Download Chapter 12: Intermolecular Attractions and the Properties of Liquids

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
molecules
The attractions between
molecules
Always stronger than
intermolecular forces
Always weaker than
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.
5.
6.
7.
London Dispersion forces.(Van der waals Forces)
Ion-dipole
ion-induced dipole attractions
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:
BP( o C)
F2
- 188.1
Cl2
- 34.6
Br2
58.8
BP( o C)
He
- 268.6
Ne - 245.9
Ar
- 185.7
b-London forces depend on the number of atoms in
the molecule. The larger the molecular formula the
larger the boiling point of hydrocarbons.
Formula
o
BP, 1 atm( C)
CH 4
- 161.5
C2H 6
- 88.6
C3H 8
- 42.1
C 4 H10
- 0.5
C6 H14
68.7
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
Summary of Intermolec ular Attraction s
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 .
Heats of Vaporizati on
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:
Surface area
Strength of
intermolecular
attractions.
Molar Mass
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
Formula BP, 1 atm( o C)
Formula
BP, 1 atm
CH 4
- 161.5
C2H 6
- 88.6
H 2O
100
CH 4
- 161.5
C2H 6
- 88.6
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.