureA. Determination of initial
concentrationsB. Determination of the blankC. Determination of

13. final concentrationsD. Calculation of the equilibrium
constantDetermination of an Equilibrium Constant Report
SheetA. Determination of initial concentrationsB.
Determination of the blankC. Determination of final
concentrationsD. Calculation of the equilibrium constantPost-
Laboratory Questions