GENERAL CHEMISTRY I
GENERAL CHEMISTRY I CHEM 121
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This 5 page Class Notes was uploaded by Jay O'Keefe on Monday October 19, 2015. The Class Notes belongs to CHEM 121 at Rice University taught by Staff in Fall. Since its upload, it has received 40 views. For similar materials see /class/225048/chem-121-rice-university in Chemistry at Rice University.
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Date Created: 10/19/15
Review of Hybridization Valence Bond Theory The basic idea of valence bond theory is that a covalent bond is formed by the overlap of the atomic orbitals the greater will be the degree of sharing and the stronger will be the covalent bond between them The atomic orbita can be the original atomic orbitals of the atoms but often the geometry of these orbitals is such t at eHective overlap cannot occur in the known geometry of the molecule Under these circumstances the atomic orbitals on an atom can recon gure themselves into a di erent con guration and the recon gured orbitals are said to be hybridized in 1931 the American physical chemist Linus Pauling demonstrated that the wave functions of the electrons in the orbitals of an atom could be combined mathematically essentially by adding together the amplitudes of their waveforms in threerdimensional spaceewhic by mutual constructive and destructive interference will give a different threerdimensional con guration using the equations of waves to give sets of equivalent wave functions that we now call hybridized atomic orbitals For many molecules hybri 39zation gives a set of atomic orbitals that can overlap more effectively with the atomic orbitals on other atoms in the molecule thus providing an overall molecular structure which has stronger bonds and 395 lower in energy Whi e the mathematical treatment of Pauling is beyond our scope the results of it are most useful in furthering our understanding of the bonding in molecules 1 sp Hybridization The hybridization of one s orbital and one p orbital on a central atom gives rise to two sp orbitals Hybridization as sp gives two orbitals which are in a 1 near arrangement that is 180 apart An example of sp hybridization is found in 0sz acetylene CDC This sp type of hybridization is the reason for the existence of the basic linear shape found for two electron pairs in the valence shell using the valence shell electron pair repulsion approach 2 sp2 Hybridization s on a central atom gives rise to bitals which are planar and 120 The hybridization of one s orbital and two p orbi three sp2 orbit s ybridization as sp2 gives three apart An example of sp2 hybridization is i3018 tal or Dv This sp2 type of hybridization is the reason for the existence of the basic trigonal planar shape found for three electron pairs in the valence shell using the valence shell electron pair repulsion approach if one of the orbitals is occupied by a lone pair the resulting geometry will be bent 90 This is the orbital structure dominant in ozone and other AXZE molecules sp8 Hybridization The hybridization of one s and all three p orbitals on a central atom gives rise to four sp8 orbitals Hybridization as sp8 gives four identical orbitals which are aligned in a tetrahedral con guration and so are 109 5a apart This very common structure is found in NHS NH and och it is the usual hybridization form of singlyebonded carbon atoms in organic compounds 6 in its ground state the carbon atom has the electronic structure ls72s72p2 The by hrirli ati n 39 or itals L 39 L L must reach the excited state structure of ls72s 2p3 with one electron in each of the pe pc and 17 orbitals before hybridization can occur This requires energyran energy which is more than recovered in the lowerrenergy moreroverlap structures of molecules which are possible with sp8 hybridization A Thls spz type ol hybrrdrzetron ls the reeson lor the exrstence ol the besrc tetrehedrel y perr repulsron eppro h The trrgonel pyremrdel end bent shepes exhrbrted by emmome end weter cen be unr sp d Hybrrdrzetron Atoms whlch heve d orbrtels everleble to them cen elso use them to lorm other types ol hybnd orbrtels The hybrrdrzetron ol one s one d end three p orbrtels on e centrel etom gves rrse to ve sp d orbrtels Hybrrdrzetron es sp d grves ve orbrtels three equetorrel end two exrel beceuse thrs hybrrdrzetron uses the dg the s end ell three ol the p