11.1 Chem1212K Intermolecular Forces Intermolecular forces attractive forces that exist between all molecules and atoms. o Holds many liquids and solids such as water and ice together. Thermal energy A type of kinetic energy associated with the temperature of an object, arising from the motion of individual atoms or molecules in the object. o When thermal energy is high relative to intermolecular forces, matter tends to be gaseous. o When thermal energy is low relative to intermolecular forces, matter tends to be liquid or solid. 11.2 Solids, Liquids, and Gases: A Molecular Comparison When looking at the three phases of water, notice that the densities of the solid and liquid phases are much greater than the density of the gas phase. o The solid and liquid phases are more similar in density and molar volume to one another than they are to the gas phase. o Solid is slightly less dense than liquid (atypical behavior) Most solids are slightly denser than their corresponding liquids because the molecules move closer together upon freezing. A major difference between liquids and solids is that liquids are more free in movement due to thermal energy. Properties of Liquids: o Liquids have high densities in comparison to gases. o Liquids have an indefinite shape and assume the shape of their container. o Liquids have a definite volume, they are not easily compressed. Properties of Solids: o Solids have high densities in comparison to gases. o Solids have a definite shape; they do not assume the shape of their container. o Solids have a definite volume; they are not easily compressible. o Solids may be crystalline (ordered) or amorphous (disordered). Phase Gas Liquid Solid Table 11.2 Properties of the Phases of Matter Density Shape Volume Strength of Intermolecular Forces* Low Indefinite Indefinite Weak High Indefinite Definite Moderate High Definite Definite Strong *Relative to thermal energy
Crystalline the atoms or molecules that compose them are arranged in a wellordered three-dimensional array (diamond). Amorphous the atoms or molecules that compose them have no long-range order (charcoal). One phase of matter can be transformed to another by changing the temperature, pressure or both.
11.3 Intermolecular Forces: The Forces That Hold Condensed Phases Together Intermolecular forces originate from the interactions between charges, partial charges, and temporary charges on molecules, much as bonding forces originate from interactions between charged particles in atoms. Intermolecular forces, even the strongest ones, are generally much weaker than bonding forces. o Bonding forces are the result of large charges interacting at very close distances. Intermolecular forces are the result of smaller charges interacting at greater distances. Dispersion forces, the weakest of the intermolecular forces, are present in all molecules and atoms and increase with increasing molar mass. These forces are always weak in small molecules but can be significant in molecules with high molar masses. Dipole-dipole forces are present in polar molecules. Hydrogen bonds, the strongest of the intermolecular forces that can occur in pure substances (second only to ion-dipole forces), are present in molecules containing hydrogen bonded directly to fluorine, oxygen, or nitrogen. Ion-dipole forces are present in mixtures of ionic compounds and polar compounds. These are very strong and are especially important in aqueous solutions of ionic compounds. 11.4 Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action The surface tension of a liquid is the energy required to increase the surface area by a unit amount. o Molecules at the surface are inherently less stable-they have higher potential energy-than those in the interior. o In order to increase the surface area of the liquid, some molecules from the interior have to be moved to the surface, a process requiring energy. Another manifestation of intermolecular forces is viscosity, the resistance of a liquid to flow. o Motor oil is more viscous than gasoline. o Viscosity is measured in a unit called the poise, defined as 1g/cm*s. Water is approximately 1centipoise (cP). o Viscosity is greater in substances with stronger intermolecular forces because molecules are more strongly attracted to each other, preventing them from flowing around each other as freely. o Viscosity also depends on molecular shape, increasing in longer molecules that can interact over a greater area and possibly become entangled. o Viscosity increases with increasing molar mass and with increasing length. (inc. magnitude of dispersion forces) o Viscosity with decreases increasing temperature. (flows more free as its temp. is increased) Capillary action the ability of a liquid to flow against gravity up a narrow tube. o Results from a combination of two forces
The attraction between molecules in a liquid is called cohesive forces. Cause liquid to stay together The attraction between these molecules and the surface of the tube is called adhesive forces. Cause the liquid to spread out over the surface of the tube. o If adhesive forces are greater than cohesive (water) then the attraction to the surface draws the liquid up the tube, while the cohesive forces pull along the molecules not in direct contact with the tube walls. The water rises up until the force of gravity balances the capillary action. (Thinner tube, higher rise) (concave meniscus) o If the adhesive forces are smaller than the cohesive (mercury), the liquid does not rise up the tube at all (it will drop to a level below the surrounding liquid) (convex meniscus due to metallic bonding).
11.5 Vaporization and Vapor Pressure Vaporization the phase transition from liquid to gas (endothermic process, takes energy). Condensation the phase transition from gas to liquid (exothermic process, gives off heat). Liquids that vaporize easily are termed volatile. Liquids that do not vaporize easily are termed nonvolatile. The rate of vaporization increases with increasing temperature. The rate of vaporization increases with increasing surface area. The rate of vaporization increases with decreasing strength of intermolecular forces. The amount of heat required to vaporize one mole of a liquid to gas is called the heat of vaporization. o Always positive because the process is endothermic-energy must be absorbed to vaporize. o Somewhat temperature dependent (lower temp, more energy required). o When something is condensed it releases heat making the heat of vaporization negative. Pg. 479 When the rate of condensation equals the rate of vaporization, dynamic equilibrium has been reached. The pressure of a gas in dynamic equilibrium with its liquid is called its vapor pressure. o Depends on the intermolecular forces present in the liquid and the temperature. Weak intermolecular forces = volatile Strong intermolecular forces = nonvolatile. Independent of surface area. When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium.
When the temperature of a liquid is increased, its vapor pressure rises because the higher thermal energy increases the number of molecules that have enough energy to vaporize. The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. o The normal boiling point of a liquids is the temperature at which its vapor pressure equals 1 atm. Clausius-Clapeyron equation: 483-485 Critical temperature represents the temperature above which liquid cannot exists. Critical pressure represents the pressure required to bring about a transition to a liquid at the critical temperature.
11.6 Sublimation and Fusion Sublimation phase transition from solid to gas. Deposition phase transition from gas to solid. At the melting point, the molecules have enough thermal energy to overcome the intermolecular forces that hold them at their stationary points, and the solid turns into a liquid. Melting or fusion the phase transition from solid to liquid. o Endothermic process Freezing the phase transition from liquid to solid. o Exothermic process The amount of heat required to melt 1 mol of a solid is called the heat of fusion. o Positive because melting is endothermic 11.8 Phase Diagrams A phase diagram is simply a map of the phase of a substance as a function of pressure and temperature. 12.1 Solutions Solution a homogeneous mixture of two or more substances. Also has at least two components. o Solvent the majority component. o Solute the minority component. Substance that dissolves 12.2 Types of Solutions and Solubility Aqueous solutions contain water as the solvent, and a solid, liquid, or gas as the solute. The solubility of a substance is the amount of the substance that will dissolve in a given amount of solvent. Solution Phase Gaseous solution Common Types of Solutions Solute Phase Solvent Phase Example Gas Gas Air (mainly oxygen and nitrogen)
Liquid solution Solid solution
Gas Liquid Solid Solid
Liquid Liquid Liquid Solid
Club Soda (CO2 and water) Vodka (ethanol and water) Seawater (salt and water) Brass (copper and zinc) and other alloys
Equilibrium: rate of dissolving = rate of cystallization