Chapter 13
Ions in aqueous Solutions
And
Colligative Properties
Compounds in Aqueous Solution
• Dissociation
– The separation of ions that occurs when an ionic
compound dissolves
–
H2O
NaCl (s)  Na+
(aq)
+ Cl- (aq)
H2O
MgCl2 (s)  Mg2+
(aq)
+ 2 Cl- (aq)
• Assuming 100% dissociation
1 mole NaCl in water -> 1 mole Na+ and 1 mole Cl=2 mole of solute ions
1 mole CaCl2 -> 1 mole of Ca and 2 mol of Cl- = 3
mole of solute ions
• Do practice page 436
• Remember must represent the facts when
writing an equation
• Some substances do not remain soluble in
solution.
• Use table 437 and 860 to determine solubility
Net Ionic Equations
• Show the compounds and ions that undergo a
chemical change in a reaction in an aq soln.
– To create an ionic equation, must convert a
balanced chemical equation into an ionic eq.
• ALL soluble ionic compounds are shown as
dissociated ions in soln.
Ionic equations
• 2HCl (aq) + Na2O (aq) -> 2NaCl (aq) + H20 (l)
• 2H+ (aq) + 2Cl- (aq) + 2Na+ (aq) + O2- (aq)  2 Na+
(aq)
+ 2 Cl- (aq) + H20 (l)
• Spectator Ions
– Ions that do not take part in a chemical reaction
and are found in solution both before and after
the reaction
• To convert an ionic equation into a net ionic
equation, remove the spectator ions from
both sides of the equation
– The ions and compounds left after the spectator
ions are removed is called the net ionic equation
• See practice page 440
Ionization
• Molecular compounds can also form ions in
solution
• Ionization means creating ions where there
were none
H2O
• HCl (g) - H+ (aq) + Cl- (aq)
Hydronium Ion
• Molecular compounds often contain Hydrogen
bonded by a polar covalent bond.
• Some of the compounds ionize in an aq.
Solution to release H+ , the H+ ion has such a
high reactivity that it attaches itself to water
to form H3O+ or the hydronium ion
• HCl (g) + H2O H3O+ (aq) + Cl- (aq)
Electrolytes
• Strong electrolytes
– Any compound whose dilute aqueous solutions
conduct electricity well
– Caused by all (almost all) solute existing as ions in
solution
• Weak electrolytes
– Any compound whose dilute aqueous solutions
conduct electricity poorly
– Caused by very small amounts of solute existing as
ions in solution
• Do section review page 443
• Homework: page 458 #2, 8-13
Colligative Properties of Soln.
• Properties that depend on the concentration
of solute particles but not on their identity
• These include: freezing point, boiling point,
vapor pressure, osmotic pressure,
Raoult's Law and Vapor Pressure
Lowering
• Bp and Fp of soln differ from those of pure
solvents
• When Nonvolatile solutes (a substance that
has little tendency to become a gas under
existing conditions) added to a liquid to form a
solution, the vapor pressure above that
solution decreases.
• Liquid molecules at the surface of a liquid can escape to the
gas phase when they have a sufficient amount of energy to
break free of the liquid's intermolecular forces.
• That vaporization process is reversible. Gaseous molecules
coming into contact with the surface of a liquid can be
trapped by intermolecular forces in the liquid.
• Eventually the rate of escape will equal the rate of capture
to establish a constant, equilibrium vapor pressure above
the pure liquid.
• If add a nonvolatile solute to that liquid, the
amount of surface area available for the
escaping solvent molecules is reduced
because some of that area is occupied by
solute particles.
• the solvent molecules will have a lower
probability to escape the solution than the
pure solvent.
The Vapor Pressure of a Solution is Lower than
that of the Pure Solvent
• The French chemist Francois Raoult discovered
the law that mathematically describes the vapor
pressure lowering phenomenon.
• Raoult's law states:
– the vapor pressure of an ideal solution is dependent
on the vapor pressure of each chemical component
and the mole fraction of the component present in the
solution.
– Once the components in the solution have reached
equilibrium, the total vapor pressure p of the solution
is:
• and the individual vapor pressure for each
component is:
• Where p* is the vapor pressure of the pure
component x is the mole fraction of the
component in solution
• Solutions that obey Raoult's law are called ideal
solutions because they behave exactly as we would
predict.
• Solutions that show a deviation from Raoult's law are
called non-ideal solutions because they deviate from
the expected behavior.
• Very few solutions actually approach ideality, but
Raoult's law for the ideal solution is a good enough
approximation for the non- ideal solutions
Freezing Point Depression
• describes the phenomenon that the freezing point of a
liquid (a solvent) is depressed when another compound is
added, meaning that a solution has a lower freezing point
than a pure solvent. This happens whenever a solute is
added to a pure solvent, such as water.
• The phenomenon may be observed in sea water, which
due to its salt content remains liquid at temperatures
below 0°C, the freezing point of pure water.
• The freezing point depression happens both when the
solute is an electrolyte, such as various salts, and a
nonelectrolyte
ΔTf = Kf · mB
•
ΔTf, the freezing point depression, is defined as Tf (pure solvent) − Tf (solution), the
difference between the freezing point of the pure solvent and the solution. It is
defined to assume positive values when the freezing point depression takes
place.
•
Kf, the cryoscopic constant, which is dependent on the properties of the solvent.
It can be calculated as Kf = RTm2M/ΔHf, where R is the gas constant, Tm is the
melting point of the pure solvent in kelvin, M is the molar mass of the solvent,
and ΔHf is the heat of fusion per mole of the solvent. (LOOK THESE UP)
•
mB is the molality of the solution, van 't Hoff factor i as mB = msolute · i. The factor i
accounts for the number of individual particles (typically ions) formed by a
compound in solution.
Examples:
•
–
–
–
–
i = 1 for sugar in water
i = 2 for sodium chloride in water, due to dissociation of NaCl into Na+ and Cli = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2 Cli = 2 for hydrogen chloride in water, due to complete dissociation of HCl into H+ and Cl-
• Kf of nonelectrolytes is -1.86 *C/m
– *C is celsius degree
– m is mol solute/kg solvent
• Kf of electrolytes may be looked up
– See page 448 for examples
Boiling Point Elevation
• Nonvolatile solutes elevate the bp of the
solvent
• Molal boiling point constant is the boiling
point elevation of the solvent in a 1-molal
solution of a nonvolatile, nonelectrolyte solute
– Water is .51 *C/m
ΔTb = Kb · mB
•
•
ΔTb, the boiling point elevation, is defined as Tb (solution) - Tb (pure solvent).
Kb, the ebullioscopic constant, which is dependent on the properties of the
solvent. It can be calculated as Kb = RTb2M/ΔHv, where R is the gas constant, and
Tb is the boiling temperature of the pure solvent, M is the molar mass of the
solvent, and ΔHv is the heat of vaporization per mole of the solvent.
•
mB is the molality of the solution, calculated by taking dissociation into account
since the boiling point elevation is a colligative property, dependent on the
number of particles in solution. This is most easily done by using the van 't Hoff
factor i as mB = msolute · i. The factor i accounts for the number of individual
particles (typically ions) formed by a compound in solution.
•
Examples:
– i = 1 for sugar in water
– i = 2 for sodium chloride in water, due to the full dissociation of NaCl into Na+ and Cl– i = 3 for calcium chloride in water, due to dissociation of CaCl2 into Ca2+ and 2Cl-
• Do practice page 450 and 451