select the molecule that will display stronger intermolecular dipole-dipole forces?

Question

Q1) PCl3
SO3

Q2) CF4
CH2F2

Q3) SO3
SO2

Q4) OCS
CO2

Answer 1

PCl3 has the same shape as NH3 (tetrahedral with P in the center, the 3 Cl at the 3 corners and a lone pair at the fourth corner) So the polar covalent bonds don't cancel out each other

SO3 is planar trigonal and thus the dipole moments cancel out each other and it is non-polar

CF4 has a tetrahedral shape and since all bonds are the same they cancel each others dipole moment;the molecule is nonpolar

CH2F2 has the approximately the same shape as CF4, but since not all bonds are the same their angle is a bit different and the sum of the dipole moments is not zero; thus the molecule is polar

SO2 is a bent molecule (because of the sulphur's lone pair), think of it in a way as water with S in the place of O and O in the place of H so it is polar.

CO2 is a linear molecule (O=C=O) so the dipole moments of the two identical polar bonds cancel out each other; the molecule is nonpolar

O=C=S has different polar bonds, so although they have opposite directions the result will be a slight dipole moment, so the molecule is polar.

In all your questions you have to compare a polar and a nonpolar molecule. Of course the one with the strongest dipole-dipole interactions in each case is the polar molecule.

Answer 2

Ignore the two symmetric molecules c) and e) Read your class notes to see if the teacher or book lists H-bonds as a separate force from dipole-dipole forces. If so, eliminate NH3 Put the remaining polar molecules in order by differences in electronegativities.