Demonstration with Magnets Valuable prize for balancing suspended magnet between sets of attracting ma gnt! Chemistry 125: Lecture 2 Sept 4, 2009 Force Laws, Lewis Structures, and Resonance Congratulations to Saybrook! (first to complete their directory on the Wiki)
For copyright notice see final page of this file Are There Atoms & Molecules? What Force Holds Atoms Together? What Holds Atoms Together? Hooks Clips? Friction?
Springs? Bolts? Hormones? The Hands of the Deity? What Holds Atoms Together? Gravity Quarks Electrical Forces
The Strong Force Magnetic Forces The Weak Force Kinetic Energy Strange Attractors Quantum Forces Exchange of Photons 0
Lets Vote Shared Electron Pairs 58 5 Robert Boyle P V = const Air Pump built by his servant Robt. Hooke Robert Boyle (1627-1691)
1661 1678 Hookes Law Hookes Force Law F = -k x force Scale energy
extension Hookes Law Ut tensio sic vis Force x Potential 2 x Energy Isaac Newton (1643-1727) Force : Gravity
Attraction at a Distance vs. Cartesian blocked repulsion Newton : Force r-2 (How about mass?) Isaac Newton (1643-1727) Query 31 in Opticks (1717) Have not the small Particles of Bodies certain Powers, Virtues, or Forces by which they act at a distance, not only upon the Rays of Light for reflecting, refracting and inflecting them, but also upon one another for producing a great part of the Phaenomena of Nature? For it's well know that
Bodies act one upon another by the Attractions of Gravity, Magnetism and Electricity; and these Instances shew the Tenor and Course of Nature, and make it not improbable but that there may be more attractive Powers than these. For Nature is very consonant and conformable to her self. Query 31 How these Attractions may be perform'd, I do not here consider. What I call Attraction may be perform'd by impulse, or by some other means unknown to me. I use that Word here to signify only in general any Force by which Bodies tend towards one another, whatsoever be the Cause. For we must learn from the Phaenomena of Nature what Bodies attract one another, and what are the Laws and Properties of the attraction, before we enquire the Cause by which the Attraction is perform'd, The Attractions of Gravity, Magnetism and Electricity, react to very sensible distances, and so
have been observed by vulgar Eyes, and there may be others which reach to so small distances as hitherto escape observation; and perhaps electrical Attraction may react to such small distances, even without being excited by Friction. Query 31 The Parts of all homogeneal hard Bodies which fully touch one another, stick together very strongly. And for explaining how this may be, some have invented hooked Atoms, which is begging the Question; and others tell us that Bodies are glued together by rest, that is, by an occult Quality, or rather by nothing; and others that they stick together byMaybe conspiringFMotions, that is, by>2relative rest
1/rr ? chymical amongst themselves. I had rather infer from their Cohesion, that their Particles attract one another by some Force, which in immediate Contact is exceeding strong, at small distances performs the chymical Operations above mention'd, and reaches not far from the Particles with any sensible Effect. Query 31 the Attraction [between glass plates separated by a thin film of Oil of Oranges] may be proportionally
greater, and continue to increase until the thickness do not exceed that of a single Particle of the Oil. Query 31 There are therefore Agents in Nature able to make the Particles of Bodies stick together by very strong Attractions. And it is the business of experimental Philosophy to find them out. (This business will take us nearly five weeks) Fgravity =
mass1 mass2 r 2 Astronomy Kepler, Newton Felectrical = charge1 charge2 r 2
(?) Net Deflection () Pointer Twist () Meyzires Engineering School Initially wanted to be a 1760-1761 mathematician
(1732-1799) pith ball pp. 569-577 (0) gilded pith ball Charles Augustin Coulomb (1736-1806) needle deflection
scale charge pinhead by rubbing National Maritime Museum, Greenwich Royal ofpointer Engineers torsion ~1751Corps Charles Augustin with deg. scale continued1760-1790 to
deny his mothers 0 desire 36 that he study medicine Acadmie Royale des Sciences electrical and therefore temporarily 126 18ofwas in charge building Ft. Bourbon, Martinique, 1764-1772 1781-1793
1793 funds, he disowned. Without torsion balance silver wire 567 8.5 (~20join m thick was forced to his father in 1785 ~1/4 of a
hair) Institut de France Mmoires de lAcadmie Montpellier. gilded Royale des Sciences, 1795-1806 (Gillmour. 1971, p. 5) data to derive his law for repulsion of like charges: F 1/r 2
How certain could he be that the exponent for r is exactly 2, and not 2 + ? That is, how large a could be consistent with his data? (Modern experiments, relevant to the rest mass of the photon and to the dimensionality of space, show < 10-17) Hints: One approach would be to make a plot based on numbers derived from these data. You might want to consider experimental error and geometry. Detail on experiment and calculation is available in the translation of Coulombs paper on the course website.
