midterm 2

• Electronegativity is similar to radii periodic trend
• Gas is the only state that can be compressed
• phase names

  • melting: solid to liquid
  • sublimation: solid to gas
  • freezing: liquid to solid
  • boiling: liquid to gas
  • deposition: gas to solid
  • condensing: gas to liquid
• Intermolecular forces

  • occur between different molecules
  • can be temporary, permanent and weak, and permanent and strong
• Density of states

  • gas: low
  • liquid: high
  • solid: high
• Volume of states

  • gas: container
  • liquid: definite
  • solid: definite
• shape of states

  • gas: container
  • liquid: container
  • solid: definite
• Dipole-dipole intermolecular forces occur only in polar compounds
• molecules with stronger intermolecular forces tend to have higher boiling points
• ion-dipole forces are stronger than dipole-dipole forces
• hydrogen bonds occur with F, O, N
• For DNA the intermolecular force between G and C with 3 H-bond acceptors and donors are stronger
• All molecules have London Dispersion Forces (LDFs)
• London dispersion forces

  • weakest of the intermolecular forces
  • caused by instantaneous uneven distribution of electron density within an atom or molecule
• ranking of intermolecular forces from weakest to strongest

  1. London dispersion forces
  2. dipole-dipole
  3. hydrogen bonding
  4. Ion-dipole
• Molecules with weaker intermolecular forces have lower melting points, they also have lower boiling points
• Exception to general trends for melting points: London Dispersion forces can overwhelm weak dipole-dipole forces
• water's special case

  • low molar mass
  • strong dipole forces and hydrogen bonding
  • high melting point, high boiling point
  • high polarity allows it to dissolve polar and ionic compounds
  • high heat capacity
  • expands when it freezes
• Vaporization

  • transition between liquid and gas
  • partial pressure caused by liquid vaporizing
  • vapor pressure
• Phase change

  • occurs at a dynamic equilibrium
  • the reaction happens in both directions
  • Kinetic energy increases as temperature increases
  • rate of vaporization also increases
• Ranking by vapor pressure: hydrogen bonds are the least, ionic bonds are the most, nonpolar bonds are the most
• Exothermic or endothermic phases

  • Endothermic: vaporization, fusion, melting, sublimation
  • exothermic: deposition, condensation
• Phase diagrams

  • show compounds phase changes as a function of temperature and pressure
  • solid lines represent equilibrium between 2 phases
  • triple points show the equilibrium phase between 3 phases
  • critical point occurs at high pressures and high temperatures
  • critical point: where gas and liquid phases have the same density
• Molecular solids: form when covalent compounds are grouped together
• network covalent solids: a type of atomic solid connected by covalent bonds
• non-bonding atomic solid: noble gasses
• ionic solids: formed by a combination of cations and anions
• metallic atomic solid: held together by a sea of electron clouds, metals
• type of interaction for each solid

  • network covalent: covalent bond
  • molecular solids: intermolecular, (london dispersion, H-H, Dipole-dipole)
  • non-bonding atomic: London dispersion
  • ionic solids: ionic bonds
• Solids that have high melting points: metallic, metals
• solids with lowest melting points: noble gasses
• Number of atoms per cell (crystalline structures)

  • simple cubic: 1
  • body centered cubic: 2
  • Face Centered Cubic: 4
• Coordination number (Crystalline solid)

  • simple cubic: 6
  • Body centered cubic: 8
  • Face centered cubic: 12
• Closed packing of cells

  • most efficient simple cubic cell has a packing efficiency of 52%
  • closest packing structures have 74% efficiency
  • Hexagonal closest packing: A-B-A layering
  • Cubic closest packing (face centered cubic): A-B-C layering
• elements that form network covalent bonds: C, Si, As or oxides of nonmetallic elements
• Carbon

  • has many different allotropes or forms
  • main forms: graphite and diamond, they have very different structures and properties
• forms of carbon

  • graphite: relatively soft solid made of sheets of molecules that can slide across each other
  • Diamond: very hard solid formed by a series of tetrahedral bonds
  • bucky ball: ball shape with carbon atoms connect in intersecting rings that form a soccer ball like structure
  • Carbon nanotubes: tubes of carbon atoms connected in hexagonal shapes that are known for their use in electronics and light weight materials
• ionic solids have the highest melting point out of all molecular solids