8dlt.d by
WALTER A. WOLF Eisenhower College
Seneca Falls. New Y o h 13148
a Styrofoam ball on a ring stand and a few sticks. In actually generating a hybrid orbital, each contributing " P vector is represented with a Styrofoam hall placed at the end of the appropriateaxis. Since thestudents can generally do simple vector algebra, they can readily see orientation and magnitude of the hvhrid vector. After all four so3vectors have been generated, the Cartesian system is quite cluttered, but if the sticks representing the axes are then removed. leavine only the '.hyhridMvwtors, the students are quite impressed with the o 0 v I . o tetrahedral ~~ Eeomrlrs of t h ~hvhrid . set. In fact, it is remarkable, given the relative crudeness of construction, how close the angle is to 109.5°. Also, since the student readily sees that each of the vectors is identical in length, it is possible to relate this fact to the equivalency of the hybrid orbitals. I will gladly supply a more detailed discussion of this approach to any who are interested.
Compact Compact Kenneth M. Long, of Westminster College, New Wilmington, PA, points out that most frwhmnn chern~stryrc.xts continue to use the early I'auling values ior the ~lectronegativities of the elements. ~ o d a t e d .recalculated values are available in almost all current inorganic textbooks. One example of the differences between the values is that the newlv calculated electronegativities of nitrogen and chlorine are no longer the same. Joseph A. Solomon of the Philadelphia College of Pbarmacy and Science, Philadelphia, Pennsylvania, reports a way to combine two short experiments. Instead of using an eudiometer to collect the gas while determining the equivalent weight of a metal, a large cylinder is employed as a water resewoir. This allows the pressure to be varied and made equal to the external pressure, if desired. Thus a Boyle's Law experiment mav be performed with the same lab setuo. G. F. ~ t k h s o nof the University of Waterloo, waterloo, Ontario. Canada. has found that in large undergraduate classes it is very difficult to keep the heavyly used f10-120" C drvine ovens uo to tem~erature,due to the constant ooenine of the doors by &udenti. This difficulty has been ovekome with a simple traffic control svstem. Three ovens are set up side by sidk and adjusted to the same temperature. ~ t t a c b e d t o the case of each is a small chassis bearing two pilot lights. one red and one green. The leads are connected tb a box containing a 2-rph motor drivina three cam-actuated microswitches. The system is set to gi\.e at any time a green signal on oneoven, and red un the other two. Every 10 min thr npxt green signal appears. Students are advised that ovens may only be opened on a green signal (except in the last 1C. min of the lab period). Thus a sample may be left in the oven slightly less than 30,60, or 90min with theassurance that for a t least of the elasped time the door has been closed and the temperature has been approaching or is a t the set point. ~~
Hybrid Orbitals in General Chemistry: a Simple Vector Approach George R. Wiger California State Uniuersity Dominguez Hills Carson, 90747 In discussing the structureof molecules in terms of orbital overlap, the concept of orbital hybridization is one of the most important in chemistry. However, due to the rigorous mathemntics involved in the exact solutions for asetof hvbrid orbitals, the usual treatment is very cursory. Thus, the student is told that CH4 is tetrahedral and that such a geometry requires a hybrid set of atomic orbitals which we call sps. The actual process whereby the hybrids are generated is commonly the two step, "promotion" followed by "mixing" as presented in most chemistry textbooks. In attempting to avoid the presentation of facts without some sort of supporting illustratiou, I have found the following method useful. Waser [Waser, J., J. CHEM. EDUC., 48,603 (1971)] has pointed out very effectively how the hybrid orbitals of sp" type can be simplified with regard t o their directional characteristics by consideration of only the p contribution. The hybrid can then be seen as the simple vector sum of the contributingp orbitals. Adaptation of this technique to the classroom requires only a three-dimensional Cartesian coordinate system which is easily improvised with
~
~
Volume 55, Number 10. October 1978 / 655
