Experiment 4(97 2003)Format

8/4/2019 Experiment 4(97 2003)Format

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Experiment 4: Homogeneous GaseousEquilibriumThe Dissociation of N2O4

CHM 3011



Preparation

To make dry ice, take the small coolerinto Rm 212.

Keeping the door ajar, close the dry icemaker, making sure the latch is secure.

Open the CO2 tank by turning the handlecounterclockwise for several revolutions.

You should hear a hiss and see a whitefog seeping out of the maker.

When the sound changes from a hiss toa loud sound through the safety valve ontop of the maker, quickly close the tank.

Open the dry ice maker and remove the

dry ice cake with paper towels*Caution: Do not allow bare skin to touch thedry ice as this will result in a severe burn.

Place the cake into the cooler.*Click on picture for videoPlease note that it will take approximately 30-

40s for the dry ice to be made. The videodoes not show this waiting period.



Preparation

Break the dry ice cake intosmall pieces and add itslowly to the Dewar flask

containing isopropyl alcohol. The aim is to cool the trap

so that the N2O4 /NO2 mixture is condensed beforereaching the pump.

Dry ice is warm enough that

any air in the system ispumped through the trap.

The alcohol provides goodthermal contact between thedry ice and trap.

*Click on picture for video



Procedure

Take the Dumas (gas) bulband, if necessary, lubricate

the stopcock with aminimum quantity of Corninggrease.

Connect the bulb to thevacuum manifold andexhaust to a pressure of 1-

2mm.*Click on picture for video



Procedure

Remove the evacuated bulb and weighit to 0.1mg.

a. Clean the bulb with a Kimwipe andplace in the balance.

b. Turn the left side knob to Preweigh.c. The illuminated number on the

front of the balance should beapproximately 70. Turn the left sideknob to the off position.

d. Enter the value of the illuminatednumber using the front-left knob forthe tens of grams and the front-

right knob for unit grams.e. Turn the left-side knob to FullRelease.

f. Use the right side knob to align theblack bar with the illuminatednumber.

g. Record the mass.*Click on picture for video



Procedure

The partially evacuated bulb has buoyancy that causes themass to drift somewhat; for this reason, take at least four

readings for each mass measurement throughout theexperiment.



Procedure

Fill the bulb with dry air atatmospheric pressure by

allowing the air to enterthrough a CaSO4 dryingtube.

Close the stopcock andreweigh the bulb.

*Don’t forget to replace theblack cap and pinch collaron the CaSO4 drying tube.




Procedure

*Refer to the next slide for pictures.

Measure the atmospheric pressure on the barometer and record theroom temperature using the thermometer on the barometer.

Adjust the level of the reservoir by using the cistern to adjust the screwuntil the level just touches the tip of the white pointer in the cistern.

Move the sliding vernier scale until the front and back of its bottom areeven with the top of the convex mercury meniscus.

Read the height to the nearest millimeter from the position of the

vernier bottom on the fixed scale. Use the vernier to read to the nearest 0.1mm by noting which line on

the vernier matches a line on the fixed scale. Correct the mercury height to what it would be at 0 C by using the

chart to the side of the barometer to determine the amount to subtractfrom the reading at room temperature and the pressure reading.



Use the sliding vernier tomeasure the height of the

mercury column.

Cistern adjusting screw

Adjust the screw until themercury reservoir justtouches the tip of the whitepointer in the cistern.



Procedure

Under the supervision of theinstructor, evacuate the

Dumas bulb again and fillthe bulb with the N2O4 /NO2 mixture to atmosphericpressure.

*CAUTION: Dinitrogen tetroxideis corrosive and very

poisonous; all operationsshould be carried out withcare.




Procedure

Attach the capillary tip withtygon tubing to the top of the

bulb before placing it in thebath.




Procedure

Immerse the bulb in a waterbath regulated to about40°C with the stopcock andstem of the bulb heldvertically above the water.

As soon as the bulb is in thebath, place the funnel, whichis attached to house

vacuum, a centimeter abovethe end of the capillary.

Keep the temperatureconstant to within 0.1°C forat least ten minutes.




Procedure

Using the funnel to catch theescaping gas, repeatedlyopen the stopcock

momentarily until the loss ofbrown fumes is negligibleand the gas mixture is atatmospheric pressure; thenbe certain the stopcock isclosed.

