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Chapter 12 Intermolecular Attractions and the Properties of Liquids

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Chapter 12 Intermolecular Attractions and the Properties of Liquids Powered By Docstoc
					  Chapter 12: Intermolecular
Attractions and the Properties of
       Liquids and solids
• There are important differences between
  gases, solids, and liquids:
  – Gases -
  – Liquids -
  – Solids –
Properties can be understood in terms of how tightly the
molecules are packed together and the strength of the
intermolecular attractions between them.
• Intermolecular forces are:




• Intramolecular forces are:
Strong intramolecular attractions exist between H and Cl
within HCl molecules. These attractions control the chemical
properties of HCl. Weaker intermolecular attractions exist
between neighboring HCl molecules. Intermolecular
attractions control the physical properties of this substance.
• There are only a few important types of
  intermolecular forces
LIST OF INTERMOLECULAR FORCES
• Dipole-dipole attractions
  – Polar molecules tend to align their partial
    charges
  – The attractive force is about 1% of a covalent
    bond and drops off as 1/d3 (d=distance between
    dipoles)
                                    The net
                                    interaction of the
                                    imperfectly
                                    aligned
                                    molecules is
                                    attractive.
• Hydrogen bonds
  – Very strong dipole-dipole attraction that occur
    when H is covalently bonded to to a small,
    highly electronegative atom (usually F, O, or
    N)
  – Typically about ten times stronger than other
    dipole-dipole attractions
  – Are responsible for the expansion of water as it
    freezes
(a) Polar water molecule. (b) Hydrogen bonding produces
strong attractions in the liquid. (c) Hydrogen bonding
(dotted lines) between water molecules in ice form a
tetrahedral configuration.
http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/waterdens.html
10 cups of liquid water into the
freezer and what do you get…

11 cups of ice!!!
• London forces
  – The (very) weak attractions between nonpolar
    molecules
  – Arise from the interactions of instantaneous
    dipoles on neighboring molecules

                           An instantaneous dipole
                           on one molecule can
                           produce and induced
                           dipole on another. The
                           net interaction of these
                           over time is attractive.
–   These instantaneous dipole-induced dipole
    attractions are called London dispersion
    forces, London forces, or dispersion forces
–   London forces decrease as 1/d6 (d=distance
    between molecules)
–   Polarizability is a measure of the ease with
    which the electron cloud on a particle is
    distorted
–   It tends to increase as the electron cloud
    volume increases
                                 Large electron clouds are more
                                 easily deformed than small
                                 ones. The magnitude of the
                                 resulting partial charge is also
                                 larger. The larger molecules
                                 experience larger London
                                 forces than small molecules.


• The boiling point of the halogens and noble
  gases demonstrate this:
       BP( o C)       BP( o C)       BP( o C)         BP( o C)
  F2   - 188.1    Br 2 58.8      He - 268.6     Ar    - 185.7
 Cl 2 - 34.6      I2   184.4     Ne - 245.9      Xe - 107.1
                                                 Rn    - 61.8
     – London forces depend on the number of atoms
       in the molecule
     – The boiling point of hydrocarbons demonstrates
       this trend
Formula    BP at 1 atm ( o C) Formula    BP at 1 atm ( o C)
CH 4        - 161.5           C 5 H12       36.1
C2H6          - 88.6        C 6 H14         68.7
C3H8         - 42.1                          
C 4 H10        - 0.5         C 22 H 46       327
– Hexane, C6H14, (left) has a BP of 68.7oC while
  the BP propane, C3H8, (right) is –42.1oC
  because hexane has more sites (marked with *)
  along its chain where attraction to other
  molecules can occur.
– Molecular shape affects the strength of London
  forces
– More compact molecules tend to have lower
  London forces than longer chain-like molecules
– For example the more compact neopentane
  molecule (CH3)4C has a lower boiling point
  than n-pentane, CH3CH2CH2CH2CH3
– Presumably this is because the hydrogens on
  neopentane cannot interact as well as those on
  n-pentane with neighboring molecules
Space filling models of two molecules with
 formula C5H12. The H atoms in the more
 compact neopentane cannot interact as well
 with neighboring molecules as the H atoms in
 the more chain-like n-pentane.
 • Ion-dipole and ion-induced dipole attractions are
   the attractions between an ion and the dipole or
   induced dipole of neighboring molecules




(a) The negative ends of water dipoles surround a cation. (b) The
positive ends of water dipoles surround an anion. The attractions
can be quite strong because the ions have full charges.
Ion-dipole forces of attraction




             http://www.chem.purdue.edu/gchelp/liquids/ions.gif
                                          Ion-dipole
                                          attractions hold
                                          water molecules in
                                          a hydrate. Water
                                          molecules are
                                          found at the
                                          vertices of an
                                          octahedron around
                                          the aluminum ion
                                          in AlCl3·6H2O.

• It is sometimes possible to predict physical properties (like
  BP and MP) by comparing the strengths of intermolecular
  attractions
Ion-induced dipole forces of attraction
Summaryof Intermolecular Attractions
 Dipole-dipole: occur between molecules with permanent
   dipoles; about 1% - 5% of a covalent bond.
 Hydrogen bonding: occur when molecules contain N-H
   and O-H bonds; about 5% to 10% of a covalent bond.
 London dispersion: present in all substances; are weak,
   but can lead to large net attractions.
 Ion-dipole: occur when ions interact with polar molecules;
   can lead to large net attractions.
 Ion-induced dipole: occur when an ion induces a dipole
   on neighboring particle; depend on ion charge and the
   polarizability of its neighbor

				
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posted:1/28/2012
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