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Intermolecular Forces: Liquids, and Solids. Chapter 13. A Molecular Comparison of Liquids and Solids. Intermolecular Forces. Intermolecular Forces. Ion-Dipole Forces Interaction between an ion (Na + ) and a dipole (water). Strongest of all intermolecular forces. Intermolecular Forces.

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slide4

Intermolecular Forces

Ion-Dipole Forces

  • Interaction between an ion (Na+) and a dipole (water).
  • Strongest of all intermolecular forces

Chapter 13

slide6

Intermolecular Forces

Dipole-Dipole Forces

  • Interaction between an dipole on one molecule and a dipole on an adjacent molecule.
  • Dipole-dipole forces exist between neutral polar molecules.
  • Weaker than ion-dipole forces

Chapter 13

slide8

Intermolecular Forces

London Dispersion ForcesInduced Dipole – Induced Dipole

  • Weakest of all intermolecular forces.
  • It is possible for two adjacent nonpolar molecules to affect each other.
  • The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).
  • This attraction causes the electron clouds become distorted.
  • In that instant a polar molecule (dipole) is formed (called an instantaneous dipole).

Chapter 13

slide9

Intermolecular Forces

London Dispersion Forces

Chapter 13

slide10

Intermolecular Forces

Hydrogen Bonding

  • A special case of dipole-dipole forces.
  • This intermolecular force is very strong.
  • Strongest of the three Van der Waal’s forces (Hydrogen bonding, Dipole-dipole, London forces,)
  • H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N).

Chapter 13

slide11

Intermolecular Forces

Hydrogen Bonding

Chapter 13

slide12

Some Properties of Liquids

Viscosity

  • 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.

Chapter 13

slide13

Some Properties of Liquids

Surface Tension

  • The surface of a liquid behaves as a membrane or barrier.
  • This is due to the unequal attractive forces on molecules as the surface.
  • Surface molecules are only attracted inwards towards the bulk molecules.

Chapter 13

slide14

Some Properties of Liquids

Surface Tension

  • Cohesive forces bind molecules to each other.
  • Adhesive forces bind molecules to a surface.

Chapter 13

slide15

Some Properties of Liquids

Surface Tension

  • Meniscus is the shape of the liquid surface.
    • If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass).
    • If cohesive forces are greater than adhesive forces, the meniscus is curved downwards.

Chapter 13

slide16

Some Properties of Liquids

Surface Tension

Capillary Action - When a narrow glass tube is placed in water, the meniscus pulls the water up the tube.

Chapter 13

slide17

Properties of Liquids

Vaporization

  • Also called evaporation
    • A process in which a substance is transfromed from a liquid to a gas.
  • Standard molar enthalpy of vaporization (DHovap)
    • The energy required to convert one mole of a liquid at its boiling point to a gas.
  • The resulting gas will exert a pressure on a system.

Chapter 13

slide18

Properties of Liquids

Vapor Pressure

  • This is the pressure exerted by a substance in the gas phase.
  • As a liquid’s temperature increases, its vapor pressure increases.

Chapter 13

slide19

Properties of Liquids

Vapor Pressure

Volatile – A substance which has a low boiling point

Or

A substance which has a high vapor pressure at a low temperature

Chapter 13

slide20

Properties of Liquids

Vapor Pressure and Boiling Point

  • Liquids boil when the external pressure equals the vapor pressure.
  • Two ways to get a liquid to boil: increase temperature or decrease pressure.
  • Normal boiling point is the boiling point at 760 mmHg (1 atm).

Chapter 13

slide21

Properties of Liquids

Vapor Pressure and Boiling Point

  • Vapor pressure, temperature and enthalpy of vaporization can be related to each other using: Clausius-Clapeyron equation:

P = pressure

T = temperature

R = gas law

DHovap = enthalpy of vaporization

Chapter 13

slide22

Properties of Liquids

Vapor Pressure and Boiling Point

The Clausius-Clapeyron equation makes more sense when it is rearranged into the slope intercept form.

Chapter 13

slide23

Properties of Liquids

Vapor Pressure and Boiling Point

Chapter 13

slide24

Structures of Solids

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.
  • Three-dimensional stacking of unit cells is the crystal lattice.

Chapter 13

slide25

Structures of Solids

Unit Cells

Chapter 13

slide26

Structures of Solids

Unit Cells

Chapter 13

slide27

Structures of Solids

Cell Occupancy

Chapter 13

slide28

Structures of Solids

Cell Occupancy

Chapter 13

slide29

Structures of Solids

Cell Occupancy

Chapter 13

slide30

Structures of Solids

Cell Occupancy

Zn4S4 ZnS

Chapter 13

slide31

Structures of Solids

Close Packing of Spheres

  • A crystal is built up by placing close packed layers of spheres on top of each other.
  • There is only one place for the second layer of spheres.
  • There are two choices for the third layer of spheres:
    • Third layer eclipses the first (ABAB arrangement). This is called hexagonal close packing (hcp).
    • Third layer is in a different position relative to the first (ABCABC arrangement). This is called cubic close packing (ccp).

Chapter 13

slide32

Structures of Solids

Close Packing of Spheres

Chapter 13

slide33

Structures of Solids

Close Packing of Spheres

  • Each sphere is surrounded by 12 other spheres (6 in one plane, 3 above and 3 below).
  • Coordination number: the number of spheres directly surrounding a central sphere.

Chapter 13

slide34

Structures of Solids

Other Kinds of Solid Materials

Molecular Solids

These are crystalline substances in which the “building blocks” are composed of molecules in place of ions.

Example: Table Sugar

Chapter 13

slide35

Structures of Solids

Other Kinds of Solid Materials

Network Solids

These are crystalline substances in which the “building blocks” are atoms and all the atoms are connected by covalent bonds.

Example: Diamond

Chapter 13

slide36

Phase Diagrams

  • Phase diagram: plot of pressure vs. temperature summarizing all equilibria between phases.

Chapter 13

slide37

Phase Diagrams

Chapter 13

slide38

Phase Diagrams

Triple point - Temperature and pressure at which all three phases are in equilibrium.

Critical point – Point above which the liquid and gas phases are indistinguishable.

Critical temperature - The minimum temperature for liquefaction of a gas using pressure

Critical pressure - Pressure required for liquefaction

Chapter 13

slide39

Homework

2, 16, 18, 24, 30, 32, 40, 44

Chapter 13