Chapter 12 - Intermolecular Forces: Liquids, Solids, and Phase Changes

• The interactions between the particles' potential energy and kinetic energy give birth to the features of each phase. The potential energy in a sample of matter is determined by two types of electrostatic forces:

• Each molecule has intramolecular (bonding) forces. For example, the chemical behavior of the three states of water is identical because they are all made up of the same bent, polar HOH molecules bound together by identical covalent bonding forces.

• The molecules are held together by intermolecular (nonbonding) interactions (or, more broadly, interparticle forces). Because the intensities of these pressures differ, the physical behavior of the states differs.

• The potential energy in the form of intermolecular forces tends to attract molecules together. Coulomb's law states that the electrostatic potential energy is proportional to the charges of the particles and the distances between them.

• The kinetic energy associated with the molecules' random motion tends to scatter them. It is proportional to the absolute temperature and connected to their average speed.

• The interaction of potential and kinetic energy determines whether a material exists as a gas, liquid, or solid. As we progress from gas to liquid to solid, the potential energy effect grows while the kinetic energy influence diminishes.

• The potential energy (energy of attraction between gas particles) in gas is minimal in comparison to the kinetic energy (energy of motion); hence, Gas particles, on average, are wide apart and travel randomly throughout the container.

• Because of the vast distance between gas particles, gas is extremely compressible. Because gas particles are free to move, a gas fills its container and readily passes through another gas, as illustrated in the table below.

• The van der Waals radius, which is always greater than the covalent radius, specifies the minimum distance across which intermolecular forces can act. Bonding (intramolecular) forces are significantly stronger than intermolecular interactions.

• Between ions and polar molecules, ion-dipole forces exist. On polar molecules, dipole-dipole forces exist between oppositely charged poles.

• When H is linked to N, O, or F, it is attracted to the lone pair of N, O, or F in another molecule, resulting in hydrogen bonding, a kind of dipole-dipole force. In an electric field, electron clouds can be deformed (polarized).

• Between a charge and the dipole, it produces in another molecule, ion– and dipole–induced dipole forces form

• The attractions between particles in a liquid are strong enough to draw them closer together. As a result, liquid particles collide and have restricted freedom of motion.

• Because the particles still have enough kinetic energy to move around randomly, a liquid conforms to the shape of its container, but it has a surface (it does not fill the container).

• Because liquids resist applied forces, they only minimally compress. A liquid does flow, but it does so significantly more slowly than a gas.

• The attractions of the intermolecular interactions obviously outweigh the kinetic energy of the particles in a solid. As a result, the particles are locked in their relative positions to one another, only jiggling in situ.

• A solid has its own form and does not flow appreciably because the particles have very limited freedom of motion. Because the particles are so close together, a solid compresses less than a liquid.

• The environment exemplifies these variations in flowability perfectly. The gases in the atmosphere mix so efficiently that the bottom 80 km of air has a homogeneous composition. Because there is less mixing in the seas, the composition at different depths can sustain different species.

• The potential and kinetic energies in each phase explain phase shifts or physical transitions from one phase to another; temperature also has an impact: As the temperature rises, so does the average kinetic energy, allowing particles to travel quicker and more readily overcome attractions.

• As the temperature falls, so does average kinetic energy, causing particles to travel more slowly and attractor forces to pull them together more readily.

• Pressure also has an effect on phase transitions, most notably when gases are present and, to a lesser extent, when liquids are involved. Each phase transition is accompanied by an enthalpy change, H, measured in kilojoules per mole (at 1 atm and the temperature of the change).

  • The term Exothermic changes refer to the molecules of a gas attracting each other into a liquid, and then becoming fixed in a solid, the system of particles loses energy, which is released as heat.

  • The term Endothermic changes refer to heat that must be absorbed by the system to overcome the attractive forces that keep the particles fixed in place in a solid or near each other in a liquid.

• Because of the relative magnitudes of intermolecular forces (potential energy) and average speed (kinetic energy), particles in a gas are far apart and moving randomly, while particles in a liquid are in contact and moving relative to each other, and particles in a solid are in contact and fixed positions.

• These molecular variations explain macroscopic variances in form, compressibility, and flowability.

• The nonlinear relationship between vapor pressure and temperature is converted to a linear one with the ClausiusClapeyron equation:

  • ln P = −ΔHvap/R (1/T) + C

y = m x + b

• where ln P is the vapor pressure's natural logarithm, Hvap is the heat of vaporization, R is the universal gas constant (8.314 J/molK), T is the absolute temperature, and C is a constant (not related to heat capacity).

• The equation is frequently used to calculate the heat of vaporization. Under it, in blue, is the equation for a straight line, with y = ln P, x = 1/T, m (the slope) = Hvap/R, and b (the y-axis intercept) = C. A plot of ln P vs. 1/T yields a straight line, as illustrated in Figure 12.8 for diethyl ether and water.

• If we know the vapor pressures at two temperatures, we can compute Hvap using a two-point variant of the Clausius-Clapeyron equation.

• Doping improves semiconductor conductivity and is crucial in contemporary electronic materials. Doping silicon with Group 5A(15) atoms introduces negative sites (resulting in an n-type semiconductor) by adding valence electrons to the conduction band, whereas doping with Group 3A(13) atoms introduces positive holes (resulting in a p-type semiconductor) by emptying some orbitals in the valence band.

• When these two types of semiconductors come into touch with each other, they produce a p-n junction. A transistor is formed by sandwiching a p-type component between two n-type sections. Liquid crystal phases feature molecules that are organized like crystalline solids yet flow like liquids.

• The molecules are typically rod-shaped, and

• Liquid crystal phases feature molecules that are organized like crystalline solids yet flow like liquids.

• The molecules often have rod-like forms, and intermolecular forces maintain them aligned. Thermotropic phases are formed by heating the solid; lyotropic phases are formed by varying the solvent content. The molecular order of liquid crystals differs between the nematic, cholesteric, and smectic phases. Controlling the orientation of the molecules is critical in liquid crystal applications.

• Ceramics are extremely heat and chemical-resistant. The majority are high-temperature network covalent solids produced from simple reactants. Other materials benefit from their lightweight strength.

• Liquid crystal phases feature molecules that are organized like crystalline solids yet flow like liquids.

• The molecules often have rod-like forms, and intermolecular forces maintain them aligned. Thermotropic phases are formed by heating the solid; lyotropic phases are formed by varying the solvent content. The molecular order of liquid crystals differs between the nematic, cholesteric, and smectic phases.

• Controlling the orientation of the molecules is critical in liquid crystal applications. Ceramics are extremely heat and chemical-resistant. The majority are high-temperature network covalent solids produced from simple reactants. Other materials benefit from their lightweight strength.