Chapter 14 Ions in Aqueous Solutions & Colligative Properties
Section 14-1 • Compounds in Aqueous Solutions
Dissociation • note how equation charges balance as well:
Sample Problem 14-1 • Write the equation for the dissolution of aluminum sulfate, Al2(SO4)3, in water. How many moles of aluminum ions and sulfate ions are produced by dissolving 1 mol of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mol of aluminum sulfate? • nsolute=1mol Al2(SO4)3 solvent=water • nAl3+=? nSO42-=?
Precipitate Reactions • net ionic equations • anything that is water soluble should be shown as separate aqueous ions. So a compound would be written as two separate ions. • things that are not soluble are written as a single compound in solid form, the precipitate. • ions present in reactants and products that don’t change in reaction are called spectator ions and they cancel each other out.
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 write the formula equation, overall ionic equation, net ionic equation for the reaction.
Strong & weak electrolytes • Hydronium ion=H3O1+ • all strong acids (most binary acids) and soluble ionic compounds are strong electrolytes. • Weak electrolytes do not dissociate 100%. It is in equilibrium and never fully ionizes. Weakly soluble compounds can’t be strong electrolytes.
Section 14-2 • Colligative Properties of Solutions
Vapor pressure lowering • Non-volatile solutes raise boiling point and lower melting point • They get in between the water molecules and “distract” them from vaporizing, thus raising boiling points. They also are at lower energy than the solvent alone so it would require additional energy to freeze them
Freezing point depression • Molal freezing point constant (Kf), freezing point depression of the solvent in a 1 molal of a nonvolatile nonelectrolyte solute • Freezing point depression, DTf, is the amount the usual freezing point is lowered by the addition of the solute.
Sample Problem 14-3 • What is the freezing point depression of water in a solution of 17.1 g of sucrose, C12H22O11, with 200g of water. What is the actual freezing point of the solution?
Boiling point elevation • Molal boiling point constant, Kb • Boiling point elevation, DTb
Osmotic Pressure • Semipermeable membranes allow some things to pass but not others • Osmotic pressure-pressure required to stop osmosis
Strong electrolytes and colligative properties • While a molecule is surrounded by water molecules leading to colligative properties, strong electrolytes ionize and each molecule forms several ions. Thus strong electrolytes have extreme colligative properties.