applications anJ analogies Put the Body to Them! Robert R. Perkins Kwantlen College PO. Box 9030 Surrey, BC, Canada V3T 5H8 As societv has become more aware of the oossible hazards associated with some chemical substances, it is not possible to perform safely all the lecture demonstrations in our classrooms that we once did. We should not cease doing demonstrations but merely try harder to make effective use of appropriate demonstrations and analogies that illustrate chemical orinciples. A wide range of chemical principles can be demonstrated using t h i human body along with a few simple devices. A recent article ( I ) described a series of analogical demonstrations t h a t the author uses to provide a visual trigger for students to associate a particular chemical principle. Wherever possible I try to involve students in my demonstrations and I would like to describe several that they have found to be useful. Quantized System A handful of change pulled from your pocket provides a perfect example of a quantized state. Like the modern view of the atom, our monetary system can have only certain allowed states: pennies, nickels, dimes, quarters, but no 23-cent, 72-cent, nor other denomination coins. One also can pull bills from a wallet: only integer dollar values will be possible. Quantized States One can jump from the floor of a classroom to a chair and then to the top of a table. There are no stable levels in between these two. If one has the necessary daring (and ability), a jump from the floor directly to the top of the table may he attempted. Obviously more energy is required to make this more difficult transition. Once atop the table, there are two possible routes back to the floor of the classroom: a single direct transition, or a two-step process involving a drop to the chair and then from the chair to the floor. A total of three different downward transitions are oossible. corresoondine to the three different emission fo; hydrogen atoms with their electrons in iines the second excited state (i.e., n = 3 to n = 1,n = 3 to n = 2,
RONDELORENZO edited by Middle Georgia College Cochan, GA31014
One could illustrate the three types of photons by tossing out differently sized (or colored) halls (or sponges) a s one drops from the table to the floor or from the table to the chair to the floor. This emission spectrum will consist of only three lines (colors), a much different situation from the continuous spectrum obtained by passing visible light through a prism. The whole process gives new meaning to the term "Quantum Leap" (a former TV program) as another example of a term or phrase that is well-known to the public a t large but actually has a scientific basis of which most people are unaware. In the TV program, the leading character "leaps" through time to live the lives of other i n d ~ v ~ d u ; hut-the ~ii, iemporal s h f t s are totally random and unprcd~ctobleMuch the same sltuatlon was portraved rn "The Time Tunnel". anothrr TV nromnm from ~the late 1960's where two scientists were trapped in a series of random jumps in time. ~
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Boiling Point Trends Liquids boil when the forces between adjacent molecules are overcome. These weak forces may be illustrated by placing one finger of the left hand on top of the same finger of the right hand. The two fingers can be separated quite easily. If one places the right hand on top of the left hand, more force is necessary to slide the two hands apart as a result of the larger surface area of contact. This corresponds to the differences i n the magnitude of the intermolecular forces (London, dispersion, or Van der Waals deoendinp t h e textbook u s e d ) between adiacent u unon . nonpolar molecules. These forces arise from the temporary dipoles induced in the molecules, and these furces generally increase in magnitude as the molar mass of the molecule increases. For example, chlorine (Clp) has a boiling point of 239 K while bromine (BrJ has a boiling point of 332 K. Where two compounds have the same molar mass, stronger temporary dipoles could be expected in the molecules with the larger area of contact. There will be stronger dispersion forces between adjacent molecules of pentane (boiling point 309 K) than between adjacent molecules of 2,2-dimethylpropane (boiling point 273 K). The pentane molecules are attracted to each other over a n elongated surface rather than the small interactions between the more spherical molecules of 2,2-dimethylpropane. In order to illustrate the "extra" forces present in polar molecules, the instructor can shake hands with a student. More force will be required to separate the two hands in a
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handshake rather than when the two hands are merely i n contact with each other. The boiling point of water (373 K) is much higher than that of hydrogen sulfide (213 K) because of the very strong dipole forces (known as hydrogen bonds) between adjacent water molecules. Intermolecular versus Intramolecular Changes When asked, "What is produced when water boils?", the occasional student may say hydrogen gas and oxygen gas; i.e.. confnsinz intermolecular and intramolecular forces. his confusion has been mentioned elsewhere ( 2 ) .As just discussed in the previous example, the mamitude of the intermolecular fdrces between adjacent water molecules corresponds to separating "shaking hands". The instructor now can take one of the fingers of the student and indicate that a much larger force would need to be applied to separate that finger from the others on that hand! The intramolecular force between the oxygen atom and the hydrogen atoms i n a water molecule is much greater than the intermolecular force between adjacent water molecules. More energy i s required to break the covalent bond between the oxygen atom and the hydrogen atom than to convert liquid water to gaseous water. PolarlNonpolar Molecules The concept of bond polarity can be illustrated easily by handing a length of rubber tubing to a pair of students and having them pull. The student that wins will have the greater attraction for the tubing. A linear covalent molecule like H-C1 is polar as the chlorine atom has a greater attraction for the pair of bonding electrons. This greater attraction i s known a s the greater electronegativity of chlorine a s compared to hydrogen. For a non-linear molecule, one must sum all the individual bond moments to determine whether the overall molecule has a permanent dipole m o m e n t . T h i s resolution of forces c a n be demonstrated easily by using a model of PFs (a trigonal hiovramid). or SFf f a n octahedron). Both models will spin very nicely because the molecules are perfectly symmetrical fall bond moments cancel). If one removes one of the F atoms in the equatorial plane, the models will no longer spin a s evenly (all bond moments do not cancel). The point now can be made to the students that one must consider the shape of a molecule to determine whether one is dealing with a polar or nonpolar compound.
