Announcements - 11/5/02
•Exam #3
•Wed, 11/13, 7pm, HERE
•See Exam #3 Info Page
•New Online Quiz (#7): Thurs - Tue
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1
Lattice Energies
„ Lattice energies vary with:
E = k Q1 Q2
d
d
2
1
The Born-Haber Cycle
„ Use Hess’s Law to arrive at ∆Hlattice
First, convert elements to gas phase atoms:
1) Na(s) → Na(g)
2) ½Cl2(g) → Cl(g)
∆Hof[Na(g)] = 108 kJ
∆Hof[Cl(g)] = 122 kJ
Next, ionize the atoms:
3) Na(g) → Na+(g) + e4) Cl(g) + e- → Cl-(g)
∆H = I1(Na) = 496 kJ
∆H = EA(Cl) = -349 kJ
Finally, bring the ions together to form the lattice:
5) Na+(g) + Cl-(g) → NaCl(s) ∆H = - ∆Hlattice
Knowing ∆Hof[NaCl(s)] = -411 kJ, we can solve for ∆Hlattice
3
NaCl BornHaber Cycle
1) Atomize Elements
2) Ionize Elements
3) Form Lattice
∆Hlattice = ∆Hatomize + I1(Na) +
EA(Cl) + (-∆Hof[NaCl])
= 122 kJ + 108 kJ +496 kJ +
(-349 kJ) + (-(-411 kJ))
= 788 kJ
4
2
Ion Electron Configurations
„ Usually add or remove electrons to reach nearest Noble Gas
configuration:
Na+ [Ne]
Cl- [Ar]
„ What about Transition Metal Ions?
-can’t accommodate the loss/addition of enough electrons to
reach Noble Gas configuration
-electrons removed from ns orbitals first, and then from
(n-1)d orbitals:
Fe [Ar]3d64s2
Fe2+ [Ar]3d6
electrons removed from 4s
Fe3+ [Ar]3d5
electrons removed from 3d
5
Ionic Sizes
•Cations: smaller
•Anions:
larger
-increasing Z gives
decreasing size for
isoelectronic ions:
O2- > F- > Na+ > Mg2+ > Al3+
6
3
Covalent Bonds
„ When two non-metals form a bond, neither can
attain Noble Gas configuration by complete
electron exchange, so they share:
H2
H : H
both atoms attain Noble Gas
configuration
7
Multiple Bonds
„ More than just two electrons can be shared (in
order to reach Noble Gas electron configuration):
Example: O2
-each oxygen has 6 valence e-:
:O:
..
-each needs to share TWO e- to reach octet:
:O::O:
.. ..
-multiple bonds are: shorter and stronger than single
bonds
8
4
Electronegativity
„ Electronegativity (EN)
-combines IE and EA terms to give the relative
ability of an atom to attract e-’s to itself when
bonded to another atom
-developed by Linus Pauling (based on thermochemical data)
Examples:
Na: 0.9
Cl: 3.0
←unitless quantity
-makes sense: Na has lower ionization energy and lower
electron affinity than Cl, so Na should have less ability
to attract electrons from Cl
9
Electronegativity:
Periodic Trends
Decrease
down group
In
cre
ase
ac
r
os
sp
er
io
d
10
5