11 bp- boiling point elevation by solid

BP: Boiling Point Elevation by a Solid

147

Chapter the Eleventh

11.1 INTRODUCTION

A colligative property is a physical property that depends solely on the number ofparticles of solute present and not on their nature (big, small, ionic, light, heavy,etc.). This experiment studies one of the four colligative properties of dilute systems:boiling point elevation, that is, the increase in the boiling point of some volatilematerial as small amounts of nonvolatile solute are added. Boiling point elevationdata can be used to calculate the molecular weight of the added solute; however, thisrequires an extremely accurate measurement of the temperature change. We use aspecial thermometer for this purpose. In addition, we have to take steps to avoid theeffects of superheating on our measurement of the boiling point. The technique ofdetermining molecular weight by boiling point elevation is called ebullioscopy.

Figure 11.1 Experimental Setup

11. KH_Wilson_84756_CH11.qxd 9/8/08 4:08 PM 47

It is important to remember that boiling point elevation is dependent on thenumber of solute particles. An interesting application of this experiment is to studysolutions in which the number of solute particles is not necessarily the number pre-dicted solely from the number of moles of solute in the solution. For example, anionic solid in an aqueous solution will dissociate, resulting in an increased numberof particles in solution. In this experiment, you will investigate the alternative sit-uation where a solute dimerizes, decreasing the number of particles in solution.

11.2 TEXTBOOK REFERENCES

Daniels, F.; Alberty, R. A.; Williams, J. W.; Cornwell, C. D.; Bender, P.; Harriman,J. E. Experimental Physical Chemistry; McGraw-Hill: New York, 1970; p. 86.(available in the library in the pchem reference folder, and through E-Reserves)

11.3 CHEMICALS

148 Hooked on PChem Lab

Above your bench

Benzoic acid

Chloroform

Naphthalene

11.4 EQUIPMENT

Refer to Sections 4.2, Analytical Balance, 4.10, Pellet Press, 4.11.2, Thermistorand 4.14, Volumetric Pipette.

Stockroom In the experiment drawer

Condenser Two beakers (one large, onemedium)

Cottrell flask Heat-protecting gloves

Glass pumping mechanism Boiling chips

50 mL volumetric pipette Hot/Stir plate

Digital thermometer Magnetic stir bar

Thermistor probe with digital readout Pipette bulb

One one-holed rubber stopper Two spatulas

Supply bench Large tweezers

Analytical balance Hoses and hose clamps

Pellet press Lab stand and clamps

11. KH_Wilson_84756_CH11.qxd 9/8/08 4:08 PM Page 148

11.5 SAFETY

149Chapter 11 BP: Boiling Point Elevation by a Solid

Physical Hazards Health Hazards

Benzoic acid None Severe irritant, sensitizer

Chloroform None Highly toxic, carcinogenic, mutagenic

Naphthalene Flammable Toxic, irritant, sensitizer, carcinogenic

Wear gloves and use a fume hood while measuring these chemicals. Keep the con-denser pointing toward the back of the bench at all times. (The boiling solution can‘bump’, blowing the hot solution out of the condenser with considerable force.)

11.6 PROCEDURE

The boiling point elevation experiment is the most sensitive experiment in this lab.Pay attention to the hints in the procedure warning you of potential problem areas.A small experimental error will result in a large error in your final results.

Cottrell

Flask

Condenser

Rubber

stopper

Glass

Pumping

Mechanism

Thermistor

Figure 11.2 Boiling Point Elevation Apparatus


11.6.1 Setup

1. The glassware supplied by the stockroom will be clean. Thoroughly clean anddry the probe of the thermistor. Assemble the flask and condenser. (Do notuse grease for the ground glass joint, unless you are interested in the boilingpoint elevation of a solution of vacuum grease in chloroform.)

2. Set up a beaker on the hot plate to be used as a water bath. Positionone clamp to hold the Cottrell flask so that the water bath can be used to heatthe contents of the flask, positioning the clamp as high up the flask body assafely possible (to maximize the length of flask able to be in the water bath,minimizing the amount of water needed in the water bath, and thus minimiz-ing the amount of time needed to heat the water bath). Attach the condenser,taking care to point the condenser away from you and toward the back of thebench. (This is important. We’ve had several instances where boiling liquid hasbumped out of the condenser, for reasons that we think we understand and areavoiding now.) Make sure that it is possible to disconnect the flask from thecondenser, so that you can change flasks later in the experiment. (Note that thepumping mechanism should not yet be in the flask.)

