intermolecular forces
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Intermolecular forces. Liquids and Solids Chapter 11. Liquids vs. Solids. Physical properties are due to intermolecular forces Understood in terms of kinetic-molecular theory Gases are highly compressible and assume the shape and volume of their container - PowerPoint PPT PresentationTRANSCRIPT
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Liquids and SolidsChapter 11
INTERMOLECULAR FORCES
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Liquids vs. SolidsPhysical properties are due to
intermolecular forcesUnderstood in terms of kinetic-molecular
theoryGases are highly compressible and assume
the shape and volume of their containerLiquids are almost incompressible, assume
the shape but not the volume of the containerSolids are incompressible and have a definite
shape and volume
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Liquids vs. SolidsSolids and liquids are condensed phases
Converting a gas into a liquid or solid requires the molecules to get closer to each other
Forces holding solids and liquids together are called intermolecular forces
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INTERMOLECULAR FORCES
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Intermolecular ForcesAttraction between molecules
Weaker than ionic or covalent bonds (16 kJ/mol vs. 431 kJ/mol for HCl)
Melting or boiling breaks intermolecular forces
Condensing forms intermolecular forcesMelting points / Boiling points reflect
strength of intermolecular forcesHigh melting/boiling points indicates strong
attractive forces
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Intermolecular ForcesVan der Waals forces exist between
neutral moleculesIncludes London-dispersion forces, dipole-
dipole forces, and hydrogen-bonding forcesIon-dipole interactions are important in
solutionsALL are WEAK electrostatic interactions
(~15% as strong as a covalent or ionic bond)
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Van der Waals ForcesIon-Dipole
Interaction between an ion and the partial charge on the end of a polar molecule (dipole)
Important in formation of solution between ionic substances in polar liquids (ex. NaCl in water)
Dipole-DipoleExist between neutral polar moleculesPolar molecules attract each otherNeed to be close together to form strong
attractionsWeaker than ion-dipole forces
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Van der Waals ForcesLondon Dispersion ForcesWeakest of all intermolecular forcesPossible for two adjacent neutral
molecules to affect each otherNucleus of one molecule (atom) attracts the
electrons in an adjacent molecule (atom)Electron “clouds” become distorted –
temporaryTemporary distortion creates an
instantaneous dipoleOne instantaneous dipole can create an
instantaneous dipole in a nearby molecule (atom)
Temporary dipoles attract each other
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Van der Waals ForcesLondon Dispersion ForcesMolecules must be very close together for
these attractive forces to occurPolarizability is the ease with which an
electron cloud can be deformedThe larger the molecule- the more polarizable
it isForces increase as molecular weight
increasesForces depend on the shape of the
molecule
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Van der Waals ForcesHydrogen BondsBoiling points of compounds with hydrogen
bonded to an electronegative atom are abnormally high
Special case of dipole-dipole interactionsRequires:
H bonded to a small electronegative elementAn unshared pair of electrons on a nearby
small electronegative atom/ionHydrogen only has one electron, so in an
electronegative bond it is “electron bare”
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PROPERTIES IN LIQUIDS
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Properties in LiquidsViscosity
Viscosity is the resistance of a liquid to flow.A liquid flows by sliding molecules over each
other.The stronger the intermolecular forces, the
higher the viscosity.Surface Tension
Bulk molecules (those in the liquid) are equally attracted to their neighbors.
Surface molecules are only attracted inwards towards the bulk molecules
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Surface Tension
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Surface Tension Surface tension is the amount of energy required
to increase the surface area of a liquid.
Cohesive forces bind molecules to each other. Adhesive forces bind molecules to a surface
Meniscus is the shape of the liquid surface. Adhesive > Cohesive : U-shaped meniscus (water)
Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube
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PHASE CHANGES
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Phase Changes
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Enthalpy of Phase ChangesSublimation: Hsub > 0 (endothermic). Vaporization: Hvap > 0 (endothermic).Melting or Fusion: Hfus > 0
(endothermic).
Deposition: Hdep < 0 (exothermic). Condensation: Hcon < 0 (exothermic).Freezing: Hfre < 0 (exothermic).
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Heating Curves Plot of temperature change versus heat added is a
heating curve.
During a phase change, adding heat causes no temperature change.These points are used to calculate Hfus and Hvap.
Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. Achieved by keeping the temperature low and
increasing kinetic energy to break intermolecular forces.
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Critical Temperature and PressureGases liquefied by increasing pressure at
some temperature.
Critical temperature: the minimum temperature for liquefaction of a gas using pressure.
Critical pressure: pressure required for liquefaction.
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Vapor Pressure on a Molecular Level
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Vapor PressureDynamic Equilibrium: the point when as many
molecules escape the surface as strike the surface. Vapor pressure is the pressure exerted when the
liquid and vapor are in dynamic equilibrium.
Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid
evaporates. Volatile substances evaporate rapidly. The higher the temperature, the higher the average
kinetic energy, the faster the liquid evaporates.
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Vapor Pressure and Boiling PointLiquids boil when
the external pressure equals the vapor pressure.
Temperature of boiling point increases as pressure increases.
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PHASE DIAGRAMS
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Phase Diagrams
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Phase DiagramsWater vs. Carbon Dioxide
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SOLIDS
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Unit Cells Crystalline solid: well-ordered, definite
arrangements of molecules, atoms or ions. Crystals have an ordered, repeated structure.
The smallest repeating unit in a crystal is a unit cell. Unit cell is the smallest unit with all the symmetry
of the entire crystal.
Three-dimensional stacking of unit cells is the crystal lattice.
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Unit Cell vs. Lattice
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Three common types of unit cell.Primitive cubic, atoms at the corners of a simple
cube, each atom shared by 8 unit cells;
Body-centered cubic (bcc), atoms at the corners of a cube plus one in the center of the body of the cube, corner atoms shared by 8 unit cells, center atom
completely enclosed in one unit cell;Face-centered cubic (fcc), atoms at the corners of a
cube plus one atom in the center of each face of the cube, corner atoms shared by 8 unit cells, face atoms shared by 2
unit cells.
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Unit Cells
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Solids: Four Types Molecular (formed from molecules) - usually soft
with low melting points and poor conductivity.
Covalent network (formed from atoms) - very hard with very high melting points and poor conductivity.
Ions (formed from ions) - hard, brittle, high melting points and poor conductivity.
Metallic (formed from metal atoms) - soft or hard, high melting points, good conductivity, malleable and ductile.
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Covalent Network Solid
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Ionic Lattice CsCl Structure
Cs+ has a coordination number of 8.
Cation to anion ratio is 1:1. Zinc Blende Structure (ZnS).
S2- ions adopt a fcc arrangement. Zn2+ ions have a coordination
number of 4. The S2- ions are placed in a
tetrahedron around the Zn2+ ions. Fluorite Structure (CaF2).
Ca2+ ions in a fcc arrangement. There are twice as many F- per
Ca2+ ions in each unit cell.