Intermolecular Forces in Liquids
When a substance is in the liquid state, its molecules or atoms are held together by mutual attraction; without these forces, the molecules or atoms would expand to fill all the space available, becoming a gas. However, these forces are not as strong as those holding solids together, giving liquid molecules or atoms freedom to move about. We can measure the strength of the intermolecular (or interatomic) forces in liquids through the boiling point: the more tightly the units are held together, the more heat energy will be needed to separate them into a gas. Conversely, if intermolecular attractions are weak, the boiling point will be low.
There are four main types of intermolecular forces, from strongest to weakest: ion-dipole, dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole (also called dispersion or London forces). Collectively, they are referred to as van der Waals' forces, after the scientist who also investigated their effects on gases.
When we refer to dipoles, we mean an electrically asymmetrical molecule. If a molecule is not electrically symmetrical, a positive charge will accumulate on one side, and a negative will build up on the other. Molecules with dipoles are said to be polar; nonpolar molecules do not have dipoles. Water, on the right, is a polar molecule because the oxygen atom "wants" electrons more than the hydrogen, pulling the molecule's electrons towards the oxygen and creating a charge imbalance. Since electrical charges can attract or repel each other, dipoles are important in intermolecular forces.
The first type of attraction is ion-dipole. In these situations, a charged ion is attracted to the dipole of a polar molecule. These are by far the most powerful types of attraction. Examples include the dissolution of salt in water; the negatively-charged Cl ions will be attracted to the positive dipole near the hydrogens, while the positively-charged sodium ions will seek the negative dipole of the oxygen atom. These attractions are powerful enough to tear apart the NaCl crystal when it enters water, meaning that salt dissolves. If the crystal structure is too strong to be broken by attractions between a solvent and the ionic solid, then the solid will not dissolve.
The next type of attraction is dipole-dipole, in which the dipoles of two molecules are mutually attracted. For example, the molecule FI has a permanent dipole because fluorine "wants" electrons more than iodine, which leads to a positive charge on the iodine and a negative charge on the fluorine. These molecules will attract each other, because the negatively-charged fluorine will be drawn to the positive iodine atom of another molecule.
A subtype of dipole-dipole interactions is hydrogen bonding, which occurs when a nitrogen, oxygen, or fluorine atom is bonded with one or more hydrogens. Since each of these atoms "wants" electrons more than hydrogen, a dipole will result. These dipoles result in strong attractions between molecules. The intermolecular attractions in water (H2O), menthanol (CH3OH), ammonia (NH3), and hydrogen fluoride (HF) are all examples of hydrogen bonding. All of these substances have conspicuously high boiling points, due to the unusual strength of hydrogen bonding.
Another type of intermolecular attraction is dipole-induced dipole. This occurs when a polar molecule, such as water, is attracted to a non-polar molecule, such as pentane (C5H12). This type of attraction occurs because the charge on the water molecule distorts the electron clouds of pentane's component atoms, causing them to be either attracted to the positive dipole or repelled by the negative dipole. These interactions are very weak, but explain the solubility of non-polar compounds in polar solvents, such as oxygen dissolved in water.
Note that the first three types of forces generally decrease with size; the larger the atoms or molecules, the less attractive the intermolecular forces will be.
The final type of interaction, the induced dipole-induced dipole forces, is also known as dispersion forces or London forces. Since the electron clouds whirling about an atom are not always perfectly symmetrical, there will be occasional attractions between non-polar molecules because of the anomalies in their electron clouds. These forces are usually weak, but increase with the atom's or molecule's sizes, and can become fairly strong in large molecules or atoms (because there are more opportunities for electron cloud distortion). Therefore, methanol (CH42, because bromine is a comparatively large atom) and pentane (C5H12, a relatively large molecule) are liquids. Dispersion forces occur between all molecules, but are most evident in non-polar molecules, because they are the only attractions holding the molecules together.
Ion-ion attractive forces also occur when ionic solids are melted, and are even stronger than ion-dipole interactions. These forces only occur at very high temperatures; trying to melt an ionic solid requires a lot of energy!
These interactions lead us to postulate a solubility law for liquids: like dissolves like. For example, polar liquids will dissolve other polar liquids because of dipole-dipole interactions, and non-polar liquids will also usually dissolve non-polar liquids due to dispersion forces. However, non-polar liquids are not sufficiently attracted to polar liquids to break the strong dipole-dipole interactions between solvent molecules. Therefore, liquids with like polarities will dissolve, whereas a non-polar liquid will not dissolve in a polar liquid.
Intermolecular attractions have some other effects on liquids. Viscosity is a measure of how fluid, or "runny," a liquid is. For example, water has low viscosity and runs easily. Cold maple syrup flows slowly, so it has high viscosity. Intermolecular forces play some role in viscosity, because stronger attractions between molecules cause them to resist flow more strongly. Molecule size is also an important factor in viscosity; longer molecules can become tangled and flow slowly. Surface tension is also a result of intermolecular forces. Molecules at the surface of a liquid are attracted to the molecules beneath and beside them, leading to an inward force on the liquid and a kind of skin on the surface. This tension also causes drops of water to contract into spheres, minimizing surface area. Vapor pressure, the subject of the next page, is also affected by intermolecular attractions, as you will see.