Chapter 10 - Liquids and Solids

10.1 - Intermolecular Forces

• Remember that temperature is a measure of the random motions of the particles in a substance
• If energy is added, the motions of the molecules increase, and they eventually achieve the greater movement and disorder characteristic of liquid water
• Dipole-dipole forces are forces that act between a polar molecule
  • Molecules with dipole moments can attract each other electrostatically by lining up so that the positive and negative ends are close to each other
  • Dipole-dipole forces are typically only about 1% as strong as covalent or ionic bonds, and they rapidly become weaker as the distance between the dipoles increases
  • Strong dipole forces are seen among molecules in which hydrogen is bound to a highly electronegative atom, such as nitrogen, oxygen, or fluorine
  • Two factors account for the strengths of these interactions: the great polarity of the bond and the close approach of the dipoles
• Hydrogen bonding has a very important effect on physical properties
  • One factor is the relatively large electronegativity values of the lightest elements in each group,
  • The second factor is the small size of the first element of each group
  • Hydrogen bonding is also important in organic molecules
  • The alcohols methanol and ethanol have much higher boiling points than would be expected from their molar masses because of the polar O---H bonds in these molecules, which produce hydrogen bonding
• London dispersion forces: Forces that exist among noble gas atoms and non-polar molecules
• The dispersion forces in molecules with large atoms are quite significant and are often actually more important than dipole-dipole forces

10.2 - The Liquid State

• Liquids exhibit many characteristics that help us understand their nature. We have already mentioned their low compressibility, lack of rigidity and high density compared with gases
• Many of the properties of liquids give us direct information about the forces that exist among the particles
• For a given volume, a sphere has a smaller surface area than any other shape
• Surface tension: The resistance of a liquid to an increase in its surface area
• A non-polar liquid such as mercury shows a convex meniscus
• The characteristic of a liquid, in which the cohesive forces, are stronger than the adhesive forces toward the glass
  • Another property of liquids strongly dependent on intermolecular forces is viscosity, a measure of a liquid’s resistance to flow
  • Molecular complexity leads to higher viscosity because very large molecules can become entangled with each other
• The liquid state has strong intermolecular forces and significant molecular motions
• As a starting point, a typical liquid might best be viewed as containing a large number of regions where the arrangements of the components are similar to those found in the solid, but with more disorder, and a smaller number of regions where holes are present
• We can use a relatively simple model for gases

10.3 - An Introduction to Structures and Types of Solids

• Crystalline solids: Highly regular arrangement of their component
• Amorphous solids: Considerable disorder in their structures.
• The positions of the components in a crystalline solid are usually represented by a lattice
  • A particular lattice can be generated by repeating the unit cell in all three dimensions to form the extended structure
  • The smallest repeating unit of the lattice is called the unit cell
  • The structures of crystalline solids are most commonly determined by X-ray diffraction
  • Diffraction is due to constructive interference when the waves of parallel beams are in phase and to destructive interference when the waves are out of phase
  • Diffractometer: A computer-controlled instrument used for carrying out the X-ray analysis of crystals
• There are many different types of crystalline solids
  • When solid sodium chloride dissolves in the polar water, sodium and chloride ions are distributed throughout the resulting solution and are free to conduct electric current
  • Table sugar, on the other hand, is composed of neutral molecules that are dispersed throughout the water when the solid dissolves
• The third type of solid is represented by elements such as carbon, boron, silicon, and all metals
• These substances all have atoms at the lattice points that describe the structure of the solid
• In the Group 8A solids, the noble gas elements are attracted to each other with London dispersion forces
• The internal forces in a solid determine the properties of the solid

10.4 - Structure and Bonding in Metals

• Metals are characterized by high thermal and electrical conductivity, malleability, and ductility
• The closest packing model for metallic crystals assumes that metal atoms are uniform, hard spheres.
• Knowing the net number of spheres (atoms) in a particular unit cell is important for many applications involving solids
• Examples of metals that form cubic closest packed solids are aluminum, iron, copper, cobalt, and nickel.
• Malleable: Can be pounded into thin sheets
• Ductile: Can be drawn to form a wire
• A related model that gives a more detailed view of the electron energies and motions is the band model, or molecular orbital (MO) model, for metals
  • In this model, the electrons are assumed to travel around the metal crystal in molecular orbitals formed from the valence atomic orbitals of the metal atoms
• The existence of empty molecular orbitals close in energy to filled molecular orbitals explains the thermal and electrical conductivity of metal crystals
• Metals conduct electricity and heat very efficiently because of the availability of highly mobile electrons
• Alloy: a substance that contains a mixture of elements and has metallic properties
  • In a substitutional alloy, some of the host metal atoms are replaced by other metal atoms of similar size.
  • An interstitial alloy is formed when some of the interstices (holes) in the closest packed metal structure are occupied by small atoms

