Modern Chemistry Chapter 13

• Ions in Aqueous Solutionsand Colligative Properties

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

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

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.

Ionic Equations

• An example of an ionic equation.Na+ +

(aq)(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.

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.

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.

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?

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.

Colligative Properties

The freezing-point depression ( Δtf ) 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 = -1.86°C/m

Δtf = Kfm

Colligative Properties

• boiling-point elevation ( Δtb ) 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 = 0.51°C/m

Do problems #1-4 on page 451.

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.

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.

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