Dispersion forces

Dispersion forces are a basic force between two molecules or atoms but it is the weakest attractive force in between them called dispersion forces. Dispersion forces occur in between the two atoms which have low molecular weight or two substances which are non-polar in type i.e. Hydrogen, neon, etc. 

Hence, we can say that dispersion forces purely depend on the molecular weight of an atom and the distribution of electron pairs in between two atoms or substances. So, throughout the article, we will see more information about the dispersion forces like characteristics, effects, and many more things.

Dispersion forces: Characteristics

• Based on the polarity, dispersion forces are also categorized into two types called polar dispersion forces and non-polar dispersion forces. Where polar dispersion forces are created when both molecules have some partial charge but in non-polar dispersion forces, there are no charges on both the molecules.
• Covalent bonds are framed based on the equal sharing electrons so we can say that there are no charges available that are still considered as polar dispersion forces. The reason behind that is they are connected because of electronegativity. 
• Remember that, “Dispersion forces are inversely proportional to the sixth power of the distance between interacting atoms or molecules”.

Dispersion forces: Effects

1. Effects on boiling point: 

Dispersion forces are framed in liquid states. So, the stronger dispersion forces create more energy. So, we can say that more energy is required to separate the two molecules which are connected by dispersion forces.

2. Effects on solubility:

The stronger the dispersion force in between solute molecules and solvent molecules, the greater solubility in between the solvent and solute.

Dispersion forces: Polarity

• Dispersion forces are always present whether the dipole moment is present or not.
• For example, we can say that HCl has a lower boiling point than HI but HCl has a more dipole moment than the HI.
• The reason behind it is that I atoms have more electrons than the Cl atoms and that’s the reason behind the greater dispersion forces.

Dispersion forces: Consequences 

• The main principle behind the dispersion forces is the order of the magnitude of attractive forces.
• Dispersion forces are present in between the long-range molecules greater than nanometer distances.
• Dispersion forces may be attractive or repulsive types of forces.

Dispersion forces: Formula

The tendency of the molecules to form induced dipoles is known as Polarizability. It is represented by ‘μ’ and the expression for the same is:

μ = α * E

E is the electric field

α is the polarizability

μ is the induced dipole moment

The interaction energy can be calculated using the London dispersion force formula as follow:

V₁₁= -3α₂ I / 4r⁶ …………. eq(1)

Equation 1 was designed for 2 identical atoms/ molecules which was further modified by a German Physicist for 2 un identical atoms/molecules as follows:

V₁₂= - 3I₁I₂α₁α₂ / (2I₁ + I₂r⁶)

I is the ionization energy

α is the polarization

r is the distance between the molecules

Dispersion forces: Examples

As we know dispersion forces are framed on the small surface area. And that’s the reason for decreasing the boiling point of the molecules. The alkene isomers whose branches are less developed have more boiling points and whose branches are more developed having more less boiling points. Examples of those types of alkenes are shown in the table below.

Things to Remember

• Dispersion forces occur in between the two atoms which have low molecular weight.
• The stronger the dispersion forces, higher is the boiling point and higher is the solubility.
• Dispersion forces may be attractive or repulsive depending upon the polarity due to which the forces have been enacted.
• The formula to calculate the polarizability is μ = α * E.
• Dispersion forces are always present whether or not the dipole moment is present or not.