Chapter 12 Intermolecular Attractions and the Properties of Liquids by hcj

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									                 Intermolecular Forces
Important differences between gases, solids, &liquids:
  – Gases - expand to fill their container.
  – Liquids - retain volume, but not shape.
  – Solids – retain volume and shape.

  Question :Why ??

Properties can be understood in terms of how tightly
  the molecules are packed together and the strength
  of attractions between them.
State the differences between
      Intermolecular &
   Intramolecular forces.
The differences between Intermolecular & Intramolecular forces


 Intramolecular forces          Intermolecular forces


 The attractions within         The attractions between
 molecules                      molecules

 Always stronger than           Always weaker than
 intermolecular forces          Intramolecular forces

 Control the chemical         Control the physical properties
 properties of the substance. of the substance.
Which of these are intermolecular forces ???
1.Metallic bond
2.Ionic bonds
3.Dipole-dipole attractions.
  – 1 % - 5% of strength of covalent bond.
4.London Dispersion forces.(Van der waals Forces)
5.Ion-dipole
6.ion-induced dipole attractions
7. Hydrogen bonding .
  – 5- 10 % of strength of covalent bond
1- Metallic Bond




2-Ionic Bond
3-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/d 3 (d=distance between dipoles).
4-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/d 6 (d=distance between
    molecules)
Strength of London forces depends on three factors:
a. 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.
b. Number of atoms in the molecule
c. Molecular shape
                              a- Polarizability :Large electron
                              clouds are more easily deformed
                              than small ones. The magnitude
                              of the resulting partial charge is
                              larger. The larger molecules
                              experience larger London forces
                              than small molecules.


Boiling point of halogens and noble gases demonstrate this:
b-London forces depend on the number of atoms in
  the molecule. The larger the molecular formula the
  larger the boiling point of hydrocarbons.
Hexane, C6H14, (right) has a BP of 68.7oC while the BP propane,
  C3H8, (left) is –42.1oC because hexane has more sites (marked
  with *) along its chain where attraction to other molecules can
  occur.
c-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 . Both have 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.
5-Ion-dipole and
6-ion-induced dipole attractions are the attractions between
  an ion and the dipole or induced dipole of neighboring
  polar 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.
7-Hydrogen bonding
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 10 times stronger than other
    dipole-dipole attractions .
  – Are responsible for making water a liquid , for
    high boiling point of water & 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.
Electronegativity
describe the relative attraction of an atom for the
 electrons in a bond. The element with the larger
 electronegativity will carry the partial negative charge.
 In general, electronegativity increases bottom to top in
 a group and left to right in a period. Order of
 electronegativity F > O >N
1. Dipole-dipole: occur between molecules with
   permanent dipoles; about 1% - 5% of a covalent
   bond.
2. London dispersion: present in all substances; are
   weak, but can lead to large net attractions.
3. Ion-dipole: occur when ions interact with polar
   molecules; can lead to large net attractions.
4. Ion-induced dipole: occur when an ion induces a
   dipole on neighboring molecule; depend on ion
   charge and the polarizability of its neighbor.
5. Hydrogen bonding: occur when molecules contain
  N-H , O-H & F-H bonds; about 5% to 10% of a
   covalent bond.
Heat of vaporization (∆Hvap ) The liquid begins to
  evaporate in the closed container. Avg. kinetic energy
  of remaining molecules is less, so the temperature is
  lower. Dynamic Equilibrium .
The molar heat of vaporization or enthalpy (∆Hvap )
 heat absorbed when 1 mole of liquid is changed to 1mole of
  vapor at constant T&P. It is measured at the normal
  boiling point of a substance

• The rate of evaporation depends on:
Thank you Wikipedia encyclopedia
Thank you Wikipedia encyclopedia
It is directly proportional to strength of intermolecular
   attractions between molecules. It increases with
   molecular mass.
Example:
The molar heat of vaporization of water is 40.6 kJ/mol.
   How many kJ of heat energy are required to convert
   1.0L of water to steam?
Solution:
1.0L=1000 mL x1.0 g/mL = 1000 g
1000 g x 1mol x 40.6 kJ = 2260 kJ of heat .
             18g         mol
Factors that affect Vapor pressures :
1-Increasing temperature. It increases the amount of vapor
   and decreases the amount of liquid. At higher
   temperature, the total fraction of molecules with kinetic
   energy large enough to escape to vapor phase is larger
   so the rate of evaporation is larger.
2- Vapor pressure increases with decreasing intermolecular
   forces.
Factors that do not affect Vapor pressures:
 Volume changes can effect vapor pressure for a short time
   then equilibrium is re-established and the vapor pressure
   returns to its initial value
 Vapor pressures of Solids:
• Solids also have vapor pressures. The pressure of the
  vapor that in equilibrium with the solid is called the
  equilibrium vapor pressure of the solid.
• At a given temperature, some of the solid particles have
  enough kinetic energy and escape into the vapor phase
• The molar heat of fusion(∆Hfus) is the heat absorbed
  by one mole of solid when it melts to give a liquid at the
  same temperature and pressure.
• The molar heat of sublimation (∆Hsub) is the heat
  absorbed by one mole of a solid when it sublimes to
  give one mole of vapor at constant T & P.
• All of these quantities tend to increase with increasing
  intermolecular forces.

								
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