chem

• Kinetic Molecular Theory (KMT)


  A model used to explain the behavior of matter
• Postulates of KMT


  • Matter is made of particles that are constantly in motion
  • The amount of kinetic energy in a substance is related to its temperature
  • There is space between particles
  • Phase changes happen when the temperature of the substance changes sufficiently
  • There are attractive forces in between particles called intermolecular forces

  
• Intermolecular forces


  • The strength of these forces increases as particles get closer together
• States of matter


  • Solid
  • Liquid
  • Gas
• Condensed phases


  Solids and liquids
• Intermolecular forces (IMFs)


  Hold the particles of liquid and solid close together
• Thermal energy (heat)


  Overcomes IMFs (liquids → gases)
• When a substance changes from solid to liquid to gas, the molecules remain intact. The changes in state are due to changes in the forces among molecules rather than in those within the molecules.
• Intermolecular forces (IMFs)

  Forces of attraction BETWEEN atoms, molecules, and ions when they are placed close to each other

  
• Intramolecular forces

  Covalent bonds inside molecules
• Intermolecular forces

  When strong, atoms, molecules or ions are strongly attracted to each other and draw closer together. More likely to be found in condensed states such as liquid or solid.
• Intermolecular forces


  When weak, atoms, molecules or ions do not have a strong attraction for each other and move far apart
• Van der Waals forces

  A general term used to define the attraction of intermolecular forces between molecules
• Types of Van der Waals forces

  • Weak London Dispersion Forces
  • Stronger dipole-dipole forces
• Van der Waals distance


  The distance between two nonbonded atoms in adjacent molecules
• Van der Waals radius


  One-half the closest distance between the nuclei of two nonbonded atoms. Larger than the covalent radius.
• Molecule size, similarity, molar mass

  Hydrogen bonding > dipole-dipole > London Dispersion Forces

  
• Molecule size variation


  London Dispersion Forces could become more significant than dipole-dipole and even hydrogen bonding
• London Dispersion Forces (LDF)


  Exist in all molecules and atoms. Arise from the formation of temporary instantaneous polarities across a molecule from the circulations of electrons.
• Molecular weight


  Higher molecular weights have more electrons, making their electron clouds more deformable. This increases polarizability and results in higher LDF, higher melting points, boiling points, and enthalpies of vaporization.
• Comparison of n-Pentane and neopentane

  • n-Pentane has a higher boiling point than neopentane

  
• Dipole-dipole forces


  Exist between molecules that are polar - those that have a permanent dipole moment due to uneven sharing of electrons
• Dipole-dipole attractions


  Substances with dipole-dipole attractions tend to have higher melting and boiling points compared to nonpolar molecules, which only have LDF
• Hydrogen bonding


  A special type of dipole-dipole force, the attraction between the H atom of one molecule and a lone pair of the N, O, or F atom of another molecule
• Hydrogen bonding


  Relatively strong force of attraction between molecules, considerable energy is required to break hydrogen bonds. Explains the exceptionally high boiling points and melting points of compounds like water and hydrogen fluoride. Plays an important role in biology, e.g. holding nucleotide bases together in DNA and RNA.
• Polarizability


  The ease with which the electron cloud of an atom, ion, or molecule is distorted by an electric field
• Polarizability


  Smaller particles are less polarizable than larger ones because their electrons are held more tightly. Polarizability increases down a group and decreases across a period. Cations are smaller than their parent atoms and less polarizable; anions show the opposite trend.
• Ion-dipole forces


  Especially important in aqueous solutions of ionic compounds
• Types of intermolecular forces

  • Dispersion forces
  • Dipole-dipole forces
  • Hydrogen bonding
  • Ion-dipole forces
• Fluid


  A gas or a liquid. A substance that can flow.
• Properties of liquids

  • Surface tension
  • Viscosity
  • Vapor pressure
  • Boiling point
  • Molar heat of vaporization

  
• Surface tension

  The cohesive forces between liquid molecules that cause the surface of a liquid to behave like an elastic sheet
• Surface tension


  Stronger intermolecular forces result in higher surface tension. Surface tension decreases with increasing temperature.
• Capillary action

  The tendency of a liquid to rise in narrow tubes or be drawn into small openings, due to cohesive and adhesive forces
• Viscosity

  A measure of a fluid's resistance to flow. The greater the viscosity, the slower the liquid flows.
• Water has a viscosity of 1 centipoise or 0.001 Pa-s at 20°C.
• Cohesion


  Intermolecular attraction between like molecules (the liquid molecules)
• Adhesion


  Attraction between unlike molecules (such as those in water and in the particles that make up the glass tube)
• These forces (cohesion and adhesion) also define the shape of the surface of a liquid in a cylindrical container (the meniscus)
• Meniscus shape


  • Water displays a concave meniscus (Adhesive > Cohesive)
  • Mercury displays a convex meniscus (Cohesive > Adhesive)