honors chemistry chap 13
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Honors Chemistry Chap 13Molecular Structure
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13.1 Electron DistributionConsider 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
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13.1 Electron DistributionIt’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
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13.2 Electron Pair RepulsionAlso called VESPR Theory
◦Valence Electron Shared Pair Repulsion Theory
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13.2 Electron Pair RepulsionOne 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
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13.2 Electron Pair RepulsionRepulsion betw chg clouds
determine arrangement of orbitals & \ the shape of the molec
Electron prs spead 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)
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13.2 Electron Pair Repulsion3 e- prs – axes of chg clouds will
be 120o apart◦Trigonal Planar E- prs lie in the same plane as the
nucleus4 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
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13.2 Electron Pair RepulsionUnshared 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
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13.2 Electron Pair RepulsionCH4, NH3, H2O, & HF – all have 4
clouds around them◦\ expect all 4 clouds to pt to corners
of tetrahedronCH4 – all clouds are shared prs –
size & repulsions are =◦\ bond angle is 109.5o – perfect
tetrahedron◦Shape of molecule is tetrahedral
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13.2 Electron Pair RepulsionNH3 – 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
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13.2 Electron Pair RepulsionH2O – 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)
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13.3 Hybrid OrbitalsThe 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
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13.3 Hybrid OrbitalsHowever, C undergoes hybridization
during bondingThe 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 shapeEa can bond to another atom
If ea bonds to an identical atom, the 4 bonds are equivalent
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13.4Geometry of Carbon CompoundsMethane – CH4 – 1 C atom & 4 H
atoms◦Bonds involve s orbital of ea H atom
w/ 1 sp3 hybrid orbital of C 109.5o betw ea 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
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13.5 Sigma & Pi BondsA 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”
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13.5 Sigma & Pi BondsDifferent ways to form a sigma
bond:1. 2 s orbitals2. An s & a p orbital3. 2 p orbitals (overlapping end-to-
end)4. 2 hybrid orbitals ex) sp3’s5. A hybrid orbital & an s orbital
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13.5 Sigma & Pi BondsSince p orbitals are not spherical,
when 2 half-filled p orbitals overlap, they can form 1 of 2 types of bonds1. Overlap end-to-end & form a s
bond2. Overlap sideways (parallel) & form
a Pi Bond (p)
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13.5 Sigma & Pi BondsEthylene (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
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13.5 Sigma & Pi BondsThe 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
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13.5 Sigma & Pi BondsAcetylene (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
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13.5 Sigma & Pi BondsDouble & triple bonds are less flexible,
shorter, & stronger than a single bondp 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
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13.6 Organic NamesNames 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
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13.6 Organic NamesPrefixes show # of C atoms in chain
or ring◦H2C CH2 - ethene
◦H C C H - ethyneMolecs whose C atoms form a ring
begin w/ cyclo◦Simplified diagrams can be used to
represent cyclic comps C atoms are @ the vertices
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13.6 Organic NamesC 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
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13.7 Multiple Bond Molecular ShapesFormaldehyde contains a double
bond betw C & O
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13.7 Multiple Bond Molecular ShapesN2 contains a triple bond
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13.7 Multiple Bond Molecular ShapesUsing 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
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13.7 Multiple Bond Molecular ShapesIn 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
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13.7 Multiple Bond Molecular ShapesWhen C has 2 double bonds, the
molec will be linearCO2 -
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13.7 Multiple Bond Molecular ShapesKetene:
◦ 2 dbl bonds on 1 C atom – that part is linear
◦ Other C atom has 2 single & 2 dbl bond like formaldehyde
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13.7 Multiple Bond Molecular ShapesWhen C is triple bonded to
another atom, molec is linear
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13.7 Multiple Bond Molecular ShapesIn 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
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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
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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
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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
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13.8 Benzene C6H6 Many ways to represent benzene:
Represent delocalized e-s from p bonds
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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
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13.9 IsomersIsomerism – the existence of 2 or
more subst w/ the same molecular formula, but diff stuctures◦These structures are isomers Very common in organic chem
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13.9 IsomersC4H10 – butane – 2 structures
can be drawn for this formulaButane methyl propane
(isobutane)
◦These are structural isomers or skeleton isomers – C chain is altered
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13.9 IsomersGeometric 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
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13.9 Isomers◦Cis 2 butene
◦Trans 2 butene
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13.9 IsomersCis – 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)
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13.9 IsomersPositional Isomers – occurs w/ a
3rd elem or mult bond where the 3rd elem or mult bond can occupy 2 or more diff positions
Functional Isomers – Formed when a 3rd elem 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
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13.10 Inorganic CompoundsHybridize like organic comps
◦Be ends in 2s2 – hybridizes 2 orbitals 2 sp orbitals
Linear molec
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13.10 Inorganic CompoundsB ends in 2s2 2p1 - 3 orbitals
hybridize◦3 sp2 orbitals
Trigonal planar
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13.11 Bond summaryBCl3 – trigonal planar
◦Used to produce high-purity metalsHigher 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.