Final answer:
Among the molecules listed, HBr and CH3F have dipole-dipole forces due to their polar nature with uneven electron distribution, whereas CI4 is nonpolar with no net dipole.
Step-by-step explanation:
Dipole-dipole forces are attractive forces between polar molecules with permanent dipoles. Polar molecules have a net dipole due to an unequal distribution of electrons, resulting in a partial positive charge on one side and a partial negative charge on the other side of the molecule. An example is hydrogen bromide (HBr), where the more electronegative Br atom has a partial negative charge, and the H atom has a partial positive charge, allowing it to engage in dipole-dipole interactions. Carbon tetraiodide (CI4) is a nonpolar molecule with a symmetrical tetrahedral structure, leading to the cancellation of dipole moments and consequently to no net dipole. Lastly, methyl fluoride (CH3F) is a polar molecule because the electronegative fluorine atom creates a significant dipole with its bonded carbon.
The molecules that have dipole-dipole forces are HBr and CH3F. These molecules have a difference in electronegativity between the bonded atoms, leading to a partial positive and partial negative charge within the molecule. The dipole-dipole forces occur when the positive end of one polar molecule attracts the negative end of another polar molecule, creating an electrostatic attraction between them.