Types of Forces in Chemistry

Different Types of Forces

Intermolecular Forces:

                             These are forces of attraction and/or repulsion between the interacting particles i.e. atom or molecules.

Dutch Scientist J. Van der Waals (1837-1923) explains deviation of the real gases from ideal behavior with intermolecular forces, so intermolecular forces are also called as van der waals forces.

Example: Hydrogen bonding which is strong dipole-dipole interaction.

Dispersion Forces: 

                             If an atom gets instantaneous dipole (i.e. Atom has more electron density in right or left hand side) then its nearby atom become induced dipole, so these two temporary dipole attract each other. This attraction force is known as dispersion forces.

·       As these forces were first proposed by F. London so these forces are also known as London forces.

Dipole-Dipole Forces: 

                                    This type of force act between the molecules which have permanent dipole. Dipole of these molecule possess some partial charges (denoted by delta that is delta positive or delta negative)

Example: HCl molecule, where H possess delta positive and Cl possess delta negative.

Dipole-Induced Dipole Forces: 

                                             These attractive forces act between polar and non-polar molecules where polar molecules have permanent dipole, which induced the dipole and non-polar molecule by deforming electronic cloud of non-polar molecule.

·       As polarisability increases, strength of the attractive interaction also increases.

Hydrogen-Bond:  

                           It is a type of dipole-dipole interaction present in molecules with high polar N-H, O-H and H-F bonds.

Thermal Energy: 

                           It is the energy of the body arise due to the motion of its atoms and molecules.

·       Thermal energy is directly proportional to temperature of the substances.

Intermolecular forces v/s Thermal interactions

·       Intermolecular forces make molecules of the substance keep together.

·       While thermal energy of the substance make molecules keep apart.

·       These two (thermal energy and intermolecular forces) decides collectively the states of matter.

·       If intermolecular forces predominance then

                                      Gas->Liquid->Solid

·       If thermal energy predominance then

                                      Solid->Liquid->Gas