Boiling points are a function of the intermolecular forces.
The ethanoic acid dimer has an effective RMM = 120. This means that considerable Van der Waals force would be expected between particles. Compare with a totally non-polar molecule with a similar relative mass such as decane. The boiling point of decane is 174ºC.
Were ethanoic acid simple monomers (RMM = 60) then comparison with pentane (bp=36ºC) suggests a much higher degree of intermolecular force.
It seems that the boiling point of ethanoic acid (118ºC) is somewhere between that expected from a simple monomer (with hydrogen bonds) and a dimer.
Clearly the situation is not simple and there will be contributions from hydrogen bonding, Van der Waals forces, monomers and dimers. It is difficult to state categorically which is the most important.
May 21st, 2007
All liquids contain particles in which the energy distribution is governed by the laws of statistics. This energy distribution may be plotted as a curve, called the Maxwell Boltzmann graph that shows some particles with very little energy, some with large amounts of energy and the bulk of the particles with energy somewhere in between. Those particles with large amounts of energy can escape from the body of the liquid producing a gas formed of liquid particles above the surface. This is called the vapour and the pressure that it exerts is called the vapour pressure.
The boiling point of a liquid is the temperature at which the vapour pressure equals the outside (atmospheric pressure). At this point bubbles of vapour form in the body of the liquid at this temperature – we call this “boiling”
The bubbles of vapour forming in the liquid can literally push the water apart as they have the same pressure as the atmosphere.
If the external pressure decreases then the vapour pressure will be able to equal it at a lower temperature. i.e. the liquid boils at a lower temperature.