3. When you are satisfied that all parts of your apparatus are securely positionedand clamped, connect the hoses to and from the condenser, making sure touse hose clamps, and ensure that there are no leaks. Add enough warm waterto your water bath to immerse the bottom inch of the Cottrell flask. Set up thesmall digital thermometer to monitor the bath temperature. Use a stir bar todistribute the heat throughout the bath, making sure the stir bar rotates with-out bumping the flask or the thermometer. Start heating the water.

11.6.2 Boiling Point of the Pure Liquid

4. Add enough chloroform to the flask to submerge the bottom of the ther-mistor probe. Place the pumping mechanism in the flask. Add three or fourlarge boiling chips. (Too many, or too small, boiling chips will prevent thepumping mechanism from functioning properly. You do not want to use boil-ing chips that are small enough to fit inside the main arm of the pumpingmechanism, to prevent possible bumping hazards.) Use the rubber stopper toposition the tip of the thermistor probe inside the flask about below theliquid surface. Make sure there is a good seal between the thermistor and thestopper, so that no vapour escapes. Make sure the level of liquid in the waterbath is slightly higher than that of the liquid in the flask. (Lowering the flaskinto the bath, rather than increasing the volume of water, will reduce the timeyou will spend waiting for the bath to heat up.) Plug the thermistor probe cableinto the digital thermometer readout, and keep it away from the hot plate. Pressthe upper half of the rubber rocker switch marked On/Off to turn the unit on.A second press will turn the unit off. Note that the thermometer will shut it-self off after 10 minutes of operation. Make sure the thermometer is workingin degrees . To toggle between and , press the lower half of the rubberrocker switch down for seconds, while the HOLD indicator flashes, andthen the units indicator will switch to show which units are now in use.

5. Heat the water bath up to its boiling point. As heat transfers from the bath tothe chloroform, the chloroform will begin to boil and reflux in the condenser.

,4°F°CC

1 cm

1 cm

500 mL



(You will see liquid drops forming inside the condenser. If this is occurring tooclose to the exit end of the condenser, increase the water flow to the condenserwater jacket, or reduce the heat of the water bath, so that you don’t lose yourchloroform out the end of your condenser and gas your neighbours.)

6. Wait until the chloroform has established a good rolling boil. You shouldclearly see reflux in the condenser, and the liquid surface should be agitated,not just an occasional bubble rising to the surface. Record the temperaturereading on the thermistor at regular intervals. The resolution of the thermis-tor probe is very sensitive ( at this temperature range), so the temper-ature reading will not remain stationary but will vary around some averageposition, due to temperature and pressure fluctuations in the laboratory.

7. The temperature of the refluxing liquid will approach a constant value as thesystem approaches equilibrium. Wait long enough to be satisfied that it hasequilibrated. (A perfectly constant boiling point cannot be expected, but atequilibrium the observed temperature will vary slightly around a mean value.)When your system has reached equilibrium, record your estimated averagetemperature of the refluxing liquid. (Care must be exercised in all temperaturereadings, since a small error here will lead to large errors in your dataanalysis.) Because boiling point is sensitive to changes in pressure, record thebarometric pressure of the room just after taking your final temperaturereading.

11.6.3 Boiling Point of the Calibration Solution

8. While you are waiting for the chloroform to boil, weigh of naphtha-lene. Use the pellet press to compact the naphthalene into several tablets. Ac-curately record the total weight of your tablets. (Making a pellet makes iteasier not to lose some of your sample when adding it to the flask. Naphtha-lene flakes easily stick to the sides of the flask.)

9. After recording the boiling point of pure chloroform, turn the heating plateoff. Remove the thermistor probe. Carefully, using protective gloves, discon-nect the flask from the condenser and clamps. Pour the hot chloroform into abeaker in the hood, and leave it to cool.

10. DO NOT leave the beaker of waste chloroform/benzoic acid/naphthalene inthe beaker in the hood. Dispose of it appropriately, clean the beaker, and re-turn it to your drawer.

11. You do not have to clean the flask. Simply put two or three large boiling chipsand your naphthalene tablets into the bottom of the flask, followed by thepumping mechanism. Connect the flask to the condenser and clamp it intoplace.

