Intermolecular forces are forces of interaction between molecules. These forces influence properties of substances such as boiling and melting points, phases at room temperature, viscosity and many more. We will be talking primarily about forces of attraction between molecules.
Intermolecular forces can be divided into 4 major categories:
1. ION-ION
This is the strongest intermolecular force. It is present between ions. Ions are easily recognized because they have a charge. Ex: Na+, Cl-, OH-...
2. ION-DIPOLE
This force is present in a mixture that has both ions and polar molecules. Example: Na+ and H2O
3. HYDROGEN BONDING
Even though there is a word bonding in its name, it is still a force and not a bond. Hydrogen bonding is present in molecules that have OH, NH or FH. It is the strongest type of dipole-dipole force and is usually written separately. It occurs when a hydrogen from OH, NH or FH is attracted to the lone pair of an O, N or F of a separate molecule. Example: H2O with H2O
4. DIPOLE-DIPOLE
This type of force is present in polar molecules. Ex: H2S, CH3Cl...
A polar molecule has an asymmetrical distribution of charge. In order to recognize a polar molecule, we must draw out the Lewis dot structure first.
Mayya's Trick
A Molecule is NONPOLAR if
It consists of two of the same atoms. Example: H2, O2, N2
OR
The central atom is attached to the same atoms AND the shape of the molecule is one of the following: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, square planar.
Example: CCl4(carbon is attached to all the same atoms, chlorines, and the shape of the molecule is tetrahedral)
5. LONDON DISPERSION
This type of force is present in all substances. However, nonpolar molecules have ONLY dispersion force while others have dispersion force and other forces.
It is also important to note that dispersion force is the weakest force. The strength of dispersion forces is connected to the molar mass. The higher the molar mass, the stronger the dispersion force will be.
For example: H2 should have weaker dispersion forces than Cl2 since it has a lower molar mass.
There are also the following forces:
Ion-induced dipole = Present when we have an ionic compound and nonpolar molecule such as Na+ and H2
Dipole-induced dipole = Present when we have a polar molecule and nonpolar molecule such I2 dissolved in HBr
Attractive intermolecular forces hold molecules together and are responsible for many properties observed in substances.
Stronger the intermolecular forces lead to:
higher the boiling point
higher the melting point
greater surface tension (the energy required to increase the surface area of a liquid by a unit amount.)
greater viscosity (the resistance to flow )
lower the vapor pressure
Example: State all types of intermolecular forces operate between the following pairs of species:
a. H2
b. NH3
c. NaCl dissolved in H2O
Solution: All of them will have dispersion forces since these are present in all substances.
a. H2 consists of two atoms of the same element and is nonpolar. Only dispersion forces will be present.
b. NH3 has hydrogen bonding because it has NH.
c. NaCl dissolved in H2O will be an ion-dipole force because NaCl is ionic and water is polar.