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Honors Chemistry Chap 13

Honors Chemistry Chap 13. Molecular Structure. 13.1 Electron Distribution. Consider 2 models of molec structure which account for their shape 1 st model takes into account the repulsive forces of e- pairs

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Honors Chemistry Chap 13

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  1. Honors Chemistry Chap 13 Molecular Structure

  2. 13.1 Electron Distribution • Consider 2 models of molec structure which account for their shape • 1st model takes into account the repulsive forces of e- pairs • 2nd model considers ways in which orbitals can overlap to form orbitals around more than 1 nucleus • E-’s in these orbitals bind the atoms together

  3. 13.1 Electron Distribution • It’s useful to use Lewis e- dot diagrams to describe the shape of molecs. or polyatomic ions • Shaired Pairs – prs of e-’s involved in bonding • Unshared Pairs – prs. of e-’s not involved in bonding • Lone pairs

  4. 13.2 Electron Pair Repulsion • Also called VESPR Theory • Valence Electron Shared Pair Repulsion Theory

  5. 13.2 Electron Pair Repulsion • One way to account for molec shape is to look @ e- repulsion • Ea bond & lone pair in outer level for a charge cloud that repels other chg clouds – due to like charges • Also due to Pauli Exclusion Principle – e- of like spins may not occupy the same vole of space • Repulsion due to like spins is much greater than repulsion due to like charges

  6. 13.2 Electron Pair Repulsion • Repulsion betwchg clouds determine arrangement of orbitals & \ the shape of the molec • Electron prsspead as far apart as possible to minimize repulsive forces. • If there are 2 e- prs, they will be on opp sides of the nucleus • Linear (180o apart)

  7. 13.2 Electron Pair Repulsion • 3 e- prs – axes of chg clouds will be 120o apart • Trigonal Planar • E- prs lie in the same plane as the nucleus • 4 prs – axes of chg clouds will be as far apart as possible – 109.5o • Tetrahedral • Will not lie in the same plane • 4 faces; ea is an equilateral triangle w/ the nucleus @ the center

  8. 13.2 Electron Pair Repulsion • Unshared pr is pear-shaped w/ stem end @ nucleus • Acted upon by 1 nucleus • Shared pr is more slender bec it’s atracted by 2 nuclei • Less repulsion bec it takes up less space • lone prs – most repulsion – take up the most space • Repulsion betw unshared & shared pr is intermediate

  9. 13.2 Electron Pair Repulsion • CH4, NH3, H2O, & HF – all have 4 clouds around them • \ expect all 4 clouds to pt to corners of tetrahedron • CH4 – all clouds are shared prs – size & repulsions are = • \ bond angle is 109.5o – perfect tetrahedron • Shape of molecule is tetrahedral

  10. 13.2 Electron Pair Repulsion • NH3 – 1 lone pr & 3 shared prs – since lone pr occupies more space, shared pairs are pushed together • \ bond < is 107o • E- clouds form tetrahedron; but atoms of the molec for trigonal pyramid • Shape of molecule is trigonal pyramidal

  11. 13.2 Electron Pair Repulsion • H2O – 2 unshared prs & 2 shared prs • Add’l cloud size of unshared prs causes even greater reduction in bond < - 104.5o • E- clouds are tetrahedral, but molec is bent • HF – only 1 bond axis, \ no bond angle • 180o – molec is linear • The diff in molec shape results from unequal space occupied by unshared prs & bonds (shared prs)

  12. 13.3 Hybrid Orbitals • The 2nd model of molec shape considers the diff ways 2 & p orbitals ma overlap when e-’s are shared • C has 4 outer e-’s • Expect 2 half-filled p orbitals avail for bonding

  13. 13.3 Hybrid Orbitals • However, C undergoes hybridization during bonding • The 1s orbital & 3 p orbitals combine into 4 equivalent hybrid orbitals. • Called sp3 hybrids or hybrid orbitals • The 4 orbitals are degenerate – same energy • Ea contains 1 e- • The sp3 hybrids are arranged in tetrahedral shape • Ea can bond to another atom • If ea bonds to an identical atom, the 4 bonds are equivalent

