Dipole-Dipole Interactions

Key Questions

Dipole-dipole forces are the attraction between the positive end of one molecule and the negative end of another.

Explanation:

Dipoles form when there is a large difference in electronegativity between two atoms joined by a covalent bond.

The atoms share the electrons unequally because the more electronegative atom pulls the shared electrons toward itself.

That means that one atom has a partial negative charge, and the other atom has a partial negative charge.

For example, H-Cl has a dipole. The electrons spend more of their time near the Cl atom (the red end in the diagram below).

A dipole-dipole interaction is the attraction between two polar molecules. When they approach each other, the negative end one molecule attracts the positive end of the other.

The molecules arrange themselves to increase their attraction and reduce their potential energy.

• The first thing required for there to be dipole-dipole interactions between molecules is for both molecules to have a dipole. These dipoles may be permanent (polar molecules) or temporary (instantaneous or induced dipoles in non-polar molecules).

Permanent dipole-permanent dipole interactions

Polar molecules have an asymmetrical electron cloud/charge distribution. This is due to an asymmetrical shape (due to lone pairs of electrons around the central atom) and/or due to the presence of polar-covalent intra-molecular bonds (electronegativity difference between the two atoms of 0.5+), which do not "cancel out" each other.

As there is an asymmetrical charge distribution, one side ("pole") of the molecule has a slight ("delta") negative charge, while the other pole has a delta positive charge. The attraction between the delta negative pole of one molecule and the delta positive pole of a neighbouring molecule is called a permanent dipole-permanent dipole attraction.

Temporary dipoles

Non-polar molecules can also have temporary dipoles. These form more often when the molecule has a large electron cloud (you can predict this from its molar mass). At any one instant, the electrons may be unevenly distributed, creating an instantaneous dipole within the molecule: one pole is delta negative, while the other is delta positive. This can induce at dipole in a neighbouring molecule, creating a temporary dipole-temporary dipole attraction.

This idea can be used to explain why $C {l}_{2}$ is a gas, $B {r}_{2}$ is a liquid (more dipole-dipole attractions), and ${I}_{2}$ is a solid that sublimes (even more dipole-dipole attractions).

It is caused by forces exerted over a polar molecule -such as solubilisation, voltage- which re-orient the charges of the polar molecule.

Explanation:

This question is a picking brain.

Here is the answer: Dipole interaction of substance are caused by the surrounded media (such as solvent, temperature, ...) and inductive forces (such as voltage, ...) as well.

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