Modern Chemistry Chapter 13

1
Modern Chemistry Chapter 13

• Ions in Aqueous Solutionsand Colligative
  Properties

2
Section 1- Compounds 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

3
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

4
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.

5
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.

6
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.

7
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.

8
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?

9
Section 2-Colligative 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.

10
Colligative Properties

• The freezing-point depression ( ?tf ) 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 -1.86C/m
• ?tf Kfm

11
Colligative Properties

• boiling-point elevation ( ?tb ) 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 0.51C/m
• Do problems 1-4 on page 451.

12
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.

13
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.

14
Chemistry Chapter 13 Test Review

• multiple choice (20)
• 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