Solubilidade e Interaçao Intermolecular

1332 Journal of Chemical Education_

Vol. 87 No. 12 December 2010

_pubs.acs.org/jchemeduc

_r2010 American Chemical Society and Division of Chemical Education, Inc.

10.1021/ed100315r Published on Web 10/08/2010

In the Classroom

Salting Effects as an Illustration of the RelativeStrength of Intermolecular ForcesEric C. Person* and Donnie R. GoldenDepartment of Chemistry, California State University, Fresno, Fresno, California 93740-8034,United States*[email protected]

Brenda R. RoyceUniversity High School, Fresno, California 93740, United States

Understanding intermolecular forces and their relativestrengths is an essential learning objective for any high schoolor general chemistry course. There are a wide variety of demon-strations illustrating the impact of intermolecular forces on theproperties ofmaterials including viscosity (1), surface tension (2),and vapor pressure (3), but few look at the relative strength ofthese forces. Solubility offers another way to illustrate theseconcepts and introduce the important role that solubility willplay in upper-division chemistry courses.

In the simplest terms, a solute will be soluble in a solventif the strength of the intermolecular forces formed between thesolute and solvent molecules are stronger, resulting in a lowertotal potential energy, than the intermolecular forces found inthe pure substances. In this way, the relative solubility of speciescan be used as a means to compare the strength of intermolecularforces that are formed in solutions.

An example of this relative solubility is the salting outof nonelectrolytes from aqueous solutions upon the addition ofelectrolytes. In this demonstration isopropyl alcohol, a none-lectrolyte, becomes immiscible with water after the addition ofammonium sulfate, a strong electrolyte. At a simple level, once asufficient quantity of the electrolyte is added to the solution,watermolecules must choose between forming ion-dipole inter-actions with the dissolved electrolytes and weaker dipole-dipoleor hydrogen-bonding interactions with the nonelectrolytes. Asthe water molecules solvate the ions, the nonelectrolytes arepushed out of solution and will transfer to or form separatephases.

Several demonstrations of these salting effects have beendescribed in the literature. Shakhashiri describes salting ofmethanol from an aqueous solution using potassium carbonateleading to a discussion of phase diagrams and the Gibbs phaserule (4). Smith modified this procedure using ethanol, sodiumcarbonate, and bromthymol blue to help visualize the formationof separate phases in a classroom setting (5). This demonstrationmodifies Smith's procedure in three substantive ways. First, theacid-base color change is removed, as the concept may not havebeen covered in lecture prior to the discussion of intermolec-ular forces. Second, the demonstration uses materials studentsare already familiar with: rubbing alcohol, food coloring, andfertilizer. Third, an additional portion of water is added reform-ing a single phase to emphasize that the separation of layers is theresult of a competition of the relative strength of the intermo-lecular forces that can form between two solutes and a limitednumber of solvent molecules.

Demonstrations of salting interactions can provide a use-ful connection to organic chemistry, as salting effects are usedextensively in liquid-liquid and acid-base extractions. Forexample, the use of saturated salt solutions (brine) to washorganic extracts stems in part from two effects related to salting:first, the brine solution partially dries the organic layer bydrawing dissolved water out to form more favorable interactionswith dissolved ions, and second, the high salt concentra-tion significantly reduces the solubility of any organic solutesdissolved in the aqueous phase.

Procedure

Add 15 mL of water and one drop of food coloring to a50 mL test tube (25 200 mm), cap with a rubber stopper, andmix by inverting. Though most colors of food coloring can workfor this demonstration, green or blue food coloring are recom-mended as they provide a nice contrast and partition morecompletely than yellow and red colors. Add 15 mL of rubbingalcohol (70% isopropyl alcohol), cap, and mix by inverting(Figure 1A). Add 7 g of ammonium sulfate that has been groundto a powder (Figure 1B), cap, and mix by shaking vigorouslyfor 5-10 s (Figure 1C). Two distinct layers should form inapproximately 10-20 s on standing (Figure 2). A colorless layeris observed forming from the bottom and increasing in size untilit is approximately 70% of the total solution volume. The foodcoloring is dissolved in the top alcohol layer while the bottom

edited byTodd P. SilversteinWillamette University

Salem, OR 97301-3922

Figure 1. Images showing the steps of the main procedure described:(A) 30 mL of the 35% isopropyl alcohol solution with one drop of bluefood coloring, (B) the alcohol solution after adding 7 g of powderedammonium sulfate, (C) the solution after shaking vigorously to aid the saltin dissolving, (D) the solution after the layers have settled, and (E) thesolution after adding an additional 15 mL of water.