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Enantiomers Introductory organic textbooks indicate that the maximum number of possible stereoisomers for a compound containing n chiral centers is equal to 2". Most instructors probably have used a left hand and a right hand to illustrate the pair of enantiomers arising from a single chiral atom in a molecule. By including a pair of gloves, one can illustrate the presence of two chiral atoms and the two pairs of possible enantiomers: t h e RRILL a n d RLILR hanaglove combinations. By using a pair of gloves and a pair of shoes, the four sets of possible enantiomeric pairs can he demonstrated: the RRR'LLL, RRWLLR, RLWLRL, and LRFVRLL hand/glove/shoe combinations. Diastereomers Two students can be used to illustrate optical isomers that are and are not related by mirror image. Have each student bend over and grab left ankle with left hand and right ankle with right hand. The two students now represent a pair of enantiomers (optical isomers related by mirror image). Next have one of the students reverse hands; i.e., right hand/left ankle and left hanuright ankle. The
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two students now represent a pair of diastereomers (optical isomers not related by mirror image). Chromatography Compounds that have different polarities can be easily separated by column or gas-liquid chromatography. It is useful to have a simple model of this process because it is used extensively in chemistry and biology to separate the individual components in a mixture. Have three students represent molecules of pentane, 1-butanol, and butanoic acid (boiling points of 35 "C, 118 "C, and 164 "C, respectively). The three students now pass along a line made up of the rest of the students in the class. The student representing pentane ignores everyone, merely moving along to the end of the line. The student representing 1-butanol must stop and shake the right hand of each person in the line. The student representing butanoic acid must stop and shake the right hand and then the left hand of each person in the line. The time required for each student to move to the end of the line increases as the number of interactions with the other students increases. The elution time of polar molecules on a column in which a polar support material is used will he longer than that of nonpolar molecules Chromatographic Resolution Optical isomers are much more difficult to separate using chromatography than compounds that have very different polarities. The compound 3-bromo-2-butanol will have four ~ o s s i b l estereoisomers: the RWSS enantiomeric pair and the RSISR enantiomeric pair. Any one of the comoounds from the first enantiomeric pair with anv one of the compounds from the second enantiomeric pair would result in a oair of diastereomers. The separation of a pair of enantiomers (a process known as resolution) is much more difficult than the separation of a pair of diastereomers. This may he illustrated by having two students walk across the classroom. A student walking while holding left ankle with left hand and right ankle with right hand should be able to cross the classroom faster than a student holding left ankle with right hand and right ankle with left hand. The two students represent a pair of diastereomers (optical isomers not related by mirror image). As diastereomers have different physical properties, the elution times of these com~oundsfrom a column will be different. The diastereomeis R,R-3-bromo-2-butanoI and R,S-3bromo-2-butanol would corres~ondto this tvoe -. of mixture. Two students walking while each holding left ankle with left hand and right ankle with right hand will be able to cross the classroom a t a~proximatelythe same rate. Only if the room is long enough will one student eventually win the race. The two students represent a pair of enantiomers (optical isomers related by mirror image). As enantiomers have the same physical properties (except for the direction of rotation of plane-polarized light), the elution times of these compounds from a n ordinary column will be very similar. The seoaration of the mixture into the individual compounds (resolution) will require a more specialized column where the s u o ~ o r material t will interact differentlv with the right-handed and left-handed forms of the cornpound under consideration. The enantiomers R,R-3bromo-2-butanol and S,S-3-bromo-2-butanoI would correspond to this type of mixture. Literature Cited I. Foltman, J. J J Chzm Edvr 1992.69.32&324. 2 Rayner-Canham. G. Chsmtech 1992,329332.
