Chemistry intermolecular forces

Intermolecular forces such as dipole dipole, H-bonds, dispersion etc

Intermolecular forces

-Intermolecular forces exist between covalent molecules

-There are three types of IMF

-The type of IMF that forms between the molecules depends on the shape of the molecule.

-We determine the shape of the molecule by Valence shell Electron Pair Repulsion

VSEPR

-The Valence electron pairs in the shell of an atom will repel eachother and position themselves as far apart as possible

-Shape is therefore determined by the number of shared and lone electrons electron pairs.

-Lone pairs will take up more space than bonded pairs so it will change molecule shape

-Lone pairs repel 3x more than bonded pairs with eachother

Polarity molecules

-The type of IMF that exists between a covalent molecule depends on the shape and polarity of the molecule

-Polar and Non-Polar refers to the weak positive or negative charge that can exist for a covalent molecule only

-Polar Molecules exist when electrons are not shared equally between elements in the covalent molecule. This is due to differences in Electronegativity

-These occur in asymmetrical molecules

Dipole-Dipole forces

-These form between polar molecules due to the charged polar ends of the molecules (dipole)

-the charged ends molecules attract eachother and form a bond

-These bonds vary in strength based on the polarity of the molecule

-More polar/ Higher electronegative diff means higher melting/boiling point.

Hydrogen Bonds

-Particular type of dipole-dipole when hydrogen is attached to N,O,F

-This is because these connections have very high electronegative difference

-Hydrogen bonds are approximatley 10x stronger than dipole

-These occur in any molecule that have hydrogen bonded with N,O,F

Dispersion forces

-These exist in all covalent molecules

-Electrons shared in covalent molecules are constantly in motion, they will clump together temporarily and form temporary dipoles

-thus they have temporary charged areas of the molecule, these form attractive force

-These temp dipoles bond non-polar molecules together

-Dispersion forces increase as you increase in molecule size/mass

-As mass increases, dispersion forces become dominant IMF in covalent molecules.

Key properties relating to IMF

*MP/BP

-Increases with molecule size ( dispersion forces)

-If similar mass, it depends on type of IMF present

*Vapor

Low MP/BP means higher vapour pressure

High vapour pressure means low IMF strength