Intermolecular Attractions

Document Sample
Intermolecular Attractions Powered By Docstoc
					Intermolecular Attractions
-- Liquids and Solids

    L. Scheffler
    IB Chemistry 1-2




                         1
     Intermolecular Forces

 Intermolecular  forces are the forces of
  attractions that exist between molecules
 The strength of these forces determine:

    • The state of matter: solid, liquid, or gas
    • The melting and boiling points of
      compounds
    • The solubilities of one substance in
      another.


                                                   2
Types of Intermolecular
Forces
Intermolecular forces include:
   Hydrogen bonding

   Dipole to dipole interactions

   van der Waals forces or
    dispersion forces



                                    3
     Hydrogen Bonding
Hydrogen bonding occurs between polar covalent
molecules that possess a hydrogen atom that is
bonded to an extremely electronegative element;
specifically - N, O, and F.


                       Weak attractions occur
                       between the hydrogen
                       atoms of one molecule
                       and the oxygen atom of
                       another.


                                                  4
      Hydrogen Bonding
The weak attractions that result form hydrogen
bonding cause molecules to stick together.
                          As a result molecules
                          with significant
                          hydrogen bonding
                          have higher melting
                          points and boiling
                          points than they
                          would otherwise
                          have.

                                                  5
     Hydrogen Bonding
Hydrogen bonds are the strongest of all of the
intermolecular forces. They are about one-
tenth the strength of a covalent bond .



Because hydrogen bonds must be overcome
for a substance to melt or evaporate,
substances that have significant hydrogen
bonding have higher than normal melting and
boiling temperatures

                                                 6
       Dipole-Dipole Attractions
o   If the permanent net dipole within the polar
    molecules results from a covalent bond
    between a hydrogen atom and either fluorine,
    oxygen or nitrogen, the resulting
    intermolecular force is referred to as a
    hydrogen bond
o   If this attraction occurs between other polar
    molecules it is referred to as a dipole to
    dipole interaction


                            .
                                                    7
         Dipole-Dipole Attractions
o   Dipole-Dipole attractions occur between
    molecules that have permanent net dipoles.
    (polar molecules),
o   The partial positive charge on one molecule
    is electrostatically attracted to the partial
    negative charge on a neighboring molecule.



                             .
                                                    8
         Dipole-Dipole Attractions
o   Some examples of molecules with dipole-
    dipole interactions include:
     •   SCl2
     •   PCl3
     •   CH3Cl




                         .
                                              9
           van der Waals Forces
van der Waals or dispersion forces are very
  weak forces of attraction between molecules
They result from:
1.   momentary dipoles occurring due to uneven electron
     distributions in neighboring molecules as they
     approach one another
2.   the weak residual attraction of the nuclei in one
     molecule for the electrons in a neighboring molecule.




                                                             10
       Dispersion Forces
 ..
van der Waal's Forces are named after the person who
contributed to our understanding of non-ideal gas
behavior). They are also as known dispersion forces
or as London Forces (named after Fritz London who first
described these forces theoretically in 1930)




                                                          11
       Dispersion Forces
   The more electrons that are present in the molecule,
    the stronger the dispersion forces will be.
   Dispersion forces are the only type of intermolecular
    force that operates between non-polar molecules
   Dispersion forces exist between non-polar
    molecules such as
      hydrogen (H2)
      chlorine (Cl 2)
      carbon dioxide (CO2)
      methane (CH4)



                                                            12
      Dispersion Forces
van der Waals or dispersion forces are the weakest
of the intermolecular forces
They are typically only 0.1% to 1% as strong as
covalent bonds between atoms in a molecule




                                                     13
        London Dispersion Forces
The van der Waals or
London dispersion
force is a temporary
attractive force that
occurs when the
electrons in two
adjacent atoms
occupy positions that
make the atoms form
temporary dipoles.
                        This force is sometimes called
                        an induced dipole-induced
                        dipole attraction.

                                                         1
         Effects of London
         Dispersion Forces
   London forces are the attractive
    forces that cause non-polar
    substances to condense to
    liquids and to freeze into solids
    when the temperature is
    lowered sufficiently.
   Phase changes occur when
    molecules are sufficiently close
    and dispersion forces are
    sufficiently strong to hold
    molecules together
      The Liquid State
The liquid state of a material
has a definite volume, but it
does not have a definite
shape and takes the shape
of the container, unlike that
of the solid state.
Unlike the gas state, a liquid
does not occupy the entire
volume of the container if
the container volume is
larger than the volume of
the liquid.


