Intermolecular Attractions -- Liquids and Solids

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Intermolecular Attractions -- Liquids and Solids

• L. Scheffler
• IB Chemistry 1-2

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Intermolecular Forces

• Intermolecular forces are the forces of
  attractions that exist between molecules
• The strength of these forces determine
• The state of matter solid, liquid, or gas
• The melting and boiling points of compounds
• The solubilities of one substance in another.

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Types of Intermolecular Forces

• Intermolecular forces include
• Hydrogen bonding
• Dipole to dipole interactions
• van der Waals forces or dispersion forces

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Hydrogen Bonding

• Hydrogen bonding occurs between polar covalent
  molecules that possess a hydrogen atom that is
  bonded to an extremely electronegative element
  specifically - N, O, and F.

Weak attractions occur between the hydrogen atoms
of one molecule and the oxygen atom of another.
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Hydrogen Bonding

• The weak attractions that result form hydrogen
  bonding cause molecules to stick together.

As a result molecules with significant hydrogen
bonding have higher melting points and boiling
points than they would otherwise have.
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Hydrogen Bonding

• Hydrogen bonds are the strongest of all of the
  intermolecular forces. They are about one-tenth
  the strength of a covalent bond .

Because hydrogen bonds must be overcome for a
substance to melt or evaporate, substances that
have significant hydrogen bonding have higher
than normal melting and boiling temperatures
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Dipole-Dipole Attractions

• If the permanent net dipole within the polar
  molecules results from a covalent bond between a
  hydrogen atom and either fluorine, oxygen or
  nitrogen, the resulting intermolecular force is
  referred to as a hydrogen bond
• If this attraction occurs between other polar
  molecules it is referred to as a dipole to dipole
  interaction

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Dipole-Dipole Attractions

• Dipole-Dipole attractions occur between molecules
  that have permanent net dipoles. (polar
  molecules),
• The partial positive charge on one molecule is
  electrostatically attracted to the partial
  negative charge on a neighboring molecule.

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Dipole-Dipole Attractions

• Some examples of molecules with dipole-dipole
  interactions include
• SCl2
• PCl3
• CH3Cl

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van der Waals Forces

• van der Waals or dispersion forces are very weak
  forces of attraction between molecules
• They result from
• momentary dipoles occurring due to uneven
  electron distributions in neighboring molecules
  as they approach one another
• the weak residual attraction of the nuclei in one
  molecule for the electrons in a neighboring
  molecule.

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Dispersion Forces

• ..

van der Waal's Forces are named after the person
who contributed to our understanding of non-ideal
gas behavior). They are also as known
dispersion forces or as London Forces (named
after Fritz London who first described these
forces theoretically in 1930)
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Dispersion Forces

• The more electrons that are present in the
  molecule, the stronger the dispersion forces will
  be.
• Dispersion forces are the only type of
  intermolecular force that operates between
  non-polar molecules
• Dispersion forces exist between non-polar
  molecules such as
• hydrogen (H2)
• chlorine (Cl2)
• carbon dioxide (CO2)
• methane (CH4)

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Dispersion Forces

• van der Waals or dispersion forces are the
  weakest of the intermolecular forces
• They are typically only 0.1 to 1 as strong as
  covalent bonds between atoms in a molecule

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London Dispersion Forces

• The van der Waals or London dispersion force is a
  temporary attractive force that occurs when the
  electrons in two adjacent atoms occupy positions
  that make the atoms form temporary dipoles.

This force is sometimes called an induced
dipole-induced dipole attraction.
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Effects of London Dispersion Forces

• London forces are the attractive forces that
  cause non-polar substances to condense to liquids
  and to freeze into solids when the temperature is
  lowered sufficiently.
• Phase changes occur when molecules are
  sufficiently close and dispersion forces are
  sufficiently strong to hold molecules together

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The Liquid State

• The liquid state of a material has a definite
  volume, but it does not have a definite shape and
  takes the shape of the container, unlike that of
  the solid state.
• Unlike the gas state, a liquid does not occupy
  the entire volume of the container if the
  container volume is larger than the volume of the
  liquid.
•  

