Transcript Intermolecular Forces

PHASE TRANSISTIONS
ALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE
AND/OR PRESSURE OF A SUBSTANCE.
SOLID TO LIQUID: MELTING
LIQUID TO SOLID: FREEZING
GAS TO LIQUID: CONDENSATION
LIQUID TO GAS: EVAPORATION
SOLID TO GAS: SUBLIMATION
GAS TO SOLID: DEPOSITION
Intermolecular Forces
London Dispersion Forces:
Also called Induced
dipole forces. An instantaneous dipole is created within
the atom or molecule via the instantaneous movement of
the electrons around the nucleus. All molecules have
LDF.
Dipole-Dipole Forces:
The attractive force
between molecules due to the existence of an overall
dipole moment. Polar molecules have d-d forces.
Hydrogen Bonding:
The attractive force between
a highly electronegative atom of one molecule with the
hydrogen on another molecule also containing a very
electronegative atom. N, O, F are the electronegative
atoms.
Interrogating interactions
Properties dependent on the Intermolecular Forces
SURFACE TENSION: describes the
resistance that a liquid has to an
increase in its own surface area.
Answers why bugs can walk on water
and why green strawberry plastic
baskets float on water dispite the 1”
open square “holes”.
• Intermolecular forces create surface
tension – an invisible “skin” holding the
molecules together
• Force must be applied to break the
“skin”
Properties dependent on the Intermolecular Forces
• EVAPORATION: Evaporation of a liquid
occurs when the average kinetic energy present
within the liquid is greater than the intermolecular
forces responsible for holding the substance in its
liquid state. When the particles have enough
kinetic energy to overcome these attractive forces,
the particles will escape from the surface to
become a gas.
Energy must be added to a system to overcome the attractive
forces that are exerted among liquid molecules. When an
equal quantity of vapor condenses to a liquid, an equal
amount of energy is released.
Explain the physical process of boiling.
At room temperature the water molecules have
enough energy to allow the particles to move past each
other but not enough to escape the surface tension.
As the temperature of water increases, the heat energy
(from the burner) is transferred to kinetic energy (for
the molecules) leading to an increase in the molecular
motion of the molecules. This action results in an
increase in the vapor pressure above the surface of the
liquid. When the vapor pressure of the water equals
the external pressure, boiling begins. Now a sufficient
amount of the molecules have enough energy to resist
the attractive forces. Bubbles of vapor are formed
throughout the liquid and these bubbles rise to the
surface to escape.
Properties dependent on the Intermolecular Forces
• VAPOR PRESSURE: When a liquid
evaporates in a closed container, the gaseous
vapor that forms at the surface of the liquid
eventually establishes an equilibrium with the
particles remaining in the liquid state. Equilibrium
is established when the rate of evaporation is
equal to the rate of condensation.
• A VOLATILE substance evaporates readily, has a low
surface tension, and a high vapor pressure at ambient
temperature. The Intermolecular forces are weak.
• A NONVOLATILE substance requires a large amount of
energy to evaporate, has a high surface tension, and a
low vapor pressure. The intermolecular forces are
strong.
THE DISSOLVING PROCESS
Intermolecular Forces: “Like dissolves like”

Miscible / Immiscible: Two liquids are miscible in each
other if they readily mix to form a uniform solution.
Two immiscible liquids will always separate out into
two distinct layers.
PROPERTIES ASSOCIATED WITH WATER
HYDRATES: Solids that contain water molecules as part of
their crystalline structure. The water in the hydrate is known
as the water of hydration or the water of crystallization.
HYGROSCOPIC: A substance is hygroscopic if it readily
absorbs water from the atmosphere and forms a hydrate.
DELIQUESCENT: A substance is deliquescent if it absorbs
water from the air until it forms a solution.
DESICCANTS: Compounds that absorb water and are used as
drying agents.
EFFLORESCENCE: The process by which crystalline
materials spontaneously lose water when exposed to air.
HYDROPHOBIC / HYDROPHILLIC: Hydrophobic refers to
nonpolar substances interacting with water. Hydrophillic
(water loving) is in reference to the interactions polar
substances have with water.
Some Physical Properties of Water
 Water is colorless, odorless, and tasteless.
 The normal boiling point is 100oC and the normal
melting point is 0oC.
 The heat of vaporization (DHvap) is 2259 J/g or 540
cal/g and the heat of fusion (DHfus) is 335 J/g or
80 cal/g.
 The vapor pressure of water at 20oC is 17.5 torr;
this is relatively low when compared to volatile ethyl
alcohol (43.9 torr) and very volatile ethyl ether (442.2 torr)
 The density of water at 4.0oC is 1.0 g/mL; the
density of ice at 0oC is 0.917 g/mL.
 The specific heat of water is 1.0 cal/g oC or 4.184
J/g oC.
The Unusual Properties of Water
 Water co-exists in all three states of matter naturally
on earth.
