Learning Objective: To study Vander Waals Keesom's force, interactions, and strength.
Skill Level – Intermediate
Prerequisites:
Chapter: Intermolecular Forces
Sub-topic: Van Der Waal Forces
Author's Note: Polar molecules are like magnets, attracting and orienting other polar molecules so that their combined force of interaction is much stronger than other intermolecular interactions. That is why water exists as a liquid due to polar Oxygen atoms or why sugar with polar parts dissolves in polar water. Or even why oil with hydrocarbon parts does not mix but floats on water due to zero interaction.
Here, we will learn which molecules are polar, how they interact, their strength, and finally, about hydrogen bonding – a subtype of dipole interaction.
Vander Waals Keesom (Dipole-Dipole) Interactions
Keesom force, a subtype of Vander Waals' is an attractive interaction occurring in polar molecules. For example, interaction between two HCl molecules.
Other examples of polar molecules are -CO2, SO2, NH3, H2O, HCl, etc.
How Keesom interactions occur?
A polar molecule like HCl has inherent positive and negative ends due to the electronegativity difference between the atoms H and Cl sharing the covalent bond. The more electronegative atom Cl pulls the maximum electron density creating a negative terminal and it electrostatically attracts the positive terminal of a neighbouring (similar or different) polar molecule.
Such kind of an attraction happens across the entire medium between the opposite poles of several molecules where all the molecules get oriented according to their charges.
Such an attraction happens across the entire medium between the opposite poles of several molecules, where all the molecules get oriented according to their charges. As the molecules self-orient due to their in-built polarity, the effect is called the orientation effect. The attraction is also called dipole-dipole molecular interaction.
Hydrogen bonding- A special case of dipole-dipole interaction
In molecules where one of the electronegative atoms of the covalent bond is Nitrogen (N), Oxygen (O), or Fluorine (F), in such instances, an attractive interaction in addition to dipole-dipole takes place, known as Hydrogen bonding.
However, only a limited number of elements (ones with O, N, and F atoms) show Hydrogen bonding, and such interactions are treated separately from Van der Waal forces.
Strength of Keesom Interactions
Keesom Forces are stronger than other Vander Waal forces - London forces and Debye.
At times, Hydrogen Bonding is categorized among the Van der Waal forces since it is a special case of dipole-dipole interaction; in such a situation, it would have the highest intermolecular attractive strength than all the other Vander Waal forces combined (dipole-dipole/Keesom, dipole-induced dipole/Debye, and London forces/Dispersion).
H-bonding> Keesom> Debye> London
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Related Concepts: Hydrogen Bonding, Electronegativity, London Forces