  14. 13.4Geometry of Carbon Compounds • Methane – CH4 – 1 C atom & 4 H atoms • Bonds involve s orbital of ea H atom w/ 1 sp3 hybrid orbital of C • 109.5obetwea C – H bond axis • C exhibits catenation • Occurs when 2 C atoms bond w/ ea other by overlap of an sp3 orbital from ea C atom • Other sp3 orbitals may bond w/ s orbital of H

  15. 13.5 Sigma & Pi Bonds • A covalent bond is formed when 2 orbitals from diff atoms overlap & share an e- pair • Sigma Bond (s)– formed when the 2 orbitals that overlap lie directly on the bond axis • Overlap end-to-end or “head-on”

  16. 13.5 Sigma & Pi Bonds • Different ways to form a sigma bond: • 2 s orbitals • An s & a p orbital • 2 p orbitals (overlapping end-to-end) • 2 hybrid orbitals ex) sp3’s • A hybrid orbital & an s orbital

  17. 13.5 Sigma & Pi Bonds • Since p orbitals are not spherical, when 2 half-filled p orbitals overlap, they can form 1 of 2 types of bonds • Overlap end-to-end & form a s bond • Overlap sideways (parallel) & form a PiBond (p)

  18. 13.5 Sigma & Pi Bonds • Ethylene (ethene, C2H4) shows both types of bonding • In both C atoms, 3 orbitals hybridize • 1 s & 2 p form 3 sp2 orbitals • Lie in the same plane ~ 120o bond angle • The 3rd p orbital does not hybridize • Perpendicular to plane of sp2 orbital • An sp2 orbital from ea C atom overlaps end-to-end - s bond

  19. 13.5 Sigma & Pi Bonds • The 2 remaining sp2 orbitals from ea C atom bond w/ 2 separate H atoms • sp2 to s s bond • The unhybridized p orbitals overlap sadeways - p bond • C atoms have a s bond & a p bond betw them • Double bond – 2 prs of e-’s are shared

  20. 13.5 Sigma & Pi Bonds • Acetylene (ethyne) • 1 s & 1 p orbital hybridize to form and sp hybrid orbital in ea C atom • Leaves 2 p orbitals perpendicular to ea other & perpendicular to the sp hybrids • An sp from ea C overlap to for a s bond • 2 p orbitals from ea C ovrlap to form 2 p bonds • \ acetylene has 1 s & 2 p bonds betw C atoms • Triple bond – 3 shared prs of e-’s

  21. 13.5 Sigma & Pi Bonds • Double & triple bonds are less flexible, shorter, & stronger than a single bond • p bonds – easier to break bec e-’s forming bond are farther from nuclei • \molecs containing multiple bonds are usually more reactive than similar molecs w/ only a single bond • Unsaturated Comps – comps which contain double or triple bonds betw C atoms

  22. 13.6 Organic Names • Names for organic comps have a suffix which describes how the atoms are bonded • Comps ending in a n e have all single bonds betw C atoms • Saturated Comps - CnH2n+2 • Comps ending in e n e have a double bond betw C atoms • Comps ending in y n e have a triple bond betw C atoms

  23. 13.6 Organic Names • Prefixes show # of C atoms in chain or ring • H2C CH2 - ethene • H C C H - ethyne • Molecs whose C atoms form a ring begin w/ cyclo • Simplified diagrams can be used to represent cyclic comps • C atoms are @ the vertices

  24. 13.6 Organic Names • C forms 4 bonds • 4 single bonds • 1 double & 2 single bonds • 1 triple & 1 single bond • 2 double bonds • \ assume a C atom has enough H atoms bonded to it to give it 4 bonds

  25. 13.7 Multiple Bond Molecular Shapes • Formaldehyde contains a double bond betw C & O

  26. 13.7 Multiple Bond Molecular Shapes • N2 contains a triple bond

  27. 13.7 Multiple Bond Molecular Shapes • Using VESPR Theory we can still predict the shapes of molecs containing multiple bonds • A double bond occupies more space than a single bond • 4 e-’s betw bonded atoms instead of 2 • Triple bond occupies even more space • 6 e-’s

