Namely: For each set of molecules, explain which forces you think are the most important and rationalize why the molecule with the highest ΔHvap is the highest. (i) hexane (C6H14) 31.5 (kJ mol-1), decane (C10H22) 51.4, octadecane (C18H38) 91.4 (ii) methane (CH4) 8.6, acetonitrile (CH3CN) 33.9 (iii) hexane (C6H14) 31.5, 1-pentanol (C5H11OH) 56.9
For a liquid to evaporate, the intermolecular interactions present in the liquid state must be overcome by the kinetic energy of the molecules. The enthalpy of vaporization is the energy required to overcome these interactions. Thus ΔHvap shows trends related to the types and strengths of intermolecular interactions present in particular substances. These interactions are:
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- London forces – present in all substances; due to induced-dipole-induced-dipole interactions; strength approximately proportional to number of electrons;
- Dipole-dipole interactions – present only in substances where the molecule has a net permanent electric dipole; usually stronger than London forces;
- Hydrogen bonds – interaction between an electropositive H and lone pairs on a N, O, or F atom.
Based on these definitions, consider the following three sets of molecules and discuss the role of intermolecular interactions in determining ΔHvap.
Namely: For each set of molecules, explain which forces you think are the most important and rationalize why the molecule with the highest ΔHvap is the highest.
(i) hexane (C6H14) 31.5 (kJ mol-1), decane (C10H22) 51.4, octadecane (C18H38) 91.4
(ii) methane (CH4) 8.6, acetonitrile (CH3CN) 33.9
(iii) hexane (C6H14) 31.5, 1-pentanol (C5H11OH) 56.9
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