Answer
a) NH$_{3}$ - higher dispersion forces, dipole moment
b) KCl - dipole moment
Work Step by Step
a) NH$_{3}$ requires more heat to break its stronger intermolecular forces because of its higher dispersion forces due to its larger electron cloud. In addition, it also has a dipole moment that allows stronger dipole-dipole interactions.
b) KCl, as an ionic compound, is highly polar, and compared to the very nonpolar I$_{2}$, will require more energy to break its dipole-dipole forces.
