Van der Waals vs Hydrogen Bonds
Van der Waals forces and hydrogen bonds are intermolecular attractions between molecules. Some intermolecular forces are stronger, and some are weak. These bonds determine the behavior of molecules.
Van der Waals Forces
For an intermolecular attraction, there should be a charge separation. There are some symmetrical molecules like H2, Cl2, where there are no charge separations. However, electrons are constantly moving in these molecules. Therefore, there can be instant charge separation within the molecule if the electron moves toward one end of the molecule. The end with the electron will have a negative charge temporarily, whereas the other end will have a positive charge. These temporary dipoles can induce a dipole in the neighboring molecule and thereafter, an interaction between opposing poles can occur. This kind of interaction is known as an induced dipole- induced dipole interaction. Further, there can be interactions between a permanent dipole and an induced dipole or between two permanent dipoles. All these inter molecular interactions are known as Van der Waals forces.
When hydrogen is attached to an electronegative atom like fluorine, oxygen or nitrogen, a polar bonding will result. Because of the electronegativity, the electrons in the bond will be more attracted to the electronegative atom than to the hydrogen atom. Therefore, hydrogen atom will get a positive charge partially, whereas the more electronegative atom will get a negative charge partially. When two molecules having this charge separation are close by, there will be an attraction force between hydrogen and the negatively-charged atom. This attraction is known as hydrogen bonding. Hydrogen bonds are relatively stronger than other dipole interactions, and they determine the molecular behavior. For example, water molecules have intermolecular hydrogen bonding. One water molecule can form four hydrogen bonds with another water molecule. Since oxygen has two lone pairs, it can form two hydrogen bonds with positively charged hydrogen. Then the two water molecules can be known as a dimer. Each water molecule can bond with four other molecules because of the hydrogen bonding capability. This result in a higher boiling point for water, even though a water molecule has a low molecular weight. Therefore, the energy needed to break the hydrogen bonds when they are going to the gaseous phase is high. Further, hydrogen bonds determine the crystal structure of ice. The unique arrangement of ice lattice helps it to float on water, hence protects the aquatic life in the winter period. Other than this, hydrogen bonding plays a vital role in biological systems. The three-dimensional structure of proteins and DNA are solely based on hydrogen bonds. Hydrogen bonds can be destroyed by heating and mechanical forces.
What is the difference between Van der Waals Forces and Hydrogen Bonds?
• Hydrogen bonds occur between hydrogen, which is connected to an electronegative atom and an electronegative atom of another molecule. This electronegative atom could be a fluorine, oxygen or nitrogen.
• Van der Waals forces can occur between two permanent dipoles, dipole- induced dipole, or two induced dipoles.
• For Van der Waals forces to take place, the molecule should not necessarily have a dipole, but Hydrogen bonding takes place between two permanent dipoles.
• Hydrogen bonds are much stronger than Van der Waals forces.