Honors Chemistry, Chapter 14

1
Chapter 14 Ions in Aqueous Solutions and
Colligative Properties
2
Dissociation

• When a compound that is made of ions dissolves in
  water, the ions separate from one another. This
  separation of ions that occurs when an ionic
  compound dissolves is called dissociation.
• Examples
• NaCl(s) water Na(aq) Cl-(aq)
• CaCl2(s) water Ca2(aq) 2Cl-(aq)

3
Sample Problem 14-1

• Write the equation for the dissociation of
  aluminum sulfate, Al2(SO4)3, in water. How many
  moles of aluminum ions and sulfate ions are
  produced by dissolving 1 mole of aluminum
  sulfate? What is the total number of moles of
  ions produced by dissolving 1 mole of aluminum
  sulfate?
• Al2(SO4)3(s) water 2Al3(aq) 3SO42-(aq)

4
Sample Problem 14-1
Al2(SO4)3(s) water 2Al3(aq) 3SO42-(aq) 1
mol Al2(SO4)3? 2 mol Al3 and 3 mol SO42- 2 mol
Al33 mol SO42- 5 mol of solute ions
5
General Solubility Rules

• Most Na, K, and NH4 compounds are soluble.
• Most nitrates, acetates and chlorates are
  soluble.
• Most Cl- are soluble, except for Ag, Hg(I), and
  Pb. PbCl2 is soluble in hot water.
• Most sulfates are soluble, except Ba, Sr, and Pb.
• Most carbonates, phosphates, and silicates are
  insoluble, except those of Na, K, and NH4.
• Most sulfides are insoluble, except those of Ca,
  Sr, Na, K, and NH4.

6
Soluble?

• Ca3(PO4)2?
• What about
• (NH4)2S(s) water 2NH4(aq) S2-(aq)
• And
• Cd(NO3)2(s) water Cd2(aq) 2NO3-(aq) ?
• Double Replacement gives
• NH4NO3 and CdS. Are either insoluble?

7
Net Ionic Equations

• A net ionic equation includes only those
  compounds and ions that undergo a chemical change
  in a reaction in an aqueous solution.
• Ions that do not take part in a chemical reaction
  and are found in solution both before and after
  the reaction are spectator ions.

8
Net Ionic Equations

• Example
• Cd2(aq) 2NO3-(aq) 2NH4(aq) S2-(aq)
• ? CdS(s) 2NO3-(aq) 2NH4
• Becomes
• Cd2(aq) S2-(aq) ? CdS(s)

9
Sample Problem 14-2

• Identify the precipitate that forms when aqueous
  solutions of zinc nitrate and ammonium sulfide
  are combined. Write the equation for the
  possible double-replacement reaction. Then writ
  the formula equation, overall ionic equation, and
  net ionic equation for the reaction.

10
Sample Problem 14-2

• Zn(NO3)2(aq) (NH4)2S(aq) ?
• ZnS(?)
  2NH4NO3(?)
• Are either insoluble?
• Zn(NO3)2(aq) (NH4)2S(aq) ?
• ZnS(s)
  2NH4NO3(aq)

11
Sample Problem 14-2
Zn2(aq) 2NO3-(aq) 2NH4-(aq) S2-(aq) ?
ZnS(s) 2NH4 2NO3-(aq) Net
ionic equation Zn2(aq) S2-(aq) ?ZnS(s)
12
Ionization

• Ions are formed from solute molecules by the
  action of the solvent in a process called
  ionization.
• Ionization is different than dissociation in that
  the ions do not exist before the action of the
  solvent. In dissociation ions are already
  present and are simply separated by the action of
  the solvent.

13
Hydronium Ion

• The H3O ion is known as the hydronium ion.
• Example
• H2O(l) HCl(g) ? H3O(aq) Cl-(aq)

14
Strong and Weak Electrolytes

• A strong electrolyte is 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.
• Example Aqueous solution of NaCl.
• A weak electrolyte is any compound whos dilute
  aqueous solutions conduct electricity poorly
  this is due to the presence of a small amount of
  dissolved compound in the form of ions.
• Example Aqueous solution of HF or HC2H3OO.