r2010 American Chemical Society and Division of Chemical Education, Inc.

_pubs.acs.org/jchemeduc

_Vol. 87 No. 12 December 2010

_Journal of Chemical Education 1333

In the Classroom

aqueous layer is nearly colorless (Figure 1D). If desired, use atransfer pipet to remove approximately 1 mL of each layer andplace each layer in separate watch glasses for testing by con-ductivity or flammability. Add another 15 mL of water to theremaining alcohol and water in the test tube, cap, and mix byinverting the tube. A single, uniformly colored layer will beobserved immediately and persist indefinitely (Figure 1E). Adocument camera system can be effectively used to visualize theseparation of layers if visibility of the demonstration is a concernin larger classrooms.

The identity of the two layers formed after salting can easilybe determined using either a conductivity measurement or asimple flammability test. To test for conductivity, use a simplelow voltage (9 V) conductivity indicator (e.g., Lab-Aids Kit300) or an ohmmeter after cleaning the electrodes with steelwool or emery paper to ensure good contact with the solution. Adocument camera or other projectionmethodmay be required tomake the conductivity indicator visible in a large classroom.Highvoltage conductivity indicators such as the conventional lightbulb apparatus may present a fire hazard if used with the alcoholfraction. The colored alcohol layer will show low but nonzeroconductance. The colorless aqueous layer will show relativelyhigh conductivity. The layers can then be tested for flammabilitywith a match or a butane grill lighter. We generally ignite theisopropyl alcohol layer first and then put the match out in theaqueous layer. The alcohol layer should burn for at least 1 minwith sufficient yellow color to be easily visible in a classroom. Theflame can be easily extinguished by covering with another watchglass if desired.

Variation

A variation on this procedure using sodium chloride in placeof ammonium sulfate creates some interesting effects with thepartitioning of the food coloring, which would facilitate discus-sion of relative solubility of organic molecules.

Prepare three large test tubes with 15 mL of water and15 mL of 70% isopropyl alcohol. Add one drop of yellow foodcoloring in the first test tube, one drop of green food coloringin the second test tube, and one drop of blue food coloring tothe third test tube. Add 4.5 g of sodium chloride to each tubeand shake vigorously. The sodium chloride will take longer todissolve, but as it dissolves, two layers will appear in each of thetubes (Figure 3). The green solution will split into a blue-greentop alcohol layer and a bright green aqueous layer. Both layers in

the blue tube will be relatively evenly colored, while the yellowtube will not show even color distribution. An additional 15 mLof water can be added to each tube to restore the sample to asingle, uniformly colored layer.

Commonly Available Reagents

The reagents for this demonstration are inexpensive andreadily available in commercial products. Common rubbingalcohol is a convenient source of 70% isopropyl alcohol. Ammo-nium sulfate fertilizer (21-0-0), which is essentially pure, is apotential source of the salt for this demonstration. Food coloringis available in most grocery stores. If reagent grade isopropylalcohol (99%) is used in place of rubbing alcohol, use 20 mL ofwater and 10 mL of isopropyl alcohol instead of the volumeslisted above.

