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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

5

0-mL

graduated cylinders

Two

1

2

5-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)

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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:

?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 (HC2H

3

O2) to produce ethyl acetate (CH3COOC2H5) and water according to Reac-

tion 2:

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

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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-

base titrations in which one mole of acid neutralizes one mole of

base. Forexample, itwouldnotworkfortitrationsof sulfuricacid

(H2SO

4

) 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.

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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.

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

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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 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

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

solutionneededtoneutralizetheaceticacidandthesulfuricacidcontainedinthereactionmixture. Subtract

the volume required to neutralize the sulfuric acid, determined by the blank, from this volume to obtain the

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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

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 Constant
• Purpose
• Learning Objectives
• Laboratory Skills
• Equipment-1em
• Chemicals-1em

Introduction
Procedure
A. Determination of initial concentrations
B. Determination of the blank
C. Determination of final concentrations
D. Calculation of the equilibrium constant

• Determination of an Equilibrium Constant Report Sheet

A. Determination of initial concentrations
B. Determination of the blank
C. Determination of final concentrations
D. Calculation of the equilibrium constant
Post-Laboratory Questions