Short notes on Intermolecular Forces

                                                            PHYSICAL CHEMISTRY

INTERMOLECULAR FORCES:-

• Cohesion means attraction between like molecules and adhesion means attraction between unlike molecules.
• For molecules to interact, attractive & repulsive forces must be balanced in an energetically favored arrangement.
• Knowledge of these forces and their balance (equilibrium) is important for understanding not only the properties of gases, liquids, and solids, but also interfacial phenomena, flocculation in suspensions, stabilization of emulsions, compaction of powders in capsules, dispersion of powders or liquid droplets in aerosols, and the compression of granules to form tablets.
• When molecules interact, both replusive and attractive forces operate. attractive forces are necessary for molecules to cohere, whereas repulsive forces act to prevent the molecules from interpenetrating and annihilating each other.
• The various types of attractive intermolecular forces are such as 

          1. Van der Waals Forces

          2. Ion-Dipole and Ion-Induced Dipole Forces

          3. Ion-Ion Intercations

          4.Hydrogen Bonds

          

          1.Van der Waals Forces:- They are non-ionic interactions between molecules. They are weak forces that involve the dispersion of charge across a molecule called a dipole.  

• In case of permanent dipole, the electronegative atom draws electrons from the electropositive atom thereby creating partial positive charge and partial negative charges such that a dipole is generated.
• In Keesom forces, the permanent dipoles interact with one another in an ion like fashion. But the bondind is weaker. Also known as dipole-dipole forces. 
• Debye forces show the ability of a permanent dipole to polarize charge in a neighboring non-polar molecule. Also known as dipole-induced dipole forces.
• In London forces, two neighboring non-polar molecules induce partial charge distributions/dipoles in one another. Also known as induced dipole-induced dipole forces/ dispersion forces. It is sufficient to bring the condensation of non-polar gas molecules to form liquids and solids.
• The bond energy required for van der waals forces is 1-10 kcal/mole.

     2.Hydrogen Bond:-

• It is the interaction between a moelcule containing a hydrogen atom and a strongly electronegative atom such as fluorine, oxygen, or nitrogen.
• There are two types of hydrogen bonding i.e., intermolecular and intramolecular hydrogen bonding.

• The type of hydrogen bonding seen in between two hydrogen fluoride molecules and also two formic acid molecules is intermolecular hydrogen bonding, usually called as dimers.
• In case of salicylic acid, it is both intermolecular and intramolecular hydrogen bonding.
• The hydrogen bonding in water accounts for many of its unusual properties such as its high dielectric constant, abnormally low vapor pressure, and high boiling point.
• As we can see in the given structure of ice, it is an open and well ordered 3D array of regular tetrahedron with oxygen in the center of each tetrahedron and hydrogen atoms at the 4 corners. where they are not exactly midway between oxygens.

BOND ENERGIES:-

• Van der Waal forces - 1-10 kcal/mole
• Hydrogen Bonding - 2-8 kcal/mole
• Covalent Bonding - 50-100 kcal/mole
• Ionic Bonding - >100 kcal/mole 

REFERENCE :- MARTIN