Question

At \({20^\circ }{\rm{C}}\), the vapor pressures of several alcohols are given in this table. Explain these vapor pressures in terms of types and extents of IMFs for these alcohols:

Short answer

The order of molecules according to the strength of intermolecular forces between them is, Methanol < Ethanol < Propanol < Butanol

Step-by-step solution

Define the vapour pressure   

The temperature at the physical state of the liquids starts changing to a gaseous state is called the boiling point. At this temperature, the vapor pressure of the liquid becomes equal to the external pressure.

 Identify the melting and boiling point

The vapor pressure is the pressure exerted by the molecules present in the gaseous state. The greater the number of molecules in the gaseous phase, the greater is the value of vapor pressure. The alcohol molecules contain carbon atoms bonded to the hydroxyl group. The alcohol molecules, in the liquid state, form hydrogen bonds with each other. A large amount of energy is required to overcome these hydrogen bonds. Moreover, the intermolecular force between molecules increases as the molecular mass of the compound increases.

Vapor pressure \( \propto \frac{1}{{{\rm{IMF}}}}\)

\( \propto \frac{1}{{{\rm{ Molecular mass }}}}\)

From the given compounds, methanol has the lowest molecular mass and the highest vapor pressure. On the other hand, butanol has the highest molecular mass and the lowest vapor pressure. This is because the intermolecular interactions between the molecules of butanol are strong; as the result, it has the lowest vapor pressure. The order of molecules according to the strength of intermolecular forces between them is,

Methanol < Ethanol < Propanol < Butanol