Debye Forces

Debye force, named after the Dutch-American chemist, Peter J. W. Debye, is the intermolecular force of attraction that exists between a permanent dipole and another dipole induced by the first one. So the Debye force can also be called permanent dipole-induced dipole interaction.

A polar molecule repels the electrons of a neighboring molecule, inducing a dipole. Thus, there is electrostatic attraction between the opposite charges on the two dipoles, and that is termed the Debye force.

Debye interaction

Debye Force – A Part of Van der Waals

The Debye force is one of the three constituent forces of Van der Waals, together with the Keesom force and the London dispersion force.

The Debye force exists between a polar molecule and a non-polar or symmetrical molecule. In terms of strength, it is stronger than the London dispersion force (instantaneous dipole-induced dipole) but weaker than the Keesom force (permanent dipole-dipole).

The Keesom orientation effect and the the Debye induction effect are collectively termed the polar interactions of Van der Waals forces. However, contrary to the Keesom force, the Debye force is not as temperature dependent.

This is because the induced dipole can freely shift its electron density, and hence its dipole, depending on the position it occupies relative to the polar molecule.

Debye force is only attractive and does not disappear at higher temperatures. It cannot occur between atoms, unlike the London dispersion force.Β 

Example of Debye Force

An example of Debye force is the intermolecular force of attraction between hydrogen bromide and argon. HBr, being a polar molecule, possesses a permanent dipole moment due to the electronegativity difference between hydrogen and bromine.Β 

Bromine, having a higher electronegativity, will pull the bonded electrons towards itself more than the hydrogen atom. Consequently, bromine will acquire a partial negative charge and hydrogen, a partially positive one.

The non-polar argon atom will have a dipole induced either by repulsion of its electrons by the partial negative bromine end, or by the attraction of its electrons by the partial positive hydrogen end of the HBr molecule.

This can also be explained in terms of the electrical field generated by the permanent dipole.

Average Interaction Energy

A polar molecule having a dipole moment πœ‡1 can induce a dipole πœ‡2* in a polarizable molecule near it. The two dipoles interact and orient in such a way as to attract each other.Β 

The average interaction energy at distance r of the molecules is:

V = – C / r6, where C = πœ‡12 𝛼2’ / Ο€ Ξ΅0

and 𝛼2’ is the polarizability volume of the second molecule.

Moreover, the interaction energy is independent of temperature as thermal motion does not affect the averaging process.

In addition, the potential energy of the interaction depends on 1 / r6, which comes from the 1 / r3 dependence of the field and the 1 / r3 dependence of energy of interaction between the two dipoles.

Potential Energy of Interaction

The energy of interaction of dipole (πœ‡1) – induced dipole (πœ‡2*) is given by:Β 

Β V = 2 πœ‡1 πœ‡2* / 4 Ο€ Ξ΅0 r3

Where the induced dipole moment (πœ‡2*) depends on the field generated (E) by the polar molecule and the polarizability of the second molecule (𝛼2):

πœ‡2*Β = 𝛼2 E

Substituting this into the formula given above:

Β V = 2 πœ‡1 𝛼2 E / 4 Ο€ Ξ΅0 r3

The electric field generated by the polar molecule is given by:

E =Β  2 πœ‡1 / 4 Ο€ Ξ΅0 r3

So the general form of potential energy of interaction can be written as:

Β V = (2 πœ‡1 𝛼2Β / 4 Ο€ Ξ΅0 r3) x (2 πœ‡1 / 4 Ο€ Ξ΅0 r3)

The induced dipole follows the direction of the permanent dipole, so the effects of thermal motion can be neglected. This follows that both the dipoles remain aligned no matter how fast the orientation changes.

Concepts Berg

What causes Van der Waals forces?

Van der Waals are electrostatic interactions between atoms or molecules that are in close proximity to each other. The molecules can have permanent or instantaneous or induced dipoles for Van der Waals forces to form.

Are London forces and Van der Waal forces the same?

London dispersion force is one of the three constituent forces, along with the Keesom force and the Debye force that are collectively known as Van der Waals forces.

What is Debye attraction?

Debye attraction is the attractive intermolecular force between a permanent dipole and another that has been induced by the first. This attraction is electrostatic and can also be called as “permanent dipole-induced dipole interaction”.

Which IMF is the strongest?

The strongest intermolecular force is the ion-dipole interaction. This is because it involves an ion with a strong charge as compared to other intermolecular forces that possess partial charges on atoms or molecules.

It is followed by hydrogen bond, dipole-dipole interactions and finally London dispersion forces, in order of decreasing strength.

What are the 3 types of van der Waals forces?

The three types of Van der Waals forces include the London dispersion force (instantaneous dipole-induced dipole), Debye force (permanent dipole-induced dipole), and the Keesom force (permanent dipole-dipole).

What are examples of dipole-dipole forces?

Dipole-dipole forces exist between any two molecules with dipole moment. Examples include interactions between HBr, HCl, HI etc.

How are intermolecular forces overcome?

Intermolecular forces hold molecules together and can be overcome by supplying the molecules with energy. This can be done through increasing the temperature by supplying heat. With their kinetic energy high enough, the molecules can overcome the intermolecular forces holding them together and escape.

What factors affect the strength of Intermolecular Forces?

The two primary factors that affect the strength of intermolecular forces directly are the strength of charges and the distance between them. Higher the charges, stronger will be the electrostatic attraction between them. Conversely, increasing the distance between the charges will rapidly decrease the strength of intermolecular forces.

How are intermolecular forces formed?

Intermolecular forces are essentially electrostatic forces of attraction and repulsion between molecules. The molecules can be polar, having a permanent dipole moment, or they can be non-polar initially, and have their dipole made either by fluctuation or induction.

References

  • Physical Chemistry | Fifth Edition, by P. W. Atkins (University of Oxford, Oxford, UK)
  • Intermolecular force (en.wikipedia.org)

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