Van Der Waals Forces - Types

What Are Van Der Waals Forces?

The term used to define the attraction or repulsion of intermolecular forces between molecules, surfaces, and atoms are called Van Der Waals Forces. They are the weakest of the weakest chemical forces. They have a strength between 0.4 to 4kJ/mol. There are two kinds of Van der Waals forces viz weak (London Dispersion) forces and strong (dipole-dipole forces) forces.

Properties of Van Der Waals Forces

  • Van Der Waals forces are additive.
  • The forces are weak when compared to ionic and covalent chemical bonds.
  • The forces interact only at a short-range.
  • These forces are not directional.
  • They are independent of temperature, with the irregularity of dipole-dipole interactions.

Van Der Waals Forces Example

The geckos can stick to a wide variety of surfaces, without the use of surface tension of any liquid. The secret behind this is the relatively weak intermolecular forces. The underside of the gecko’s foot is covered with millions of microscopic hair and each hair splits into hundreds. Due to van der Waals forces which is weak between the finely divided bristles and the surface the adhesion occurs. These forces are very weak but gecko can support its own body weight on just one finger.

Van Der Waals Forces Equation

Van der Waals equation is required to calculate the actual value for special cases, such as non-ideal or real gases. The equation is expressed as:

[P+(n2a/V2)][Vnb]=nRT

Where,

V is the volume in moles n

The intermolecular force of attraction is given

by (n2a/V2)

Where,

a is the specific value of the gas

P is the pressure

b is the eliminated volume per mole

R is the constant with a value = 0.08206 L atm mol-1 K-1

T is the temperature.

Types Of Van Der Waals Forces

The two types of van der walls forces discussed here are:

London Dispersion Forces

Dispersion forces are a type of van der Waals force which is the weakest of all intermolecular forces. The existence of this force was proposed by Fritz London in the year 1930. London dispersion forces occur due to electron motion. These forces are observed between atoms and nonpolar molecules.

Dipole-Dipole Forces

These forces occur between polar molecules. They are attractive forces. One molecule of hydrogen chloride has a partially negative chlorine atom and a partially positive hydrogen atom. If there is a collection of hydrogen chloride molecules, they align themselves oppositely charged neighbouring molecules are near each other.

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