Why are Hydrogen Bonds Weak?
Hydrogen bonding refers to the weak interaction between two molecules containing Hydrogen atom bonded to an electronegative atom such as Oxygen, Fluorine and Nitrogen. This only occurs in molecules that have a permanent dipole (like water and ammonia) and contains high electronegative elements like the ones mentioned above (O, F, N).
How do they form? Molecules with polar covalent bonds such as water have a charged part. When this charged part forms an electrostatic interaction with another molecule with an opposite charge for example Oxygen molecule, they form a bond. Just like when a part of hydrogen molecule sticks to the oxygen molecule, they freeze together and form an ice cube. But, only molecules which have polar covalent bonds can do this, non polar covalent bonds cannot form a hydrogen bond.
A hydrogen bond is one kind of weak bonds. Other forms of weak bonds are ionic bonds and van der Waals interaction. Weak bonds are attractions which forms easily and quickly and do not a need large amount of energy to break because they are easily broken under natural biological conditions. For hydrogen bonds, only energy of 4-5 kcal/mol is needed to break the bond. This is much lesser compared to the energy needed to break strong bonds.
Although they are weak bonds, they are very important in the biological system. Hydrogen bonds between water and other molecules determine the vital and unique attributes of water. It is also very essential to keeping the secondary structure of proteins and nucleic acids stabilized. It is also involved during enzymic catalysis. Lastly, hydrogen bonds keep the two strands of DNA double helix together.
In conclusion, hydrogen bonds are weak because they are easily formed and broken under normal conditions. Nevertheless, this weak bond is very important in our biological system.