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The Liquid State

• At the molecular level, the arrangement of the
  molecules is random, unlike that of the solid
  state in which the molecules are regular and
  periodic.
• Molecules are still closely packed but they can
  slip past each other and move around the body of
  the liquid.
• There may be some short order intermolecular
  ordering or structure, however.
•  

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Solids, Liquids and Gases

• The intermolecular forces between particles
  become stronger as particles are packed closer
  together and move less rapidly
• Energy is required to convert from solid to
  liquid to gas

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Vapor Pressure and Boiling

• Energy is required for a liquid to evaporate
• The vapor pressure of a liquid depends on the
  degree to which it will evaporate at a given
  temperature
• Liquids evaporate at the surface as long as the
  vapor pressure of the liquid is less than the
  pressure of the atmosphere above the liquid

In order to evaporate, a water molecule must have
enough energy To overcome the hydrogen bonds that
hold it in place
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Vapor Pressure curves for various liquids

• Pressure in torr

v it in place
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Vapor Pressure and Boiling

• The weaker the intermolecular forces in a
  substance, the higher its vapor pressure will be
  at any given temperature
• Volatile liquids have relatively high vapor
  pressures and hence they also have low boiling
  temperatures
• If the vapor pressure of a liquid is equal to the
  atmospheric pressure the substance will boil.

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Boiling
The boiling temperature depends on the pressure
above the liquid.
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Boiling point of water and elevation
The boiling temperature depends on the pressure
above the liquid. Atmospheric pressure decreases
with increasing elevation.
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Hydrogen bonding the boiling point
Molecules that undergo significant hydrogen
bonding tend to have much higher boiling points
than they would otherwise have.
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Characteristics of the Liquid State

• The most familiar liquid states at room
  temperature are water, alcohol, benzene, carbon
  tetrachloride, corn oil, and gasoline.
• Two elements, Bromine and Mercury are liquids at
  room temperature. A third element Gallium has a
  melting point slightly above room temperature
• Glasses, although solids, are often called frozen
  liquids, because the arrangements of molecules in
  glasses are very similar to those in liquid
  states.

Br
Ga
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Properties of liquids- Viscosity

• Viscosity of a liquid is a measure of the
  resistance of a liquid to flow,
• Viscosity is measured in N s m-2 (SI Units) or
  poise (P) or centipoise (cP).      1 P 0.1 N s
  m-2      1 cP 0.001 N s m-2

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Surface Tension

• Surface tension is the energy required to stretch
  a unit change of the surface area. Thus its units
  are N m m-2 N/m.
• There is no direct correlation between viscosity
  and surface tension. These two properties are
  independent of each other.
• The surface tension is due to the unbalanced
  force experience by molecules at the surface of a
  liquid.
• As a result of surface tension, a drop of liquid
  tends to form a sphere, because a sphere offers
  the smallest area for a definite volume.

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Surface Tension

• Substances with low surface tension have a
  tendency to form thin films.
• When detergent is added to water, it lowers the
  surface tension.
• Blowing soap water with a straw forms bubbles,
  due to the low surface tension.

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Cohesion and Adhesion

• Cohesion is the intermolecular attraction between
  like molecules,
• Adhesion is the intermolecular attraction between
  unlike molecules.
• Liquids with high surface tensions have strong
  cohesion forces, and they are poor wetting liquid
  due to low adhesion forces.
• A detergent or wetting agent is a substance that
  increases the adhesion force between two
  different materials.

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Soaps and Detergents

• Molecules of soaps and detergents have both a
  polar and an non-polar portion.

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Soaps and Detergents

• For this reason soaps and detergents are referred
  to as wetting agents.
• The wetting agent increases the wetting action of
  water with the non-polar material.
• By this action, dirt is removed when washed with
  water.

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Capillary Action

• When a small tube is dipped into a liquid, the
  level in the tube is usually higher or lower than
  that of the bulk liquid.
• If adhesion force between the tube material and
  the liquid is stronger than the cohesion force,
  the level is higher. Otherwise, the level is
  lower.
• Such phenomena are called capillary action.
• Capillary action is one of the factors
  responsible for transport of liquid and nutrients
  in plants, and sometimes in animals.

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