 The only common substance is a liquid at STP.
 As a solid, it is less dense than its liquid form, that is
“Ice floats”. Most substances contract upon
solidifying.
 It has a very high Heat Capacity. It stores a large
amount of energy with very little atomic or molecular
motion.
 It requires a lot of heat energy (enthalpy) to change
states.
 It has a high boiling point for such a low molecular
weight compound.
 It is a universal solvent, as a good dissolving medium
a large number of substances are soluble in water.
Why is Water so unusual?
The fundamental explanation for water’s unusual properties relates to the
polarity of its bonds. Polarity describes the partial charge associated with
a bond or molecule. A polar bond or molecule has a charge distribution
present (one end positively charged and the other end negatively charged)
while a nonpolar bond or molecule has no distinct charge distribution
(neutral).
Water is composed of two polar covalent O-H bonds (the difference in
electronegativity is 1.4) arranged in a “bent” molecular geometry. Each
bond has a dipole moment pointing in an overall similar direction leading to
the existence of an overall dipole moment. The oxygen atom pulls the pair
of electrons closer towards itself (making it partially negative) and further
from the hydrogen atoms (making them partially positive).
-
+
This charge distribution allows the partially positive hydrogen atoms from one
molecule to be attracted to the partially negative oxygen atom of another
molecule. This strong interlocking network between neighboring molecules is
called HYDROGEN BONDING. The ability to form strong hydrogen bonds is
the main reason for water’s unusual properties.
Why does ice float in water?
Ice floats in its own liquid due to the intermolecular force,
hydrogen bonding. As water freezes, the molecular motion
of the molecules slow down and the partial positive end
(hydrogen) of one water molecule is attracted to the partial
negative end (oxygen) of another water molecule.
Combine this event with the bent shape of water and the
molecules become arranged in a 3-D hexagonal array.
This array creates pockets of vacuum (empty space) in the
lattice structure as well as a decrease in the number of
molecules per unit volume. The mass is directly related to
the number of molecules therefore, in the solid state, since
there are less particles then there must be less mass per
unit volume therefore the solid is less dense than the
liquid.
Explain if ice will float in ethyl alcohol
(d = 0.789 g/L)?
Ice would not float in pure ethyl alcohol
because the density of water is 1.000 g/mL
which is greater than 0.789 g/mL for ethyl
alcohol. Yet since ethyl alcohol also
undergoes a small degree of hydrogen
bonding, the sinking effect is not as
dramatic as it would be with a nonpolar
substance.
Why does water have a relatively high boiling
point?
Water has a relatively high boiling
point because of the amount of
intermolecular forces present. Water
experiences LDF (London Dispersion
Forces) and d-d (dipole-dipole) forces,
along with the additional attractive
force, Hydrogen bonding. A large
amount of heat energy is required to
break all of these forces in order for a
phase transition to occur, thus the high
boiling point.
Water as a universal solvent
• Water is called the universal solvent because of its ability to
dissolve many substances. The general solubility rule is “like
dissolves like”. Since water is a polar molecule it will dissolve other
polar substances as well as ionic compounds. Water will not dissolve
or mix with nonpolar substances therefore water is immiscible in
nonpolar substances.
• Description of how water dissolves an ionic salt (like
NaCl) on the molecular level?
Although the attractive force from the partial charge of a single
polar molecule is not as strong as the charge from an ion, it is
plausible that a multitude of polar molecules could react on a single
ion effectively. The positive end (H+) of several water molecules are
attracted to the negative end of the salt crystal (Cl-) while the
negative end of several water molecules (O2-) are attracted to the
positive end of the crystal (Na+). The ionic bonds of the crystal
are weakened by the solvating effect of the water molecules and
the ions break away from the bulk crystal. The large number of
water molecules in the container prevent the salt ions from recombining.
PRACTICE PROBLEMS
1. Which contains less heat, ice at 0oC or water at 0oC?
Explain your answer.
Ice at 0oC contains less heat than liquid water at the same temperature.
Heat must be added to convert ice to water, so the water will contain that
much more additional heat energy. Also the liquid state is in motion much
more than the solid state. An increase in motion can only be accomplished
by an increase in energy.
2. On the basis of KMT, explain why vapor pressure
increases with temperature.
According to the kinetic molecular theory, the vapor pressure of a liquid should
increase with temperature because of the increase in collisions and kinetic
energy that always accompanies an increase in heat energy (temperature).
KEm = 3/2 RT. The increase in energy thus motion allows the liquid molecules to
escape (overcome the surface tension and other cohesive forces maintaining
the liquid state) from the surface of the liquid into the gas phase.
Surfactants and the laundry
• Surfactants in lungs have
two tails which makes for
a rectangular shape
• Surfactants with one tail
will arrange in a spherical
shape – micelles
• Micelles are soluble
• Detergents clean by
dissolving grease inside
the micelle