Record the temperatureimmediately before and afterthis procedure.




Procedure

Remove the capillary tip and allow the bulb to cool by placing itin a beaker of water for 5 minutes.

Read the barometer.*Click on picture for video



Procedure

Dry the bulb carefully, and thenweigh it to the nearest 0.1mg.

Since the contents of the bulbare below atmosphericpressure when the bulb isbelow the bath temperature,opening the stopcock exceptwhen the bulb is in the bath will

contaminate the gas mixturewith air and ruin the results.

If this should occur, a newsample of gas is required; seethe instructor.

*Click picture for video



Procedure

Repeat the previous steps, starting with the immersion of thebulb in the water bath, at five additional temperatures 5-7°

apart, each time holding the temperature constant to 0.1°. To save time, raise the bath temperature while the bulb is being

cooled and weighed.

Allow about 10 minutes at each new temperature to makecertain the bulb has reached equilibrium with the bath before

opening the stopcock.



Calculations and Discussion

Use the following slides as an example for the plots andcalculations required in this lab report.

The following slide shows sample data that has been inputtedinto the provided Excel spreadsheet.

Note that the columns highlighted in yellow contain thecollected experimental data. The remaining columns containvalues calculated by the program.



Florida Tech CHM 3011 Homogeneous Gas Equilibrium

Barometric Press (torr) = 762.9 Mass of empty bulb = 70.6519

Density dry air (g/mL) = 1.205E-03 Mass of bulb with air = 70.8857

Volume of bulb (L) = 1.940E-01

Do not skip or delete cells below!

Temp Mass filled bulb Mass gas T 1/T Alpha K ln(K)

41.5 71.2011 0.5492 314.7 3.178E-03 0.2636 0.2999 -1.2044

47.0 71.163 0.5111 320.2 3.123E-03 0.3345 0.5058 -0.6816

52.5 71.1325 0.4806 325.7 3.070E-03 0.3952 0.7432 -0.2967

59.5 71.1024 0.4505 332.7 3.006E-03 0.4571 1.0606 0.058963.1 71.0816 0.4297 336.3 2.974E-03 0.5113 1.4212 0.3515

71.8 71.0618 0.4099 345.0 2.899E-03 0.5443 1.6908 0.5252

slope= -6288.97566 intercept = 18.91983

std dev = 604.3139489 std dev = 1.838866

Equillibrium constant at 298 K = 0.1138

DG (kJ/mol) at 298 K = 5.39 +/-9.6

DH (kJ/mol) = 52.29 +/-5.0

DS (J/deg-mol) at 298 K 157.30 +/-15.3



To explain the computer results, provide the following sample calculations for one of yourtemperatures:

= 0.001293P(1 + 0.00367t)*760

where t = temperature in degrees Celsius

Use the density of the air and the weights of the Dumas bulb when evacuated and when filledwith dry air, to calculate the volume of the bulb.

Use the following equations to calculate a and KP:

a = MPV - 1WRT

KP = 4a2P1 –a2

where M = molecular weight of undissociated N2O4

P = the corrected barometric reading in atmospheres

V = volume of bulbW = weight of gas in bulbR = gas constantT = temperature in Kelvin

*Be sure to record your calculated values and the computer values for your temperature in thefollowing format:

V °C W T 1/T a KP lnKP



CalculationsNote: The results given here do not include the correct number of significant figures. This

will be determined by your error analysis.

• The Excel spreadsheet willautomatically create a plot of

lnK vs. 1/T and provide theslope and y-intercept.

Homogeneous Gas Equilibrium

-1.4

-1.2-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.60.8

2. 8E-03 2. 9E-03 3. 0E-03 3. 1E-03 3. 2E-03

1/T

l n ( K )



Obtain the value of KP at 298K by using the slope and intercept from the lnK vs 1/Tplot.

Then, use the following formula to obtain the value of DG°298:

DG = -RTlnKP

= ______kJ/mol

Obtain DH° by using the following formula:

slope = -DH° /R

DH = -slope*R

= ______kJ/mol

Using the Gibbs equation, calculate the entropy for dissociation:

DG° = DH° – TDS°

DS° = DH° – DG°

T

= ______J/mol



Calculations

Compare your values (in kJ/mol) with those from literature forDG°, DH°, and S°.

Experiment 4(97 2003)Format

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