Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

intermolecular forces, the forces of interaction between molecules. These forces are normally weakly attractive.

One of the most direct indications of the attraction between molecules is the heat

of vaporization of liquids. Consider a substance like neon, which consists of molecules that are single atoms. (There is no tendency for these atoms to bond chemically.) Neon is normally a gas, but it liquefies when the temperature is lowered to _246_C at 1.00 atm pressure. The heat of vaporization of the liquid at this temperature is 1.77 kJ/mol. Some of this energy (0.23 kJ/mol) is needed to push back the atmosphere when the vapor forms. The remaining energy (1.54 kJ/mol) must be supplied to overcome intermolecular attractions. Because a molecule in a liquid is surrounded by several neighbor molecules, this remaining energy is some multiple of a single molecule–molecule interaction. Typically, this multiple is about 5, so we expect the neon–neon interaction energy to be about 0.3 kJ/mol. Other experiments yield a similar value. By comparison, the energy of attraction between two hydrogen atoms in the hydrogen molecule is 432 kJ/mol. Thus, the energy of attraction between neon atoms is about a thousand times smaller than that between atoms in a chemical bond. Attractive intermolecular forces can be larger than those in Attractive intermolecular forces can be larger than those in neon. For example, chlorine, Cl2, and bromine, Br2, have intermolecular attractive energies of 3.0 kJ/mol and 4.3 kJ/mol, respectively. But even these values are much smaller than bond energies. Three types of attractive forces are known to exist between neutral molecules: dipole–dipole forces, London (or dispersion) forces, and hydrogen bonding forces.