Physical Chemistry Laboratory
Experiment I-4
GAS PHASE EQUILIBRIUM: DISSOCIATION OF N2O4
References:
See “References to Experiments”
Giauque,W. F.; Kemp J. D. J. Chem. Phys. 6 (1939) 40.
Objectives:
To measure the equilibrium constant for the dissociation of dinitrogen tetroxide at a series of
temperatures
To compute
•
the standard Gibbs free energy of the dissociation reaction at each temperature
•
the standard enthalpy and the entropy of the reaction at 25oC and at 127oC.
To know the following and their interrelations:
•
Enthalpy of Reaction
•
Entropy of Reaction
•
Gibbs Free Energy of Reaction
•
Gibbs-Helmholtz Equation
•
Equilibrium Constant, Kp
Materials and Apparatus:
Cylinder of N2O4 gas.
Gas handling manifold
Gas measurement bulbs
Tra p
Platform balance
Bul b
Mettler analytical balance
Variable temperature water bath
Pump
N 2O4
Figure 1. Gas-handling manifold
Background:
The equilibrium between nitrogen tetroxide and nitrogen dioxide is
N2O4
2NO2
If a portion α of each 1 mole of N2 O4 dissociate into 2α moles of NO2, this leaves 1–α moles of N2O4.
However, the total number of moles in the system is (1–α)+2α=1+α moles. Calculating the euilibrium
constant, assuming a total pressure P
Physical Chemistry Laboratory
Kp =
2
pNO
2
p N2O4
=
x 2NO2 P 2
xN 2 O 4 P
=
( 2! (1 + ! ))
Experiment I-4
2
(1 " ! ) (1 + ! )
P=
4! 2
1 " !2
where xA is the mole fraction of any compound A.
Procedure:
Week One
1) Determine the volume of three or four 250 mL glass bulbs, using freshly boiled distilled water
that has been cooled to about room temperature. Accurately measure the water temperature so
that you may look up the correct water density. Weigh the evacuated bulb on a platform balance,
then fill it with water with a syringe, and reweigh. Be careful to eliminate any air bubbles inside
the bulb or water drops external to the stopcock seal. Repeat this for each bulb you use in the
experiment. Record and report the I.D.# for each bulb used. Carefully empty the bulbs and place
them (without stopcocks) in an oven to dry.
Week Two
1.
Prepare the gas-handling manifold (see Figure 1) for use, under the direction of the instructor by
replacing the cold trap, evacuating the line, then filling the Dewar flask around the trap with
liquid nitrogen LN2
CAUTION: LN2 can cause severe frostbite.
CAUTION: An LN2-cooled trap can condense oxygen and argon if left open to the atmosphere
CAUTION: Evacuated glassware, including Dewar flasks are potential implosion hazards
2.
Attach the glass bulbs to the manifold by ground joints with a small amount of silicone stopcock
grease, evacuate the bulbs to less than 0.1 mm Hg and weigh on an analytical balance. Be sure to
wipe the joint as clean as possible before weighing.
3.
Reattach the bulbs to the manifold, checking each joint for leaks by isolating the pump and
monitoring the Hg manometer. If a leak is found, remove the bulb, clean both parts of the joint,
regrease lightly and reattach.
4.
Fill each bulb with N2 O4. The vapor pressure of N2O4 is about one atmosphere at room
temperature. Therefore the cylinder needle valve can be opened slowly with little danger of
over-pressurizing the system. However, always use caution with such systems to avoid damage
to glass manifolds. Once the system has been pressurized to the vapor pressure of the gas, cool
the gas bulb with ice water until the first bit of liquid is formed. At this point, shut off the bulb
and system, and separate the bulb from the system, carefully cleaning any grease from the joint.
Condense any excess N2O4 from the manifold into the cold trap.
CAUTION: N2O4 is very toxic and corrosive. All manipulations must be performed in a wellventilated fume hood.
5.
Allow the bulbs to warm to room temperature. Vent the bulbs (in the hood) several times by
opening and rapidly reclosing the stopcock until no liquid is left and no brown NO2 fumes
escape.
6.
Sequentially place the bulbs (to just below the stopcock) into a water bath to thermally equilibrate
at approximately 30°C, venting occasionally to ensure that the bulb pressure is the same as the
ambient pressure. While Venting: keep the body of the bulb immersed in the constant
temperature bath within the hood. Dry the bulbs, allow them to cool to room temperature, and
weigh on the analytical balance. Be sure to clean off any fingerprints and that no water has been
trapped around the stopcock or bulb inlet. While cooling and weighing the bulbs, increase the
temperature of the bath by ~5°C. Repeat this procedure up to 60°C.
CAUTION: Pressure can build up rapidly when the bulb is subjected to a large, rapid
temperature change.
Physical Chemistry Laboratory
7.
Experiment I-4
Shut down the gas-manifold (under direction of the instructor) by isolating the pump, venting
the manifold, and rapidly removing the LN2 Dewar. The cold trap should be disconnected and
clamped horizontally in the hood to allow frozen N2O4 to vaporize overnight.
Treatment of Data:
1.
Assume an ideal mixture of ideal gases. From your data, compute the average molecular weight
of the gas mixture at each temperature. Using these values, determine the degree of dissociation
and the equilibrium constant, Kp, ΔGo and ΔSo for the dissociation at each temperature. From an
analysis of a graph of ln Kp vs. 1/T for each bulb, determine the enthalpy of the reaction
(assumed constant over the temperature range used). From the parameters obtained from the
graph, compute α, Kp, ΔGo, and ΔSo at 127oC and at 25oC.
2.
In your report, tabulate the volume calibration data as well as the experimental values for mass,
average molar mass, α, Kp, ΔGo , ΔHo, 1/T, ln Kp, and ΔSo at each of the temperatures of the
experiment as well as at 25oC and at 127oC. Discuss the significance of the sign of ΔGo as well as
of the change in magnitude of the value of ΔGo with temperature. Treat each bulb separately, do
not average the values from the two bulbs.
3.
From the following literature values (in kJ/mol) of free energies and enthalpies of formation,
compute α, ΔHo, ΔGo, Kp and ΔSo for the reaction at 25oC and at 127oC.
NO2
N2O4
ΔH
ΔG
ΔH
ΔGfo
298.15
33.095
51.258
9.079
97.787
400.15
32.217
57.534
8.514
128.110
Temp /K
o
f
o
f
o
f
Error Analysis:
1.
From the standard deviation of slope (from linear least-squares analysis), calculate the standard
deviation of ΔH0.
2.
From the propagation of error computation of K calculate the uncertainty in ΔG0 at a temperature
in the middle of the range,
3.
From the standard deviation in (1) and the propagated error in (2), calculate the uncertainty in
ΔS0 at the same temperature in (2).
4.
Discuss sources of systematic error in this experiment, and suggest ways of eliminating as many
of them as possible.