Transcript Properties of Solutions - Baldwinsville Central School

Properties of Solutions
AP Chapter 13
The Solution Process
• Solutions form when one substance disperses
uniformly throughout another substance.
• The ability of substances to form a solution depends
on:
– the types of intermolecular attractions
– the natural tendency of substances to disperse into larger
volumes when not contained.
• The attractive interaction between solvent molecules
and solute is called solvation.
• When the solvent is water, the interaction is called
hydration.
Ionic Substances
• The dissolution of ionic substances in water is
promoted by hydration of the separate ions by
the polar water molecules.
• The overall enthalpy change upon solution
formation may be either positive or negative.
• Solution formation is favored by both a negative
enthalpy change (exothermic) and by an increase
dispersion in space of the components in the
solution (this means a positive entropy change.)
Dissolution of an ionic solid in water
Dispersion Forces
• Dispersion forces dominate when non-polar
substances dissolve in another non-polar
substance.
• Remember, like dissolves like.
Enthalpy Changes
• Involves:
– Separation of the solute molecules (∆H1)
– Separation of the solvent molecules (∆H2)
– Formation of solute-solvent interactions (∆H3)
– The overall enthalpy change is :
∆Hsoln = ∆H1 + ∆H2 + ∆H3
Entropy
• The degree of randomness in a system is called
entropy.
• Processes in which the energy content of the
system decreases tend to occur spontaneously.
• Processes occurring at a constant temperature in
which the randomness increases tend to occur
spontaneously.
• In most cases, the formation of solutions is favored
by the increase in entropy that accompanies mixing.
Saturated Solutions
• A saturated solution is present when the
number of molecules going into solution
(dissolving) equal the number of molecules
coming out of solution (crystallizing).
Solute + solvent
dissolve
solution
crystallize
This is called dynamic equilibrium.
Dynamic
Equilibrium
Solubility
• Solubility is the maximum amount of a solute
that will dissolve in a given amount of solvent
at a specified temperature, given that excess
solute is present.
• Example: the solubility of NaCl at 0°C 35.7 g
per 100 mL of water. This is the maximum
amount of NaCl that can dissolve to form a
stable equilibrium at that temperature.
Unsaturated and Supersaturated
• If less solute is dissolved than is needed to
make a saturated solution, the solution is
unsaturated.
• Under suitable conditions, it is sometimes
possible to form a solution that contains a
greater amount of solute than is needed to
make a saturated solution. This condition is
called supersaturation.
Factors Affecting Solubility
• The stronger the attraction between solute and
solvent molecules, the greater the solubility.
• Like dissolves like (the substances have similar
intermolecular attractive forces.)
• Polar substances tend to dissolve in polar
solvents. Non-polar substances do not dissolve
in polar solvents.
Miscibility
• Polar liquids that mix in polar solvents are
miscible.
• Those that do not dissolve in one another are
immiscible.
Pressure Effects
• The solubilities of liquids and solids are not
greatly affected by pressure, but the solubility
of a gas is.
• The solubility of a gas in any solvent is
increased as the pressure over the solvent
increases.
• The solubility of the gas increases in direct
proportion to its partial pressure above the
solution.
The Effect of
pressure on gas
solubility: The
concentration of solute
molecules at
equilibrium increases
in proportion to the
pressure.
Henry’s Law
• The relationship between pressure and solubility
of a gas is expressed with Henry’s Law Equation:
Sg = kPg
Sg is the solubility of the gas in the solution phase
and Pg is the partial pressure of the gas over the
solution. k is a proportionality constant (Henry’s
Law constant) which is different for each solutesolvent pair.
Temperature
• The solubility of most solid solutes in water
increases as the temperature of the solution
increases.
• The solubility of gases in water decreases with
increasing temperature.
Concentration
• Concentration of a solution can be expressed
either quantitatively or qualitatively (dilute or
concentrated).