  28. 13.7 Multiple Bond Molecular Shapes • In formaldehyde – • 3 clouds around C atom • 2 single & 1 double bond • No unshared prs; assume trigonal planar shape; 120o • However, since double bond takes up more space than single bonds, H-C-H bond angle is less than 120o - 116o • The H – C – O bond angle is more than 120o • 122o

  29. 13.7 Multiple Bond Molecular Shapes • When C has 2 double bonds, the molec will be linear • CO2 -

  30. 13.7 Multiple Bond Molecular Shapes • Ketene: • 2 dbl bonds on 1 C atom – that part is linear • Other C atom has 2 single & 2 dbl bond like formaldehyde

  31. 13.7 Multiple Bond Molecular Shapes • When C is triple bonded to another atom, molec is linear

  32. 13.7 Multiple Bond Molecular Shapes • In most comps, outer level is considered full w/ 8 e-s • If outer level is 3rd or higher, atom can contain > 8 e-’s • Mostly nonmetals (usually halogens) form comps w/ outer level containing 10, 12, or 14 e-’s • This is how Noble Gases react

  33. 13.8 Benzene C6H6 • One of the tip 20 industrial chemicals in US • Used in drugs, dyes, solvents • Highly toxic & a carcinogen • Ea C atom in the benzene ring has 3 sp2 hybrids & 1 p orbital • sp2 orbital from ea of the 6 C atoms overlap & form a ring of 6 s bonds

  34. 13.8 Benzene C6H6 • p orbitals overlap sideways & form ring of p bonds • Left over sp2 orbital from ea C overlaps w/ s orbital from H atom

  35. 13.8 Benzene C6H6 • One main characteristic of benzene is the p e-’s can be shared among all C atoms • - delocalized • Delocalization causes greater stability in benzene

  36. 13.8 Benzene C6H6 • Many ways to represent benzene: Represent delocalized e-s from p bonds

  37. 13.8 Benzene C6H6 • Conjugated system – group of atoms which contain multiple p overlap • Multiple p bonds • Multiple double or triple bonds • C C C C • Conjugated systs add special stability to the molecs

  38. 13.9 Isomers • Isomerism – the existence of 2 or more subst w/ the same molecular formula, but diff stuctures • These structures are isomers • Very common in organic chem

  39. 13.9 Isomers • C4H10 – butane – 2 structures can be drawn for this formula • Butane methyl propane (isobutane) • These are structural isomers or skeleton isomers – C chain is altered

  40. 13.9 Isomers • Geometric isomers – coposed of the same atoms bonded in the same order, but w/ diff arrangement of atoms around a double bond • p bond prevents atoms from rotating w/ respect to ea other • A diff arrangement around a dbl bond since rotation is not possible

  41. 13.9 Isomers • Cis 2 butene • Trans 2 butene

  42. 13.9 Isomers • Cis – the CH3 group (or anything other than H) are next to ea other (on same side) • Trans - the CH3 group (or anything other than H) are on opposite side (across)

  43. 13.9 Isomers • Positional Isomers – occurs w/ a 3rdelem or mult bond where the 3rdelem or mult bond can occupy 2 or more diff positions • Functional Isomers – Formed when a 3rdelem can be bonded in 2 diff ways • A mass spectrometer can be used to distinguish betw isomers having similar props. • Uses charge to mass ratios of ion fragments

  44. 13.10 Inorganic Compounds • Hybridize like organic comps • Be ends in 2s2 – hybridizes 2 orbitals • 2 sp orbitals • Linear molec

  45. 13.10 Inorganic Compounds • B ends in 2s2 2p1 - 3 orbitals hybridize • 3 sp2 orbitals • Trigonal planar

  46. 13.11 Bond summary • BCl3 – trigonal planar • Used to produce high-purity metals • Higher atomic mass elems tend to hybridize their bonding orbitals much less than lighter elems do • May be bec heavier atoms can have more bonded atoms around them bec they are larger.

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