adjustable in 1%increments to 1000 mA. I n all cases, the mass of the particle is assumed to be the electron rest mass. The use of experimental a s opposed to atomic units is intended to eive the student an intuitive feeling for the quantitiesinvolved. The center panel depicts the physical situation, using differently colored pixels to represent incident particles a s opposed to reflected particles. The speed a t which the pixels move across the screen is made aporoximatelv to the velocities of the parti"proportional .. h e s they represent. The detector icon in the ;enter panel displays the magnitude of the transmitted current, while the wave function is simultaneously displayed in the upper panel. The imaginarycomponent of the wave function is represented by the thickness of the line drawn on the screen, which allows the phase of the function to be visualized. Quantum Barrier can be used to explore one or more of the followine assienments: Plot the increase in transmis-" sion a s the particle energy appmaches the barrier height. What is the period of the cvclic variation in reflection a t particle energies in excess of the barrier energy? How does the phase of the wave function change a s the amount of reflection changes? Is transmission more sensitive to barrier height or barrier width? Why does the number of partitles in the display decrease with increasing particle-energy a t constant current?
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Animated Demonstrations II: Mass Spectrometer; Single-Crystal X-Ray Diffraction Philio . ..... I. PavlikNonnern M c y a n L n verstry Marq-ene, M 49855-53343
Thcse two prokwuns continue n series ol'animated demonstrations ~ u b l i i h r dcarlicr 16,. Muss Spectmmeter provides a n &mated view of the workings bf a simple mass spectrometer. A lecturer or student may select one of several different elements and generate its mass spectrum. Peaks appear on a chart and isotopic masses and abundances are reported in a table. Single-Crystal X-ray Diffraction shows a n X-ray source, a n animated beam of Xrays, a rotating single crystal, and a film on which spots are produced by diffraction of the beam.
About This Issue John W. Moore and Jon L. Holmes University of Wisconsin-Madison Madison. WI 53706-1396 Chemistry textbooks typically present kinetics problems of onlv a few tvoes: a table of time and concentration data for a unimolecular reaction; a table of initial rates corre" A
sponding to a few different combinations of reactant concentrations; or a table of rate constants a t two or more temperatures. Students are asked to deduce orders, rate con. problems are of stants. and activation e n e r ~ e s Such necessity "final data", and preempt the student's grasp of what had to be done in the lab to obtain the numbers. KinWORKS provides an excellent alternative, allowing students to explore a wide variety of different experimental situations much more rapidly than they could in the lab. Q.uantum Barrier addresses the problem of insufficient lecture time to explore the quantum-mechanical model of electron tunneling in detail. Often only the final expressions for the reflection and transmission coefficients are presented. The expressions for the wave functions are not so easily visualized as those for the particle in a box, and the algebra needed to obtain them is extremely laborious. Given these obstacles, few students are likely to investigate the properties of the model on their own. By allowing the student to interactively discover the various properties of quantum barriers, this program opens a much wider range of experience and intuition. Short but graphic, animated demonstrations can spice up any lecture. They are also useful for students to explore more fully on their own after a lecture is over. Mass Spectrometer will be useful when the ideas of separating isotopes and determining isotopic masses are presented. Single-Crystal X-Ray Diffraction allows students to see the basic components of an X-ray diffraction experiment and to envision how a beam of X-rays is diffracted by a crystal. Hardware Requirements Programs in this issue of JCE: Software are designed for IBM PSl2, PC, or PC-compatible microcomputers with 640K of RAM and one floppy disk drive. VGA or compatible graphics and PC- or MS-DOS 3.1 or later are also required. (CGA and EGA graphics will not work.) A disk drive larger than 360K is recommended for KinWORKS. Literature Cited 1.Pilar. F.L. Elsrnmrow Quantum Mechonles, 2nd ed.: McGraw-Hill: NewYork, 1990, pp. 73-77. 2. Atkins, E W. Molecular QuonfarnMmhmnics, 2nd ed.: Oxford Univereity Press: New York, 1983,pp. 4 1 4 . 3. Dan$,Jr., J. C. A d u o n d Physic01 Chamlsin: Ronald Ress: New York, 1965. pp. 88-43. 4. Powell, J.L.; Crasemsn. B. QvoniumMechonics, Addison-Wesley:Reading,MA.1961, pp. 107-109. 5. Rioux, F. "Numerical Solutions for Schradingeri Equation"; J. Chem. Edur: So,% wore, 19901IIB 12). 6. Pav1ik.P. 1. J Chem. Educ:Softwam 1992 58(21.
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Journal of Chemical Education: Software (often called JCE: Software)is a publication of the Journal of Chemical Education. There is an Order Form card inserted in this issue. If this card is
Volume 70 Number 9 Se~tember1993
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