Mass Percentage
• Mass percentage is expressed using the
equation:
Mass of component in soln
mass % of component =
X 100
total mass of soln
Parts per Million
• This calculation is used in dilute solutions and
uses 106 (for parts per million – ppm) or 109 (for
parts per billion – ppb) as the multiplier instead of
100.
Mass of component in soln
ppm of component =
total mass of soln
X 106
Mole Fraction
• This is how many moles of substance are in
the solution.
Moles of component
Mole fraction of component = total moles of all components
Molarity
• Relates the volume of solution to the quantity
of solute that it contains.
• Molarity =
Moles solute
Liters solution
Molality
• This is the number of moles of solute per
kilogram of solvent:
Molality =
Moles of solute
Kilograms of solvent
Colligative Properties
• Colligative properties are properties that
depend on the quantity of solute particles but
not the kind of particles in the solution.
• Adding solute particles to a solution raises the
boiling point and lowers the freezing point of
the solution.
• It also lowers the vapor pressure of the
solution.
Raoult’s Law
• The extent to which a nonvolatile solute
lowers the vapor pressure is proportional to
its concentration.
• Raoult’s Law: the partial pressure exerted by
the solvent vapor above a solution, PA, equals
the product of the mole fraction of the solvent
in the solution, XA, times the vapor pressure of
the pure solvent, P°A.
• PA = XA P°A
Raoult’s Law
• Just like ideal gases follow the ideal gas law, an
ideal solution obeys Raoult’s Law.
• Real solutions best approximate an ideal
solution when the solute concentration is low
and when the solute and solvent have similar
molecular sizes and types of intermolecular
attractions.
Boiling Point Elevation
• Normal boiling point is the temperature at
which its vapor pressure equals 1 atm.
• The boiling point of a solution will be higher
than that of the pure liquid.
• At the surface of a pure liquid, there are only
liquid molecules that can evaporate, where as
at the surface of a solution, there are a
combination of molecules that evaporate.
Boiling Point Elevation
• ΔTb = Kbm
• ΔTb is the increase in boiling point relative to
that of the pure solvent
• It is directly proportional to the concentration
of the solution expressed by its molality, m.
• Kb is called the molal boiling-point-elevation
constant.
Freezing Point Depression
• The freezing point of a solution is the point at
which the first crystals of pure solvent begin
to form in equilibrium with the solution.
• The freezing point of the solution is lower
than that of the pure liquid.
Molal Freezing-Point-Depression
Constant
• ΔTf is the decrease in freezing point, and is a
positive quantity.
• It is directly proportional to the molality of the
solute.
• ΔTf = Kfm
• Kf is the molal freezing-point-depression
constant.
• Use Table 13.4!
Osmosis
• Semipermeable materials allow some materials
to pass through the network of tiny pores, and
not others.
• Most importantly, semipermeable membranes
allow water and other small molecules to pass
through, but not larger solute molecules or ions.
Osmosis
• Osmosis is the movement of solvent
molecules through a semipermeable
membrane from a less concentrated to a more
concentrated solution.
• This net movement of solvent generates
osmotic pressure, Π, which can be measured
in units of gas pressure, like atm.
n
• Π=
RT = MRT
V
The blue arrows represent the net
movement of water molecules.
Colloids
• Colloids are particles that are large on the
molecular scale but still small enough to
remain suspended indefinitely in a solvent
system. (aka colloidal dispersions.)
• They are intermediate between solutions and
heterogeneous mixtures.
• They have the ability to scatter light; known as
the Tyndall effect.
Hydrophilic and Hydrophobic Colloids
• The most important colloids are those in which
water is the solvent, or dispersing agent.
• Hydrophilic colloids are water loving and are found
in the human body and help keep molecules
suspended in water. (enzymes, antibodies)
• Hydrophobic colloids are water fearing and must be
stabilized before they can be mixed into water.
(droplets of oil.)
How a soap molecule works.