Chapter 13: Ions in Aqueous
Solutions and Colligative
Properties
Section 1: Compounds in
Aqueous Solutions
• Standard 6.e.: Students know the
relationship between the molality of a
solute in a solution and the solution’s
depressed freezing point or elevated
boiling point.
• Objective: We will write dissolution
equations, predict if a precipitate will form
and if a precipitate forms, write a net ionic
equation.
Dissociation
• When an ionic compound dissolves in water,
the ions separate.
• To find how many moles of ions are
produced, we write a balanced dissociation
equation and look at the coefficients in front
of the ions.
NaCl  Na+ + Cl1 mol of Sodium Ion and 1 mol of Chloride Ion
• These are like decomposition reactions.
Example 1
• Write the equation for the dissolution of
aluminum sulfate, Al2(SO4)3, in water. How
many moles of Al ions and SO4 ions are
produced by dissolving 1 mol of Al2(SO4)3?
What is the total number of moles of ions
produced?
Al2(SO4)3  2Al3+ + 3SO422 mol Al3+ and 3 mol SO42Total moles = 2 + 3 = 5 moles
Example 2
• Do the same thing as the last example,
expect now you are dissolving 2 mols of
Al2(SO4)3.
2Al2(SO4)3  4Al3+ + 6SO424 mol Al3+ and 6 mol SO42Total moles = 4 + 6 = 10 moles
Precipitation Reactions
GENERAL SOLUBILITY GUIDELINES
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. Lead (II) chloride
is soluble in hot water.
Precipitation Reactions
GENERAL SOLUBILITY GUIDELINES CONT…
4. Most sulfates are soluble, except those of
barium, strontium, lead, calcium, and mercury.
5. Most carbonates, phosphates, and silicates
are insoluble, except those of sodium,
potassium, and ammonium.
6. Most sulfides are insoluble, except those of
calcium, strontium, sodium, potassium, and
ammonium.
Example 3
• Look at the solubility chart to determine if
the following are Soluble or Insoluble?
– Sodium Carbonate
– Calcium Phosphate
– Cadmium Nitrate
– Ammonium Sulfide
Example 3
• Look at the solubility chart to determine if
the following are Soluble or Insoluble?
– Sodium Carbonate
Soluble
– Calcium Phosphate
– Cadmium Nitrate
– Ammonium Sulfide
Example 3
• Look at the solubility chart to determine if
the following are Soluble or Insoluble?
– Sodium Carbonate
Soluble
– Calcium Phosphate
Insoluble
– Cadmium Nitrate
– Ammonium Sulfide
Example 3
• Look at the solubility chart to determine if
the following are Soluble or Insoluble?
– Sodium Carbonate
Soluble
– Calcium Phosphate
Insoluble
– Cadmium Nitrate
Soluble
– Ammonium Sulfide
Example 3
• Look at the solubility chart to determine if
the following are Soluble or Insoluble?
– Sodium Carbonate
Soluble
– Calcium Phosphate
Insoluble
– Cadmium Nitrate
Soluble
– Ammonium Sulfide
Soluble
Example 4
• Will a precipitate form when solutions of
cadmium nitrate and ammonium sulfide are
combined?
– Step 1: Determine if the compounds are
soluble, if soluble, continue to step 2.
Compounds are both soluble…so we
continue to step 2.
Example 4
• Will a precipitate form when solutions of
cadmium nitrate and ammonium sulfide are
combined?
– Step 2: Write double-displacement reaction
between the two compounds.
(NH4)2S + Cd(NO3)2  CdS + 2NH4NO3
Example 4
• Will a precipitate form when solutions of
cadmium nitrate and ammonium sulfide are
combined?
(NH4)2S + Cd(NO3)2  CdS + 2NH4NO3
– Step 3: Determine if the newly formed
compounds are soluble. If one is insoluble,
then it is a precipitate.
CdS or Cadmium Sulfide is insoluble,
so it is the precipitate.
Net Ionic Equations
• Includes only those compounds and ions that
undergo a chemical change in a reaction in an
aqueous solution.
• Basically, if the ions are part of a soluble
product, they don’t end up in the final equation,
only the ions for the precipitate that is formed,
remain in the equation.
• The ions that do not take part in the chemical
reaction are called spectator ions.
