Intermolecular Forces; Definition, types and examples
Definition of Intermolecular Forces(van der Wall’s forces)
The attractive forces that exist between the individual particles molecules, atoms or ions are called intermolecular forces. These forces are also called van der Wall’s forces.
The physical state of a substance depends on the strength of intermolecular forces. The properties of liquids such as boiling point, heat of vaporization, vapors pressure, surface tension and viscosity depend on the strength of intermolecular forces. There are four types of intermolecular forces (van der Wall’s forces)
List of Intermolecular forces
- Dipole dipole forces
- Ion dipole forces
- Dipole induced dipole forces
- Instantaneous dipole induced dipole forces (i.e. London dispersion forces.
- Dipole -dipole forces
The attractive forces between the positive end of one molecule with the negative end of the other molecule are called dipole-dipole forces. There is greater electronegativity difference between hydrogen and chlorine. Due to this, chlorine being more electronegative acquires a partial negative charge whereas hydrogen acquires partial positive charge. Hence, the molecules are close to each other and the positive end of one molecule attracts the negative end of the other molecule. These forces are called dipole-dipole forces and they are approximately one percent as effective as covalent bond. The strength of these forces depends upon the electronegativity difference between the bonded atoms and the distance between the molecules. The distances between molecules in the gaseous phase are greater so these forces are very weak in this phase. An example of the molecules showing dipole-dipole attractions is given below:
Dipole- Dipole Forces (Type of Inter molecular Forces)
Stronger dipole- dipole forces are responsible for higher values of thermodynamic parameters such as meting point, boiling point, heat of vaporization and heat of sublimation.
- Ion- Dipole forces
The forces of attraction between ions and the molecule of the polar solvent are called ion dipole forces. Ionic compounds like M+X– are normally soluble in polar solvent such as water. Water molecules break the crystal lattice and the ions are set free. These positive and negative ions are then surrounded and enveloped by water molecules. The negative ends of the dipole of water are attracted towards the cation M+ while the positive ends orient towards the anion X– that’s why ionic compounds get solubilized in water. The forces of attraction between ions and water molecules are named as ion dipole forces.
Ion dipole interaction ((Intermolecular Forces) of cation and anion with H2O.
iii. Dipole induced dipole forces
The forces of attraction between polar molecule and temporarily induced molecule (neutral) are called dipole induced dipole or Debye forces. If a mixture of substances containing polar and non-polar molecules are present. Then positive end of the polar molecule attracts the mobile electrons of the nearby non polar molecules. In this was polarity is induced in non-polar molecule and thus, both molecules become dipoles that attract each other. These forces are also called as Debye forces. The following figure makes the idea clear.
Dipole induced dipole interaction ((Type of Intermolecular Forces)
Instantaneous dipole-induced dipole forces or London dispersion forces
The forces of attraction between non-polar molecules in which polarity is induced for an instant are called instantaneous dipole induced dipole forces or London dispersion forces. These forces of attraction present among the non-polar molecules like neon, argon, chlorine, methane etc need special attention because under normal conditions such molecules don’t have dipoles. We know that helium gas can be liquefied under appropriate conditions. In other words forces of attraction operate among the atoms of helium which cause them to cling together in the liquid state.