The Goals of General ChemistryA SYMPOSIUM
General Chemistry for Engineers B. D. Kybett University of Regina. Regina. Saskatchewan S4S 0 A 2 Canada Teachers of chemistry do not face the 80's with the same optimism that they faced the 70's. Enrollments are down. Graduating chemists find it difficult to obtain jobs as chemists or in industry. The gap in starting salaries between chemists and chemical engineers is widening ( I ) . Fewer chemistry classes are often being required for a bachelor's degree in engineering. Why? One reason, which has been given many times, is the new curricula for high schools and universities that were introduced in the 60's and early 70's. They were a tremendous improvement over their predecessors, but were largely desiened htentionallv or not) bv chemists for chemists: heaut i h theoretical explanations of bonding, hut no direct aoolications of these conceots to the world outside the laho.. ratory. Courses. includine our own. have been modified since then to include'the technological applications of chemistry. The theme for the first-year university courses a t the University of Regina is "Molecules, their structure and reactions." This recoenizes that chemistw is a studv of the nature of molecules. A discussion of the theoiies of atomic and molecular structure leads into the major part of the course-how and why, and how fast, do molecules react with each other? One problem we faced with a section of the course taken mainly, but not exclusively, by engineering students, was how to include a discussion of the properties of polymers and plastics. In 1968 M. Morton (2) wrote about the ". . . astonishing lack of emphasis. . ."on polymer chemistry in the undergraduate curriculum, despite the fact that a t that time over 25'70 of the chemists in the USA (excluding analytical chemists) were polymer chemists. The proportion is higher today, but the "emphasis" on polymer chemistry is still almost non-existent, especially a t the first-year level. A brief survey of first-year chemistry texts (on my bookshelf) showed that between 1to 3 percent of the contents were devoted directly to polymers, including biopolymers. T h e nroblem is that the chemistw .of .oolvmers - is comolex. and not easily rationalized in terms of simple theories.-yet; if this is not done. one is reduced to a boring catalog of names. formulae, and properties. Polymers, and plastics, can be introduced. however. into the aeneral chemistry course as an forces. ;ipplic;atinn d the concept uf ~nt~!rmoler~llsr One of the most striking. and useful. umoerties of olastics is their high strength-to-weight ratio. This isusually expressed as the tenacity, in grams per denier, hut is more easily visualized as the "self-supporting length," the length of material, as a wire of uniform cross section which, if held vertically,
Table 1. Self-supporting Lengths of Some Common Materials Steel Aluminum
Rayon Nylon
Polwrn~~lene
4 miles 10 miles 15 miles 36 miles 42 miles
Table 2. Typlcal Tensile Strengths of Some Plastlcs (In p.s.i.) Polyethylene [-CHs-Isoo Polyethylene [-CHr-Irooo Polyvinylchloride Polyvinylidenechloride Nylon 6.6
500 2,000
7.500 20.000 10.000
Table 3. Lattlce Energy Increments (kcal mol-') C-H C-F C-CI
10 2.5 4.0
C-Br C-l
5.0 6.5
C-CH3 C-NHI
2.5
c=o C=N
6.5 5.5 3.5
could just support its own weight. Polypropylene's self-supporting length is ten times that of steel and four times that of aluminum (Table 1).Other comnarisons of the nerformance of materials have hken given by 'Platzer (3). T h e tensile streneths of some nlastics are shown in Table 2. Plastics consist of long molecules parkrd together, and they break not hv thr breakinr of thr chemirnl hond; in the ool\.mer molec;le, hut by themolecules sliding over each dth&, i.e., the intermolecular forces define the tensile strength. I cannot recollect the source, but an excellent analogy has been described, a bowl of cooked spaghetti. Pick up a handful and pull-the strands slide over each other, instead of breaking. Less messv are bundles of pipe . . :leaners held loosely. hv. rubber hands (4): Longer molecules will have larger intermolecular forces per molecule and are stronger. The tensile strength of polyethylene with 500 methylene groups in the polymer chain is 500 psi; with 1,000 methylene groups it has increased to 2,000 A
psi.