10.5 - Carbon and Silicon: Network Atomic Solids

• Many atomic solids contain strong directional covalent bonds to form a solid that might best be viewed as a “giant molecule”
• The two most common forms of carbon, diamond, and graphite, are typical network solids
• In diamond, the hardest naturally occurring substance, each carbon atom is surrounded by a tetrahedral arrangement of other carbon atoms to form a huge molecule
  • In the energy-level diagram for diamond, there is a large gap between the filled and the empty levels
• Silicon is also an important constituent of the compounds that make up the earth’s crust
  • Silicon compounds are fundamental to most of the rocks, sands, and soils
  • Like glass, ceramics are based on silicates, but with that, the resemblance ends
• We can enhance the conductivity of silicon by doping the crystal with an element such as boron, which has only three valence electrons, one less than silicon
• A ceramic contains two phases: minute crystals of silicates that are suspended in glassy cement.
• Ceramics seem an obvious choice for constructing jet and automobile engines in which the greatest fuel efficiencies are possible at very high temperatures
  • But, ceramics are brittle—they break rather than bend—which limits their usefulness
• Electrons must be in singly occupied molecular orbitals to conduct a current
• The p–n junction has revolutionized electronics; modern solid-state components contain p–n junctions in printed circuit

10.6 - Molecular Solids

• There are many types of solids that contain discrete molecular units at each lattice position
• A common example is an ice, where the lattice positions are occupied by water molecules
• These substances are characterized by strong covalent bonding within the molecules but relatively weak forces between the molecules.
• The forces that exist among the molecules in a molecular solid depend on the nature of the molecules
• Water molecules are particularly well suited to interact with each other because each molecule has two polar O--H bonds and two lone pairs on the oxygen atom
  • This can lead to the association of four hydrogen atoms with each oxygen: two by covalent bonds and two by dipole forces

10.7 - Ionic Solids

• Ionic solids are stable, high-melting substances held together by the strong electrostatic forces that exist between oppositely charged ions
• The larger ions, usually the anions, are packed in one of the closest packing arrangements
• The smaller cations fit into holes among the closest packed anions
• trigonal holes are formed by three spheres in the same layer
  • Tetrahedral holes are formed when a sphere sits in the dimple of three spheres in an adjacent layer
• Octahedral holes are formed between two sets of three spheres in adjoining layers of the closest packed structures
• Closest packed structures contain the same number of octahedral holes as packed spheres
• the most useful model for explaining the structures of these solids regards the ions as hard spheres that are packed to maximize attractions and minimize repulsions

10.8 - Vapor Pressure and Changes of State

• One of the most important roles that water plays in our world is to act as a coolant
• The vaporization of water is crucial to the body’s temperature-control system through the evaporation of perspiration
  • Equilibrium: No further net change occurs in the amount of liquid or vapor because the two opposite processes exactly balance each other
  • There is no net change because the two opposite processes just balance each other
  • The pressure of the vapor present at equilibrium is called the vapor pressure of the liquid
  • When the system reaches equilibrium, the vapor pressure can be determined from the change in the height of the mercury column
  • Measurements of the vapor pressure for a given liquid at several temperatures show that vapor pressure increases significantly with temperature
• The melting and boiling points for a substance are determined by the vapor pressures of the solid and liquid states
• Sublimation: A process in which a substance goes directly from the solid to the gaseous state
• Heating curve: A plot of temperature versus time for a process where energy is added at a constant rate
• Ionic solids such as NaCl and NaF have very high melting points and enthalpies of fusion because of the strong ionic forces in these solids
• The changes of state are physical changes; although intermolecular forces have been overcome, no chemical bonds have been broken

10.9 - Phase Diagrams

• A phase diagram is a convenient way of representing the phases of a substance as a function of temperature and pressure
• The phase diagram for water shows which state exists at a given temperature and pressure
  • It describes conditions and events in a closed system of the type represented, where no material can escape into the surroundings and no air is present
• The melting point of ice decreases as the external pressure increase
  • The maximum density of water occurs at 4°C; when liquid water freezes, its volume increases
  • When water freezes in a pipe or an engine block, it will expand and break the container.
  • This is why water pipes are insulated in cold climates and antifreeze is used in water-cooled engines.
  • The lower density of ice also means that ice formed on rivers and lakes will float, providing a layer of insulation that helps prevent bodies of water from freezing solid in the winter
• Liquid carbon dioxide released from the extinguisher into the environment at 1 atm immediately changes to a vapor
• Critical temperature: The temperature above which the vapor cannot be liquefied no matter the applied pressure
• Critical pressure: The pressure required to produce liquefaction at the critical temperature