                                 16
     The Liquid State
At the molecular level, the
arrangement of the molecules
is random, unlike that of the
solid state in which the
molecules are regular and
periodic.
Molecules are still closely
packed but they can slip past
each other and move around
the body of the liquid.
There may be some short order
intermolecular ordering or
structure, however.

                                17
       Solids, Liquids and Gases

The intermolecular
forces between particles
become stronger as
particles are packed
closer together and
move less rapidly
Energy is required to
convert from solid to
liquid to gas




                                   18
          Vapor Pressure and
          Boiling
o   Energy is required for a
    liquid to evaporate
o   The vapor pressure of a
    liquid depends on the
    degree to which it will
    evaporate at a given
    temperature
o    Liquids evaporate at the
    surface as long as the vapor     In order to evaporate, a water
    pressure of the liquid is less   molecule must have enough
    than the pressure of the         energy
                                     To overcome the hydrogen
    atmosphere above the liquid
                                     bonds that hold it in place


                                                                      19
           Vapor Pressure curves
           for various liquids


Pressure
in torr




                      v it in place




                                      20
         Vapor Pressure and
         Boiling
o   The weaker the intermolecular forces in a
    substance, the higher its vapor pressure
    will be at any given temperature
o   Volatile liquids have relatively high vapor
    pressures and hence they also have low
    boiling temperatures
o   If the vapor pressure of a liquid is equal
    to the atmospheric pressure the
    substance will boil.

                                                  21
      Boiling




The boiling temperature depends on the pressure above the liquid.

                                                                    22
       Boiling point of water and
       elevation




The boiling temperature depends on the pressure above the liquid.
Atmospheric pressure decreases with increasing elevation.
                                                                    23
       Hydrogen bonding & the
       boiling point




Molecules that undergo significant hydrogen bonding tend to have
much higher boiling points than they would otherwise have.
                                                                   24
      Characteristics of the
      Liquid State
The most familiar liquid states at room
                                              Br
temperature are water, alcohol, benzene,
carbon tetrachloride, corn oil, and
gasoline.
Two elements, Bromine and Mercury are
liquids at room temperature. A third
element Gallium has a melting point
slightly above room temperature                    Ga
Glasses, although solids, are often called
frozen liquids, because the arrangements
of molecules in glasses are very similar to
those in liquid states.
                                                        25
    Properties of liquids-
    Viscosity
   Viscosity of a liquid is a measure of
    the resistance of a liquid to flow,
   Viscosity is measured in N s m -2 (SI
    Units) or poise (P) or centipoise (cP).
        1 P = 0.1 N s m-2
        1 cP = 0.001 N s m-2




                                              26
            Surface Tension

   Surface tension is the energy required to
    stretch a unit change of the surface area.
    Thus its units are N * m m-2 = N/m.
   There is no direct correlation between
    viscosity and surface tension. These two
    properties are independent of each other.
   The surface tension is due to the
    unbalanced force experience by molecules
    at the surface of a liquid.
   As a result of surface tension, a drop of
    liquid tends to form a sphere, because a
    sphere offers the smallest area for a
    definite volume.

                                                 27
        Surface Tension
   Substances with low surface
    tension have a tendency to form
    thin films.
   When detergent is added to
    water, it lowers the surface
    tension.
   Blowing soap water with a straw
    forms bubbles, due to the low
    surface tension.




                                      28
          Cohesion and Adhesion
   Cohesion is the intermolecular
    attraction between like molecules,
   Adhesion is the intermolecular
    attraction between unlike
    molecules.
   Liquids with high surface tensions
    have strong cohesion forces, and
    they are poor wetting liquid due to
    low adhesion forces.
   A detergent or wetting agent is
    a substance that increases the
    adhesion force between two
    different materials.
                                          29
         Soaps and Detergents




Molecules of soaps and
detergents have both a polar
and an non-polar portion.



                                30
        Soaps and Detergents

   For this reason soaps and
    detergents are referred to as
    wetting agents.
   The wetting agent increases
    the wetting action of water
    with the non-polar material.
   By this action, dirt is removed
    when washed with water.




                                      31
           Capillary Action
   When a small tube is dipped into a liquid,
    the level in the tube is usually higher or
    lower than that of the bulk liquid.
    If adhesion force between the tube
    material and the liquid is stronger than the
    cohesion force, the level is higher.
    Otherwise, the level is lower.
   Such phenomena are called capillary
    action.
   Capillary action is one of the factors
    responsible for transport of liquid and
    nutrients in plants, and sometimes in
    animals.
                                                   32