Questions: Molecular Polarity and Dipole Moments

Question 1

Which of the following molecules contains polar bonds but has a net dipole moment of zero?

• A. H₂O
• B. NH₃
• C. CO₂
• D. HCl

Answer: (C)

CO₂ is linear, so its two identical C=O bond dipoles point in exactly opposite directions and cancel, giving a zero net dipole moment. H₂O (bent), NH₃ (trigonal pyramidal), and HCl (diatomic, asymmetric) all have net dipole moments because their bond dipoles do not cancel by symmetry.

Question 2

Any molecule that contains polar bonds is a polar molecule.

• T. True
• F. False

Answer: False

Molecular polarity depends on both bond polarity AND molecular geometry. If polar bonds are arranged symmetrically, their vector dipoles cancel and the net dipole moment is zero. CCl₄ (tetrahedral) and BF₃ (trigonal planar) are classic examples: all bonds are polar, but the symmetric arrangement produces a nonpolar molecule.

Question 3

Both CO₂ and H₂O contain polar covalent bonds. Why is CO₂ nonpolar while H₂O is polar?

Model answer: CO₂ is linear — its two C=O bond dipoles point in opposite directions and cancel exactly. H₂O is bent (due to two lone pairs on oxygen), so its two O–H bond dipoles point in directions that do not cancel, producing a net dipole moment pointing toward oxygen.

Geometry is the deciding factor. The VSEPR model predicts CO₂ as linear (no lone pairs on carbon) and H₂O as bent (two lone pairs on oxygen repel the bonding pairs). In CO₂, the 180° bond angle guarantees perfect cancellation. In H₂O, the ~104.5° bond angle means the two O–H dipoles add to a net downward vector (toward O), making the molecule polar.