Features of Hydrogen Bonding

Hydrogen Bonding and Colligative properties Association

"Colligative properties" are those that depend on the number of particles and not necessarily the nature of particles. Molecular weights determined by colligative property measurements may be different from true values.

Thus the molecular weight of NaCl using such measurement will be half its actual value since NaCl exists in solution as two particles. Benzoic acid on the other hand will give twice the actual value since it exists as an association of two particles (dimer).

• In many hydrogen bonded aliphatic acids it has been observed that the experimental molecular weights are considerably higher than their true values.
• These values decrease with increase in temperature approaching the actual molecular weights at elevated temperatures.

• If the compound exists completely as dimers then the molecular weight should have been double.
• The compound exists as an equilibrium mixture containing the monomers, the dimers and even trimers wherever possible.
• Increasing temperature increase the average kinetic energy of the molecules, this disrupts hydrogen bonding inter action shifting the equilibrium in favour of the monomeric form.
• At higher temperature the value of molecular weight is closer to the actual value.

* Data on this page is from: Hydrogen bonding, Vinogradov / Linnell