ch. 8 and 9 lecture notes
DESCRIPTION
Lecture NotesTRANSCRIPT
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Chapters 8 & 9
Chemical Bonding
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Sections we are skipping & why… 8.4 – covered in Ch. 7 (good to rvw) 8.8 – covered in Ch. 6 (good to rvw) Formal charges (pp. 386-389) – don’t
need to know anymore! 9.2, 9.3, 9.4 – beyond the scope of
this course 9.6 – covered in Ch. 7 (good to rvw)
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Dogs teaching Chemistry
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Ionic bonding Electrostatic attraction
b/w oppositely charged ions
Ions form crystal lattice orderly pattern Maximizes attraction b/w
oppositely charged ions Minimizes repulsions
among like-charged ions.
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What’s an ionic bond?
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So… Technically, there are NO true ionic
bonds… wait, WHAT?!?! See graph…
However… chemists avoid this problem by using an operational definition: any compound that conducts an electric current when melted is ionic.
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Coulomb’s law Coulomb’s Law:
used to calc nrg of interaction b/w a pair of ionsQ1 and Q2 = charges
on ionsR = distance b/w ion
centers (in nm)k = 2.31 x 10-19 J•nm
If E < 0, what does that imply?
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Lattice energy Modified form of
Coulomb’s law Only difference is k
(value depends on ions in crystal lattice)
Measures stability of crystal lattice
Lattice nrg ↑ as charge of ions and radius .
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Practice Problem #1 Arrange the following ionic
compounds in order of increasing lattice energy (least exothermic to most exothermic): NaF, CsI, and CaO
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Practice Problem #2 Which substance would you expect
to have the greatest (most exothermic) lattice energy: AgCl, CuO, or CrN? Why?
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Covalent bonding E’s shared between nuclei
Shared equally: Shared unequally:
Formed b/w....
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Bonding of H2
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Network vs. Molecular Solids Network are VERY strong bonds – high melting
points, solids at room temp Some common network solids:
Molecules weaker than ionic bonds – low melting points, mostly liquids and gases at room temp
Some common molecular solids:
Properties are a reflection of their structure Examples: Graphite vs. Diamond
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Graphite vs. Diamond
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Silicon – a semiconductor Structure like diamond – why? Low conductivity so called a
“semi-conductor” Conductivity as temp
increases – why? Can change conductivity by doping
with other elements
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Doped silicon
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How doping works Let’s watch a video!
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Bonding models Chemical bonds result from a tendency
of a system to seek its lowest possible energy.
Therefore, bonding is a model proposed to explain molecular stability – an attempt to explain how atoms (something microscopic) operate based on experience in the macroscopic world.
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Fundamental properties of models Human inventions – based on
incomplete understanding of how nature works.
Often wrong – always speculations & oversimplifications
Difficult to make predictions from – many exceptions (ex: Aufbau principle)
Still very useful
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Sooo... What does this mean?
Models are still important and useful… just realize there are limitations. Bonding model assumes electrons are shared
and located between two nuclei Makes sense – atoms share electrons to
achieve stability But… electrons move freely and are
delocalized. This does NOT mean we throw out the
model!!!!
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Localized Electron (LE) Model Used to describe covalent bonds in
molecules Assumes:
Has three parts: (1) Drawing Lewis Structures, (2) VSEPR, (3) Description of orbitals used (Hybridization)
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Lewis Structures Review notes from your previous
Chemistry class if you don’t remember how to do these!
Do not worry about keeping track of which electrons come from which atoms. It’s the total number of electrons that’s important!!!!
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A few reminders… Make your octets around each atom and
THEN count. Adjust as necessary…
For Lewis structures of anions and cations:
Determining central atom:
Octet rules & exceptions:
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Practice Problem #3Draw the Lewis structures for
C2H2
ICl4-
NO2
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Review: Resonance Sometimes, more than one valid
Lewis structure can be drawn. When this happens, we say that there is resonance.
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Resonant structures for NO3
-
Which is the correct description of the bonding in NO3
-? Why?
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Practice Problem #4 How many resonance structures
are possible for NO2-?
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Practice Problem #5 Regarding the structure of NO2
- drawn in PP #4, which of the following statement(s) is/are true?
a) There is one single bond and one double bond in NO2
-
b) There are two equivalent N-O bonds, each one intermediate b/w a single and double bond
c) NO2- flips from one resonance structure to the
other.d) The actual structure is an average of the two
resonance structures.
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Practice Problem #6 Which is predicted to have shorter
sulfur-oxygen bonds, SO3 or SO32-?
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VSEPR Used to predict geometry
Molecular geometry (shape/structure) Electron-pair geometry
Based on the premise that lone e- pairs require room than bonding e- pairs and tend to bond angles.
Explain how to use handout Need to know bond angles!
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Molecular geometry vs. Electron pair geometry
Molecular geometry a.k.a. molecular structure a.k.a. shape = shape formed by
Electron-pair geometry (aka
electron-domain geometry) = shape formed by
See example
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E- pair geometry vs. Molecular geometry: Octahedral
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Practice Problem #7 Determine (a) electron-pair geometry and (b)
molecular geometry (VSEPR) for each of the following:
1) SnCl3-
2) O3
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Practice Problem #7 (cont)Determine (a) electron-pair geometry and (b)
molecular geometry (VSEPR) for each of the following:
3) SF4
4) IF5
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What about CH3OH?
How do you describe the shape when there is more than one central atom?
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What CH3OH looks like…
Bent around O
Tetrahedral around C
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Hybridization Hybridization = mixing of native
atomic orbitals (s and p, for example) to form special orbitals (sp2, for example)
Hybrid orbitals are Hybrid orbitals arrange themselves in
space such that we can determine hybridization from electron pair geometry – easiest way!
5 main types of hybridization: sp, sp2, sp3, sp3d, sp3d2
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Degenerate orbitals
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sp hybridization
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sp2 hybridization
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sp3 hybridization
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sp3d and sp3d2 hybrids
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Practice Problem #8 Give the expected hybridization of the
central atom in the following molecules:
a) SF4
b) XeF2
c) PF6-
d) CO
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Sigma (σ) bonding Centered on line
running b/w 2 atoms sharing a pair of e’s
Shared e pr occupies space b/w the two atoms
σ bond = bond Can form from
overlap of….
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Pi () bonding Represents shared e- pr
above and below the σ bond.
Formed using the p orbitals perpendicular to σ bond axis
Parallel p orbitals share an e- pr, forming a bond
Forms from sideways overlap of orbitals. Can also form from overlap of d orbitals, but we won’t consider these.
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Sigma & pi bonds in C2H4
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What about triple bonds?
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Determining sigma and pi bonds Easy as 1-2-3! Single bonds = bond Double bonds = bonds Triple bonds = bonds
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Practice problem #9Determine # of sigma & pi bonds in the
following molecules:
1. C2H2
2. C3H4
3. CH3COCOCH3
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Practice Problem #10 Consider the acetonitrile molecule:
H3C-CΞN:
a) Predict the bond angles around each carbon
b) Give the hybridizations on each carbon
c) Determine the total number of sigma and pi bonds in the molecule.
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Predicting molecular polarity What shapes will ALWAYS be
polar? Why?
What shape is an exception to this rule? Why?
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Review: Dipole moments Caused by difference in
electronegativity of the bonded atoms
Note: “covalent bond” a.k.a. “nonpolar covalent bond”
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More on dipole moments
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Ways to show polarity
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Practice problem #11Classify following as polar or nonpolar:
a) BF3: c) O2:
b) CS2: d) IF3