Condensed Phases. Explaining the physical properties of gases was simplified since the interactions between the gas molecules could be ignored.

Transcription

1 Condensed Phases Explaining the physical properties of gases was simplified since the interactions between the gas molecules could be ignored. However the particles in a liquid or solid are fairly close together and their mutual interactions cannot be ignored. Liquids and solids are called CONDENSED PHASES and intermolecular forces will play a major role in explaining their physical properties.

2 Non-Bonding Interactions Chemical Bonds: Intramolecular interactions Covalent: Sharing of atomic electrons Ionic: Transfer of atomic electrons and subsequent attraction Non-Bonding interactions do not result in the formation of a chemical bond. These are intermolecular interactions arising from intermolecular forces. Ion-dipole Dipole-dipole London dispersion forces Hydrogen bonding

3 Ion-Dipole Interactions A Coulombic Interaction between an ion and the dipole moment of a molecule. Dipole moments occur when the charge distribution in a molecule is asymmetric. Molecules with a dipole moment are called polar molecules.

4 Dipole-Dipole Interactions Occur between two polar molecules Not as strong as ion-dipole interaction Depends on the magnitude of the dipole moments Manifested in the boiling points of polar molecules

5 London Dispersion Forces Accounts for intermolecular forces in atoms and non-polar molecules Attractive force between the induced dipole moments Strength depends on the polarizability of the atom or molecule Polarizability depends on the size of the atom or molecule

6 Hydrogen Bonding Strongest of the intermolecular forces Occurs when a hydrogen atom lies between two strongly electronegative atoms Results when the positive charge on the hydrogen atom is attracted to the lone pair electrons on the electronegative atom The electronegative atoms are usually F, O, N

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8 Boiling Point The temperature at which the vapor pressure of the liquid is equal to the external pressure acting on the liquid

9 Properties of Liquids Viscosity: a measure of resistance that one part of a fluid offers to the flow of another part Depends on the magnitude of the attractive intermolecular forces in the liquid Measured in units of poise or centipoise Inversely proportional to temperature

10 Surface Tension: energy required to increase the surface area of a liquid by a given amount. Measured in J/m 2 Depends on the magnitude of the attractive intermolecular forces in the liquid

11 Adhesion: interaction between the molecules of the liquid and the surface of the container Depends on the magnitude of the intermolecular forces between the molecules in the liquid and the atoms in the surface of the container Shape of meniscus is the result of a competition between the attractive forces between the fluid molecules and those between the fluid and the surface of the container Water: meniscus curved upwards Mercury: meniscus curved downwards Capillary action

12 Vapor Pressure Vapor pressure of a liquid is the pressure exerted by the vapor in equilibrium with the liquid. Equilibrium is dynamic.

13 Vapor pressure Increases with increasing temperature Liquids that evaporate readily are volatile

14 Phase Changes Associated energy changes Enthalpy of Vaporization HVap = HVapor Hliquid Enthalpy of Fusion HFusion = Hliquid Hsolid

15 Clausius-Claperon Equation H Vap ln P = + constant RT H = slope x R Vap Vapor pressure vs T data for ethanol H Vap( CH3CH 2OH ) = kJ / mol

16 Phase Diagrams Graphical summary of the T&P at which a given phase of a substance is stable AB AC AD Vapor pressure curve of liquid Vapor pressure curve of solid Variation of melting point with Pressure Point A Triple point

17 Structure of Solids Crystalline: Atoms, ions or molecules are ordered in well defined arraignments External morphology: Flat surfaces or faces that make definite angles with one another Quartz, Diamond, Amethyst, etc. Amorphous: Atoms, ions or molecules have no orderly relationship to one another Glass, rubber, etc.

18 Unit Cell The order of a crystalline solid allows one to describe it in terms of Crystal Lattice: a three dimensional array of points, each of which represents a identical environment in the crystal Unit Cell: the repeating unit of the crystal Form the entire crystal by repeating the contents of the unit cell on the crystal lattice

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20 Unit Cells Only 7 types allowed by symmetry Simplest is cubic All sides are of the same length and all faces make angles of 90 degrees with one another Three types of cubic unit cells Primitive cube Body-centered cubic Face-centered cubic

21 Unit Cells (Continued) Some atoms in a unit cell are shared between adjacent cells

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23 Crystal Structure of NaCl

24 Density of LiF Face centered cubic unit cell Sides are 4.02 Angstroms What is the density of solid LiF?