Liquids and Solids

Intermolecular Forces  Intermolecular Forces are the attraction between molecules  They vary in strength, but are generally weaker than bonds

Intermolecular Forces  Types of intermolecular forces: Dipole-Dipole Dipole-Dipole The forces of attraction between two dipolesThe forces of attraction between two dipoles These forces are responsible for the relatively high boiling points of polar moleculesThese forces are responsible for the relatively high boiling points of polar molecules Polar molecules can also induce a dipole in a nonpolar molecule by temporarily attracting its electronsPolar molecules can also induce a dipole in a nonpolar molecule by temporarily attracting its electrons

Intermolecular Forces Hydrogen Bonds Hydrogen Bonds These are not really bonds, just strong forces in which a hydrogen atom that is bonded to a highly electronegative atom (N, O, F) is attracted to an unshared pair of electrons of an electronegative atom in a nearby moleculeThese are not really bonds, just strong forces in which a hydrogen atom that is bonded to a highly electronegative atom (N, O, F) is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule These forces account for the unusually high boiling points of HF, H 2 O, and NH 3These forces account for the unusually high boiling points of HF, H 2 O, and NH 3

Intermolecular Forces London Dispersion London Dispersion These are the intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipolesThese are the intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles They act between all atoms and molecules, but are the ONLY forces among noble gas atoms and nonpolar moleculesThey act between all atoms and molecules, but are the ONLY forces among noble gas atoms and nonpolar molecules This accounts for the low boiling points of these substancesThis accounts for the low boiling points of these substances Their strength increases with increasing atomic massTheir strength increases with increasing atomic mass

Properties of Liquids  Liquids are more ordered than gases because of the strong intermolecular forces and low mobility of liquid particles  The particle mobility is why liquids and gases are fluids (because they flow)

Properties of Liquids  Liquids have a relatively high density (they are hundreds of times more dense than the gas state and only about 10% less dense than the solid state)  Liquids are much less compressible than gases because the particles are closer together

Properties of Liquids  Liquids, like gases, diffuse. However, it is much slower because the particles are closer together and the attractive forces between the particles slow their movement  If temperature increases, diffusion increases

Properties of Liquids  Surface Tension – a force that tends to pull adjacent parts of a liquid’s surface together, thereby decreasing surface area to the smallest possible size  The higher the force of attraction, the higher the surface tension

Properties of Liquids  Capillary Action – the attraction of the surface of a liquid to the surface of a solid  This attraction will pull liquid molecules upward along the surface against the pull of gravity

Properties of Liquids  Viscosity – resistance of a liquid to flow  As temperature decreases, viscosity increases

Properties of Solids  Particles in solids are more tightly packed together  Therefore, intermolecular forces are much more effective in solids, holding their particles in fixed positions, making them more ordered than liquids or gases

Properties of Solids  Solids have a definite shape and a definite volume  Solids have a definite melting point, where the kinetic energies of the particles overcome the attractive forces holding them together

Properties of Solids  Solids have a very high density because their particles are more closely packed together  For particle purposes, solids can be considered to be incompressible  The rate of diffusion in solids is millions of times slower than in liquids

Crystalline Solids  Crystalline solids exist as single crystals or groups of crystals fused together  The arrangement of particles is in a lattice  The smallest portion of the lattice is called the unit cell

Crystalline Solids  Unit cells can have one of seven types of symmetry: Cubic Cubic Tetragonal Tetragonal Hexagonal Hexagonal Trigonal Trigonal Orthorhombic Orthorhombic Monoclinic Monoclinic Triclinic Triclinic

Crystalline Solids  Crystal structures can also be described by the particles in them and the bonds between the particles  There are four different types based on this classification

Crystalline Solids  Ionic Crystals Positive and negative ions arranged in a regular pattern Positive and negative ions arranged in a regular pattern They are hard, brittle, have high melting points, and are good insulators They are hard, brittle, have high melting points, and are good insulators  Covalent Network Crystals Each atom is covalently bonded to nearest neighboring atom Each atom is covalently bonded to nearest neighboring atom They are very hard, brittle, have high melting points, usually nonconductors or semiconductors They are very hard, brittle, have high melting points, usually nonconductors or semiconductors

Crystalline Solids  Metallic Crystals Consists of metal cations surround be a sea of electrons Consists of metal cations surround be a sea of electrons Have high conductivity Have high conductivity  Covalent Molecular Crystals Consists of covalently bonded molecules held together by intermolecular forces Consists of covalently bonded molecules held together by intermolecular forces Have low melting points, easily vaporized, relatively soft, good insulators Have low melting points, easily vaporized, relatively soft, good insulators

Amorphous Solids  The atoms that make up amorphous solids are not arranged in a regular pattern  They are sometimes referred to as supercooled liquids, because they retain liquid properties at temperatures at which they appear solid  Examples are glass and plastics

Changes of State  A phase is any part of a system that has uniform composition and properties  A change of state is when a substance changes from one phase to another

Changes of State  Solid  Liquid = Melting  Liquid  Solid = Freezing  Liquid  Gas = Vaporization  Gas  Liquid = Condensation  Solid  Gas = Sublimation  Gas  Solid = Deposition

Changes of State  Boiling is the conversion of a liquid to a vapor within the liquid as well as its surface  The boiling point is the temperature at which the equilibrium vapor pressure of the liquid equals the atmospheric pressure

Changes of State  The freezing point is the temperature at which the solid and liquid are in equilibrium at 1 atm of pressure  For pure crystalline solids, the freezing point and the melting point are the same

Changes of State  At sufficiently low temperatures and pressures, liquids cannot exist  Sublimation occurs when the solid and gas phases are in equilibrium with one another  Examples are CO 2 and iodine at room temperatures

Changes of State  Deposition is the opposite of sublimation  A common example is the formation of frost on a cold surface

Phase Diagram  A phase diagram is a graph of pressure versus temperature that shows the conditions under which the phases of a substance exist

Phase Diagram  The triple point indicates the temperature and pressure conditions at which the solid, liquid, and vapor phases can coexist at equilibrium  The critical point indicates the critical temperature and pressure

Phase Diagram  Critical temperature is the temperature above which the substance cannot exist in the liquid state  Critical pressure is the lowest pressure at which the substance can exist as a liquid at the critical temperature

Properties of Water  Water is a polar covalent compound  The structure and hydrogen bonding of water account for its unique properties: Relatively high melting point and boiling point Relatively high melting point and boiling point High surface tension High surface tension High heat of vaporization High heat of vaporization High Specific heat High Specific heat Universal solvent Universal solvent