Liquid and Solution

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Chapter 17

• Liquid and Solution

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Intermolecular Forces Dipole-Dipole Forces

• Molecules with dipole moments can attract each
  other electrostatically by lining up so that the
  positive and negative ends are close to each
  other.

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Dipole-Dipole Forces

• Dipole-dipole forces are typically only about 1
  as strong as covalent or ionic bonds.

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Hydrogen Bonding

• Particularly strong dipole-dipole forces are seen
  among molecules in which hydrogen is bound to a
  highly electronegative atom, such as nitrogen,
  oxygen, or fluorine.
• The hydrogen bond has only 5 or so of the
  strength of a covalent bond.

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Relatively large electronegativity value of the
lightest elements
Nonpolar tetrahedral hydrides
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Intermolecular Forces London Dispersion Forces

• Atoms can develop a momentary nonsymmetrical
  electron distribution that produces a temporary
  dipolar arrangement of charge.
• This instantaneous dipole can then induce a
  similar dipole in a neighboring atom, leading to
  an interatomic attraction .

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The Effect of Molecular Size for London
dispersion forces

• The electrons distribution around an atom or
  molecule can be distorted is called the
  polarizability (???).
• Larger and heavier atoms and molecules exhibit
  stronger dispersion forces than smaller and
  lighter ones.
• In a larger atom or molecule, the valence
  electrons are, on average, farther from the
  nuclei than in a smaller atom or molecule. They
  are less tightly held and can more easily form
  temporary dipoles.

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The Effect of Molecular Shape for London
dispersion forces

• At room temperature, neopentane (C5H12) is a gas
  whereas n-pentane (C5H12) is a liquid.
• London dispersion forces between n-pentane
  molecules are stronger than those between
  neopentane molecules.
• The cylindrical shape of n-pentane molecules
  allows them to come in contact with each other
  more effectively than the spherical neopentane
  molecules.

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n-pentane
neopentane
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Temperature Dependence of Vapor Pressure
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Vapor Pressure
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Change of State
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• Normal melting point the temperature at which
  the solid and liquid states have same vapor
  pressure under conditions where the total
  pressure is 1 atm.
• Normal boiling point the temperature at which
  the vapor pressure of liquid is exactly 1 atm.

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The supercooling of water
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Supercooled and Superheated

• Supercooled supercooling occurs because, as it
  is cooled, the water may not achieve the degree
  of organization necessary to form ice at 0oC
  thus it continues to exist as the liquid.
• Superheated superheating can occur because
  bubble formation in the interior of the liquid
  requires that many high-energy molecules gather
  in the same vicinity.

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Phase Diagram of Water
Supercritical phase ????
P218 atm T374oC
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Phase Diagram of Carbon Dioxide
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The Thermodynamics of Solution Formation

• The cardinal rule of solubility is like dissolves
  like.
• Three distinct steps for the formation of
  solutions
• Step 1 Breaking up the solute into individual
• components.
• Step 2 Overcoming intermolecular forces in the
• solvent to make room for the solute.
• Step 3 Allowing the solute and solvent to
  interact
• to form the solution.

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• Step 1 and 2 require energy, are endothermic,
  since forces must be overcome to expand the
  solute and the solvent.
• Step 3 is usually exothermic.
• The overall enthalpy change associated with the
  formation of the solution, called the enthalpy of
  solution (?Hsoln).
• ?Hsoln ?H1 ?H2 ?H3

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The Solubility of Sodium Chloride in Water

• ?H1 is large and positive because of the strong
  ionic forces in the crystal that must be
  overcome.
• ?H2 is expected to be large and positive because
  of the hydrogen bonds that must be broken in
  water.
• ?H3 is expected to be large and negative because
  of the strong interactions between the ions and
  the water molecules.

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The dissolving process requires a small amount of
energy.
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Why is NaCl so Soluble in Water?

• Consider ?G ?H-T?S
• ?H is positive and thus unfavorable. Therefore,
  ?S must be positive and large enough to make ?G
  negative.
• ?S1 and ?S2 are positive since the solute and
  solvent are expanded.
• ?S3 would be expected to be positive in general
  case.

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Ionic Compounds in Water

• The assembling of a group of water molecules
  around the ion is an order producing phenomenon
  and would be expected to make a negative
  contribution to ?S.
• The more charge density (Z/r) an ion possesses,
  the greater this hydration effect will be.
• The smaller ions presumably are able to bind the
  hydrating water molecules more firmly and thus
  show a more negative value to ?S.

