intermolecular forces

• what are intramolecular forces ?

  • Intramolecular forces are forces within a molecule and are usually covalent bonds
  • Covalent bonds are formed when the outer electrons of two atoms are shared
  • Single, double, triple and co-ordinate bonds are all types of intramolecular forces
• name and describe three types of intermolecular forces

  • Induced dipole – dipole forces are also called London dispersion forces or van der Waals’ forces 
  • Permanent dipole – dipole forces (also called van der Waals’ forces) are the attractive forces between two neighbouring molecules with a permanent dipole
  • Hydrogen Bonding are a special type of permanent dipole - permanent dipole forces
  • Intramolecular forces are stronger than intermolecular forces
  • For example, a hydrogen bond is about one tenth the strength of a covalent bond
• how do induced dipole-dipole forces form ?

  • The electron charge cloud in non-polar molecules or atoms are constantly moving
  • During this movement, the electron charge cloud can be more on one side of the atom or molecule than the other
  • This causes a temporary dipole to arise
  • This temporary dipole can induce a dipole on neighbouring molecules
  • When this happens, the δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other
  • Because the electron clouds are moving constantly, the dipoles are only temporary
• what are permanent dipole forces ?

  • Forces between two molecules that have permanent dipoles are called permanent dipole - dipole forces 
  • The δ+ end of the dipole in one molecule and the δ- end of the dipole in a neighbouring molecule are attracted towards each other
• what is hydrogen bonding ?

  • Hydrogen bonding is the strongest form of intermolecular bonding
  • Hydrogen bonding is a type of permanent dipole – permanent dipole bonding
• why is needed for hydrogen bonding to take place ?

  • A species which has an O, N or F (very electronegative) atom bonded to a hydrogen
  • When hydrogen is covalently bonded to an O, N or F, the bond becomes highly polarised
  • The H becomes so δ+ charged that it can form a bond with the lone pair of an O, N or F atom in another molecule
  • For example, in water
  • Water can form two hydrogen bonds, because the O has two lone pairs
• what are the main properties of water ?

  • Hydrogen bonding in water, causes it to have anomalous properties such as high melting and boiling points, high surface tension and anomalous density of ice compared to water
• why does water have high melting and boiling points ?

  • Water has high melting and boiling points which is caused by the strong intermolecular forces of hydrogen bonding between the molecules
  • In ice (solid H2O) and water (liquid H2O) the molecules are tightly held together by hydrogen bonds
  • A lot of energy is therefore required to break the water molecules apart and melt or boil them
• what are the properties of ice caused by intermolecular forces ?

  • Solids are denser than their liquids as the particles in solids are more closely packed together than in their liquid state
  • In ice however, the water molecules are packed in a 3Dhydrogen-bonded network in a rigid lattice
  • Each oxygen atom is surrounded by hydrogen atoms
  • This way of packing the molecules in a solid and the relatively long bond lengths of the hydrogen bonds means that the water molecules are slightly further apart than in the liquid form
  • Therefore, ice has a lower density than liquid water
• what are the properties of iodine caused by intermolecular forces ?

  • The molecular lattice of iodine consists of a neat arrangement of molecules in the crystal lattice which is held together by the weak intermolecular forces 
  • Being a non-polar molecule, the weak intermolecular bonding is due to instantaneous dipole - induced dipole interactions (the weakest of the van der Waals’ forces)
  • Iodine tends to sublime at temperatures approaching 114 oC due to weak intermolecular force
  • A purple vapour is observed when iodine sublimes
• describe solubility

  • The general principle is that 'like dissolves like' so non-polar substances mostly dissolve in non-polar solvents, like hydrocarbons and they form dispersion forces between the solvent and the solute
  • Polar covalent substances generally dissolve in polar solvents as a result of dipole-dipole interactions or the formation of hydrogen bonds between the solute and the solvent
• As covelnat molecules become larger what happens to their solubility ?

  As covalent molecules become larger their solubility can decrease as the polar part of the molecule is only a smaller part of the overall structure
• why are polar covalent substances unable to dissolve well in non-polar solvents ?

  • Polar covalent substances are unable to dissolve well in non-polar solvents as their dipole-dipole attractions are unable to interact well with the solvent
• why do giant covalent substances generally not dissolve ?

  • Giant covalent substances generally don't dissolve in any solvents as the energy needed to overcome the strong covalent bonds in the lattice structures is too great
• describe conductivity in covelant subsatnces

  • As covalent substances do not contain any freely moving charged particles, they are unable to conduct electricity in either the solid or liquid state
  • However, under certain conditions some polar covalent molecules can ionise and will conduct electricity
  • Some giant covalent structures are capable of conducting electricity due to delocalised electrons