Ions in Aqueous Solutions and Colligative Properties

Compounds in Aqueous Solutions The separation of ions that occurs when an ionic compound dissolves is called dissociation

Assuming 100% dissociation 1 mole of NaCl will dissociate into 1 mole of Na + ions and 1 mole of Cl - ions (2 moles total) 1 mole of CaCl 2 will dissociate into 1 mole of Ca + ions and 2 moles of Cl - ions (3 moles total)

No compound is completely insoluble Low solubility compounds can be considered insoluble for practical purposes

Reactions of ions are represented by net ionic equations They include 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

Ions are formed from solute molecules by the action of the solvent in a process called ionization An H + cannot normally exist by itself It is drawn to the surrounding H 2 O molecules and forms an H 3 O + molecule The is called a hydronium ion

Substances that produce ions in solution are electrolytes The ability to conduct electricity depends on the compounds ability to produce ions in solution A strong electrolyte is any compound whose dilute aqueous solutions conduct electricity well; due to the presence of all or almost all of the dissolved compound in the form of ions

A weak electrolyte is any compound whose dilute aqueous solutions conducts electricity poorly; due to the presence of a small amount of the dissolved compound in the form of ions Strong or weak electrolytes differ in the amount of ionization or dissociation Concentrated or dilute solutions refer to the amount of solute dissolved Example: HCl is always a strong electrolyte; no matter how concentrated the solution is

Colligative Properties of Solutions Properties that depend on the concentration of solute particles but not on their identity are called colligative properties 1. Vapor pressure lowering 2. Freezing point depression 3. Boiling point elevation 4. Osmotic pressure

The vapor pressure is reduced by the addition of a nonvolatile solute to a solution The solute decreases the number of solvent molecules that can go to the vapor phase (thus lowering the vapor pressure)

The freezing point is also decreased by the addition of a nonvolatile solute to a solution The molal freezing point constant (K f ) is the freezing point depression of the solvent in a 1 molal solution of a nonvolatile, non electrolyte solution Each solvent has its own characteristic value The freezing point depression,  t f, is the difference between the freezing points of the pure solvent, and it is directly proportional to the molal concentration of the solution

The molal boiling point constant (K b ) is the boiling point elevation of the solvent in a 1 molal solution of a nonvolatile, non electrolyte solution Each solvent has its own characteristic value The boiling point elevation,  t b, is the difference between the boiling points of the pure solvent, and it is directly proportional to the molal concentration of the solution

A semipermeable allows the movement of some particles while blocking the movement of others Osmosis is the movement of a solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration Osmotic pressure is the external pressure that must be applied to stop osmosis Dependent on the concentration of solute particles

Since electrolytes can produce concentration of ions greater than their original concentrations, their colligative effect can be greater than expected 1 mol NaCl produces 2 moles of ions The effect is usually less than expected due to the attractive forces between large numbers of ions The ions will tend to congregate in groups and in effect act as one ion, thus diminishing the colligative effect