Forces of Keesom

The forces of Keesom result from an intermolecular interaction between Dipôle S permanent. They in general represent the most important component of the forces of Van der Waals. They were named in the honor of Willem Hendrik Keesom which was the first required their mathematical formulation in 1912.

The forces of Keesom are mainly related to the electronegativity. They appear between at least two polar molecules (dipoles known as permanent), from where their old name of interaction “ dipole-dipôle ”. The forces of Keesom, just like the Forces of Debye (permanent dipole/induced dipole) and of London (induced dipole/induced dipole) are only one component of the forces of Van der Waals. In general the forces of Keesom represent the most significant part of these interactions, but it is not always true.

The forces of Keesom are directional . One can attach the interactions of Keesom to the ionic interactions, but as the forces of Keesom utilize only partially the loads, they are less strong.

The forces of Keesom often make it possible to explain the evolution of certain physical properties according to the dipole moment in molecules of close structure. Indeed, the forces of Keesom play for a great part in the fitting of the molecules the ones compared to the others. To overcome the forces of Keesom which exist between polar molecules, it is necessary to bring an energy larger than in the case of molar the of the same mass but non-polar molecules. Thus the liquids with polar molecules often have boiling points higher than those with non-polar molecules molar of the same mass.

Formulation

One studies the interaction between two dipoles of dipole moments permanent respective μ₁ and μ₂ distant from an average distance R . In the absence of other external forces, the dipoles are aligned to minimize energy, as two magnets would do it. Nevertheless, because of thermal agitation, they also tend to be disaligned. There thus exists an angular degree of freedom to take into account using the Statistique of Boltzmann. The potential energy E which follows with the form:

$E \ approx \,$

With:

ε₀ , relative permitivity or permittivity of the vacuum ( 8,854 10^-12 C²J^{- 1}m^{- 1} )
K , Boltzmann constant ( 1,381 10^-23 J K^{- 1} )
T , temperature ( K )

By calculating the derivative of energy compared to the average distance R , one can go back to the expression of the force of Keesom:

$F_ {Keesom} = \,$

where λ (T) , is a function of the temperature.

It is noticed that the forces of Keesom attenuate very quickly with the distance (inversely proportional to power 7 of the distance). Let us note moreover that the forces of Keesom are related on molecular arrangement and thus to the temperature. When the latter increases, the forces of Keesom weaken.

The energy of this kind of connection varies between 0,5 and 3 kJ/mol.

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