Dipole-Dipole Forces

Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together (touching or almost touching).

The figures show two arrangements of polar iodine monochloride (ICl) molecules that give rise to dipole-dipole attractions.

Note:

Polar molecules have a partial negative end and a partial positive end.
The partially positive end of a polar molecule is attracted to the partially negative end of another.
In a ICl molecule the more electronegative chlorine atom bears the partial negative charge; the less electronegative iodine atom bears the partial positive charge.
The partially positive iodine end of one ICl molecule is attracted to the partially negative chlorine end of another ICl molecule.

A dashed line is used to represent an intermolecular attraction between molecules because these forces are NOT as strong as chemical bonds.

		= I = Cl
One arrangement of ICl molecules that gives rise to a dipole-dipole attraction.	Another arrangement of ICl molecules that gives rise to a dipole-dipole attraction.

Physical Consequences of Dipole-Dipole Forces

Both ICl and Br₂ have the same number of atoms and approximately the same molecular weight, but ICl is a solid whereas Br₂ is a liquid at 0^oC. Why?

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