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

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CHEM 1110

MIDTERM EXAM

STUDY GUIDE

➢ 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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