2.2.5 [Intermolecular interactions] Flashcards
What are non bonded or intermolecular interactions?
Interactions between molecules that are considerably weaker than typical covalent and polar covalent bonds. Most important non-bonded interaction gives to forces known as ‘London Forces’
What are London forces?
Weakest type of intermolecular forces. Induced dipole between molecules also called London forces or instantaneous dipole-induced forces
How do London forces work?
When 2 non polar molecules interact the electron density is symmetrically distributed throughout the molecule.
However the electron density fluctuates over time. If at any time the electron density becomes unsymmetrical a dipole will be generated.
The electron density on left molecule has increased. Giving that end of the molecule a slightly negative charge. Causes instantaneous dipole.
The positive end of molecule closer to molecule B so electron density of electron B pulled to left generating partial negative charge on left hand of molecule, and partial positive charge on the right results in creation of induced dipole in molecule B.
How are the London forces of attraction formed?
Since it was Dipole of A led to induction of dipole B two dipoles are arranged so they interact favourably with each other responsible for the London forces of attraction between the 2 molecules.
What are the features of London forces?
The attractive forces increases with increasing number of electrons in the molecule. Stronger intermolecular forces. The stronger the forces of attraction the more energy required to separate the molecules, so boiling point temperature increases. More electrons greater fluctuations in electron density larger the instantaneous and induced dipoles created.
London forces depend on the shape and size of the molecules. The more points of contact between the molecules greater overall London forces
London forces are always present between molecules regardless of permanent dipole or not.
Straight chain molecules have stronger IM forces than branched chain molecules as can pack closer together. reduced distance forces act over.
How do molecules with permanent dipoles interact with each other?
If molecules possess permanent dipoles also interact with one another. If dipoles are aligned correctly then a favourable interaction and 2 molecules will attract.
Random movement of the molecules causes them to not always be in a favourable position.
So interaction between permanent dipoles is less than interactions between instantaneous and induced dipoles.
Permanent dipoles can induce nearby molecules to form permanent dipole permanent. and permanent dipole induced dipole.
