Transcript Intermolecular Forces - Los Angeles City College

Summary of the three States of Matter
ALSO CALLED PHASES,
HAPPENS BY CHANGING THE TEMPERATURE AND/OR PRESSURE OF A SUBSTANCE.
GAS: total disorder; mostly empty space; particles have
complete freedom of motion (vibrational, rotational, &
translational); particles are very far apart.
 Cool or compress (increase pressure) a gas to make a liquid
 Heat or reduce pressure of a liquid to make a gas
LIQUID: Disorder; particles or clusters of particles are
free to move relative to each other (vibrational &
rotational); particles are relatively close to each other.
 Cool or compress (increase pressure) a liquid to make a solid
 Heat or reduce pressure of a solid to make a liquid
SOLID: order ranges from amorphous(slightly
disordered) to crystalline (ordered); particles are
essentially in fixed positions (vibrational only); particles
are close to each other.
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.
Intermolecular Forces
Hydrogen Bonding: A special case dipoledipole force. 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.
Ion-dipole forces: This describes the attraction
between the dipole of a polar molecule and the
charge of an ion. The positive ion of the salt is
attracted to the partially negative end of a polar
molecules while the negative ion is attracted to the
positive end of the polar molecule. This force is
weaker than ionic bonding but if the amount of
ion-dipole attractions are collectively large then it
may disrupt the ionic bond of the crystal lattice.
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”.
If the intermolecular forces between liquid particles are strong then the particles will be
more attracted to each other, the forces at the surface pull inward, leading to a reduction
in the liquids surface area and thus a more spherical shape. This type of liquid is
referred to as having a high surface tension. Hg > H2O > O2
A substance with a low surface tension does not exhibit much curvature because the
forces between particles is weak and less energy is needed to move these particles
elsewhere. A substance with a low surface tension will evaporate readily.
CAPILLARY ACTION: describes the attraction the liquid particles have to itself
relative to the attraction the liquid particles have to the wall of the tube. If the
liquid particles are more attracted to the wall particles than to itself, then a
meniscus (curvature similar to upside down contact lenses) is formed in the
tube.
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.
• 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.
Properties dependent on the Intermolecular Forces
•
Compare the relative surface tension, vapor pressure, volatility, and capillary action of
the following substances. Justify your answers in terms of intermolecular forces.
a) H2O vs. CH4
The first step is to draw the Lewis Structure, determine the VSPER geometry,
and then draw the dipole moments.
H2O is bent with two pairs of nonbonding electron and two bonding pairs of
electrons and a bond angle less than 109.5o. A dipole moment exists between
each H-O bond in the same direction thus an overall dipole can be draw with
the hydrogen end being slightly positive and the oxygen end being slightly
negative.
Now intermolecular forces can be assigned to water. Since all molecules
have LDF (London Dispersion Forces) so does water. Since and overall
dipole moment could be drawn, water also exhibit d-d (dipole -dipole) forces.
Water contains hydrogen atoms bonded to highly electronegative oxygen
atoms so this molecule also exhibits hydrogen bonding.
Methane, CH4, has a tetrahedral molecular geometry and four equivalent
dipole moments along the C-H bond, the central carbon atom is slightly more
negative than the slightly positive equidistant hydrogen atoms. An overall
dipole can not be draw due to symmetry therefore CH4 experiences only LDF.
Since liquid water exhibits more intermolecular forces
than liquid methane, the relatively nonvolatile water
would have a greater surface tension, lower vapor
pressure, and greater capillary action than methane.
b) HCN vs. NH3
Linear HCN has
LDF and d-d forces
while trigonal
pyramidal ammonia
exhibits LDF, d-d
forces, and
hydrogen bonding.
Pure liquid NH3 is
expected to have
a greater surface
tension and
capillary action,
be less volatile,
and have a lower
vapor pressure
than pure liquid
HCN.
Boiling & Boiling Point
• A pure liquid will boil when enough external energy (from the
surroundings) is applied (an increase in temperature) to the liquid
(called the system; an endothermic process) so that the vapor
pressure of the liquid is equal to the external pressure above the
liquid’s surface. Once the two pressures are equal, escape is possible
because the particles have acquired enough kinetic energy to
overcome the intermolecular forces once holding them in the liquid
state. The exact temperature at which the two pressures are equal is
referred to as the “Boiling Point” and if the pressure is 1 atm (760
mmHg) then the temperature is called the “normal Boiling Point”.
• Imagine the boiling process on a microscopic level. At ambient
temperature the liquid’s particles have a certain amount of energy
and motion. As heat energy is applied to a liquid, some of the heat
energy is transferred to the particles, leading to an increase in the
particles kinetic motion. These more energetic particles eventually
acquire sufficient energy to overcome the intermolecular forces
present and the particles break through the surface to exist as a
“gas”.
Freezing & Freezing Point
• A pure liquid will freeze when enough internal energy is removed
from the system to the surroundings, this is usually initiated by a
decrease in the surrounding’s temperature (an exothermic
process). The flow of energy from the system to the surrounding
leads to a reduction in the internal kinetic motion of the particles
so now the particles do not have adequate internal energy to
resist the intermolecular forces inherent to the system. The
attracted particles move closer together to form a rigid or semirigid arrangement with very little molecular motion (a solid).
• The exact temperature at which the solid phase is in equilibrium
with the liquid phase is referred to as the “Freezing or Melting
Point” and if the pressure is 1 atm (760 mmHg) then that
temperature is called the “normal Freezing/Melting Point”.
PRACTICE PROBLEMS
1. List the intermolecular force(s) that pertain to the following
compounds.
A) C6H6
B) NH2CH2NH2
C) CH3CH2NH2
D) HCN
E) CH4
A) C6H6
B) NH2CH2NH2
C) CH3CH2NH2
D) HCN
E) CH4
LDF LDF, d-d, Hbond LDF, d-d, Hbond LDF, d-d LDF
2. Predict the relative surface tension, vapor pressure, volatility, and
boiling point of the following substances by placing them in order of
most to least.
surface tension:
most:
B
______
C
______
D
______
A
______
E least
______
A
______
D
______
C
______
B
______
least
A
______
D
______
C
______
B least
______
______
C
______
D
______
A
______
LOW
E
vapor pressure:
most:
E
______
volatility:
most:
E
______
boiling point:
high:
______
B