Ions in Aqueous
Solutions and
Colligative
Properties
Chemistry 1-2
Mr. Chumbley
Modern Chemistry: Chapter 13
p. 410 – 419
Compounds in Aqueous
Solutions
EQ: How are solutions formed at a molecular level?
Compounds in Solution
 We
have identified two types of
compounds:


Ionic Compounds
Molecular Compounds
 When
dissolved in water, ionic and
molecular compounds behave differently
Ions in Solution
 Ions
separate from each other when ionic
compounds are dissolved in water
 Dissociation
is the separation of ions that
occurs when an ionic compound
dissolves
Dissociation
 We
can use a chemical equation to
indicate dissociation of ions in solution
NaCl 𝑠
CaCl2 𝑠
H2 O
H2 O
Na+ (𝑎𝑞) + Cl− (𝑎𝑞)
Ca2+ (𝑎𝑞) + 2Cl− (𝑎𝑞)
Sample Problem 13A
Write the equation for the dissolution of aluminum
sulfate, Al2(SO4)3, in water.
A.
How many moles of aluminum ions and sulfate
ions are produced by dissolving 1 mol of
aluminum sulfate?
B.
What is the total number of moles of ions
produced by dissolving 1 mol of aluminum
sulfate?
Practice!
 Practice

#1
A (p. 412)
Solubility Guidelines
(Figure 1.3 on p. 413)
1
Sodium, potassium, and ammonium compounds are
soluble in water.
2 Nitrates, acetates, and chlorates are soluble.
3
Most chlorides are soluble, except those of silver,
mercury(I), and lead.
Most sulfates are soluble, except those of barium,
4
strontium, lead, calcium, and mercury.
Most carbonates, phosphates, and silicates are
5 insoluble are insoluble, except those of sodium,
potassium, and ammonium.
6
Most sulfides are insoluble, except those of calcium,
strontium, sodium, potassium, and ammonium.
Precipitation Reactions

The solubility guidelines can be used to
predict what precipitate, if any, will be
formed when combining two solutions

Consider the combination of ammonium
sulfide and cadmium nitrate
NH4 2 S 𝑠
Cd NO3
2
𝑠
H2 O
H2 O
2NH4+ 𝑎𝑞 + S 2− (𝑎𝑞)
Cd2+ 𝑎𝑞 + 2NO−
3 (𝑎𝑞)
Precipitation Reactions
NH4 2 S 𝑎𝑞 + Cd NO3
2
𝑎𝑞
2NH4 NO3 ? + CdS(? )
 By
consulting the solubility guidelines, it can be
determined that cadmium sulfide is insoluble, while
ammonium nitrate is soluble, giving:
NH4 2 S 𝑎𝑞 + Cd NO3
2
𝑎𝑞
2NH4 NO3 𝑎𝑞 + CdS(𝑠)
Net Ionic Equations
A
net ionic equation includes only those
compounds and ions that undergo a
chemical change in a reaction in
aqueous solution
Net Ionic Equations
 Consider
the previous reaction:
NH4 2 S 𝑎𝑞 + Cd NO3
 The
2
𝑎𝑞
2NH4 NO3 𝑎𝑞 + CdS(𝑠)
net ionic equations would look like:
+
2− 𝑎𝑞
Cd2+ 𝑎𝑞 + 2NO−
3 𝑎𝑞 + 2NH4 𝑎𝑞 + S
+
CdS 𝑠 + 2NO−
𝑎𝑞
+
2NH
3
4 𝑎𝑞
Spectator Ions
 Ions
that do not take part in a chemical
reaction and are found in solution both
before and after the reaction are
spectator ions
 Therefore,
the ammonium ion and nitrate
ion are both spectator ions in the
example
Sample Problem 13B
Identify the precipitate that forms when aqueous solutions of
zinc nitrate and ammonium sulfide are combined. Write the
equation for the possible double displacement reaction. Then
write the net ionic equation for the reaction.
Molecular Compounds in
Solution
A
molecular compound ionizes in a polar
solvent
 Ionization
occurs when ions are formed
from solute molecules by the action of the
solvent
 Ionization
is different from dissociation
Hydronium Ion

Many molecular compounds have a
hydrogen atom bonded by a polar covalent
bond

The ionization of these compound is enough
to transfer the H+ ion to the water molecule
making it H3O+

The H3O+ ion is known as the hydronium ion
Strength of Electrolytes
 The
strength of an electrolyte depends on
how many dissolved ions it contains
A
strong electrolyte has almost all of the
dissolved compound in the form of ions
A
weak electrolyte has small amounts of
the dissolved compound in the form of
ions
Colligative Properties of
Solutions
EQ: How does concentration affect the properties
of solutions?
Colligative Properties
 The
amount of solute sometimes affects
the properties of solutions
 Colligative
properties are properties that
depend on the concentration of solute
particles but not on their identity
Molality
 Molality
(m) is the concentration of a
solution expressed in moles of solute per
kilogram of solvent
moles solute
Molality =
mass of solvent (kg)
Freezing Point Depression

Solutes affect how the solvent undergoes phase
changes

Freezing point depression (Δtf )is the difference
between the freezing points of the pure solvent and
a solution

The change in temperature of that freezing point is
dependent on the molal concentration (m) and a
value called the molal freezing point constant (Kf)
∆𝑡𝑓 = 𝐾𝑓 𝑚

For water, Kf = -1.86 ˚C/m
Sample 13C (p. 425)
 What
is the expected change in the freezing
point of water in a solution of 0.050 mol of sucrose
(C12H22O11) in 0.20 kg of water?
Sample 13F (p. 430)
What is the expected change in the freezing point
of water in a solution of 62.5 g of barium nitrate,
Ba(NO3)2 in 1.0 kg of water?
Boiling Point Elevation

Boiling point elevation (Δtb) is the difference
between the boiling points of the pure solvent and
a nonelectrolyte solution of that solvent

The change in temperature of that boiling point is
dependent on the molal concentration (m) and a
value called the molal boiling point constant (Kb)
∆𝑡𝑏 = 𝐾𝑏 𝑚
For water, KB = 0.51 ˚C/m
Sample 13D (p. 425)
 What
is the boiling point elevation of a solution
made from 20.1 g of a molecular solute with a
molar mass of 62.0 g/mol and 400 g of water?
Effect of Electrolytes
 Since
electrolytes dissociate in solution,
they have a greater impact on colligative
properties
 An
estimation for the effect of an
electrolyte on the freezing-point or
boiling-point can be found using the
concentration of total ions
Sample Problem 13F (p. 430)
What is the expected change in the
freezing point of water in a solution of 62.5 g
of barium nitrate in 1.00 kg of water?
Unused Slides
Sample Problem 12D (p. 401)
A solution was prepared by dissolving 17.1 g
of sucrose (C12H22O11) in 125 g of water. Find
the molal concentration of this solution.
(molar mass C12H22O11 = 342.30 g/mol)