Intermolecular Attractions and the Properties of Liquids and Solids

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Chapter 12

• Intermolecular Attractions and the Properties of
  Liquids and Solids

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Gases, Liquids and Solids
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Intermolecular Attractions

• Intermolecular forces depend on distance
• Gases have very small attractive forces
• Solids/liquids have larger attractive forces
  since molecules are closer to each other
• Intermolecular forces - attractions between two
  molecules
• Intramolecular forces - chemical bonds that hold
  molecule together
• Intermolecular forces weaker than Intramolecular
  forces

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van der Waals Forces

• HCl molecules
• H and Cl atoms held tightly by covalent bond
• Strength of chemical bond keeps molecule intact
• Attractions between HCl molecules are weaker (4
  as strong)
• Attraction between molecules determine physical
  properties
• Notice disorientation!

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Dipole-dipole attractions

• HCl(g) - polar molecule with partial charges
• Polar molecules tend to line up so partial
  negative and near partial positive
• Still net attraction!! (Dipole-dipole!)
• Why weak?
• Charges associated are only partial charges
• Ordinary temperatures (Thermal energy) causes the
  dipoles to be somewhat misaligned reducing
  effectiveness of attractions

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Hydrogen Bonds

• Important Dipole-dipole attraction when hydrogen
  bonds to very small, highly electronegative atom
• Think FON (HF, OH, and NH)
• Why Hydrogen bonding?
• Ends of bond contain substantial positive and
  negative charges
• Charges highly concentrated due to small size
• Positive ends can get very close to negative of
  another molecule due to small size

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Hydrogen Bonds in Water

• In Liquid water - molecules experience hydrogen
  bonds that continually break and re-form
• As water freezes, molecules become locked and
  participate in 4 hydrogen bond
• Resulting structure has larger volume than liquid
  water
• Ice cubes float in more dense liquid

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London Dispersion forces

• Nonpolar molecules still have attraction
  (although weak) to hold substance together
• 1930 - Fritz London, German Scientist
• Nonpolar substances can still have attraction
• Atoms constantly moving
• Motion in one particle affects neighboring
  particles
• Electrons repel and push away
• At any given moment, the electron density of
  molecule can be unsymmetrical
• At particular instant, instantaneous dipole!

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Induced Dipoles

• As instantaneous dipole forms, causes electron
  density of neighbor to be unsymmetrical
• Also forms a dipole (called INDUCED DIPOLE)
• Always causes positive of one to be near negative
  of another

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• Very short lived attraction
• Dipoles vanish as they are formed but will form
  in other location
• Over period of time, there is a net, overall
  attraction but relatively weak
• Called London dispersion forces or instantaneous
  dipole-induced dipole attractions
• Distinguished from permanent dipole-dipole

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Strengths of London forces

• Measure using boiling point
• Polarizability
• Measure of the ease the electron cloud is
  distorted
• As volume of electron cloud increases,
  polarizability increases
• As atom size increase, higher London forces
• Number
• For molecules containing same elecments, London
  forces increase with number of atoms
• BPhexane gt BPpropane
• Molecular Shape

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Intermolecular forces and tightness of packing
affect the properties of liquids and solids

• Compressibility and diffusion depend primarily on
  tightness of packing
• Most physical properties depend primarily on
  strengths of intermolecular attractions
• Rate of evaporating depends on surface area,
  temperature, and strengths of intermolecular
  attractions

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Compressibility and Diffusion

• Compressibility - measure of the ability of
  substance to be forced into smaller volume
• Incompressible
• Solids and liquids have no empty volume
• Diffusion
• Quick in gases
• Slow in liquids
• Almost nonexistent in solids

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Surface tension

• A property related to the tendency of a liquid to
  seek a shape that yields the minimum surface area
• The shape with minimum surface area is sphere
• Why?
• Molecules within liquid surrounded by densely
  packed molecules
• Whereas surface molecules have neighbors beside
  and below it
• Surface molecules are attracted to fewer neighbors

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• A molecule at the surface has a higher potential
  energy than a molecule in the bulk of the liquid
• Remember a system becomes more stable when its
  potential energy decreases
• For a liquid, reducing surface area (reducing the
  number molecules at surface area) lowers
  potential energy
• Lowest energy achieved when liquid has smallest
  surface area
• Surface tension of a liquid is proportional to
  energy needed to expand surface area

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Surface tension every day

• Water in rim
• Invisible Skin
• Soap, Pepper and Water

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Wetting of surface by a liquid

• Wetting - spreading of liquid across a surface to
  form a thin film
• To occur, the intermolecular attractive forces
  between the liquid and the surface must be of
  about the same strength as forces within liquid
  itself
• Think glass coated
• SURFACTANTS - drastically lower the surface
  tension of water
• Water is wetter

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Viscosity

• Viscosity - resistance to a change in form of a
  liquid
• Internal friction of material
• Factors
• Temperature (Temp decreases viscosity increases)
• Molecular size
• Tangling
• Attractions
• Acetone vs. Ethlyene glycol

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Evaporation and sublimation

• Important factor Change of State!
• Evaporation - for liquid, tendency to undergo
  change to gas
• Sublimation - solid to gas change of state

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Evaporation and cooling

• Evaporation causes cooling effect
• Rate of evaporation per unit of surface area of a
  given liquid is greater at a higher temperature
• The weaker the intermolecular attractive forces,
  the faster the rate of evaporation at a given
  temperature

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• Change of state - substance is transformed from
  one physical state to another
• Physical equilibrium similar to chemical
  equilibrium
• Evaporation and condensation (change of vapor to
  liquid) cause equilibrium
• Evaporation increases number of molecules in
  vapor
• Condensation decreases number of molecules in
  vapor
• Melting Point - solid to liquid

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Vapor Pressure

• Vapor pressure - the pressure that molecules
  exert when a liquid evaporates
• Equilibrium vapor pressure
• Occurs in closed container
• Rates of evaporation and condensation are equal
• Concentrations of molecules in vapor remains
  constant and the vapor exerts constant pressure

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Factors that affect Vapor Pressure

• VP is solely function of evaporation per unit
  area of liquids surface
• If rate large
• Large conc of molecules in vapor state necessary
  for eq.
• Also, VP is high, and evaporation rate high
• As temp increases, rate and VP increase
• Can use VP as indication of relative strengths of
  attractive forces in liquids

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Boiling Point

• How would you check for boiling?
• Increasing heat just increases amt of bubbles
• Any pure liquid remains at constant temperature
  at boiling point
• Why do liquids boil?
• Bubbles contain vapor!!
• As liquid evaporates, pressure pushes
• Opposing force is pressure of atmosphere
• The temp at which vp of liquid is equal to
  prevailing atmospheric pressure

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• Why does water boil at lower temp in Denver than
  NY?
• Normal boiling point - boiling point of liquid at
  1 atm
• Relates to intermolecular attractions
• When attractive forces are strong, the liquid has
  low vp and therefore, must be heated to higher
  temp
• High boiling points result from strong
  intermolecular attractions

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Heating and cooling curves

• Heating Curve Application
• Heat at constant rate
• Diagram

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Le Châteliers Principle

• When a dynamic equilibrium in a system is upset
  by a disturbance, the system responds in a
  direction that tends to counteract the
  disturbance and, if possible, restore equilibrium
• Heat Liquid ? vapor
• Position of equilibrium

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Phase Diagrams

• Graphical representation of phase equilibria
• Triple point - all three phases exist
• Critical point

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Phase Diagrams for Water