Test: Intermolecular Forces - States Of Matter

NH₃ and NH₃. Both polar, asymmetric molecules. CH₄ is nonpolar. CO₂ is symetrical.

Intermolecular force can only be Vaan der Waals force acting b/w two molecules.

Thermal energy is the energy of a body arising from motion of its atoms or molecules. It is directly proportional to the temperature of the substance. It is the measure of average kinetic energy of the particles of the matter and is thus responsible for movement of particles. This movement of particles is called thermal motion. We have already learnt that intermolecular forces tend to keep the molecules together but thermal energy of the molecules tends to keep them apart. Three states of matter are the result of balance between intermolecular forces and the thermal energy of the molecules.

In a gas, the distance between molecules, whether monatomic or polyatomic, is very large compared with the size of the molecules; thus gases have a low density and are highly compressible.

Density: The molecules of a liquid are packed relatively close together. Consequently, liquids are much denser than gases.

Van der Waals forces are distance-dependent forces between atoms and molecule not associated with covalent or ionic chemical bonds. Sometimes the term is used to encompass all intermolecular forces, although some scientists only include London dispersion force, Debye force, and Keeson force.

Dipole-dipole interaction energy between stationary polar molecules is proportional to 1/ r³ and that between rotating polar molecules is proportional to 1/ r⁶ where ‘r’ is the distance between polar molecules

Besides dipole - dipole interaction, polar molecules can interact by London forces also.

All of the above are the properties of gases.

London dispersion forces operate only over very short distance. The energy of interaction varies as
1/(distance between two interacting particles)⁶ 

Large or more complex the molecules, greater is the magnitude of London forces.

This is obviously due to the fact that the large electron clouds are easily distorted or polarised.
Hence, greater the polarisability of the interacting particles, greater is the magnitude of the interaction energy.

The formation of hydrogen bonds is an important quality of the water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids and, since living things have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are constantly formed and broken as the water molecules slide past each other.

A hydrogen bond is a partially electrostatic attraction b/w a hydrogen atom which is bound to a more electronegative atom such as N, O and F etc. And also hydrogen bonding occurs only b/w molecules in which H atom bounded to a electronegative atom.

Partial charge is a small charge developed by displacement of electrons. It is less than unit electronic charge and is represented as δ⁺ or δ^–  

HF - Hydrogen bonding HCl,HBr,HI→ Dipole-dipole interaction, London-dispersion force.

It is because of hydrogen bonding . Since F is the most electronegative element , its hydrogen bonding tendency is strongest which makes boiling point of HF increase.