Intermolecular forces
Intermolecular forces are the forces of attraction and repulsion
between interacting particles like atoms and molecules when
they are not bonded with ionic or covalent bond.
Intermolecular forces are also known as van der Waals forces
Dispersion Forces
or London Forces
01.
Hydrogen Types of van Dipole-dipole
04. 02.
bonding der waals (Keesom force)
forces
03.
Dipole-Induced dipole
(Debye force)
London Forces/ dispersion Forces
Instantaneous Dipole – Induced Dipole Interaction
The weak attractive forces between molecules resulting from the
small, instantaneous dipoles that occur because of varying
positions of the electrons during their motion about nuclei.
Interaction between non-polar species is explained by this
interaction.
Example Ar and Ar
+ + + – + –
Atom Ar Atom Ar Atom Ar Atom Ar
Symmetrical distribution of with instantaneous dipole, Induced
electronic charge cloud more electron density on dipole
the right hand side
– + – +
Atom Ar Atom Ar
More electron density Induced dipole
on the left hand side
Dipole-Dipole Forces (Keesom force)
Interaction between polar molecules is called as dipole-dipole
interaction .
+ – + –
H Cl H Cl
This interaction is stronger than the London forces but is weaker
than ion-ion interaction because only the partial charges are
involved.
Dipole–Induced Dipole Forces (Debye Force)
Interaction between polar and non-polar species is explained
by dipole-induced dipole interaction.
When the dipole comes near the non polar molecule it can induce
polarity in non polar molecule resulting in Dipole–Induced Dipole
interactions
Induced dipole moment depends upon the dipole moment
present in the permanent dipole and the polarisability of the
electrically neutral molecule.
In this case also cumulative effect of dispersion forces and
dipole-induced dipole interactions exists.
+ –
A B
Permanent dipole non-polar molecule
(a polar molecule)
+ – + –
A B
Permanent dipole Induced dipole in a
(a polar molecule) non-polar molecule
Example
H2O and Xe , HCl and Argon
Hydrogen bonding
In the case of some liquids, there is a strong interaction
between the molecules.
If a molecule is constituted of hydrogen and an electronegative
atom like, fluorine oxygen, nitrogen or sulphur, such a molecule
will show intermolecular interactions.
Hydrogen-bonding in water is represented as ;
......H O ...... H O ......H O ......
H H H
Hydrogen bonding
Due to electronegativity difference, there is a weak bond
between hydrogen of one molecule and oxygen of the other.
This is called hydrogen bond.
Such a bond continues indefinitely through a hundred or
thousand molecules, thus, displaying strong interaction.
It is also displayed by ammonia as :
H H
NH3 N—H N—H
H H
QUESTION
London dispersion forces are present in
(1) HCl
(2) Oxygen
(3) He
(4) NaCl
SOLUTION
Ans 3
QUESTION
Dipole – induced dipoles are also known as :
(1) London forces
(2) Keesom force
(3) Debye forces
(4) Hydrogen bonding
SOLUTION
Ans 3
Home Work
QUESTION
Which one of the following intermolecular forces is the
strongest?
(1) Hydrogen-bonding
(2) Keesom forces
(3) London forces
(4) Debye forces
SOLUTION
Ans 1
QUESTION
What are the forces of attraction and repulsion between
interacting molecules known as _________
(1) Attractive forces
(2) Repulsive forces
(3) Intermolecular forces
(4) Intramolecular forces
SOLUTION
Ans 3
QUESTION
London force is also known as _____________
(1) Dispersion force
(2) Van der Waals forces
(3) Hydrogen bonding
(4) Covalent bonds
SOLUTION
Ans 1
