Modern Chemistry Chapter 13
• Ions in Aqueous Solutions
and Colligative Properties
Section 1Compounds in Aqueous Solutions
• dissociation is the separation of ions that occurs when
an ionic compound dissolves
– see sample problem A on page 436
– do practice problem #1 on page 436
Solubitity
• precipitation reactions occur when
combinations of ions in a solution have an
extremely low solubility and a precipitate
forms
– see table 1 on page 437 of the textbook for
general solubility guidelines
Net Ionic Equations
• An ionic equation lists all of the ions in the
solutions of both the reactants and the products.
• A net ionic equation includes ONLY those
compounds and ions that undergo a chemical
change (usually the formation of a precipitate) in
a reaction in an aqueous solution.
• Spectator ions are ions that do not take part in
a chemical reaction and are found in solution
both before and after the reaction.
Ionic Equations
• An example of an ionic equation.
Na+ (aq) + Cl-(aq) + Ag+(aq) + NO3- (aq) 
Na+ (aq) + NO3- (aq) + AgCl (ppt)
• By removing the spectator ions (in italics), we
end up with the following net ionic equation.
Ag+ (aq) + Cl- (aq)  AgCl (ppt)
Do problems #1, 2, 3, & 4 on page 440 of the text.
Terms referring to solutions
• ionization occurs when ions are formed
from solute molecules by the action of the
solvent
HCl (dissolved in H2O)  H+ + Cl-
• The hydronium ion ( H3O+ ) forms when
a H+ ion combines with a water molecule.
This is the ion commonly associated with
acids.
Terms
• A strong electrolyte is any compound whose
dilute aqueous solutions conduct electricity well
due to all or almost all of the dissolved
compound forming ions.
• A weak electrolyte is any compound whose
dilute solutions conduct electricity poorly due to
only small amounts of the dissolved compound
forming ions.
– Do section review questions #1 & 2 on page 443.
Historical Chemistry
• Read the Historical
Chemistry feature on
“The Riddle of
Electrolysis”.
• Answer the two questions
at the end of the feature.
• What role might
electrolysis play in the
production of fuel for
hydrogen fuel cells?
Section 2Colligative Properties
• Colligative properties are properties that
depend on the concentration of solute
particles, not their identity.
• A nonvolatile substance is one that has
little tendency to become a gas under its
existing conditions.
Colligative Properties
The freezing-point depression ( Δtf ) is the
difference between the freezing points of the
pure solvent and a solution of a nonelectrolyte in
that solvent.
• is directly proportional to the molal concentration of the
solution
• The molal freezing-point constant (Kf ) is the amount the
freezing point of a 1.0 molal solution is depressed = 1.86°C/m
Δtf = Kfm
Colligative Properties
• boiling-point elevation ( Δtb ) is the difference
between the boiling points of a pure solvent and
a nonelectrolyte solution of that solvent.
• molal boiling-point constant ( Kb ) is the
boiling point elevation of a solvent in a 1.0 molal
solution of a nonvolatile, nonelectrolyte solute
Kb = 0.51°C/m
Do problems #1-4 on page 451.
Osmosis
• semipermeable membrane allows the passage of some
particles while blocking the passage of others
• osmosis is the movement of solvent through a
semipermeable membrane from the side of lower solute
concentration to the side of higher concentration.
• osmotic pressure is the external pressure that must be
applied to stop osmosis
• ***Read Chemistry in Action on page 453.
Electrolytes & Colligative
Properties
• Because electrolytes dissolve in aqueous
solution to produce more than one ion per
molecule, electrolytes produce colligative
properties that are almost equal to the
molality of the dissolved ions.
1 mole NaCl  1 mole Na+ + 1 mole Cl- = 2 moles of ions
Do section Review problems #1, 3, & 5 on page 456.
Chemistry Chapter 13 Test Review
• multiple choice (20)
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relationship of moles of ions to molecules
recognize net ionic equations
definition of precipitation reaction
use solubility guidelines to identify precipitates
definitions of dissociation & ionization
the hydronium ion formula & its anions
definitions of weak & strong electrolytes
colligative properties
how nonelectrolytes, electrolytes, and nonvolatile solutes affect
colligative properties
– calculate molality using freezing point depression & molal
freezing point constant