3) Explain why the standard enthalpy of vaporization, (Delta) Hvap, values for each set of compounds below are not the same.

a. CH4 and H2O

b. PH3 and NH3

c. C2H6 and C3H8

d. BH3 and OF2

a. CH4 is only held together by London dispersion forces since any dipoles cancel out due to symmetry. On the other hand H2O has strong hydrogen bonds

b. PH3 has dipole-dipole forces while NH3 is held together by stronger hydrogen bonding

c. C2H6 and C3H8. Both substances are held together by London dispersion forces since any dipoles cancel out due to symmetry. However C3H8 has the stronger London dispersion forces of the two because it is a larger molecule,

d. BH3 is held together by London dispersion forces since any dipoles cancel out due to symmetry while OF2 is held together by stronger dipole-dipole forces