Example 5
• Write the net ionic equation for the production
of ammonium nitrate and cadmium sulfide.
2NH4+ + 2NO3- + Cd2+ + S2- 
CdS + 2NO3- + 2NH4+
• If ions show up on both sides of the equation,
cross them out and rewrite the equation
without them.
Cd2+ + S2- CdS
Ionization
• Ions are formed from solute molecules by the
action of the solvent.
• Different from dissociation because it involves
molecular compounds rather than ionic
compounds.
• In order for ions to form, the strength of the
bond within the solute molecule must be
weaker than the attractive forces of the
solvent molecules.
The Hydronium Ion
H3O+
• When a compound ionizes in a solution
and releases a H+ ion, it binds to the H2O
and forms H3O+.
H2O + HCl  H3O+ + Cl-
Electrolytes and Nonelectrolytes
• Electrolyte: a compound that conducts an
electric current when it is in an aqueous
solution or in the molten (liquid) state.
• All ionic compounds are electrolytes because
they dissociate into ions.
• Nonelectrolyte: a compound that does not
conduct an electric current in either aqueous
solution or the molten state.
• Strong Electrolyte: nearly all the ionic
compound exists as separate ions.
• Weak Electrolyte: only a fraction of the ionic
compound exists as separate ions.
• Standard 6.e.: Students know the
relationship between the molality of a
solute in a solution and the solution’s
depressed freezing point or elevated
boiling point.
• Objective: We will write dissolution
equations, predict if a precipitate will form
and if a precipitate forms, write a net ionic
equation.
Homework
• Ch 13.1 page 458 # 2-7
Section 2: Colligative
Properties of Solutions
• Standard 6.e.: Students know the
relationship between the molality of a
solute in a solution and the solution’s
depressed freezing point or elevated
boiling point.
• Objective: We will list the four colligative
properties, and calculate freezing-point
depression and boiling-point elevation.
Vapor-Pressure Lowering
• The addition of a nonvolatile substance
will raise the boiling point and lower the
freezing point.
• This has to do with vapor pressure of the
solvent.
• As the number of solute particles
increase, the proportion of solvent
molecules decreases.
Freezing-Point Depression.
• When 1 mol of a nonelectrolyte solution is
dissolved in 1 kg of water, the freezing point is
-1.86°C instead of 0.0°C.
• If 2 mols are dissolved, it is 2 x -1.86°C.
• This is called the molal freezing-point constant
(Kf) and changes for different solvents.
• Freezing-point depression Δtf is the difference
between the two freezing points. Changes
according to concentration.
Example 6
• What is the freezing-point depression of
water in a solution of 20.54 g of sucrose,
C12H22O11, in 200 g of water? What is the
actual freezing point of the solution?
• Find molality of the sugar solution.
• Multiply the Kf (of water) by the molality.
• Take normal freezing point 0°C + Δtf.
Boiling-Point Elevation
• When 1 mol of a nonelectrolyte solution is
dissolved in 1 kg of water, the boiling point is
100.51°C instead of 100.0°C. An increase of
0.51°C
• This is called the molal boiling-point constant
(Kb) and changes for different solvents.
• Boiling-point elevation Δtb is the difference
between the two boiling points. Changes
according to concentration.
Example 7
• What is the boiling-point elevation of a
solution made from 20.1 g of a
nonelectrolyte solute and 400.0 grams of
water? The molar mass of the solute is
62.0 g/mol.
Osmotic Pressure
• A semipermeable membrane allows only
water molecules to pass through during
osmosis. This can cause an increase in
volume on one side of the membrane.
• Osmotic pressure is the external pressure
that must be applied to stop osmosis.
• The higher the concentration of a
solution, the greater the osmotic pressure.
Electrolytes & Colligative Properties
• When electrolytes are dissolved in a
solvent, the effects are greater than that
of nonelectrolytes. This is because more
moles of solute particles are formed when
the compounds dissolve.
• Standard 6.e.: Students know the
relationship between the molality of a
solute in a solution and the solution’s
depressed freezing point or elevated
boiling point.
• Objective: We will list the four colligative
properties, and calculate freezing-point
depression and boiling-point elevation.
Homework
Ch 13.2 pg 456 #1-4 and 458 #14 and 16