0 Net Deflection () (1) Coulomb used these Pointer Twist () Two Problems 36 126 18 567 8.5
(2) Two years later (1787) Coulomb extended this law for repulsion to include attraction between opposite charges. Explain why Coulomb would need to develop a new apparatus for this experiment. That is, why couldnt he just use the same apparatus with different charges on the two gilded pith balls? Hint: Remember that the torsional force is approximately linear in the displacement. It might help to graph the Coulombic and torsional energies through a region that includes the point where they balance.
Binding Energies from Various Sources Coulombic q1q2/rr : Proton-Electron at 1.54 (216 kcal/rmol) Magnetic 12/rr3 : Electron Spins at 1.54 (0.0014 kcal/rmol) Strong Binding : Proton-Neutron in Deuterium Nucleus (5 x 107 kcal/rmol) Gravitational m1m2/rr : C atoms at 1.54 (3 x 10-32 kcal/rmol) Chemical Bond : C-C at 1.54 (90 kcal/rmol) (similar to 1 e Coulombic) -3 0 3
6 (What of Kinetic Energy?) Log (Potential Energy) kcal/rmol Is there a Chemical Force Law? How far can you Stretch a Chain of Atoms before it Snaps? Demonstration with Magnets Valuable prize
for balancing suspended magnet between sets of attracting ma gnt! Force Laws & Molecular Structure Spring (ut tensio sic vis) F = -k x E = k/2 (x)2 nd =F 2Slope Spring (weaker, opposing)
x minimum Balanced Single Minimum Potential Energy sum 0 0 Electrical Charges (gravity, etc.)
F = k / (x)2 E = -k/(2 |x|) sum Balancedx minimum ! rd 3 Stronger DoubleBody Minimum Thus with springs you might make a stable polyatomic molecule from point atoms.
(but not with ions or magnets) However, if bonds obeyed Hookes Law, they could never break. Morse Potential Mathematically convenient approximation for realistic bond energies (proposed 1929) Fixed Neighbor Sum
Second Fixed Neighbor Morse Potential Snaps at Inflection Point (Change from direct to inverse force) What ARE bonds? 19 Century Experiments led to VALENCE numbers th
1861 Different # for different atoms: H(1), C(4), O(2), N(3)or 5?) NH3 and NH4Cl Why do Elements Differ? Gertrude and Robert Robinson (1917) Such slippery concepts explain so much convinceScheme you of nothing. Why/Whenthat theyReaction latent valence loop ?
Why/When partial dissociation? reaction What does the loop mean? How Many? Might Latent Valence Loop explain trivalence of pentavalent N? Might Partial Dissociation explain amine/HCl reactivity? product
Electron Discovered 1897 The Cubic Octet of G. N. Lewis as as Harvard Harvard Undergraduate Instructor ~1894 ~1902
E. S. Lewis, by permission (1875-1946) Octet to "Explain" Periodicity & Electron Transfer (1902 teaching notes) Octet Predicts Shared Pair Bonding shared edge shared face ? Cubic Octet to
to Tetrahedral TetrahedralOctet Octet (G. N. Lewis 1916) :N N: Tetrahedral distribution of the bonds from C had already been known in organic chemistry for 40 years!
Good Theory should be as possible as Realistic & Simple In regard to Facts it should allow: Prediction Suggestion Explanation Classification & Remembering Postdiction: Realm of Lore
End of Lecture 2 Sept 4, 2009 Copyright J. M. McBride 2009. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0). Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol . Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0
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