Sections 3.1-3.3

Water, pH, and Non-Covalent Bonding

Non-covalent bonding

Non-covalent bonds are not at strong as covalent bonds, but they are important in the stabilization of molecules. In contrast to covalent bonds, non-covalent bonds do not share electrons. Noncovalent bonds include:

  • Electrostatic interactions
  • van der Waals forces
  • Hydrophobic interactions
  • Hydrogen bonds

Electrostatic interactions are formed between positive and negative ions. The bond is non-directional, meaning that the pull of the electrons does not favor one atom over another. An example is NaCl, which is formed between the positively charged Na+ ion and the negatively charged Cl- ion. The average strength of an electrostatic interaction is 15 kilojoules/mol (kJ/mol). The bond strength lessens when the distance between the two ions increases.

van der Waals forces (aka London forces) are weak forces between temporary dipoles. These forces may be attractive or repulsive. They are also non-directional. The average bond energy for van der Waals forces is on the order of 10 kJ/mol.

Hydrophobic interactions result when non-polar molecules are in a polar solvent, e.g. H2O. The non-polar molecules group together to exclude water (hydrophobic means water fearing). By doing so they minimize the surface area in contact with the polar solvent.

A hydrogen bond results when a hydrogen atom that is covalently bound to an electronegative atom (e.g. O, N, S) is shared with another electronegative atom. A hydrogen bond is directional toward the electronegative atom. An example of this is the hydrogen bonds formed in water. Hydrogen bonds are constantly being made and remade. The half-life of a hydrogen bond is about 10 seconds. A hydrogen bond has an energy of 21 kJ/mol.

As stated above, non-covalent bonds are not as strong as covalent bonds, but the additive effect of many non-covalent bonds can stabilize a molecule. Non-covalent bonds are very important in the structure of proteins.

Bond type Energy (kJ/mol)
Covalent, e.g. C-C 350
Electrostatic 15
van der Waal's 10
Hydrogen 21

Copyright 2002, John Wiley & Sons Publishers, Inc.