Question

All compounds in a chemical series have the same commonality of structure, therefore the same predominant IMF. Using concepts of intermolecular forces, please explain the trends of decreasing ΔT with increasing molar mass within a chemical series.

Answer 1

Solution:

The are three major types of intermolecular forces; 1. London dispersion forces, 2.dipole–dipole interactions, and H-bonds. 1 & 2 are often referred to collectively as van der Waals forces.

London dispersion forces (LDF) are due to the formation of instantaneous dipole moments in polar or nonpolar molecules as a result of short-lived fluctuations of electron charge distribution. This in turn cause the temporary induced dipole with neighboring molecules. LDF is proportional to # of sigma bonds in the molecule.

Dipole–dipole interactions (DDI) arise from the electrostatic interactions of the positive and negative ends of molecules with permanent dipole moments. Strength of DDI is proportional to the magnitude of the dipole moment and inversely proportional to the distance between dipoles.

H - bonds (HB) are especially strong dipole–dipole interactions between molecules that have hydrogen bonded to a highly electronegative atom, such as O, N, or F. The resulting partially positively charged H atom on one molecule (the hydrogen bond donor) can interact strongly with a lone pair of electrons of a partially negatively charged O, N, or F atom on adjacent molecules (the hydrogen bond acceptor).

Strength of IMF are in the following increasing order; LDF < DDI < HB.

As the molar mass of a chemical series increase, the # of sigma bonds increases (non-polar group) and the functional group remain unchanged. This in turn increase the LDF. Energy required to break the LDF is less. Hence, the trends of decreasing ΔT with increasing molar mass within a chemical series is observed.