1.4 Intermolecular Forces CCEA

What are intermolecualr forces

the attractive forces between neighbouring molecules in simple covalent molecules

Intramolecular forces

the forces within the moleucles i.e covalent bonds

Three types of intermolecular forces

1. Van der Waals forces: between all molecules

2. Permanent dipole-dipole attractions: between polar molecules

3. Hydrogen bonding: between certian polar molecules contiaing H,N,O,F

What are Van der Waals 

Van der Waals forces are attractions between instantenous and induced dipoles on neighbouring molecules

the size of the molecule (number of electrons)

more electrons create larger induced dipoles will experience greater van der Waals attraction. The m.p and b.p of noble gases increase down group, AN increase in van der Waals attraction between halogen molecules explain why F2 and Cl2 are gases with m.p and b.p and the heavier Br₂ and I₂ are liquids and solids

points of contacts between molecules

surface area increases strength of V.D.W attraction between molecules by increasing contact between electrons in neighboruing molecules.

What are permanent dipole-dipole attractions

the attractions between the positive end S+, on a molecule with the negative end S-, of the permanent dipole of a neighbouring molecule

where are permament dipoles found

in molecules with polar bonds (difference in electronegativity)

covalent bond is polar when electrons are shared unevenly between two atoms due to differeing electronegativity values

Hydrogen bonding

an attraction between the lone pair of electrons on a very electronegative atom (N,O,F) on one molecule and a hydrogen on a neighbouring molecule in which the hydrogen is covalently bonded to the very electronegative atom

What does hydrogen bonding do to melting point and boiling point

A large amount of energy is needed to break the strong hydrogen bonds, water ammonia and hydrogen fluoride high M.P and B.P

What does hydrogen bonding do to solubility

Covalent compounds which can form hydrogen bonds are generally soluble in water l e,g ammonia is soluble in water because the nitrogen in ammonia forms hydrogen bonds with the hydrogen in water, and the hydrogen on ammonia forms hydrogen bonds with oxygen in water.

What can polar substances dissolve

Polar solvents e.g water

What can non polar substances dissolve in

Non polar solvents e.g hexane

why do group 4 have a low boiling point

Group 4 hydrides have a tetrahedral shape so bond polarities cancel pit, no permanent dipole only weak gander walls forces exist between molecules which increase down group because more electrons

Why is the density of ice lower than liquid water

Liquid water hydrogen bonds are constantly breaking and forking

Solid water hydrogen bonds holds the molecules in fixed positions, this creates highly ordered structure where the molecules are more widely space, so more open structure

Why is hydrogen ass

Below 0C, molecules have so little energy that the hydrogen bonds force them to from a crystal structure, when ice melts the fixed hydrogen bonds break, allowing water molecules to pack together more closely