Formation of Earth's Oceans and Atmospheres

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Formation of Earth's Oceans and Atmospheres

• Intra- and intermolecular Forces

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Forces that Determine States of Matter

• Intramolecular forces - strong forces holding
  atoms together in a molecule or an ionic
  substance - covalent and ionic bonds
  respectively
• Intermolecular forces - weak forces of attraction
  between molecules
• State of a substance depends on strengths of
  these forces

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Ionic Substances

• Force of attraction between oppositely charged
  ions (intramolecular force) is the strong
  electrostatic force
• Ionic substances tend to exist as solids with MP
  gt 500 K
• Ionic substances tend to be components of the
  solid portion of Earth called the lithosphere

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Covalent Substances

• Covalent bonds holding atoms together in a single
  molecule (intramolecular force) are very strong
• However, force of attraction between molecules
  (intermolecular force) is weak
• Small molecules with lowest inter-molecular
  forces became components of the atmosphere of
  primitive Earth

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Continued...

• The small molecule, water, with strongest
  intermolecular force became the component of the
  oceans (hydrosphere)
• To understand atmospheres and oceans, need to
  understand the factors that determine the
  strengths of intermolecular forces

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London Dispersion Force (LDF)

• Intermolecular force present between all
  molecules
• Results from the force of attraction between
  temporary fluctuating dipoles consisting of an
  instantaneous dipole and induced dipoles

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London Dispersion Force (LDF)
Force of attraction between a temporary
instantaneous dipole and an induced dipole
http//wine1.sb.fsu.edu/chm1045/notes/Forces/inter
mol/Forces02.htm
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London Dispersion Force (LDF)
LDF increases with increasing size (molecular
weight) for similar molecules
Tro, 295
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Hydrogen Bonding

• Molecules containing H bonded to either F, O, or
  N exhibit LDF and an additional intermolecular
  force called H-bonding
• Force of attraction between H bonded to N, O, or
  F in one molecule and a lone pair of electrons on
  O, N, or F in an adjacent molecule

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Hydrogen Bonding
Each water molecule can form H-bonds with 4 other
water molecules
http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/H/HydrogenBonds.html
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Effect of H-Bonding
As size decreases, LDF decreases, and B.P.
decreases. However, the smallest molecule, water,
has the highest B.P. due to a strong additional
force H-bonding
Ebbing/Gammon, General Chemistry,6th,Houghton
Mifflin, 1999, 461
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Effect of H-Bonding
HF and NH3 exhibit H-bonding while CH4 does not
Ebbing/Gammon, General Chemistry,6th,Houghton
Mifflin, 1999, 461
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Primitive Atmospheres of Earth

• Original atmosphere when Earth formed 4.5 billion
  years ago was H2 and He
• When solar fusion started, these light gases
  escaped earths gravity
• Second atmosphere came from volcano outgassing
  (Big Burp) N2, H2, H2O, NH3, and CH4 and from
  collisions with comets, which consist primarily
  of ice

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Formation of Oceans

• Of the molecules in the second atmosphere of
  primitive Earth, water was most abundant and had
  the strongest intermolecular forces (LDF, and
  H-bonding)
• As Earth cooled, water was the first to liquefy
  and it came down as rain which formed the
  hydrosphere - oceans, etc

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Electronegativities of Atoms

• Ability of an atom to attract electrons in a bond
  is called the electronegativity (EN) of the
  element
• EN of Main Group elements (A-families) generally
  increase in going upward in a family or across a
  period from left to right

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http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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Electronegativity and Bonding

• Ionic bonding occurs between atoms having widely
  differing EN (metal/non-metal)
• Covalent bonding occurs between atoms having high
  EN (non-metals)
• Two kinds of covalent bonds possible depending on
  the differences in the EN of the bonding atoms

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Predicting Bond Types
If the difference in electronegativities between
the two bonded atoms is 0 (two atoms of the same
element), the bond is non-polar covalent
Bonding pair of electrons is equally shared by
two F atoms
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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If the difference in electronegativities between
the two bonded atoms is greater than 0, but less
than 2.0, the bond is polar covalent
Atom having higher EN has a small negative charge
while atom with lower EN, a small positive
charge
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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If the difference in electronegativities between
the two bonded atoms is 2.0 or greater, the bond
is ionic
Atom having higher EN has a full negative charge,
while atom with lower EN has a full positive
charge
Li, F-
http//wine1.sb.fsu.edu/chm1045/notes/Bonding/Pola
rity/Bond05.htm
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Polarities of Molecules

• A molecule is polar if it contains polar bonds
  in geometries that do not cancel the bond
  polarities
• Geometries that can cancel polarities of
  identical bonds are linear, trigonal planar and
  tetrahedral

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Geometries and Polarities of Molecules

• Molecules having one central atom and two
  terminal atoms are linear if there are no lone
  pairs on central atom or bent if there are lone
  pair(s) on central atom

Polar
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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Molecules having one central atom and 3 terminal
atoms are trigonal planar if there are no lone
pairs on the central atom or trigonal pyramidal
if there is a lone pair on the central atom
Non-polar for AB3
Polar
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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• Simple molecules having one central atom and four
  terminal atoms are tetrahedral

Non-polar for AB4
http//wine1.sb.fsu.edu/chm1045/notes/Geometry/VSE
PR/Geom02.htm
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Non-Polar Molecules

• A molecule is non-polar if it contains no polar
  bonds or if it contains identical polar bonds in
  geometries that cancel their polarities
• Geometries that can cancel polarities of
  identical bonds are linear, trigonal planar or
  tetrahedral

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Polarity and Intermolecular Forces

• Forces between molecules are called
  intermolecular forces
• Intermolecular forces depend, in part, on the
  polarities of molecules
• Three types of intermolecular forces
  Dipole-Dipole Force, London Dispersion Force, and
  Hydrogen Bonding

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Dipole-Dipole Force
Force of attraction between opposite charges of
permanent dipoles of adjacent polar molecules
Most readily observed in properties of molecules
having same size (molecular wt.)
http//wine1.sb.fsu.edu/chm1045/notes/Forces/inter
mol/Forces02.htm
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Dipole Forces
Masterton/Hurley, Chemistry, 3rd, Saunders, 1997.
240
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States of Earth's Components

• Substances composed of ions have strong
  intramolecular forces, tend to be solids
  (lithosphere)
• Substances composed of small molecules have weak
  intermolecular forces, tend to be gases
  (atmosphere)
• Water has strongest intermolecular forces, tends
  to liquefy (hydrosphere)

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Relationships of Topics in Lecture