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Nonpolar Compounds in Water

• The dispersal of nonpolar solute particles in
  water can also produce negative value to ?S.
• The polar water molecules will not strongly
  hydrate the nonpolar molecules.
• Water forms a cage to isolate the nonpolar solute
  from the water bulk.

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Factors Affecting SolubilityStructure Effects
Fat soluble hydrophobic
Water soluble hydrophilic
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Factors Affecting SolubilityPressure Effect
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Henrys Law

• PkHx
• P the partial pressure of the gaseous solute
• x the mole fraction of the dissolved gas.
• kH constant
• The amount of gas dissolved in a solution is
• directly proportional to the pressure of the
• gas above the solution.

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Factors Affecting SolubilityTemperature Effect
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The Vapor Pressure of Solutions

• Nonvolatile Solute/Solvent

Raoults Law
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Raoults Law with Multiple Volatile compounds

• Ideal solution A solution, obeys Raoults law,
  formed with no accompanying energy change, when
  the intermolecular attractive forces between the
  molecules of the solvent are the same as those
  between the molecules in the separate components.

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Rauolts Law
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• 1.What is the total vapor pressure in a mixture
  of
• 50.0 g CH3OH (P 93.3 torr) and 25.0 g H2O
• (P 17.5 torr)?
• 2.In a mixture of 86.0 g C6H6 (P 93.96 torr)
• and 90.0 g C2H4Cl2 (P 224.9 torr), what is
• the total vapor pressure?

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Nonideal Solution

• Solutions that do not obey Raoults Law are
  called nonideal solutions.
• Solute-solvent interactions are significantly
  different from solute-solute and solvent-solvent
  interactions, the solution is likely to be a
  nonideal solution.
• Intermolecular forces between components in a
  dissolved solution can cause deviations from the
  calculated vapor pressure.

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Negative Deviation from Raoults Law

• Both components have a lower escaping tendency in
  the solution than in the pure liquids.

d- d
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Positive Deviation from Raoults Law

• If two liquids mix endothermically, this
  indicates that the solute-solvent interactions
  are weaker than the interactions among the
  molecules in the pure liquids.
• More energy is required to expand the liquid than
  is released when the liquids are mixed. The
  molecules in the solution have a higher tendency
  to escape than expected.

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ideal system
positive deviation
negative deviation
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Colligative Properties

• In the dilute solution, this change in solvent
• chemical potential leads to a change in the vapor
  
• pressure, the normal boiling point and the
• normal freezing point and causes the phenomenon
• of osmotic pressure.
• They depend only on the number of the solute
• particles in an ideal solution

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Phase diagrams for water
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Freezing Point Depression
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Boiling Point Elevation
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Molal Freezing Point Depression Constant
Molal Boiling Point Elevation Constant
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Osmotic Pressure

• ?MRT
• ? the osmotic pressure in atmospheres
• M molarity of the solute
• R gas law constant
• T Kelvin temperature

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ArtificialKidney
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ReverseOsmosis
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Colligative Properties of Electrolyte Solutions

• The relationship between the moles of solute
  dissolved and the moles of particles in solution
  is usually expressed by the
• vant Hoff factor

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Colloids

• A suspension of tiny particles in some medium.
• Colloidal suspensions exhibit light scattering. A
  beam of light or laser, invisible in clear air or
  pure water, will trace a visible path through a
  genuine colloidal suspension.
• Tyndall scattering is caused by reflection of the
  incident radiation from the surfaces of the
  particles, reflection from the interior walls of
  the particles, and refraction and diffraction of
  the radiation as it passes through the particles.

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• Liquid-Vapor Equilibrium at Fixed Pressure

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Distillation

• Lever Rule

X
Tie Line
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Liquid
LV
Vapor
E
D
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Dew-point(??) the temperature at which the
saturated vapor starts to condense Bubble-point(
??) the temperature at which the liquid starts
to boil
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Azeotrope(??)

• A mixture of two or more components
• which has a constant boiling point at a
  particular composition.

ideal system negative deviation
positive deviation
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Component 1 Component 2
Component Water Ethanol
Component Boiling Point (K) 373.15 351.65
Azeotrope Mol. Frac. 0.1053 0.8947
Azeotrope Temperature (K) 351.35 351.35