Intermolecular Forces Flashcards
What are intermolecular forces
Weak attractive forces between molecules which are overcome when a substance melts/boils
Which molecules do London forces occur between
Exist between all molecules
How do London forces arise
Arise from attractions between temporary instantaneous dipoles and the fleeting dipoles they induce in neighbouring molecules
-when non-polar molecules meet there are fleeting repulsions and attractions between the nuclei of atoms and surrounding electron clouds causing temporary displacement of electrons creating temporary dipoles which induce dipoles in neighbouring molecules
What is the weakest intermolecular force and how many times weaker is it than covalent bonds
London forces are the weakest IMF being 100x weaker than covalent bonds
Why do larger molecules have stronger London forces
As they have more electrons there is a higher chance of temporary dipoles forming as they have larger electron clouds
What is polarisability
The extent to which the electron cloud in a molecule can be distorted by a nearby electric charge
Why do bigger molecules have a higher polarisability
Have more electrons so greater possibility for temporary induced dipoles
What are dipole-dipole interactions
Molecules with permanent dipoles attract one of their poles with opposite pole of another dipole molecule. These attract more strongly than in non-polar molecules and acts in addition to London forces
What is hydrogen bonding
Strong intermolecular force between a delta positive hydrogen atom covalently bonded to F/O/N and a lone pair of electrons on a delta negative F/O/N atom of a nearby molecule. The highly electronegative atom attracts electrons strongly away from the hydrogen so polar bond and hydrogen very electron deficient
-causes strong intermolecular bond between this delta positive hydrogen atom and lone pair of electrons on delta negative F/O/N of nearby molecule
What physical properties depend on strength of intermolecular forces
-melting and boiling temperature
-state
-solubility
-viscosity
Two factors that affect the strength of London forces
-number of electrons= more electrons means greater chance of temporary dipoles forming as more possibility of electron clouds shifting
-surface area of the contact between molecules, greater the surface area stronger the forces
Why is ice less dense than water
the hydrogen bonding in ice forms a 3D lattice which pushes molecules further apart from each other than in the liquid form meaning more space between molecules so lower density
