Intermolecular forces (IMFs) are the attractive or repulsive forces that exist between separate molecules. These forces are significantly weaker than the intramolecular bonds holding atoms together within a single molecule, but they are responsible for the physical properties of substances, such as melting points and boiling points. Dipole-dipole forces are a type of permanent attraction, arising from a fixed separation of charge within the interacting molecules.
The Foundation: What Makes a Dipole Permanent?
A molecule must satisfy two structural requirements to possess a permanent dipole moment. The first involves electronegativity, which measures an atom’s ability to attract electrons within a chemical bond. When atoms bond unequally, the more electronegative atom pulls the electron density closer. This unequal sharing creates partial charges, denoted as partial positive (\(\delta+\)) and partial negative (\(\delta-\)).
The second requirement is the molecule’s overall geometry, which must be asymmetrical. A molecule may contain multiple polar bonds, but if the molecular shape is highly symmetrical, these individual bond dipoles can cancel each other out. For instance, carbon dioxide (\(\text{CO}_2\)) has two polar bonds, but its linear structure causes the dipoles to pull equally in opposite directions, resulting in no net permanent dipole. Molecules with an asymmetrical shape, like water (\(\text{H}_2\text{O}\)) or hydrogen chloride (HCl), ensure the partial charges are fixed at opposite ends, creating a constant separation of charge.
How Permanent Dipoles Create Intermolecular Forces
The separation of charge within polar molecules means the resulting attractive force is continuous. Since the \(\delta+\) and \(\delta-\) regions are fixed by the molecular structure, the molecule always behaves like a tiny, two-poled magnet. When these polar molecules are near each other, the positive end of one molecule is electrostatically attracted to the negative end of a neighboring molecule.
This attraction causes the molecules to orient themselves to maximize favorable positive-to-negative interactions, reducing the overall potential energy. Because this force is always present, polar substances require more energy to overcome these continuous attractions and change state. This constant attraction is why polar substances often exhibit higher melting and boiling points than non-polar substances of comparable size.
Permanent vs. Transient: Comparing Dipole-Dipole and London Dispersion Forces
The permanent nature of dipole-dipole forces is best highlighted by contrasting them with the London Dispersion Force (LDF). Dipole-dipole forces arise from a fixed charge distribution built into the molecule’s structure. In contrast, LDFs originate from the random, momentary fluctuations of electron clouds around a molecule’s nucleus.
These electron movements momentarily skew the charge distribution, creating a transient dipole that quickly vanishes and reforms elsewhere. While LDFs are present in all molecules, they become the dominant force only in non-polar molecules that lack a permanent dipole. Dipole-dipole forces, being constant, are generally stronger than LDFs when comparing molecules of similar mass and size. The strength difference exists because the attraction in a dipole-dipole interaction is steady, whereas the attraction in LDF is fleeting and intermittent.