Liquids, Solids and
Intermolecular Forces
The forces that hold molecules together are called intermolecular forces.
ion –ion (interactions)
forces
ion –dipole forces
exist between an ion and a polar molecule
Intermolecular forces as a whole are usually called van der Waals forces
Dipol –dipol forces
exist between polar molecules
London dispersion
forces
exist between nonpolar molecules
Hydrogen bonding
exist in the molecules that have a hydrogen atom bonded
to an electronegative atom having unshared pair of
electron like nitrogen, oxygen and fluorine.
Intermolecular forces as a whole are usually called van der Waals forces
Dipol –dipol forces
exist between polar molecules
London dispersion
forces
exist between nonpolar molecules
Hydrogen bonding
exist in the molecules that have a hydrogen atom bonded
to an electronegative atom having unshared pair of
electron like nitrogen, oxygen and fluorine.
• Question
What types of molecular forces are present in H2
CCl4 SCO NH3
-The stronger intermolecular forces, the higher the melting/boiling point of
subtances
-Dispersion (london) forces exist between all molecules
-As molar mases increases dispersion forces becomes stronger and the
boiling point of subtances increase
• Question
List the following subtances in order of
increasing boiling points?
BaCl2 H2 HF CO Ne
List the following subtances in order of increasing
boiling points?
CCl4 Cl2 HF ClNO N2
Compare the boiling points of ethanol and dimetileter (they
both have the same simple formula C2H6O , MA=46,07)
CH3CH2OH
CH3OCH3
Compare the boiling points of CH3OCH3 (MA=46) and H2O (MA=18)
Some Properties of Liquids and solids
Solids are incompressible, do not flow. In a solid the structural
particles (atoms, ions or molecules) are in direct contact and
intermolecular forces hold them into a fixed volume and
shape.
Liquids are incompressible, flow readily. In a liquid atoms or
molecules are close together and intermolecular forces are
strong enough to hold them in a fixed volume but not a
definite shape.
Phase changes.
Melting
solid
Liquid
Freezing
liquid
Solid
Vaporization
liquid
Gas
Condensation
gas
Liquid
Sublimation
solid
Gas
Deposition
gas
Solid
Phase diagrams
The Triple point
is a point on a phase diagram which all three phases
(liquid, solid and vapor) are in equilibrium with each
other
The critical point
is a point at which the liquid and vapor become
indistiguishable. The density of liquid decrease, the
surface tension of the liquid approaches zero. The
meniscus between the liquid and vapor disappears.
The critical
temperature
The temperature at the critical point Tc. above Tc the
liquid phase of a pure subtances can not exist.
The critical
pressure
The pressure at the critical point Pc
Supercritical Fluids
Carbon dioxide
Iodine
Some Properties of Liquids
Cohesive Forces
Intermolecular forces between like molecules.
Adhesive Forces
Intermolecular forces between unlike molecules.
Surface Tension
The resistance of a liquid to spread out and increase its surface
area. Energy or work required to increase the surface area of a
liquid.
Viscosity
is an internal resistance to flow liquids with strong intermolecular
attractions are generally more viscous than liquids with weak
intermolecular attractions.
Surface Tension
The resistance of a liquid to spread out
and increase its surface area.
Energy or work required to increase the
surface area of a liquid.
liquid contains some molecules in the gaseous or
vapor state,
the partial pressure of these gaseous molecules in
equilibrium with a liquid is called vapor
pressure.
The vapor pressure of a liquid always increases as temperature
rises
• Question: an organic compound has a vapor pressure 0f 91.0
mmHg at 40°C. Calculate its vapor pressure at 25 °C, taking
the heat of vaporization to be 5.31 x 104 J.mol
• Question: the vapor pressure of napyhalene (C10H8)at 25 °C
is 0.300mmHg,
How many napthalene will sublime into an evacutaed 500.0ml
flask?