IMFs

• Permanent dipole

  A small charge difference that does not change across a bond, with positive and negative partial charges on the bonded atoms: the result of the bonded atoms having different electronegativities.
• Permanent dipole-dipole interactions

  Attractive forces between the permanent dipoles in different molecules.
• London forces


  Attractive forces between Induced dipoles in different molecules: also known as induced dipole-dipole interactions
• Dipole

  A separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge, δ+, and the other has a small negative charge, δ-
• Dipole-dipole force

  An attractive force between permanent dipoles in neighbouring polar molecules
• Induced dipole-dipole interactions

  • Movement of electrons produces a changing dipole in a molecule
  • At any instant, an instantaneous dipole will exist, but its position is constantly shifting
  • The instantaneous dipole induces a dipole on a neighbouring molecule
  • The induced dipole induces further dipoles on neighbouring molecules, which then attract one another
• induced dipoles are temporary
• Induced dipoles result from interactions of electrons between molecules. The more electrons in each molecule:

  • the larger the instantaneous and induced dipoles
  • the greater the induced dipole-dipole interactions
  • the stronger the attractive forces between molecules
• Permanent dipole-dipole interactions act between the permanent

  dipoles in different polar molecules.
• A simple molecular substance is made up of simple molecules - small units containing a definite number of atoms with a definite molecular formula, such as Ne, H2, H2O and CO2
• In the solid state, simple molecules form a regular structure called a simple molecular lattice.
• In the simple molecular lattice:

  • the molecules are held in place by weak intermolecular forces
  • the atoms within each molecule are bonded together strongly by covalent bonds
• Non-polar simple molecular substances are soluble in non-polar solvents
• simple molecular substances tend to be insoluble in polar solvents
• Explain the solubility of non-polar simple molecular substances

  • When a simple molecular compound is added to a non-polar solvent, such as hexane, intermolecular forces form between the molecules and the solvent
  • The interactions weaken the intermolecular forces in the simple molecular lattice
  • The intermolecular forces break and the compound dissolves
• Simple molecular substances tend to be insoluble in polar solvents

  • When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules
  • The intermolecular bonding within the polar solvent is too strong to be broken
• Polar substances may dissolve in polar solvents as the polar solute molecules and the polar solvent can attract each other
• Simple molecular structures are non-conductors of electricity because they have no mobile charged particles