Hazards

As with all laboratory experiments and instructionaldemonstrations utilizing chemicals and other hazardous materi-als, proper personal safety equipment including protective eye-wear should always be used. Ammonium sulfate (CAS # 7783-20-2) is recognized as a mild irritant to human eyes and skin.Caution should be taken when exposed because it can beabsorbed through the skin. Inhalation of the compound maylead to respiratory tract infection. Isopropyl alcohol, also knownas isopropanol, does have some notable hazards associated withits use. Isopropyl alcohol (CAS # 67-63-0) is a highly flammableliquid having a flash point of only 12 C. Breathing of vaporsshould be avoided. Inhalation can cause drowsiness, dizziness, andrespiratory infection. Isopropyl alcohol can also cause skin irrita-tion, be absorbed through the skin, and may be harmful ifswallowed. Target organs include the kidneys, liver, nervous, car-diovascular and gastrointestinal systems. Appropriate care shouldbe taken to avoid fire hazards if using the flammability test on theseparate layers. At a minimum, the area should be free of othercombustible or flammable materials, the experiment should beperformed in an area with adequate ventilation, and fire suppres-sion equipment such as fire extinguishers should be available.

Discussion

In general, a solute will be soluble in a solvent if the strengthof the intermolecular forces between the solute and solvent

Figure 3. Images showing the colors formed by yellow, green, and bluefood coloring in 35% isopropyl alcohol before (A) and after (B) addingsodium chloride as described in the variation. The blue-green colorformed in the alcohol layer for the green coloring results from the differentextent of partitioning of the blue and yellow dyes that make up the greencoloring.

Figure 2. Images showing the rapid separation of the aqueous andisopropyl alcohol layers. The time required for the separation variesbetween approximately 10 and 20 s after vigorous shaking of thealcohol solution and added ammonium sulfate.

1334 Journal of Chemical Education_

Vol. 87 No. 12 December 2010

_pubs.acs.org/jchemeduc

_r2010 American Chemical Society and Division of Chemical Education, Inc.

In the Classroom

particles is stronger than that of the interactions between theseparate pure substances. In this case, hydrogen bonds betweenthe alcohol and water are sufficiently strong to allow isopropylalcohol and water to be miscible in all proportions. Most water-based food coloring dyes contain polar functional groups thatform strong dipole-dipole interactions with water includinghydrogen bonds that allow them to be quite soluble in water.Take time before adding the salt to emphasize these solubilityproperties. Adding food coloring to water before adding theisopropyl alcohol demonstrates the solubility of the visible foodcoloring dye in both water and alcohol, creating an opportunityfor a deeper discussion surrounding which layer is colored aftersalting.

Ammonium sulfate is also soluble in water due to thestrength of the ion-dipole attractions between ammoniumand sulfate ions with the water molecules. Small quantities ofammonium sulfate (3 g for the 30 mL volume described) willdissolve in the 35% isopropyl alcohol solution formed in thedemonstration without forming a second layer. As additional saltis added, there is insufficient water to completely solvate both theammonium sulfate and the isopropyl alcohol. In this situation,water will favor the interactions with the lower potential energyresulting from stronger intermolecular forces. The displacementof isopropyl alcohol to form a separate layer while the ammo-nium sulfate dissolves completely illustrates that the ion-dipoleinteractions formed between water and ammonium sulfate arestronger than the hydrogen bonds formed between water andisopropyl alcohol. The significant reduction of solubility ofnonelectrolytes such as isopropyl alcohol on the addition of highsalt concentrations is typically referred to as salting. The solubi-lity of the salt in the solution is also significantly reduced by thepresence of isopropyl alcohol in the aqueous layer. Food coloringprovides a convenient way to visualize the formation of theselayers in part because it is also subject to the salting effectspushing it out of the aqueous layer. The isopropyl alcohol layerwill have a density of approximately 0.87 g/mL causing it to siton top of the aqueous layer with a density of approximately1.16 g/mL. The addition of a second portion of water providessufficient water to solvate both the ammonium sulfate andisopropyl alcohol allowing a single layer to form again.

