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Liquids and solids

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					LIQUIDS AND SOLIDS
Chapter 10
INTERMOLECULAR FORCES
 The attractive and repulsive force between the
  atoms in the molecule are called as
  intermolecular forces.
 London forces
 London forces exist in nonpolar molecules.
 These forces result from temporary charge
  imbalances. The temporary charges exist because
  the electrons in a molecule or ion move randomly
  in the structure. The nucleus of one atom attracts
  electrons form the neighboring atom. At the same
  time, the electrons in one particle repel the
  electrons in the neighbor and create a short lived
  charge imbalance.
   These temporary charges in one molecule or atom
    attract opposite charges in nearby molecules or
    atoms. A local slight positive charge d+ in one
    molecule will be attracted to a temporary slight d-
    negative charge in a neighboring molecule.
   Dipole-dipole interactions Dipole-dipole interactions
    exist between molecules that are polar. This requires the
    presence of polar bonds and a un symmetric molecule.
    These molecules have a permanent separation of positive
    and negative charge. In the illustration the H end of HCl is
    permanently slightly positive charge. The Cl end of HCl
    has a permanent slight negative charge. the "H" in one
    molecule is attracted to the "Cl" in a neighbor. The
    intermolecular force is weak compared to a covalent bond.
    But this dipole-dipole interaction is one of the stronger
    intermolecular attractions.
HYDROGEN BONDING
   The polar molecules, such as water molecules, have a weak,
    partial negative charge at one region of the molecule, and a
    partial positive charge where the hydrogen atoms are.
   Thus when water molecules are close together, their
    positive and negative regions are attracted to the
    oppositely-charged regions of nearby molecules. This force
    of attraction, is called a hydrogen bond. Each water
    molecule is hydrogen bonded to four others.
LIQUID STRUCTURE
 Viscosity is a quantity that describes a fluid's
  resistance to flow.
 The higher the viscosity of a liquid, the more
  slowly it flows; hydrogen bonded liquids typically
  have high viscosities. Viscosity usually decreases
  with increasing temperatures.
SURFACE TENSION




   The cohesive forces between molecules down into
    a liquid are shared with all neighboring atoms.
    Those on the surface have no neighboring atoms
    above, and exhibit stronger attractive forces upon
    their nearest neighbors on the surface. This
    cause an imbalance of the intermolecular forces
    at the surface which causes surface tension.
STRUCTURES OF SOLIDS
 The ordered arrangement of atoms, molecules or
  ions in a crystalline solid means that we can
  describe a crystal as being constructed by the
  repetition of a simple structural unit.
 The crystal structure of a material or the
  arrangement of atoms in a crystal can be
  described in terms of its unit cell.
 Spheres can pack in close-packed and open (non
  close-packed) structures.
 In the cubic crystal system, there are, besides the
  close-packed structure (face-centered cubic) two
  important packings;the simple cubic structure
  and body-centered cubic structure.
                   b.c.c
                           f.c.c
    Simple cubic
COUNTING THE NUMBER OF ATOMS IN A
UNIT CELL

   The atom in a unit cell are counted by determining
    what fraction of each atom resides within the cell.
   The number of atoms in a unit cell is counted by
    noting how they are shared between neighboring
    cells.An atom at the center of a cell belongs entirely to
    that cell. For an fcc structure each of the eight corner
    atoms is shared by eight cells, so overall they
    contribute 8x 1/8=1atom to the cell.
   Each atom at the center of each of the six faces
    contributes ½ an atom so jointly they contribute
    6x1/2=3 atoms.
   Total number of atoms in a fcc unit cell is 1+3=4and
    the mass of the unit cell is 4 times the mass of one
    atom.
 Atom Location    Fraction Inside Unit Cell
 Corner          1/8
  Edge            1/4
  Face            1/2
  Anywhere else    1
CLASS PRACTICE
   The atomic radius of copper is 128pm,and the
    density of copper is 8.93g/cm³. Is the copper metal
    close packed?
BAND THEORY OF SOLIDS
    In insulators the electrons in the valence band
    are separated by a large gap from the conduction
    band, in conductors like metals the valence band
    overlaps the conduction band, and in
    semiconductors there is a small enough gap
    between the valence and conduction bands that
    thermal or other excitations can bridge the gap.
    With such a small gap, the presence of a small
    percentage of a doping material can increase
    conductivity dramatically.
DOPING OF SEMICONDUCTORS
 The addition of a small percentage of foreign
  atoms in the regular crystal lattice of silicon or
  germanium produces dramatic changes in their
  electrical properties, producing n-type and p-type
  semiconductors.
 Impurity atom with 5 valence electrons produce

