6. Intermolecular forces

• Intermolecular forces are forces between molecules. They’re much weaker than covalent, ionic or metallic bonds. There are 3 types:

  • Induced dipole-dipole (VdW) forces
  • permanent dipole-dipole forces
  • hydrogen bonding
• VdW forces causes all atoms and molecules to be attracted to each other. Electrons in charge clouds are always moving quickly. Electrons in atom are likely to be more to one side than the other. At this moment, atom would have a temporary dipole. This dipole can cause another temporary dipole in the opposite direction on neighbouring atom. 2 dipoles are then attracted to each other. Second dipole can cause another dipole in 3rd atom. Because electrons are constantly moving, dipoles are being created/ destroyed all the time. Overall effect is for the atoms to be attracted to each other

  
• Not all VdW forges are the same strength - larger molecules have larger electron clouds, meaning stronger VdW forces. Shape of molecule also affects strength of VdW forces. Long, straight molecules can lie closer together than branched ones - closer together 2 molecules are, stronger the forces between them
• As you go down Nobel gases, number of electrons increases. So the VdW forces increases, and so do the boiling points
• As Alkane chains get longer, number of electrons in molecules increase. Area over which VdW forces can act also increases. This means VdW forces are stronger, and so boiling points increase
• In substance made up of molecules that have permanent dipoles, there will be weak electrostatic forces of attraction between the delta + and delta - charges in neighbouring molecules. Called permanent dipole dipole forces
• If you put an electrostatic ally charged rod next to a jet of polar liquid, like water, the liquid will move towards rod. It’s because the polar liquids contain molecules with permanent dipoles. even if the rod is positively or negatively charged. Polar molecules in liquid can turn around so oppositely charged end is attracted towards rod. More polar the liquid, stronger electrostatic attraction between rod and jet, greater deflection. However, liquids made up of non-polar molecules, such as hexane , will not be affected at all when placed near charged rod

  
• Hydrogen bonding is strongest intermolecular forces. Only happens when hydrogen is covalently bonded to fluorine , nitrogen or oxygen. Fluorine nitrogen and oxygen are very electronegative, so they draw bonding electrons away from hydrogen atoms
• Hydrogen bonding is so polarised, and hydrogen has such a high charge density because it’s so small, that hydrogen atoms form weak bonds with line pairs of electrons on fluorine, nitrogen or oxygen atoms of other molecules. Molecules which have hydrogen bonding usually organic, containing -OH or -NH groups
• Substances with hydrogen bonding has higher boiling points and melting points than other similar molecules because of the extra energy needed to break hydrogen bonds.
• As liquid water cools to form Ice, molecules make more hydrogen bonds and arrange themselves into regular lattice structure. Since hydrogen bonds are relatively long, average distance between H2O molecules is greater in ice than in liquid water - so ice is less dense than liquid water
• Simple covalent compounds- have strong covalent bonds within molecules, but weak forces between molecules. Their physical properties, such as electrical conductivity, melting point And solubility are determined by bonding in compound
• Electrical conductivity - simple covalent compounds don’t conduct electricity because there are no free ions or electrons to carry charge
• Melting point- simple covalent compounds have low melting points because the weak forces between molecules are easily broken
• Solubility - some simple covalent compounds dissolve in water depending on how polarised molecules are