Intermolecular forces
Intermolecular forces are forces of attraction and repulsion between interacting particles (atoms and molecules). They are weaker than intermolecular forces such as covalent or ionic bonds within a molecule. In chemistry, especially at the high school level, understanding these forces is important because they explain how and why matter behaves differently in different states - solid, liquid and gas.
Observation of states of matter
The three main states of matter are solid, liquid, and gas. Each state has different characteristics based on how the particles interact with each other:
- Solid: Particles are adjacent to each other in a definite arrangement. Strong intermolecular forces hold the particles together, giving solids a definite shape and volume.
- Liquid: Particles are in close contact but not in a rigid structure, allowing them to flow. Liquids have a definite volume but no definite shape, conforming to the shape of their container.
- Gas: Particles are far apart and move around freely. Gases have neither a definite shape nor a definite volume, they expand to fill their container.
Types of intermolecular forces
There are many types of intermolecular forces, each of which varies in strength and ability to act under specific circumstances.
1. London dispersion force
London dispersion forces, also known as dispersion forces, are the weakest intermolecular forces and arise from the temporary polarization of electron clouds in atoms or molecules.
Imagine for a moment that the electrons revolving around the nucleus become momentarily concentrated on one side of the atom, creating an instantaneous dipole. This dipole can induce a dipole in the neighboring atom, producing a weak, temporary attractive force.
Random electron distribution: - O oo O o
2. Dipole-dipole force
Dipole-dipole forces occur between polar molecules — molecules that have permanent dipoles. A polar molecule has a partial positive charge at one end and a partial negative charge at the other end, due to differences in electronegativities between the atoms.
The positive end of one polar molecule attracts the negative end of another polar molecule, resulting in a stronger type of intermolecular force than London dispersion forces.
Permanent dipole interactions: + -- oo -- +
3. Hydrogen bonding
Hydrogen bonding is a special type of dipole-dipole interaction. It arises when hydrogen is covalently bonded to highly electronegative atoms such as nitrogen, oxygen or fluorine. The larger electronegative difference creates a stronger dipole. If another electronegative atom with a lone pair of electrons approaches the hydrogen atom, a hydrogen bond is formed.
Water is the most common example of a substance with strong hydrogen bonds, which accounts for its high boiling point and surface tension.
Hydrogen bond example: O - H -- O | N - H -- O
4. Ion-dipole force
Ion-dipole forces are forces of attraction between an ion and a polar molecule. These forces are particularly strong, much stronger than hydrogen bonds, and are typically found in solutions where ionic compounds are dissolved in polar solvents, such as salt in water.
Example of ion-dipole interaction: Na+ -- O(-)H2O
Intermolecular forces and physical properties
The strength and type of intermolecular forces directly affect the physical properties of substances, including boiling point, melting point, solubility, and vapor pressure.
1. Boiling point and melting point
Generally, substances with stronger intermolecular forces have higher boiling and melting points. This is because more energy is needed to overcome these forces. For example, hydrogen fluoride (HF
), which has hydrogen bonding, has a much higher boiling point than hydrogen chloride (HCl
), which has only dipole-dipole forces.
2. Solubility
Like dissolves like - polar substances dissolve well in polar solvents, and nonpolar substances dissolve well in nonpolar solvents. Water, a polar solvent, can effectively dissolve ionic compounds and other polar substances, thanks largely to its ability to form hydrogen bonds.
3. Vapour pressure
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase. Substances with weak intermolecular forces have higher vapor pressures because the particles easily escape from the liquid phase. For example, diethyl ether, which has weak London dispersion forces, has a higher vapor pressure than water at the same temperature.
Role of intermolecular forces in everyday life
Understanding intermolecular forces goes beyond theoretical knowledge; it has real-world applications. These forces explain why bubbles form in boiling water, why ice is less dense than liquid water, and how lizards walk on walls.
For example, the ability of water to form droplets and beads on a surface is due to its high surface tension that results from strong hydrogen bonding. This property is important in the water transport mechanisms of plants. Meanwhile, some substances stick to surfaces due to adhesive forces (a type of intermolecular force).
Conclusion
In short, intermolecular forces are essential to understanding the behavior of substances in different states of matter. London dispersion forces, dipole-dipole interactions, hydrogen bonding, and ion-dipole forces all contribute to determining the physical properties of substances. Their importance is evident in both natural processes and practical applications, making them an integral part of chemistry.