UNIVERSITY OF MINNESOTA DULUTH
DEPARTMENT OF CHEMICAL ENGINEERING
ChE 3211-4211
CHEMICAL EQUILIBRIUM IN A LIQUID PHASE
OBJECTIVE
The objective of this experiment is to determine the equilibrium constant for a chemical
reaction from laboratory data. The experiment is designed to approach equilibrium from both
the forward and reverse directions in order to determine if there is a difference in the equilibrium
constant.
INTRODUCTION
There are many chemical reactions that do not go to completion. The products formed by
these reactions, in turn, react to give the reactants. In other words, these chemical reactions are
reversible. In these reactions, a point is reached where the rate of formation of the products by
the forward reaction is equal to the rate of formation of the reactants by the reverse reaction.
When a reaction reaches this stage, it is said to be in chemical equilibrium.
For a general reversible reaction:
mM + nN = rR + sS
the thermodynamic equilibrium constant is defined as:
s
a rRe a Se
Ka = m n
a Me a Ne
where
Ka = Thermodynamic equilibrium constant
aie = activity species i at equilibrium
The thermodynamic equilibrium constant Ka is constant for a given temperature. Addition
of a catalyst in a chemical reaction does not change the value of the equilibrium constant, it
merely decreases the time it takes for the reaction to reach equilibrium.
In the liquid phase, the activity, aie, of a species i in a mixture at equilibrium is given by the
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product of its equilibrium concentration Cie and its activity coefficient γie. The thermodynamic
equilibrium constant can then be written as:
s
γ rRe γ Se
Ka = m n
γ Me γ Ne
s
C rRe CSe
n
Cm
Me C Ne
Ka = Kγ Kc
where
s
γ rRe γ Se
Kγ = m n
γ Me γ Ne
s
C rRe CSe
Kc = m n
C Me C Ne
Kc is defined as the concentration equilibrium constant.
The chemical equilibrium to be studied in this experiment is the esterification reaction.
C2H5OH + CH3COOH = CH3COOC2H5 + H2O
At ordinary temperatures and in the absence of a catalyst, this reaction approaches equilibrium
very slowly. Equilibrium may be attained in a reasonable time, either by allowing the reaction
mixture to stand (in thick-walled sealed tubes) at a high temperature (about 200E C), or at room
temperature in the presence of a strong acid which acts as a catalyst. The concentration
equilibrium constant can be determined by analyzing the equilibrium mixture.
In the present experiment, 6 N HCl is to be used as a catalyst to avoid the danger of an
explosion of the heated sealed tubes. The equilibrium is to be approached from both directions,
starting with CH3COOC2H5 and with a mixture of C2H5OH and CH3COOH. The equilibrium
concentrations can be determined by using known weights of the reactants and of the
standardized 6 N HCl and titrating the total acid present in the equilibrium mixture. The
mixtures are placed in flasks with ground glass stoppers, to avoid loss by volatilization.
The relatively concentrated acid, which is used as catalyst in this experiment, affects the
concentration equilibrium constant by changing the activity coefficients of the reactants. A
smaller value of the concentration equilibrium constant would be obtained if the reaction were
carried out in the absence of the catalyst. It is unnecessary to thermostat the reaction mixture,
since the heat of the reaction is practically zero.
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REFERENCES
Atkins, P.W., "Physical Chemistry", 3rd Edition, W.H. Freeman & Company, New York, 1986,
pp. 217-223
EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
50 mL buret
20 mL transfer pipet
10 mL transfer pipet
5 mL graduated pipet.
2 mL graduated pipet
6-125 mL erlenmeyer flask
3-150 mL beaker
3-250 mL beaker
8-glass stoppered weighing bottles
8-25 mL volumetric flasks/ground glass stoppers.
CHEMICALS/MATERIALS
1.
2.
3.
4.
5.
6.
6 N HCL.
1 N NaOH.
CH3COOC2H5.
Glacial CH3COOH.
Absolute C2H5OH.
