Ch. 13 Properties of Solutions PDF

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An encompassing synopsis of Chapter 13 of "Chemistry: The Central Science." Lecture notes may be filled in as you listen to lecture to ensure completeness since lecturing may change slightly year to year....

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Ch. 13: Properties of Solutions The Solution Process ● Solution is formed when one substance disperses uniformly throughout another ● Ability of substances to form solutions depends on 2 factors: ○ Natural tendency of substances to mix and spread into larger volumes when not restrained ■ The mixing of gases is a spontaneous process ● When molecules mix and become more randomly distributed, entropy increases ○ More mixing = More entropy (ΔS) ■ The formation of solutions is favored by the increase in entropy that accompanies mixing ● 3 types of intermolecular interactions involved in solution formation: ○ Solute-solute interactions must be overcome to disperse solute particles in solvent ○ Solvent-solvent interactions must be overcome to make roome for solute particles in solvent ○ Solvent-solute interactions between solvent and solute particles occur as particles mix (known as solvation) ■ When solvent is water, interactions are referred to as hydration ○ The types of intermolecular interactions involved in the solution process (enthalpy, ΔH)

● Energetics of Solution Formation ○ Solution processes are accompanied by changes in enthalpy ○ Separation of solute particles from one another always requires energy input → endothermic process ○ Separation of solvent particles to accommodate the solute always requires energy input → endothermic process ○ Attractive mixing interactions between solute and solvent particles always releases energy → exothermic process (proceed spontaneously) Saturated Solutions and Solubility ○ Crystallization: process in which molecules, ions, or atoms come together to form a crystalline solid (When these rates become equal to each other, a dynamic equilibrium establishes and no further increase in the amount of solute in solution occurs) ○ A solution that is in equilibrium w/ undissolved solute is saturated ■ Additional solute will not dissolve if added to saturated soln ○ The amount of solute needed to form saturated solution is known as solubility ■ The solubility of a particular solute in a particular solvent is the maximum amount of solute that can dissolve in a given amount of the solvent at a certain temperature, assuming that excess solute is present ○ If a smaller amount of solute is dissolved than needed to form a saturated soln, the soln will be unsaturated ○ Some solns may be supersaturated, but they are very unstable Factors Affecting Solubility ● Solvent-Solvent Interactions ○ The stronger the attractions between solute and solvent molecules, the greater the solubility of the solute in that solvent ■ Polar liquids tend to dissolve in polar solvents ( like dissolves like) ■ Polar molecules tend to be soluble in water ■ Ionic solutes dissolve in polar solvents ■ Nonpolar liquids tend to be insoluble in polar liquids ○ Liquids that mix in all circumstances are miscible; those that do not are immiscible ○ As the number of carbons in an alcohol increases, the polar OH group of that alcohol becomes smaller → less soluble in water ■ Smaller number of carbons on alcohol = higher solubility in water

○ Pressure Effects ■ Solubility of a gas in any solvent increases as partial pressure of the gas above the solvent increases (Henry’s Law) ■ Heavier gases are more soluble than lighter gases (O2(g) is more soluble than He(g)) ○ Temperature Effects ■ Solubility of most solid solutes in water increases as the solution temperature increases ■ Solubility of gases in water decreases as temperature increases ○ For endothermic rxns, solubility increases as temperature increases ■ HEAT + A + B → Solution ○ For exothermic rxns, solubility decreases as temperature increases ■ A + B → Solution + HEAT (heat counteracts solution formation)

Expressing Solution Concentration ● Mass Percentage, parts per million (ppm), and parts per billion (ppb) ● Mole Fraction, Molarity, and Molality Colligative Properties ● Colligative Properties: property of a solvent (vapor-pressure lowering, freezingpoint lowering, boiling-point elevation, osmotic pressure) that depends on the total concentration of solute particles present ○ Depend only on the number of solute particles present, not on the kind or identity of the solute particles ● Vapor-Pressure Lowering ○ Vapor pressure is the pressure exerted by the vapor when it is at equilibrium w/ the liquid (when rate of vaporization = rate of condensation) ■ A substance with no measurable vapor pressure is nonvolatile, one that exhibits a vapor pressure is volatile ○ A solution with a volatile liquid and a nonvolatile solute forms spontaneously ■ Presence of nonvolatile solute in liquid solvent results in reduction of vapor pressure above the liquid

○ Raoult’s Law ■ The resulting value will give a number which represents the new vapor pressure of the volatile solvent after adding a nonvolatile solute ■ An ideal solution is defined as one that obeys Raoult’s Law ○ Boiling-Point Elevation ■ The boiling point of the solution is higher than that of the pure solvent ○ Freezing-Point Depression ■ Freezing Point: temperature at which vapor pressure of solid equals vapor pressure of liquid ■ Freezing-Point Depression: nonvolatile solute-solvent interactions also cause solutions to have lower freezing points than pure solvent

Boiling-Point Elevation and Freezing-Point Depression

● Change in temperature is directly proportional to molality using van’t Hoff Factor Van’t Hoff Factor (i) ● Takes into account dissociation in solution and compensates for the number of particles dissolved ● Because colligative properties depend on number of particles dissolved, solns of electrolytes show larger changes than those of nonelectrolytes ○ Strong Electrolytes: dissociate into ions ■ Ionic, Strong Acid, Strong Base ○ Non-Electrolytes: dissolve as entire molecules ■ Molecular Compounds Osmosis ● Net movement of solvent molecules from solution of low to high concentration across a semipermeable membrane ○ Semipermeable membrane allows smaller particles (solvent) to pass through, while blocking larger particles (solute) ● Net movement of solvent from high solvent (low solute) concentration to low solvent (high solute) concentration ● Osmotic Pressure ● If osmotic pressure is same on both sides of membrane, solutions are isotonic ● Hypotonic Solution ○ Solute concentration outside of cell is less than inside of cell (inside is more concentrated) ○ Water flows into cell (hemolysis) ● Hypertonic Solution ○ Solute concentration outside the cell is greater than inside the cell (outside is more concentrated) ○ Water flows out of cell (crenation)...