The quantity of ammonium sulfate recommended is below itssolubility limit in the alcohol solution so that it will all dissolvequickly and so that a single layer can be formed upon addition ofthe second portion of water. If toomuch color is still evident in theaqueous layer for the instructor's preference, additional ammoniumsulfate will essentially eliminate any detectable color in the aqueouslayer. Blue food coloring will show more complete transfer to thealcohol fraction than the green coloring and significantly morethan the red or yellow coloring in a typical box of McCormickbrand food coloring. If more salt is added, more water may berequired to reform a single layer and some solid ammonium sulfatemay appear as a third layer at the bottom of the tube.

The type of salt added to this demonstration is not criticaland can have interesting effects on the partitioning of the color.All electrolytes may show salting effects to some degree. Ammo-nium sulfate was used in this demonstration because it isinexpensive, dissolves rapidly, and has the relatively strong saltingeffects necessary to displace the hydrogen bonding of isopropylalcohol. The strength of the salting effects depends on the ionicstrength of the electrolyte solution. This in turn depends on thecharges of the ions and the overall solubility of the salt. As shown

in the described variation, sodium chloride also shows saltingeffects and will form separate layers, but concentrations nearsaturation are necessary and do not form as quickly as thedemonstration with ammonium sulfate.

The described variation using sodium chloride takes slightlymore time to form separate layers, but illustrates an interestingdifference in the effect of salt on the two dyes that make upMcCormick's green food coloring: FD&C Blue 1 and FD&CYellow 5 (6). The chemical structures of these two dyes areshown in Figure 4. Whereas the sodium chloride is able tosufficiently displace the isopropyl alcohol to form two layers, ithas different effects on these two dyes as observed in Figure 3.The top layer of the center tube appears blue instead of greenbecause the alcohol layer contains significantly more of the bluedye than the yellow dye due to differences in their partitioningbetween the brine and alcohol layers, likely caused by a combina-tion of differences in their relative hydrophobic character andhydrogen bonding potential. FD&C Yellow #5 has two chargedsulfonate groups, two azo nitrogens, and a phenol with goodpotential to form hydrogen bonds with water due to minimalsteric hindrance. FD&C Blue #1 shows increased hydrocarboncharacter with five benzene rings instead of only three. AlthoughFD&C Blue #1 has four charged groups, the resonance structurewith the positive charge on the central carbon atom is consider-ably stabilized as a tight ion pair with the neighboring orthosulfonate group (supported by molecular modeling). Finally, thenitrogen atoms in FD&C Blue #1 are expected to have reducedhydrogen bonding potential due to steric hindrance and theresonance structures involving their lone pairs of electrons.

Acknowledgment

The authors would like to thank Angie Person for assistancewith the photography and digital photo processing. CatherineBanks (Department of Chemistry, Peace College, Raleigh, NC)is thanked for checking this demonstration.

Literature Cited

1. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook forTeachers of Chemistry; The University of Wisconsin Press: Madison,WI, 1989; Vol. 3, p 313-316.

Figure 4. The chemical structures of FD&C Yellow #5 and FD&CBlue #1, the two dyes used in McCormick green food coloring.

r2010 American Chemical Society and Division of Chemical Education, Inc.

_pubs.acs.org/jchemeduc

_Vol. 87 No. 12 December 2010

_Journal of Chemical Education 1335

In the Classroom

2. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook forTeachers of Chemistry; The University of Wisconsin Press: Madison,WI, 1989; Vol. 3, p 301-304.

3. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook forTeachers of Chemistry; The University of Wisconsin Press: Madison,WI, 1989; Vol. 3, p 242-248.

4. Shakhashiri, B. Z. Chemical Demonstrations;A Handbook forTeachers of Chemistry; The University of Wisconsin Press: Madison,WI, 1989; Vol. 3, p 266-268.

5. Smith, E. T. Chem. Educator 1996, 1; http://chemeducator.org/sbibs/samples/spapers/11smi897.htm (accessed May 2009).

6. McCormick Product Detail - Green Food Color. http://www.mccormickgourmet.com/ productdetail.cfm?id=6036 (accessed May2009).

Supporting Information Available

Videoof thedemonstrationusing green food coloring and ammoniumsulfate. This material is available via the Internet at http://pubs.acs.org.