Modern Chemistry Chapter 13 Ions in Aqueous Solutions

Modern Chemistry Chapter 13 • Ions in Aqueous Solutions and 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) + Cl-(aq) + Ag+(aq) + NO 3 - (aq) Na+ (aq) + NO 3 - (aq) + Ag. Cl (ppt) • By removing the spectator ions (in italics), we end up with the following net ionic equation. Ag+ (aq) + Cl- (aq) Ag. Cl (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 H 2 O) H+ + Cl- • The hydronium ion ( H 3 O+ ) 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 Na. Cl 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, and nonvolatile solutes affect colligative properties – calculate molality using freezing point depression & molal freezing point constant