Intermolecular forces are attractions between particles, especially molecules, that affect physical properties without changing chemical bonds inside each molecule. This reference helps students compare force types, predict boiling points, and explain solubility patterns. It is useful for chemistry topics involving phase changes, molecular polarity, and solutions.
The main intermolecular forces are London dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole attractions. In general, stronger attractions require more energy to separate particles, so boiling point, melting point, surface tension, and viscosity increase. Polarity, molar mass, molecular shape, and the presence of , , or bonds are the most important clues.
Key Facts
- London dispersion forces occur in all particles and generally increase as molar mass and polarizability increase.
- Dipole-dipole forces occur between polar molecules with permanent partial charges, such as attraction between and ends.
- Hydrogen bonding occurs when is bonded to , , or and is attracted to a lone pair on , , or nearby.
- Ion-dipole forces occur between an ion and a polar molecule, and they are important when ionic compounds dissolve in water.
- A useful strength trend is London dispersion dipole-dipole hydrogen bonding ion-dipole, but size and shape can change comparisons.
- For similar molecules, stronger intermolecular forces usually mean a higher boiling point because more energy is needed to separate particles.
- Like dissolves like means polar solutes usually dissolve best in polar solvents, while nonpolar solutes usually dissolve best in nonpolar solvents.
- Coulombic attraction can be summarized by , so greater charge and shorter distance increase attraction.
Vocabulary
- Intermolecular force
- An attraction between separate particles or molecules that affects physical properties such as boiling point and solubility.
- London dispersion force
- A temporary attraction caused by shifting electrons that create instantaneous and induced dipoles.
- Dipole-dipole force
- An attraction between the positive end of one polar molecule and the negative end of another polar molecule.
- Hydrogen bond
- A strong dipole-dipole attraction involving bonded to , , or and a lone pair on , , or .
- Ion-dipole force
- An attraction between an ion and the partial charge on a polar molecule.
- Polarizability
- The ease with which an electron cloud can be distorted to form temporary dipoles.
Common Mistakes to Avoid
- Calling hydrogen bonding a covalent bond is wrong because it is an attraction between molecules or between parts of large molecules, not a shared electron pair.
- Assuming every molecule with has hydrogen bonding is wrong because hydrogen bonding requires directly bonded to , , or .
- Ignoring London dispersion forces is wrong because every atom and molecule has them, including nonpolar substances like and .
- Ranking boiling points by polarity only is wrong because molar mass, surface area, and hydrogen bonding can strongly affect intermolecular force strength.
- Saying ionic compounds dissolve in water because of hydrogen bonding is wrong because the main attraction between ions and water is ion-dipole force.
Practice Questions
- 1 Rank , , and from lowest to highest expected boiling point, and identify the strongest intermolecular force in each substance.
- 2 Which substance should have stronger London dispersion forces, or ? Explain using molar mass and polarizability.
- 3 For dissolving in , name the main intermolecular attraction between ions and water molecules.
- 4 A student says nonpolar molecules have no intermolecular forces, so they cannot become liquids. Explain why this claim is incorrect.
Understanding Intermolecular Forces Reference
At the particle level, these attractions come from uneven electron behavior. Electrons move constantly, so even a nonpolar particle can develop a brief uneven charge distribution. A nearby particle responds to that distribution, creating a temporary attraction.
Larger electron clouds are easier to distort, which is why atoms or molecules with more electrons often attract more strongly. Shape matters because long, flat molecules can touch over a larger area than compact, rounded molecules.
More contact points usually mean a stronger total pull. This helps explain why straight chain hydrocarbons often boil at higher temperatures than branched hydrocarbons with the same formula.
Hydrogen bonding is especially important because it can link many molecules into a loose network. Water is a major example. Each water molecule can form attractions in several directions, giving liquid water unusually high surface tension and a relatively high boiling point for its small size.
Alcohols can form similar attractions, so ethanol boils much higher than an ether with the same molecular formula. A strength ranking is useful, but it is not a substitute for careful comparison.
A very large nonpolar molecule may have stronger total attractions than a small polar molecule. Students should first compare size and shape, then identify permanent polarity and possible hydrogen bonding sites.
When a substance dissolves, particles must separate from their original neighbors before new attractions form with the solvent. This is why dissolving an ionic solid in water depends on more than attraction between an ion and a water molecule. The water molecules must provide enough stabilizing attraction to pull ions away from the crystal lattice.
Around each dissolved ion, water molecules arrange in an orientation that places the oppositely charged end nearest the ion. These groups of surrounding water molecules are called hydration shells.
Oil does not mix with water because water molecules would need to lose favorable attractions to make room for nonpolar oil molecules. Soap works because one end of a soap molecule interacts with water while its long nonpolar end can surround grease.
Intermolecular forces appear in many observations outside a chemistry diagram. Perfume and rubbing alcohol evaporate quickly because their particles can escape the liquid more easily than particles in thicker liquids. Motor oil flows slowly because its large molecules have many contact areas.
In chromatography, substances travel different distances because they are attracted by different amounts to the paper or coating and to the moving solvent. Common mistakes include calling every molecule with hydrogen a hydrogen bonding molecule, forgetting that hydrogen must be directly bonded to nitrogen, oxygen, or fluorine, and judging boiling point from molar mass alone. Drawing the molecule, marking polar bonds, and comparing similar substances gives more reliable predictions.