12. Use the volumetric pipette to add of chloroform to the flask. Use therubber stopper to position the thermistor probe inside the flask so that the endof the probe falls between the arms of the pumping mechanism and downabout 3/4 of their length. Make sure there is a good seal between the thermis-tor and the stopper so that no vapour escapes and so the level of liquid in thewater bath is slightly higher than that of the liquid in the flask.

13. Heat the mixture, dissolving the naphthalene. Do not leave your experimentunattended. Check periodically to ensure that no boiling chips have becomelodged up the main arm of the pumping mechanism. This is potentially

50.0 mL

, 5 g

60.01K



dangerous, because if pressure that is built up behind the boiling chip is sud-denly released, hot liquid can bump out the end of the condenser. If you seesuch a situation developing, stop the experiment and remove that boiling chipfrom your system. As the solution boils, it will start bubbling up through thearms of the pumping mechanism and spraying out over the thermistor probe.(If the liquid does not flow steadily through the pumping mechanism, adjustthe water bath heat. Do not allow the liquid to get hot enough to bubble up overthe pumping mechanism. You may have to reposition things so that the boil-ing chips are beneath the pumping mechanism.) Record the temperature read-ing on the thermistor at regular intervals. Wait until the temperature hasfinished rising and is relatively stable. Wait longer because you probablyhaven’t waited long enough. When your system has reached equilibrium,record your estimated average boiling point of the chloroform/naphthalenemixture as read by the thermistor. The difference between this reading andthat taken for the pure chloroform is the boiling point elevation for thissolution.

14. While you are waiting for the naphthalene/chloroform solution to boil, weighof benzoic acid. Use the pellet press to compact this into several tablets,

and accurately record the total weight of your tablets.15. After recording the boiling point of the naphthalene/chloroform solution, add

the hot solution to the beaker in the hood to cool and thoroughly clean anddry the flask and the lower portion of the thermistor probe. (You don’t wantto study a benzoic acid/naphthalene/chloroform mixture.)

11.6.4 Boiling Point of the Benzoic Acid Solution

16. Repeat the experiment, investigating a benzoic acid/chloroform solution. It isunnecessary to again measure the boiling point of chloroform.

, 5 g


Table 11.1 Waste Disposal

Waste Chemicals Waste Container

Excess solid naphthalene, benzoic acid Solid waste

Chloroform-containing waste Organic halogenated

11.7 WASTE DISPOSAL

Do not pour hot chloroform into the waste bottle. Wait until it has cooled. Speed upcooling by placing the beaker in a bucket of ice water.

11.8 SHUT DOWN

1. Turn off water to the condenser. Turn off the hot plate/magnetic stirrer.2. Make sure the pellet press/balance area is clean and that you have returned your

chemicals to where they belong.


3. Put all waste, including the chloroform you left in the hood to cool, in the ap-propriate waste containers. Clear up any spills that have occurred. Clean and dryall glassware. Put away all equipment used. Return appropriate items to thestockroom.

4. Ask your TA to check and sign for your equipment drawer, initial your lab bookand fill in the appropriate Report Summary Sheet.

11.9 LITERATURE VALUES

Chloroform: Boiling point Density

11.10 DATA ANALYSIS

11.10.1 Boiling Point of the Calibration Solution

1. Calculate the molality of your solution of naphthalene in chloroform.

(11.1)

2. For dilute solutions, the size of the boiling point elevation depends upon thesolute concentration:

(11.2)

where is the elevation of the boiling point (in Kelvin) for a solution of mo-lality m over that of the pure solvent and is a constant characteristic of thesolvent called the boiling point constant, or the molal elevation of the boilingpoint. Find the experimental for chloroform from your observed boilingpoint elevation. Compare to the supplied literature value.

11.10.2 Boiling Point of the Benzoic Acid Solution

3. Equation 11.2 can be converted to calculate the molecular weight of the solute in , where grams of solute dissolved in grams of solvent of

boiling point constant gives a boiling point elevation :

(11.3)

Using your experimentally determined for chloroform, your observed boil-ing point elevation for the benzoic acid/chloroform mixture, and your massesof solute (benzoic acid) and solvent (chloroform), calculate the apparent mo-lecular weight of benzoic acid for your solution.

