### Chapter 13

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Chapter 13
Ions in Aqueous Solutions and
Colligative Properties
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Compounds in Aqueous Solutions

When a compound that is
water, the ions separate
from one another.

This separation of ions that
occurs when an ionic
compound dissolves is
called dissociation.

The opposite reaction is
called ionization
NaCl (s)
H2O
Na+ (aq) + Cl – (aq)
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Remember: Coefficients in a Balanced
Equation = Number of moles!
Problem:
Write the equation for the
Al2(SO4)3 (s) H2O 2Al 3+ (aq) + 3SO42– (aq)
dissolution of aluminum sulfate,
Al2(SO4)3, in water. How many
moles of aluminum ions and
2 mol Al 3+ + 3 mol SO42– = 5 mol of solute
sulfate ions are produced by
ions
dissolving 1 mol of aluminum
sulfate? What is the total number
of moles of ions produced by
dissolving 1 mol of aluminum
sulfate?
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Solubility Rules for Ionic
Compounds
Compounds
Solubility
Exceptions
1. Salts of alkali
metals and ammonia
Soluble
Some lithium
compounds
2. Nitrate salts and
chlorate salts
Soluble
Few Exceptions
3. Sulfate salts
Soluble
Compounds of Pb, Ag,
Hg, Ba, Sr, and Ca
4. Chloride salts
Soluble
Compounds of Ag and
some compounds of
Hg and Pb
5. Carbonates,
phosphates,
chromates, sulfides,
and hydroxides
Most are insoluble
Compounds of the
alkali metals and of
ammonia
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Net Ionic Equations

Write a complete ionic equation that shows dissolved ionic
compounds as their free ions.

Eliminate ions that do not participate in the reaction by canceling
ions that appear on both sides of the equation. These are called
spectator ions.

Ions that are not directly involved in a reaction are called spectator
ions.

Rewrite the equation, leaving out the canceled spectator ions.

Balance the atoms and the charges of the ions.

A net ionic equation indicates only those particles that actually take
part in the reaction.
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Net Ionic Equations
Cd2+ (aq) +2NO3- (aq) + 2NH4+ (aq) + S2- (aq)
CdS (s) + 2NO3- (aq) + 2NH4+ (aq)
**Get rid of spectator ions
** Find the solid and then the ions that make up that solid. Those are the only
molecules that are used in the net ionic equations.
Cd2+ (aq) + S2- (aq)
CdS (s)
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Practice Problem

Write a balanced net ionic equation for the following reaction:
Pb(s) + AgNO3 (aq)  Ag (s) + Pb(NO3)2 (aq)
1. The nitrate ion is the spectator ion.
2. The number of atoms balance, but the charges on
the ions do not balance.
3. Place a coefficient 2 in front of Ag+ (aq) to balance
the charges.
4. A coefficient of 2 in front of Ag (s) rebalances the
atoms.
5. Pb(s) + 2Ag+ (aq)  2Ag (s) + Pb2+ (aq) is the balanced net ionic
equation
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Practice Using Net Ionic Equations &
Solubility Rules

Identify the precipitate formed and write the net ionic equation for the
reaction of aqueous potassium carbonate with aqueous strontium
chloride.
1. Write the formula for the beginning compounds (reactants). Look up
the symbols and balance the charges.
2. Double-displacement
a) list the products- change partners
..cation listed first
b) balance charges in the products
c) balance the equation (count the atoms)
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Continued…
3.
Using solubility rules, look at possible new pairings of cation and
anion that give an insoluble substance.
4.
Dissociation: separate the ions
Include symbol, charge, and state (aq)
Do not separate the solid
5. Eliminate the spectator ions and write the net ionic equation.
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
1. Reactants as dissociated free ions
2K+ (aq) + CO32- (aq) + Sr2+ (aq) + 2Cl- (aq)
Charges must be balanced to equal 0.

2. Of the two possible combinations, KCl is soluble (Rules 1 and 4) and
SrCO3 is insoluble (Rule 5)

3. The net ionic equation must be balanced for the number of atoms of
each element and the charges on the ions.
Sr2+ (aq) + CO32- (aq)  SrCO3 (s)
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
Hydronium Ion
The H3O+ ion is known as the
hydronium ion. The reaction of the
H+ ion to form the hydronium ion
produces much of the energy
needed to ionize a molecular
solute.
H2O(l) + HCl (g)
H3O+ +Cl- (aq)
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Electrolytes

Strong Electrolytes:
Any compound whose dilute
aqueous solutions conduct
electricity well; this is due to the
presence of all or almost all of
the dissolved compound in the
form of ions.

Weak Electrolytes:
Any compound whose dilute
aqueous solutions conduct
electricity poorly; this is due to
the presence of a small amount
of the dissolved compound in
the form of ions.

Hydriodic acid HI

Acetic acid (CH3COOH)

Hydrobromic acid HBr

Carbonic acid (H2CO3)

Hydrochloric acid HCl

Amonnia (NH3)

Sulfuric acid H2SO4

Sodium Hydroxide (NaOH)
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Colligative Properties of Solutions
 Colligative
Properties- Properties that depend on
the concentration of solute particles but not on
their identity
 We
will discuss:
 Vapor-Pressure Lowering
 Freezing-Point Depression
 Boiling-Point Elevation
 Osmotic Pressure
 Relationship between Colligative Properties and
Electrolytes
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Vapor-Pressure Lowering

Nonvolatile Substance- A substance that has little tendency to
become a gas under existing conditions.

Adding a nonvolatile solute to a solvent always lowers the
vapor pressure.
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Freezing-Point Depression

Freezing-Point Depression(Δtf)- the difference between the
freezing points of the pure solvent and a solution of a nonelectrolyte in that solvent, and it is directly proportional to
the molal concentration of the solution
Δtf = Kfm
Molal freezing point constant (Kf)- the freezing point depression of
the solvent in a 1-molal solution of a nonvolatile, nonelectrolyte
solute.
Molality (m) – mol solute/kg of solvent
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Freezing-Point Depression Con’t
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Practice Problem

A water solution containing an unknown quantity of a nonelectrolyte
solute is found to have a freezing point of -0.23°C. What is the molal
concentration of the solution?

m=
Δtf / Kf
Δtf = f.p of solution – f.p of pure solvent = -0.23°C – 0.00°C = -0.23°C
m= 0.12m
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Boiling-Point Elevation

Boiling-Point Elevation- the difference between the boiling
points of the pure solvent and a solution of a non-electrolyte
in that solvent, and it is directly proportional to the molal
concentration of the solution
Δtb= Kbm
*Molal freezing point constant (Kb)
Example - antifreeze
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Osmotic Pressure

Osmosis- The movement of solvent
through a semipermeable membrane
from the side of lower solute
concentration to the side of higher solute
concentration

Semipermeable membrane- allows the
passage of some particles while blocking
the passage of others

Osmotic Pressure- the external pressure
that must be applied to stop osmosis
 Because osmotic pressure is dependent
on the concentration of solute particles
and not on the type of solute particles,
it is a colligative property
 The greater the concentration of a
solution, the greater the osmotic
pressure
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