Ions in Aqueous Solutions and
Colligative Properties
Compounds in Aqueous
Solutions
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The separation of ions that occurs when an
ionic compound dissolves is called
dissociation

Assuming 100% dissociation
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1 mole of NaCl will dissociate into 1 mole
of Na+ ions and 1 mole of Cl- ions (2 moles
total)
1 mole of CaCl2 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

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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

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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 H2O molecules
and forms an H3O+ molecule
The is called a hydronium ion
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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
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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
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1.
2.
3.
4.
Vapor pressure lowering
Freezing point depression
Boiling point elevation
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)

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The freezing point is also decreased by the
addition of a nonvolatile solute to a solution
The molal freezing point constant (Kf ) is the
freezing point depression of the solvent in a
1 molal solution of a nonvolatile, non
electrolyte solution

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Each solvent has its own characteristic value
The freezing point depression, Dtf, is the
difference between the freezing points of the
pure solvent, and it is directly proportional to
the molal concentration of the solution
Dt f  K f m

The molal boiling point constant (Kb) is the
boiling point elevation of the solvent in a 1
molal solution of a nonvolatile, non
electrolyte solution

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Each solvent has its own characteristic value
The boiling point elevation, Dtb, is the
difference between the boiling points of the
pure solvent, and it is directly proportional
to the molal concentration of the solution
Dtb  Kb m
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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