Dipole-Dipole?

Question

In which compounds are there dipole-dipole attractions?

a. HBr
b. H2Se
c. AsH3
d. GeH4
e. AsF5

Answer 1

In order for a molecule to have dipole moment (and thus have dipole-dipole attractions between its molecules) it must have polar covalent bonds (that is bonds between atoms of different electronegativities) and the shape of the molecule should such that the sum of the dipole moments of the bonds do not cancel out.

a) HBr has a single bond between atoms with different electronegativites so it has a dipole moment

b) Se belongs in the same group of the periodic table as O and S. So the molecule has a similar bent shape as water, the dipole moments of the bonds do not cancel out and the molecule has a net dipole moment.

c) As belongs to the same group as N. So the shape of the molecule will be similar to NH3 (trigonal pyramid) and it wil have a net dipole moment

d) Ge belongs to the same group as C. So the shape of the molecule is like that of CH4 (tetrahedral). in this case the geometry is such that the dipole moments of the bonds cancel out each other and the molecule has no dipole moment.

e)AsF5. The molecule has the shape of a trigonal bipyramid. All bonds are identical and the geometry is such that their dipole moments cancel out so the molecule has no dipole moment.

To summarize,dipole-dipole attractions are found in a,b and c.

Answer 2

a. HBr