Transcript Intermolecular Forces, Liquids,and Solids

Intermolecular Forces,
Liquids, and Solids
Chapter 11 Brown-LeMay
I. Kinetic Molecular Description
Intermolecular forces (I.F.) – attractive
electrostatic interactions that occur
between molecules, atoms, or ions of a
substance.
Liquids – I.F. are strong enough to hold
sub together but weak enough to allow
movement
Solids – I.F. are strong so no movement of
molecules occurs (they vibrate)
Types of solids
Crystalline solids – molecules and ions
arranged in repeating patterns
Amorphous solids – molecules and ions
arranges in random fashion
II. Intermolecular Forces tend to be weak
Type of Interaction
Aprox. E kg/mol
Van der Waals (dd,ld)
I.F.
Hydrogen Bonding
I.F.
Ionic Chem Bond
0.1 - 10
Covalent Chem Bond
100 - 1000
10 - 40
100 - 1000
A. Importance of I.F.
The stronger the I.F. the greater the boiling
and melting point and the lower the vapor
pressure – the weaker the I.F. the lower
the melting and boiling point and the
higher the vapor pressure
Woops Debbie Deeeeeeeee’s
B. Van der Walls forces
– are all electrostatic2-types
1. Dipole-Dipole (D.D.)
Debbie De – result
from the tendency of
polar molecules that
position themselves
so that the pos. and
neg. ends of different
molecules are near to
each other
Debbieeeeeeeee D
For molecules with the same approximate
mass and size I.F. increases with
increasing polarity
2. London forces (dispersion forces-L.D.) in
non-polar molecules no D.D. forces exist –
small dipoles exist because of electron
movement – position of electrons in
clouds- all molecules and compounds
have these forces.
London Dispersion Forces
London Dispersion Forces
L.D. forces ten to increase with increasing
molecular weight
Polarizable- larger atoms have electron
clouds that are easily distorted
3. Hydrogen Bonding (x – oxygen very
negative atom – y- hydrogen)
 Exists between a
hydrogen atom
covalently bonded to
a very electronegative
atom X, and a lone
pair of electrons on
another small,
electronegative atom
Y.
Hydrogen Bonding
 The hydrogen bond in
water may be
explained in part on
the basis of the dipole
moment of the –O—H
when the electrons in
the hydrogen atom
are furthest away
from the oxygen atom
 X & Y atoms are
usually F, N, or O
Strength of Hydrogen Bonding
 H---N < H---O < H--F
4. Ion-dipole forces –
attraction between an
ion and an opposing
charge pole of a
molecule
III. Properties of Liquids
 A. Viscosity – resistance of a liquid to flow
Ex. Motor oil – SAE (society of automotive
engineers) viscosity of motor oil increases as
temperature increases – SAE ratings are for 0oF
or -18oC increased ratings indicate greater
viscosity for a given tempt.
The greater the rating the thicker the oil.
SAE 10w/40 behaves like a 10 in the winter and
a 40 in the summer. The thicker grade is needed
in the summer because the oil is heated and
becomes thinner.
B. Surface Tension
 The energy required
to increase the
surface area of a
liquid by a unit
amount (1m2)
 Why do liquids have
this property?
 molecules at the
surface experience a
net force towards the
middle
 Molecules at the
center experience no
net force
Surface Tension Forces
 Cohesive - bind like
molecules together
 Adhesive – bind a
substance to a
surface
 Meniscus-curved
surface of a liquid
caused by the
combination of
cohesive forces
between the liquid
and container
Surface Tension Forces
 Convex Meniscus ex
mercury cohesive
forces are greater
 Concave Meniscus ex
water adhesive forces
greater
IV. Changes of State
A. Phase changes or transitions –
substance structure is altered
 solid liquidgas
More ordered to less ordered state energy
is supplied to overcome IF (endothermic)
Less ordered to more ordered state
energy is released (exothermic)
B. Energy Changes and Changes of State
liquid-gas- vaporization
solid- liquid- melting
solid- gas- sublimation
endothermic processes
gas- Liquid condensation
liquid-solid- freezing
gas – solid deposition
exothermic processes
Gas
Gas
liquid
Solid
Transitions between solid, liquid, and
gaseous phases typically involve large
amounts of energy compared to the
specific heat. If heat were added at a
constant rate to a mass of ice to take it
through its phase changes to liquid water
and then to steam, the energies required
to accomplish the phase changes (called
the latent heat of fusion and latent heat of
vaporization ) would lead to plateaus in the
temperature vs. time graph. The graph
below presumes that the pressure is one
standard atmosphere.
B-C melting, D-E boiling, heat is added
but no change in temperature occurs
because all the energy is used to break IF.
Less energy is required to freeze water
(delta H fusion) than to vaporize it (delta H
vaporization)
The energy required by the 5 processes
warming the solid(1), melting(2), warming
the liquid(3), boiling(4), and warming the
gas(5) is determined by the identity of the
substance ant the amount of the sample
present.
Warming processes q=s x m x delt t
s= specific heat- different for each
substance
Phase transitions q = (number of moles) x
delta H
Heating curve for water