Intermolecular forces exist between independent particles, such as atoms, ions, or molecules. They can be forces of either attraction or repulsion. The amount of charge, how it is distributed, and the length of time that a charge distribution exists can affect the strength of intermolecular forces. And despite having variable force strengths, all intermolecular forces are considered weak compared to chemical bonds, or intramolecular forces. Chemical bonds are not only stronger; they are also more permanent.
The energy costs involved in breaking chemical bonds are much higher than ones needed to overcome intermolecular forces. There are five types of intermolecular forces: ion-ion forces, ion-dipole forces, ion-induced dipole/dipole-induced dipole forces, dipole-dipole forces, and London dispersion forces. Generally, we expect ion-ion forces to be the strongest, followed by ion-dipole, dipole-dipole, and then London dispersion forces. Of course, many exceptions to this hierarchy of strength exist.
In order to properly differentiate between these forces, it is important to know what must be present in order for each interaction to occur. Ion-ion forces only involve ions in mixtures of substances. Ion-ion forces can be either attractive (cation-anion) or repulsive (cation-cation/anion-anion) and the strength varies depending on charge and size. Ion-dipole forces occur in mixtures between ions and polar molecules. The anions gravitate toward positive regions of dipoles while the cations gravitate toward negative regions.
With dipoles, the strength of the forces depends upon the polarity of the molecule (or charge magnitude) and how compact the molecule is. If a molecule is more compact, there is better access to the center of charge and stronger attraction to its neighbors. Induced dipoles occur when nonpolar molecules come in the vicinity of polar or charged particles and become polar themselves. As an ion or dipole moves closer to the nonpolar molecule, a shift occurs in its electrons, throwing off its nonpolar symmetry and making it polar.
Depending on what produces this change, it will have either attractive ion-induced dipole or dipole-induced dipole forces. These may occur in pure substances or mixtures. Dipole-dipole forces may occur between the polar molecules of a pure substance, or between two different polar molecules. The positive regions of one dipole will attract the negative regions of another and vice versa. The dipoles tend to align in a way that increases the number of attractions and reduces the number of repulsions.
The strength of the force can vary with polarity: the more polar the molecules are, the more strongly they interact with each other. Hydrogen bonding is considered a special case of dipole-dipole interaction. While dipole-dipole forces are generally considered to be fairly weak, hydrogen bonding is unusually strong, especially in water. This particular type of bonding occurs when a hydrogen atom is involved in an extremely polar covalent bond, such as H-N, H-O, or H-F, and is attracted to the lone pair of a highly electronegative atom (either F, N, or O) on a separate molecule.
These may also occur in pure substances. The weakest of the intermolecular forces are the London dispersion forces. These forces occur between atoms or molecules of nonpolar substances and are present in both pure substances and mixtures. A way to predict the types of intermolecular forces present is by looking at the chemical formula, specifically whether the interacting species are polar or nonpolar. Ion-dipole forces occur between ions and polar molecules. Dipole-dipole forces (including hydrogen bonding) only occur between polar molecules.
Induced dipoles occur between polar and nonpolar molecules. If there were only nonpolar molecules, they would be London dispersion forces (but keep in mind that these forces also exist in every other kind of interaction). In the case of ion-ion forces, polarity does not matter in identifying forces, as it only involves ions and would be fairly obvious. Knowing what we do about intermolecular forces and their relative strengths, we can make a few assumptions about which forces would be present in different phases under standard conditions.
Being that solids are the most difficult to break apart, we would assume that the strongest intermolecular forces (ion-ion, hydrogen bonding) would be found within them. Liquids have a greater ability to flow because the intermolecular forces are weaker than in the solid phase, so we would assume that these would involve ion-dipole and induced dipole forces. We would also assume that the weakest intermolecular forces correspond to the gas phase, meaning dipole-dipole and London dispersion forces.
Intermolecular forces influence physical properties of each phase: gas, liquid, and solid. They can cause real gases to deviate from ideal gas behavior. They can also govern the motion of molecules; molecules in gases move constantly and randomly, they slide past each other freely in liquids, and vibrate in fixed positions in solids. The heats required to melt a solid (heat of fusion) and to vaporize a liquid (heat of vaporization) change depending on the strengths of the intermolecular forces. In liquids, water will form beads upon contact with waxed surfaces (e. . car hoods) because of the imbalance of how intermolecular forces act upon surface molecules and the symmetrical distribution of forces experienced by interior molecules.
So, the stronger the intermolecular forces, the larger the surface tension. Capillary action is another example of the effect of the imbalance of intermolecular forces. If the intermolecular interactions between the particles of a liquid and a solid are stronger than the intermolecular forces acting between the liquid’s own particles, the liquid near the walls of the solid will rise.
Other properties of liquids that can be affected by intermolecular forces are boiling point and critical temperature and pressure. In crystalline solids, the stronger the forces are, the more rigid the crystal is. This is because the stronger intermolecular forces in the solid fix the particles in place. Overall, understanding intermolecular forces is essential to understand gas, liquid, and solid phases, as well as the phase changes between them.
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 1 October 2016
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