Kb

M 51000[g ? kg21]Kbg

GDTb

DTbKb

Ggg mol21M

Kb

Kb

DTb

DTb 5 Kb m

molality 5# of moles of solute

mass of solvent in kg

Kb 5 3.63 K kg mol215 1.4832 g cm23

5 334.8 K



4. Compare your apparent molecular weight to the actual molecular weight ofbenzoic acid. Benzoic acid forms dimers in chloroform solutions,2 (i.e., thenumber of particles in solution is decreased).

S (11.4)

In this solution, the apparent molecular weight calculated using equation 11.3is an average of the molecular weights of the components of the solution. Youshould have gotten an apparent molecular weight that is higher than the molec-ular weight of benzoic acid but lower than the molecular weight of a benzoicacid dimer. (If you were way off, try recalculating your apparent molecularweight using the literature value of . If this doesn’t help, either do the calcu-lations anyway with your data, or ask your TA for some sample data, and thencomment on the experimental problems that may have specifically led to yourbad data. Suggest how you would avoid them in future.)

5. For the monomer/dimer equilibrium, the experimentally determined apparentmolecular weight is:

(11.5)

where are the solute mole fractions and are the actual molecular weights.Solute mole fractions are based on solute only (i.e., neglect solvent), thus:

(11.6)

Substituting (6) into (5):

(11.7)

Assume that benzoic acid in a chloroform solution exists in an equilibrium suchas described by equation 11.4. Using the known molecular weights of benzoicacid monomer and dimer and your experimental apparent molecular weight,calculate the mole fractions of monomer and dimer at equilibrium by solvingequations 11.6 and 11.7 simultaneously.

6. We can define as the number of moles of benzoic acid that has reacted tomake dimer. The number of moles of benzoic acid monomer and benzoic aciddimer in the final solution is given by:

(11.8)

(11.9)

Express the mole fraction of benzoic acid monomer in terms of the final numberof moles of monomer and dimer in solution, and therefore solve for and hencecalculate the final number of moles of monomer and dimer in the solution.

7. Calculate the corresponding molarities of the monomer and dimer, assumingthat the density of the solution at its boiling point is . Don’t forgetto use the total mass of the solution, including the mass of the chloroform andthe mass of the benzoic acid.

1.55 g>cm3

Nr

Final number of moles Benzoic acid dimmer 5Nr

2

Initial moles of benzoic acid 2 Nr

Final number of moles Benzoic acid monomer 5

Nr

M 5 (1 2 Xmonomer) Mdimer 1 Xmonomer Mmonomer

Xmonomer 1 Xdimer 5 1

MX

M 5 Xdimer Mdimer 1 Xmonomer Mmonomer

M

Kb

(Benzoic acid)22(Benzoic acid)



8. Use these molarities to calculate the equilibrium constant of the dimerizationreaction.

11.11 DISCUSSION QUESTIONS

1. What is superheating, and what causes it? How do we compensate for this ef-fect in our experiment?

2. Explain why your experimentally determined molecular weight from the ben-zoic acid solution is not equal to the molecular weight of benzoic acid. Drawthe structure of a benzoic acid dimer, taking note of equation 11.4. Why didn’twe worry about the formation of naphthalene dimers?

3. The thermistor you used can be read to within . Do a test calculationto determine how a small discrepancy in your temperature measurement cancreate a large change in your predicted molecular weight. We formerly used aBeckmann thermometer in this experiment that could be read at a higher reso-lution than the thermistor. (We stopped using the Beckmann thermometer be-cause it is extremely expensive, easily breakable, and difficult to calibrate.) Isour current thermistor sensitive enough, or should we use a more sensitive ther-mometer for this experiment?

4. What would you observe if you measured the boiling point elevation of benzoicacid dissolved in ethanol? You should address intermolecular forces in youranswer.

11.12 CONCLUSIONS

As part of your conclusions for this experiment, compare your equilibrium con-stant to its literature value.1

60.01K


1Tanaka, H.; Yamamoto, Y., Chuman, H. Determination of the distribution coefficients of phenol de-rivatives and others based on a volume ratiometry coupled with a shake-flask method. Analytical Sci-ences 2002, 18, 485. This paper did not measure ; however they state it clearly on page 486. Theyused a mathematical manipulation of data from another paper (Davies, M. and Griffiths, D. M. L. In-tramolecular hydrogen bonds and the association and solubilities of substituted benzoic acids J. Chem.Soc. 1955, 132.) to arrive at this value. (Keq 5 1/104K12/104) 5 1/210 3 1024 5 48 L mol21).

Keq


11 bp- boiling point elevation by solid

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