· A Solution is a homogeneous mixture
· Each of the substances in solution are called a component
· The solvent is normally the component present in the highest amount
· Other components are called solutes
· Aqueous solutions contain water as the solvent, and a solid, a liquid or a gas as a solute
13.1
· Interactions between solute and solvent molecules are known as solvation
· When the solvent is water, the interactions are also referred to as hydration.
· Enthalpy is a quantity defined by the relationship H=E+PV, the enthalpy change ΔH for a reaction that occurs at a constant pressure is the heat evolved or absorbed in the reaction ΔH=qp
· ΔH solution = ΔH1 + ΔH2 + ΔH3
· Processes in which the energy content of the system decreases tend to occur spontaneously. Spontaneous processes tend to be exothermic.
· A degree of randomness in the system, sometimes referred to as disorder, is given by a thermodynamic quantity called entropy.
· Processes occurring at constant temperature in which the randomness or disposal in space (entropy) of the system increases tends to occur spontaneously.
· Formations of solutions is favored by the increase in entropy that accompanies mixing.
13.2
· Crystallization is the process in which a dissolved solute comes out of a solution and forms a crystalline solid.
· A solution that is in equilibrium with undissolved solute is saturated.
· The amount of solute needed to form a saturated solution in a given quantity of solvent is known as a solubility of that solute.
· The solubility is the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature given that excess solute is present.
· If we dissolve less solute than that needed to form a saturated solution, the solution is unsaturated.
· Under suitable conditions it is sometimes possible to form solutions that contain a greater amount of solute than that needed to form a saturated solution. These solutions are called supersaturated.
13.3
· The stronger the attractions are between solute and solvent molecules, the greater the solubility.
· Polar liquids tend to dissolve readily in polar solvents.
· Pairs of liquids that mix in all proportions are called miscible
· Those pairs that do not dissolve in one another are called immiscible
· Nonpolar liquids tend to be insoluble in polar liquids
· All organic compounds in a series known as alcohols contain an O-H bond, known as a hydroxide bond.
· Substances with similar intermolecular attractive forces tend to be soluble in one another
· “Like dissolves like”
· The solubility of the gas increases in direct proportion to its partial pressure above the solution
· The relationship between pressure and the solubility of a gas is expressed by a simple equation known as Henry’s Law: Sg = kPg
o Sg is the solubility of the gas in the solution phase
o Pg is the partial pressure of the gas over the solution
o k is a proportionality constant known as Henry’s Law Constant
· The solubility of most solid solutes in water increases as the temperature of the solution increases
· In contrast to solid solutes, the solubility of gases in water decreases with increasing temperature
· The decreasing solubility of O2 in water as temperature increases is one of the effects of thermal pollution of lakes and streams
13.4
· The terms dilute and concentrated are used to describe a solution quantitatively
· Mass % of component =Mass of component in solution/total mass of solution × 100
· Often times concentrations of very dilute solution are expressed in parts per million (ppm) or parts per billion (ppb)
· Mole fraction of component = moles of component/total moles of all components
· Molarity = moles solute/liters solution
· The Molality of a solution, denoted m, is a unit that equals the number of moles of solute per kilogram solvent
· Molality = moles of solute/kilograms of solvent
· Molarity depends on volume of solution, whereas molality depends on the mass of solvent
13.5
· The lowering of the freezing point and the raising of the boiling point are physical properties of solutions that depend on the quantity (concentration) but not the kind or identity of the solute particles. Such properties are called colligative properties
· When the equilibrium is reached, the pressure exerted by the vapor is called the vapor pressure. A substance that has no measureable vapor pressure is nonvolatile whereas one that exhibits a vapor pressure is volatile.
· The extent to which a nonvolatile solute lowers the vapor pressure is proportional to its concentration. This relationship is expressed by Raoult’s law, which states that the partial pressure exerted by solvent vapor above a solution equals the product of the mole fraction of the solvent in the solution times the vapor pressure of the pure solvent
· PA = XAP°A
· An ideal solution obeys Raoult’s Law.
· The boiling point of the solution is higher than that of the pure liquid
· ΔTb = Kbm
· The freezing point of the solution is lower than that of the pure liquid
· ΔTf = Kfm
· The value of Kf, is the molal freezing-point-depression constant
· The concentration of solvent is higher in the solution containing less solute, however, so the rate with which the solvent passes from the less concentrated to the more concentrated solution is greater than the rate in the opposite direction.
· Osmosis: the net movement of solvent is always towards the solution with the higher solute concentration
· The pressure required to prevent osmosis by pure solvent is the osmotic pressure, II of the solution.
· II = (n/V)RT = MRT
13.6
· Intermediate types of dispersions or suspensions are called colloidal dispersions or simply colloids.
· The scattering of light by colloidal particles, known as the Tyndall effect, makes it possible to see the light beam of an automobile on a dusty dirt road or the sunlight coming through a forest canopy
· Colloids may be hydrophilic (water loving) or hydrophobic (water fearing)
· Semi-permeable membranes can also be used to separate ions from colloidal particles because the ions can pass through the membrane but the colloidal particles cannot. This type of separation is known as dialysis and is used to purify blood in artificial kidney machines.
