Intermolecular Forces

Intermolecular forces The attraction and repulsion forces between molecules – (intramolecular forces are the forces within the molecule – bonding) Cotton vs plastic raincoat Broken down into three types

Van der Waals Forces Intermolecular forces Forces holding molecules in order But very weak compared to the covalent bonding between elements Energy to boil water vs the amount to decompose water

Dipole-Dipole Force Force between dipoles or polar molecules Strength of the force depends on the polarity of the molecule

London Force Force between nonpolar molecules – While the overall molecule does not have a dipole the electrons moving within the molecule will create a momentary dipole – The more electrons the molecule has the higher the London Force

Predicting boiling points Can use the number of electrons as a prediction of the boiling point. This is because the intermolecular forces are related to how difficult the molecules are to separate

Isoelectronic Molecules Iso – meaning the same electronic – electrons These molecules would have the same number of electrons and the London forces will be similar However this does not mean that the boiling points would be the same. Remember the boiling point depends on the London forces AND the dipole-dipole

Summary More polar -> stronger dipole-dipole -> higher boiling point More electrons -> larger London Forces -> higher boiling point Using this knowledge to predict relative boiling points for certain cases

Lab Exercise 3.4

Hydrogen bonding Hydrogen (proton) is shared with a lone pair. Hydrogen needs to be bonded with an atom with a high electronegativity Need to have at least one lone pair on the atom bonded to Hydrogen to give hydrogen bonding to another molecule Three possible structures: HF (g), anything with – OH and –NH. Example is sugar C 12 H 22 O 11(s), or better written as C 12 H 14 O 3 (OH) 8, we can see there are 8 –OH groups to hydrogen bond

Examples DNA and proteins Water, ice – Reason for different densities

Physical Properties of Liquids Intermolecular forces (IMF) contribute to various Surface tension – in the liquid IMF are in all directions but at the surface it is only away from the surface. Cohesion – attraction between like molecules Adhesion – attraction between unlike molecules

Properties Cont’d How easily the liquid evaporates – Weak forces – easy to evaporate – volatility

Summary Intermolecular forces are the attraction and repulsion of positive and negative charges All molecules have London forces – with momentary diploes Polar molecules have dipole-dipole forces Hydrogen bonding occurs when a hydrogen is attracted to the lone pair from an adjacent molecule IMF affect many various physical properties