0.1N KHC8H4O4.
EXPERIMENTAL PROCEDURE
The reaction will be carried out in 25 ml volumetric flasks with ground glass stoppers.
Pipet 3 mL of 6 N HCl into each of 8 vol flasks, being careful to get all of the solution below the
neck of the volumetric flask. To each of the first two volumetric flasks add 3 mL of distilled
water. To the third and fourth, add 3 mL of pure CH3COOC2H5. To the fifth and sixth, add 1.8
mL of glacial CH3COOH and 1.2 mL of absolute C2H5OH. To the seventh and eighth, add 1.2
mL of glacial CH3COOH and 1.8 mL of absolute C2H5OH. Twist the ground glass stoppers on
the volumetric flasks slightly to create a tight seal. Tubes 3 and 4 originally contain a
heterogeneous mixture. To promote the reaction, shake these tubes occasionally (several times a
day). Record temperature at the start and end of your experiment.
Standardize the 1 N NaOH by titrating 3 replicates of 20 mL of standard 0.1 N potassium
hydrogen phthalate (actual standard concentration will be found on the bottle) with the 1 N
NaOH using phenolphthalein as an indicator. Titrate at least 3 - 2.0 mL samples of glacial
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CH3COOH and 3 - 3.0 mL samples of 6 N HCl with the standardized 1 N NaOH using
phenolphthalein as an indicator.
Using the small stoppered weighing bottles and an analytical balance, determine the weights
of the following liquids discharged by the same pipets which were used in measuring out the
samples: 3.0 mL of 6 N HCl; 3.0 mL of distilled water; 3 mL of pure CH3COOC2H5; 1.2 mL, 1.8
mL, and 2.0 mL of glacial CH3COOH; and 1.2 mL and 1.8 mL of absolute C2H5OH.
After 2 days or more, analyze the contents of each volumetric flask as follows: 1)
Restandardize the 1 N NaOH using the standard 0.1 N potassium hydrogen phthalate; 2) remove
the ground glass stopper on the volumetric flask and pour the solution into the 125 mL E-flask;
3) with a wash bottle, rinse the inside of the volumetric flask, adding the rinse water to the Eflask; and 4) titrate the excess of acid with the standardized 1 N NaOH.
SAFETY NOTES
1. Before starting the experiment, review the Material Safety Data Sheets (MSDS) on
HCl, absolute C2H5OH, and CH3COOC2H5. The sheets can be found in the MSDS
notebook located in the laboratory.
2. Personal protective equipment shall include safety goggles and nitrile gloves. Safety
glasses with side shields can be worn if you do not have access to safety goggles.
There is a chance of chemicals being splashed during this experiment and safety
glasses with side shields do not provide as high a level of protection as safety goggles.
3. A pipet bulb shall be used to pipet liquids.
4. Notify your instructor in case of a spill.
WASTE DISPOSAL PROCEDURES
All sample wastes shall be discarded in the waste container provided. The wastes from the
standardization procedure can be disposed of into the sewer system. Wastes from the cleaning of
glassware can also be disposed of into the sewer system.
07/2012
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Department of Chemical Engineering
Stockroom Checkout slip
Chemical Equilibrium In a Liquid Phase
ChE 3211
Name:
Date:
(print name)
Lab No.: Lab 1 Tuesday 12:00 - 4:50 PM
Lab No.:
Lab 3
Lab 2: Thursday
12:00 - 4:50 PM
Tuesday and Thursday morning (9:30 - 11:50 AM)
(circle one)
Equipment
Out
In
Equipment
20 mL transfer pipet
3-150 mL beaker
10 mL transfer pipet
8-glass stop weigh bottle
3 mL transfer pipet
8-25 mL vol flask
10 mL graduated pipet
2-black pipet bulb
2 mL graduated pipet
2 - 1.5" magnetic stir bars
6-125 mL erlenmeyer flask
3-250 mL beaker
Name:
(Signature)
5
Out
In