oxbllels one ol the p orbrtels pt ls perpendrculer to the n plene ln the trrgonel brpyremrdel geometry ol thrs hybrrdrzetron ls the trrgonel plene ol spz hybrrdrzetron wrth the eddrtron ol the z exls component due to the plend d olsp d brpyremrd The trrgonel brpyremrd ls the besrc shepe essumed by ve bondrng pens ol electrons m the outer velence shell ol en etom m the velence shell electron pen repulsron epproech Occupyrngthe orbrtels equetorrel rst wrth lone peus leeds to the VSEPR geometrres mmmm 5 sprd2 Hybrrdrzetron Atoms whlch heve two or more d orbrtels everleble to them cen use more then one d n rbll 1 nd WV n gves slx orbrtels whlch ere equrvelent m energy end geometry All edrecent orbrtels ere 9m epert end ell nonedrecent orbrtels ere 1m epert Thls lorm ol hybrrdrzetron hes the geometrrc structure ol en octehedron wrth slx vertrces end ergnt srdes The even um H y 0 H the octehedron ere the s pet pv end dzy etomrc oxbltals the z exls component ls egem due to the pi end the d etomrc orbrtels Molecules or rons whlch heve thrs structure d to heve octehedrel structures or symmelxy sullur hexe uonde end urenrum hexe uonde ere exemples olsuch molecules usrng the velence shell electron pen repulsron epproech Describe the bonding in acetonltrlle CHSCN Molecular Orbital Theory The prlhclple of the molecular orhltal theory ls that the electrohlc structures of molecules are determlhed prlmarlly hy the huclel of the atoms whlch cohstltute them ahd the arrahgemeht of those huclel lh space eh t e electrohs are added lhto tlme ah the e ects of those added electrohs oh the ehtlre molecule are cohsldered as they are added sequehtlally If those e eats are hegleglhle as they most often are a oualltatlye plcture cah be developed reasohahly readlly The Paull exclusion prlhclple stated that each electroh lh ah atom had to have a unique set of ouahtum numbers slhce two values of splh ouahtum humher are posslhle each atomlc orhltal could cohtalh 39 39 l l 39l u lr p39 p 39 l T 39 39 39 applles also lh molecular orhltal theory each molecular orhltal cah cohtalh two electrohs if ahd ohly if thelr splhs are palred Agalh wheh deallhg wlth atomlc orhltals dl erent orhltals could have the same energy For example the three 2p orhltals oh ah lsolated nitrogen atom all have the same energy but they have dl erent orlehtatlohs lh space Molecular orhltals whlch are dl ereht haylhg dl erent orlehtatlohs lh space cah also ha e the same e ergy Molecular orhltal theory ls to lsolated molecules what atomlc orhltal theory ls to lsolated lll lt Prlhclples 1 The total humher of molecular orhltals produced ls always equal to the total humher of atomlc orhltals cohtrlhuted hy the atoms that have comhlhe 2 Wheh t 39 w theylorma tahlll d39 l39 quot 39ahdadestahlllzed higher energy ahtlhohdlhg orhlta Electrohs of the molecular are asslghed to orhltals of successlyely higher energy accordr lug to the Paull prlhclp e ahd Hund s rule 4 Atomlc orhltals comhlhe to form molecular orhltals most e ectlvely wheh the atomlc orhltals are of slmllar energy e hydrogen molecule ls the slmplest molecule we know Wheh molecular hydrogen ls formed from two hydrogen atoms the two ls atomlc orhltals oh the two hydrogen atoms overlap ahd lhteract T 39 39 39 39 h A pp of L 439 39 ard each other If the electroh patterhs of these orhltals are vlewed as waves the lhteractloh ls hoth cohstructlve ahd destructlve The cohstructlve interference produces a bonding orhltal whlch ls located prlmarlly hetweeh the two ydrogen atoms thus providing a locatloh lh whl h a palr of electrohs cah be shared hetweeh them The energy of the bonding molecular orhltal ls lower t ah the energy of the orlglhal atomlc orhltals oh the lhdlvldual atoms The destructlve interference produces ah ahtlhohdlhg orhltal whlch avolds the space hetweeh the two hydrogen atoms The energy of the antibonding molecular orhltal ls higher thah the energy oltheorlglhal 39 quot l l 39 quot quot l a oms 39 lh the molecule wlll prefer to occupy the bonding molecular orhltal ahd the difference lh energy hetweeh the bonding molecular orhltal ahd the two orlglhal atomlc orhltals ls the hohd ehthalpy of the hydrogen molecule Energy level diagram How about He Bond order p INCREASING ENERG 291A 2973 29793 2pr In 295 zpaA 2er ZDrA 2w 25A 2a 25A 2sB 2 25 Why is 02 paramagnetic Resonance and MO Theory Ozone
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