[CHEM 1110] - Midterm Exam Guide - Comprehensive Notes for the exam (16 pages long!)

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7 Feb 2017
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CHEM 1110
MIDTERM EXAM
STUDY GUIDE
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Lewis Theory
Chemical bonds form by pairing valence electrons such that atoms get a ng config
(octet rule)
Nh3 forms, nh4 does not. Doesnt tell us 3d
2h+ O = h20: tells us why water is stable
Combinbin lewis theory and vespr
Valence shell electron pair reulsipon: e pairs repel each toher
Electrostatic or quantum mechanical: pauli exclusion: 1 orbital 2 electrons
Hard to generalize to more complex structures
Predicts trig pyramial for ch3 but its planar
Valence bond and molecular orbital throey
1st qm model: valence bond theory: energy of atomic or hybrid orbitals centered
on atoms that make molecule: calculated as atoms get closer together
Nuclei get closer: s orbitals interact: can be energetically favoralb (h) or
repulsive
S orbitals overlap: favorable
Have to stay at a finite distance so energy rapidly increases as you
compress the bond
H2s: h 1s, h 1s, s 3s2 3p4
Half filled orbitals overlap: 1s and 1s: 3p4
Ch2: h 1s2, c 2s2 2p2
Valence bond theory with hybridization
C: 2s2, 2p2: hybridization: 4 sp3 orbitals
Methane: correctly predicts chem structure and molecular geo if hybridized
orbitals on carbons are used
Energies of atomic orbitals overlap but remaind intact in bond formation
Molecular orbital theory
Second qm model: atomic orbitals are single electron wave functions of an atom
Atomic orbitals are single e wave functions that are solutions to the schrodinger
equation
Mo theory finds approx soltions to schrodingers
Multi electron systems: no exact soltion to schrodinger
E arb> or = to true energy
Molecular orbital theory variation principle
Variation principle: arbitrary wave fnction is used to calculate energy, value
calculated is never lower than the true energy
Choose trial wave function: variable parameters: model true unkown wave
function
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Systematically vary parameters of trial wave function such that calculated
energy becomes smaller with each adjustment, will approach the true
wave function
True wave approx by linear combination of a: lcao
Linear combo: add or subtract atomic orbitals to yield molecular
orbitals
Molecular orbitals
Atomic wave constructive combination: seulting mo has less energy than original:
bonding mo: no node between nuclei
Sigma pi, most electron density between nuclei
Combine destructively: antibonding mo: node btw nuclei
sigma* pi*: most electron density outside nuclei
Orbital energy diagram: h2
One bonding one antibonding
No dissociation: if orbital is occupied or not
Bond order: # electrons in bonding mo’s - electrons in antibonding mo’s / 2
2 e in bonding orbital: bond order: 1
Hydrogen molecular cation: h2+
One electron i bonding, 0 in antibonding: ½
Strength of bond in h2+ is less than bond strnegth in h2
Dihelium: he2: 2 in bonding 2 in antibodning: 0
Mo diagrams for second period
H and he have only one occupid orbital: 1s
Dilithium: 1 valence
Bo: ¼ (4-2)=1
Since more electrons are in bonding o than in antibonidng o: net
bonding interaction
Any filled energy level genrate filled bonding and antibonding mos: only
consider valence shell
Core e remaind in atomic orbitals and dont form sigma or sigma* mo
Interaction of p orbitals: sigma bond: not centrosemantic
Valence e in o2: partially occupied 2 p orbitals:
Smalle magentic spin with unpaired electrons: dioxygen is paramagnetic: have
unpaired electrons
Neither lewis nor valence predict this result
N2: no unapired so diamagnetic
Nothing in ab: leads to triple bond:
Heteronuclear diatomics
Invole atoms nearby in same row have nearly equal valence orbital energies
Mo diagram for homonuclear diatomics can be sed to predit e sturcture
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