15
Colligative Properties

• Properties that depend upon the concentration of
  solute particles but not on their identity are
  called colligative properties.

16
Vapor Pressure Lowering

• A non-volatile substance is one that has little
  tendency to become a gas under existing
  conditions.
• When a non-volatile substance is dissolved in a
  solvent, some solvent at the surface of the
  liquid is displaced by the solute. This reduces
  the probability that solvent molecules can escape
  from the surface thus lowers the vapor pressure.

17
Freeze-Point Depression

• The freeze-point depression, Dtf, is the
  difference between the freezing points of the
  pure solvent and a solution of non-electrolyte in
  that solvent, and is directly proportional to the
  molal concentration of the solution.
• The molal freezing-point constant (Kf), is the
  freezing-point depression of the solvent in a 1
  molal solution of a nonvolatile, nonelectrolyte
  solute.
• Dtf Kf m

18
Sample Problem 14-3

• What is the freezing-point depression of water in
  a solution of 17.1 g of sucrose, C12H22O11, and
  200 g of water? What is the actual freezing
  point of the solution?
• Given 17.1 g sucrose
• 200 g water
• Kf -1.86oC/m
• C12H22O11 342.34 g/mol

19
Sample Problem 14-3
17.1g sucrose x1 mol sucrose 0.0500 mol
342.34 g 0.0500mol sucrose
x 1000 g water 0.250 m 200 g water
1 kg water Dtf 0.250 m x (-1.86oC/m)
-0.465oC f.p. solution 0oC (-0.465oC)
-0.465oC
20
Sample Problem 14-4

• A water solution containing an unknown quantity
  of a nonelectrolyte solute is found to have a
  freezing point of -0.23oC. What is the molal
  concentration of the solution?
• T/P/S
• 0.12 m

21
Boiling Point elevation

• The boiling-point elevation, Dtb, is the
  difference between the boiling points of the pure
  solvent and a nonelectrolye solution of that
  solvent, and it is proportional to the molal
  concentration of the solution.
• The molal boiling-point constant (Kb) is the
  boiling-point elevation of the solvent in a
  1-molal solution of a nonvolatile, nonelectrolyte
  solute.
• Dtb Kb m

22
Sample Problem 14-5

• What is the boiling-point elevation of a solution
  made from 20.0 g of a nonelectrolyte solute and
  400 g of water? The molar mass of the solute is
  62.0 g/mol
• Given 20.0 g of solute
• 400 g of water
• 62.0 g/mol molar mass

23
Sample Problem 14-5
Dtb Kb m Dtb Kb m 20.0 g solute x 1 mol
solute 0.323 mol
62.0 g 0.323 mol solute x 1000 g water 0.808 m
400 g water 1 kg water Dtb
0.51oC/m x 8.08 m 0.41oC
24
Osmotic Pressure

• Semipermeable membranes allow the movement of
  some particles while blocking the movement of
  others.
• The movement of solvent through a semipermeable
  membrane from the side of lower solute
  concentration to the side of higher solute
  concentration is osmosis.
• Osmotic pressure is the external pressure tat
  must be applied to stop osmosis.

25
Electrolytes and Colligative Properties

• How many moles of ions are presented by a 1 molar
  solution of barium nitrate?
• Ba(NO3)2 (s) ? Ba2(aq) 2NO3-(aq)
• Answer 3 molar

26
Sample Problem 14-6

• What is the expected change in the freezing point
  of water in a solution of 62.5 g of barrium
  nitrate, Ba(NO3)2, in 1.00 kg of water?
• Given 62.5 g Ba(NO3)2
• Ba(NO3)2 is 261.35 g/mol
• 1.00 Kg of water
• Dtf -1.86oC/m

27
Sample Problem 14-6
Given 62.5 g Ba(NO3)2 Ba(NO3)2 is 261.35
g/mol 1.00 Kg of water Dtf
-1.86oC/m T/P/S -1.33oC