The magnitude of a molecular dipole moment can be approximated by vector addition of the individual bond dipole moments.
Consider the following idealized data:
• Each N–H bond has a dipole of 1.0 D directed from H toward N.
• Each O–H bond has a dipole of 1.5 D directed from H toward O.
Geometric information (expressed in terms of a vertical z-axis that bisects the molecule):
• NH3: The three N–H bonds are symmetrically arranged around the z-axis such that each bond makes an angle of 33o with the +z direction.
• H2O: The two O–H bonds lie in the xz-plane; each bond makes an angle of 52.25o with the +z direction.
Ignore deviations from ideal geometry caused by lone-pair repulsions, and treat the bond dipoles as vectors of the given magnitude placed tail-to-tail at the central atom.
(a) Calculate the approximate magnitude of the net dipole moment of NH3. Express your answer to two significant figures in debye (D).
(b) Repeat the calculation for H2O, again reporting the magnitude to two significant figures in debye.
(c) Based on your results, predict which of the two molecules would experience the greater orientational energy when placed in a uniform external electric field of fixed strength. Support your answer quantitatively.