Presented in the Symposium on the Goals of General Chemistry at the 179th National Meeting of the American Chemical Society in Houston, Texas. March 25. 1980.
724
Journal of Chemical Education
Replacing one hydrogen atom in four in polyrthylene with will incrraw thr intermochlorine to form ~ol\vin\~lchloride lecular forces heciuie chiorine, with more electrons, will form
alarger dispersion force. Polyvinylchloride has a larger tensile strength than polyethylene of the same chain length. This could be represented in the spaghetti demonstration by a thicker tomato sauce. Polyvinylidene chloride, which is polyethylene with two hydrogens in every four replaced by chlorine atoms, will have correspondingly higher intermolecular force and tensile strength. The high tensile strengths of polyamides (e.g., nylon 6,6)and polyesters is partly due to the fact that they can form hydrogen bonds. The rationale for these predictions (which is not part of the course) is the list of lattice energy increments given in Tahle 3. These are based on measurements of the heat of sublimation of homologous series of compounds (5),and clearly show the variation of lattice energy (or intermolecular force) with chemical constitution. The long chain compounds that were used to obtain most of the data in Table 3 pack together closely and linearly in the solid phase, like matches in a box. Polymers have a more random structure and, because of the varying distances between the polymer chains, the data in Tahle 3 do not strictly apply to polymers, but do show the general effect of the different types of intermolecular forces. Once a relation between the molecular structure of the polymer, intermolecular forces, and tensile strength has heen established it is important that the student is told that there are other factors which affect the tensile strength. These include the flexibility of the polymer chain and the degree and type of crystallinity. These factors are discussed in many hooks on polymers (6). Many common plastics lose their strength a t the boiling point of water. Frazer (7) has explained how ingenious chemical strategies, such as ladder polymers, have provided materials that can survive a t temperatures as high as 900°C. This provides a useful extension of the discussion of the relation between molecular structure and strength. Polymers were also discussed in the molecular spectroscopy
part of the course. Students identified a plastic film from its infrared spectrum; the approach was similar to that descrihed by Webb, Rasmussen, and Selinger (8).We, too,had problems because the presence of copolymers and plasticizers in commercial films complicates the interpretation of the spectra. The experiment is, however, interesting and worthwhile if more than two or three lectures can he devoted to infrared sDectrosconv. ~ r ~ o r & (k9 )has descrihed an interesting experiment in whirh ~1asticsare identified from their behavior on heatine. ~ylon,.forinstance, is thermoplastic, hums with a hlue flame with a vellow ti^, and eives off basic fumes that smell of bnrning vegetati&. ~ o l ~ ~ ~ r o ~isy thermoplastic, lene hurns with a velluw llame with a hlue base. and eives off neutrnl fumes that smell like burning candle wag. ~ h e s tests e are also descrihed by Roff and Scott (10). The discussion of the relationship between molecular structure, intermolecular forces, and tensile streneths of a polymer that has been outlined above does provides logical way in which to introduce polvmers into a general chemistrv course. I t leaves out manyother factors which are involved, but a general chemistry course is only an introduction to chemical science.
Literature Cited 12) Monon, M.; J. C ~ M ~ouc:, . 45,498. 1968. 13) Platzer. N..Chem. Tech.. I.165.1971. 141 Kyktt,B. D., Can. Chem. ~duc.,7, 13,1972. ( 5 ) Dsvien, M., J. Polymer Sei.. 40.247, 1959. 16) Alirey, T. and Gumee, E. F.,"ChganicPolymers."Pmtie-Hell Inc., E n g l e w d Cliffs, NJ. 196I: Blackadder. D. A., "Some Aspects of Basic Polymer Science." Chemical Snciety, London. U.K.. 1975: Deanin, R. D.. "Polymer Structure, Properties and Applieatiuns."Cshner. Bnks. Bastan. MA, 1972. (71 Frazor. A. H..Sci.Amar.221.96. 1969.
Volume 59 Number 9 September 1982
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