  n-type semiconductors by contributing extra
  electrons.
 Impurity atoms with 3 valence electrons produce
  p-type semiconductors by producing a “hole" or
  electron deficiency.
   An impurity of valence
    five elements is added
    to a valence-four
    semiconductor in order
    to increase the number
    of free (in this case
    negative) charge
    carriers.
 An N-type semiconductor (N for Negative) is
  obtained by carrying out doping, that is, by
  adding an impurity of valence-five elements to a
  valence-four semiconductor in order to increase
  the number of free (in this case negative) charge
  carriers.
 When the doping material is added, it donates
  weakly-bound outer electrons to the
  semiconductor atoms. This type of doping agent
  is also known as donor material since it gives
  away some of its electrons.
   Consider the case of Si atom.Si atoms have four valence
    electrons, each of which is covalently bonded with one of
    four adjacent Si atoms. If an atom with five valence
    electrons, such as those from group 15 (eg. phosphorus,
    arsenic, or antimony), is incorporated into the crystal
    lattice in place of a Si atom, then that atom will have four
    covalent bonds and one un bonded electron. This extra
    electron is only weakly bound to the atom and can easily be
    excited into the conduction band. At normal temperatures,
    virtually all such electrons are excited into the conduction
    band.

   P-type semiconductor (P for Positive) is
    obtained by carrying out doping, in order to
    increase the number of free (in this case positive)
    charge carriers. When the doping material is
    added, it takes away (accepts) weakly-bound
    outer electrons from the semiconductor atoms.
    This type of doping agent is also known as
    acceptor material and the semiconductor atoms
    that have lost an electron are known as holes.
 The purpose of P-type doping is to create an
  abundance of holes. In the case of silicon, a
  trivalent atom (typically from group IIIA of the
  periodic table, such as boron or aluminium) is
  substituted into the crystal lattice. The result is
  that one electron is missing from one of the four
  covalent bonds.
 Thus the dopant atom can accept an electron
  from a neighboring atoms' covalent bond to
  complete the fourth bond. Such dopants are
  called acceptors. The dopant atom accepts an
  electron, causing the loss of half of one bond from
  the neighboring atom and resulting in the
  formation of a "hole".
PHASE CHANGES
 A phase change may be written as a chemical
  reaction. The transition from liquid water to
  steam, for example, may be written as
 H2 (l) H2 (g)
  The equilibrium constant for this reaction (the
  vaporization reaction) is
 K = Pw
 where Pw is the partial pressure of the water in
  the gas phase when the reaction is at
  equilibrium. This pressure is often called the
  vapor pressure. The vapor pressure is literally
  the partial pressure of the compound in the gas.
 The boiling point corresponds to the temperature
  at which the vapor pressure of the liquid equals
  the atmospheric pressure.
 If the liquid is open to the atmosphere, it is not
  possible to sustain a pressure greater than the
  atmospheric pressure, because the vapor will
  simply expand until its pressure equals that of
  the atmosphere.
 The temperature at which the vapor pressure
  exactly equals one atm is called the normal
  boiling point.
 In a closed container vapor is formed as the
  molecules leave the surface of the liquid. As the
  number of molecules in the vapor phase
  increases, more of them strike the surface of the
  liquid. Eventually the number of molecules
  returning to the liquid matches the number
  escaping. The liquid is in equilibrium with the
  vapor.
 H2O(l)    H2O(g)
   The van't Hoff equation provides a relationship between an
    equilibrium constant and temperature.
   ln K = - ΔHvap /R T + ΔSvap/R
   For this reaction, the equilibrium constant is simply the vapor
    pressure, P, which when substituted into the above equation
    yields the Claussius-Clapeyron equation.
   ln P = - ΔHvap /R T + ΔSvap/R
   The normal boiling point, Tbpo, corresponds to the temperature at
    which both the reactant and the product are in the standard
    state. A pure liquid under 1 atm pressure is in the standard state.
    A pure gas at 1 atm pressure is also in the standard state. Thus
    in the standard state P = 1 atm. This relation allows the
    Claussius-Clapeyron equation to be rewritten as
   ln P = - ΔHvap / R (1/T – 1/Tbp⁰)
   Tbp⁰ = - ΔHvap / ΔSvap
PHASE DIAGRAM
A phase diagram summarizes the pressures and
 temperatures at which each phase is most stable.
 The phase boundaries show the conditions under
 which two phases can coexist in equilibrium with
 each other. Three phases coexist at equilibrium at
 the triple bond. A substance cannot be converted
 to a liquid by the application of pressure if the
 temperature is above
critical temperature of
the substance.
HOME WORK
 Page 465
 10.34, 10.42,10.44, 10.66,10.72

				
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posted:3/22/2012
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