intermolecular interactions and physical properties

• alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2
• the only significant forces between alkanes are london forces
• factors effecting the increased boiling temperatures with increasing molecular mass:

  • as molecular mass increases electrons per molecule increase so the instantaneous/induced dipoles are increased
  • as the length of the carbon chain increases, there are more points of contact
• branched chain alkanes have lower boiling temperatures than their unbranched counterparts

  • this is because they cannot pack together as closely so there are less points of contact
• alcohols are a homologous series of compounds with the general formula CnH2n+1OH
• alcohols contain O-H bonds so they can form intermolecular hydrogen bonds as well as london forces
• In shorter chain alcohols, hydrogen bonding is more predominant but as the chain grows longer, london forces become more predominant
• enthalpy change - the change in heat energy of a substance at constant pressure
• the greater the enthalpy change of vaporisation, the greater the forces of attraction between the molecules
• Hydrogen halide boiling temperatures:

  • HF is significantly higher than the rest due to hydrogen bonding
  • then as you go up the group, the temperature steadily increases as the electrons increase so the strength of the london forces increase
• choosing suitable solvents:

  • like dissolves like (polar-polar, non-polar-non-polar)
  • the forces of attraction between solvent & solute must be strong enough to overcome their own intermolecular forces
• most ionic solids dissolve in water