I Chapter 13 Properties of Solutions Solutions are homogeneous mixtures Each substance in solution is called a component of the solution A The ability of a substance to form solutions depends on two factors 13 1 The Solution Process o 1 The natural tendency of substances to mix and spread into larger volumes when not restrained in some way o 2 The types of intermolecular interactions involved 1 The Natural Tendency toward Mixing The mixing of gases is a spontaneous process Spontaneous it occurs of its own accord without any input of energy from outside the system The mixing that occurs when solutions form is associated with an increase in entropy The balance of the tendencies of systems to increase their entropy and decrease their energy or enthalpy that determines whether a process is spontaneous The formation of solutions is favored by the increase in entropy that accompanies mixing 2 The Effect of Intermolecular Forces on Solution Formation Dispersion forces dominate when one nonpolar substance dissolves in another Ion dipole forces dominate in solutions of ionic substances The extent to which a substance dissolves in another depends on the relative magnitude of three kinds of intermolecular interactions o 1 Solute solute interaction between salute particles must be overcome in order to disperse the solute particles through the solvent o 2 Solvent solvent interaction between solvent particles must be overcome to make room for the solute particles in the solvent o 3 Solvent solute interactions between solvent and solute particles occur as the particles mix Solutions form when the solvent solute interactions are greater than or equal to the rest Interactions between the solute and solvent are known as solvation When water is the solvent interactions between the solute and solvent are referred to as hydration 3 Energies of Solution Formation The solution process has three components o Solute particles separate from each other H solute o Solvent particles separate from each other H solvent o Solute and solvent particles mix H mix o H soln H solute H solvent H mix 4 Solution Formation and Chemical Reactions 13 2 Saturated Solutions and Solubility B Crystallization Opposite of the solution process when the concentration of solute particles in a solution increases increasing the chances that some solute particles will collide with the surface of the solid and reattach Solute solvent dissolve crystallize Saturated solution a solution that is in equilibrium with undissolved solute Solubility of solute the amount of solute needed to forma saturated solution in a given quantity of solvent solution The solubility of a given solute in a given solvent is the maximum amount of the solute that can dissolve in a given amount of the solvent at a specified temperature given that excess solute is present Unsaturated solution a solution containing less that the amount needed to form a saturated solution Supersaturated solution a solution that contains a greater amount of solute that needed to form a saturated solution C 13 3 Factors Affecting Solubility 1 Solute Solvent Interaction Solubility of various gases in water increase as molecular mass increases or polarity increases The stronger the attractions between solute and solvent molecules the greater the solubility of the solute in that solvent Miscible the quality of being able to mix in all proportions Immiscible the quality of not being able to mix in all proportions Increased entropy can play a significant role in solution formation by increasing its chances when solute solute solvent solvent and solvent solute forces are not greatly different in a mixture As the number of carbon atoms increases in alcohol the solubility in water decreases Substances with similar intermolecular attractive forces tend to be soluble in one another 2 Pressure Effects The solubility of liquids and solids are not really affected by pressure The solubility of a gas in any solvent is increase as the pressure of the gas above the solvent The solubility of a gas in a liquid solvent increases in direct proportion to the partial pressure of increases the gas above the solution 3 Temperature Effects The solubility of most solid solutes in water increases as the solution temperature increases The solubility of gases in water decreases with increasing temperature D 13 4 Expressing Solution Concentration 1 Mass Percentage PPM and PPB mass of component solution total mass of solution 100 Mass of a component Ppm of component mass of component solution total mass of solution 106 2 Mole Fraction Molarity and Molality molesof component totalmoles of all components Mole fraction of component X component Molarity M Molality m molesof solute liters of solution moles of solute kilograms of solvent Molarity depends on volume of solution Molality depends on mass of solvent The molality of a given solution does not very with temperature because masses do not very with temperature The molarity of the solution does changes with temperature because the volume of a solution expands or contract with temperature 3 Converting Concentration Units Mass of solvent Molality mol kg solvent Mass of solute Molar mass Moles of solute Mass of solution D e n s i t y Volume of solution Molarity mol L solution 13 5 Colligative Properties E Adding a solute lowers the freezing point raises the boiling point and lowers the vapor pressure Colligative properties properties that depends of the collective effect of the number of solute particles 1 Vapor Pressure Lowering Nonvolatile a substance that has no measurable vapor pressure A solution consisting of a volatile liquid solvent and a nonvolatile solute forms spontaneously because of the increase in entropy that accompanies their mixing When a nonvolatile solute is present the vapor pressure of the solvent is lowered because solvent molecules are stabilized in their liquid state by the solute The vapor pressure of a volatile solvent above a solution containing a nonvolatile solute is proportional to the solvent s concentration in the solution Raoult s law the partial pressure exerted by solvent vapor above the solution Psolution equals the product of the mole fraction of the solvent X solvent times the vapor pressure of the pure solvent P solvent Psolution X solvent P solvent The vapor pressure lowering P is directly proportional to the mole fraction of the solute X solute P X solute P solvent Ideal Solution a solution that obeys Raoult s law Ideality