Properties of Solutions The States of Matter The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: The kinetic energy of the particles The strength of the attractions between the particles Intermolecular Forces Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together. They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities. These intermolecular forces as a group are referred to as van der Waals forces. Dipole-dipole interactions Hydrogen bonding London dispersion forces Intermolecular Forces Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution. For example, water has a very high degree of hydrogen bonding resulting in a high surface tension, a low vapor pressure, and an increased solid volume. Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. Water s surface tension is a result of its hydrogen bonding. The surface tension of a liquid can be decreased by adding a surfactant, any substance that interferes with the hydrogen bonding between water molecules Soaps and detergents are both surfactant 1
Vapor Pressure At any temperature, some molecules in a liquid have enough energy to escape. As the temperature rises, the fraction of molecules that have enough energy to escape increases. As more molecules escape the liquid, the pressure they exert increases. Vapor Pressure Due to the large amount of hydrogen bonding between water molecules, water has a very low vapor pressure: In other words, very few water molecules can escape the surface tension and evaporate from the liquid water. Solid Structure of Water Hydrogen bonding between water molecules results in an increase in the volume of water as it is converted to ice. In order for water molecules to lock into a ridged crystalline structure, they must organize their atoms to account for all possible hydrogen bonds. This results in an increase in volume. Solutions The strong intermolecular forces of water also make it a good solvent for the formation of aqueous solutions Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent. If the solvent is water, the solution is said to be an aqueous solution. Solutions The intermolecular forces between solute and solvent particles must be strong enough to overcome those that hold the solute particles together. Otherwise, the solute would be insoluble As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them. 2
Dissolution (dissolving) of solutes 1. Dissociation The breaking apart of ionic compounds into hydrated ions. (i.e. Salt in water) Electrolytes Substances that dissociate into ions when dissolved in water are called electrolytes. A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Dissolution (dissolving) of solutes 2. Mixing The surrounding of particles by solvent particles. (i.e. Sugar in water) Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes. Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases. Dissolution (dissolving) of solutes 3. Ionization The breaking apart of covalent molecules into ions by solvent interaction. (i.e. HCl gas in water) 3
A strong electrolyte dissociates completely when dissolved in water. A weak electrolyte only dissociates partially when dissolved in water. Electrolytes Strong Electrolytes Are Strong acids, Strong bases and Soluble ionic salts Factors Affecting the Solvation Process Three factors determine how fast a solute will dissolve in solvent: Agitation the stirring of solvent increases the amount of solvent reaching the solute particles. Temperature At higher temperatures the kinetic energy of the solvent is higher and can therefore increase the rate and amount of solute entering solution Surface area the smaller the solute particle size, the greater the surface area and the faster it will dissolve. Types of Solutions Saturated Solvent holds as much solute as is possible at that temperature. Dissolved solute is in dynamic equilibrium with solid solute particles. Types of Solutions Types of Solutions Unsaturated Less than the maximum amount of solute for that temperature is dissolved in the solvent. Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can usually be stimulated by adding a seed crystal or scratching the side of the flask. 4
Solubility The solubility of a substance is the amount of solute that dissolves in a given quantity of solvent at a specified temperature and pressure to produce a saturated solution. Usually expressed in grams of solute per 100g of solvent Using the reference, determine the solubility of lead (II) chloride at room temperature and lead (II) nitrate at the boiling point of water. Factors Affecting Solubility Chemists use the axiom like dissolves like : Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar solvents. Factors Affecting Solubility Temperature: Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature. Gases in Solution In general, the solubility of gases in water increases with increasing mass. Larger molecules have stronger dispersion forces. Gases in Solution The solubility of liquids and solids does not change appreciably with pressure. The solubility of a gas in a liquid is directly proportional to its pressure. S1 P 1 Where: S is the solubility of the gas; P is the pressure of the gas above the liquid. Henry s Law S P 2 2 5
Factors Affecting Solubility An increase in temperature decreases the solubility of gases: Carbonated soft drinks are more bubbly if stored in the refrigerator. Warm lakes have less O 2 dissolved in them than cool lakes. 6