2.6 - Intermolecular Forces

• intermolecular


  between molecules
• intramolecular


  within molecules
• types of intermolecular forces

  London forces

  dipole dipole interactions

  hydrogen bonds
• intermolecular forces


  weak attractive forces between molecules
• London forces


  weakest intermolecular force that exists between all molecules

  arise from attractions between temporary instantaneous dipoles
• how temporary instantaneous dipoles arise

  electrons move randomly in electron cloud and at some point will be more on one side than the other

  forms partially positive & negative regions - temporary dipoles that can induce other temporary dipoles in neighbouring Molecules
• factors affecting strength of London forces

  shape of molecule

  number of electrons
• how number of electrons affects London forces

  the higher the number of electrons the greater the distance over which they can move so the bigger the temporary dipoles so the bigger the London force

  higher polarisability
• how shape of molecule affects London force

  the more branched the molecule the less London forces because there's less molecular surface contact & less electrons interact so fewer induced dipoles so fewer London forces
• why is iodine solid at room temperature?

  it's a non-polar diatomic molecule held together by covalent bonds

  electrons gather on one side of the molecule forming temporary dipoles which induces other temporary dipoles

  so iodine molecules are held together in a lattice by Weak London forces

  simple molecular structure
• dipole dipole interactions


  polar molecules attract each other by weak electrostatic forces of attraction between partially positive and negative regions

  stronger than London forces
• hydrogen bonding


  strongest intermolecular force when H is covalently bonded to F, O or N as they are very electronegative so bond is polarised as It has such a low charge density that it's attracted to lone pairs in F, O or N
• drawing hydrogen bonds


  always in a straight line
• what affects Strength of hydrogen bond

  electronegativity of atom it's bonded to - the more electronegative the stronger the bond

  intermolecular (Weaker) or intramolecular (stronger)
• where hydrogen bonds can be found

  DNA

  Water,ice

  ammonia
• hydrogen bonding in alcohols

  all alcohols have polar - OH group so form hydrogen bonds

  So are less volatile (higher boiling points ) compared to non-polar compounds with similar number of electrons
• trend in boiling point of group 7 hydrogen halides

  all have London forces & as number of e- increases down the group so does strength of London forces which overrides decrease of strength in dipole dipole interactions so boiling point increases

  HF has a higher boiling point than HCl because F is very electronegative so forms hydrogen bonds which are the strongest intermolecular force so need the most energy to overcome
• enthalpy change of vaporisation

  measure of the amount of energy needed to completely separate the molecules of a liquid and covert it into a gas at the same temperature

  overcoming intermolecular forces of attraction between molecules to cause a change of state
• why does ice float in water?

  water molecules are arranged in a lattice Structure so the maximum number of hydrogen bonds can be formed So it's energetically stable

  but this means there's air within the lattice making it less dense than water so it floats
• structure of ice


  simple molecular
• water to ice

  volume decreases

  mass stays the same

  density decreases