Aqueous
Solutions
Thinking of the Molecules…
•Why do chemists
work mainly with
LIQUID solutions?!
Exothermic Rxns
• The temperature of the
solution rises as solute is
dissolved
• This is because the reaction
causes a loss of heat in the
system
Endothermic Rxn
• The temperature of the
solution decreases as the
solute is dissolved
• This is because the system is
absorbing heat from its
surroundings
Solution Process Model
• Simultaneous steps in solution
creation
1. Solvent particles separate
2. Solute particles separate
3. Solute particles bond to
solvent particles
Bond Breaking
• Requires energy
• Why?!
• ENDOTHERMIC REACTION
Bond Making
• Releases energy
• Why?
• EXOTHERMIC REACTION
RECAP:
• Label Exo and Endo Steps
• Simultaneous steps in solution creation
1. Solvent particles separate
2. Solute particles separate
3. Solute particles bond to solvent particles
• Since dissolving is always 3
steps, whether the reaction is
endo or exo depends on the
SUM of the energies involved
If the reaction is ENDO
• More energy is needed to
Break bonds than is given off
in formation
If the reaction is EXO
• More energy is given off in
formation of bonds than is
needed to break bonds
Electrolytes
• Conduct electricity
• In order to conduct
electricity, charged particles
(ions) must be present in
solution
Nonelectrolytes
• Do not conduct electricity
• Solutes form neutral
molecules in solution, so no
current is able to flow
Dissociation
• The separation of ions from
the crystals of ionic
compounds
• i.e. NaCl dissolves in water
forming Na+ and Cl• water surrounds the ions
(solvation aka hydration)
• negative polar side of water
attracts to the Na+, the
positive side to the Cl-
• this is represented through
the symbol (aq) meaning
aqueous solution
• i.e.
K2SO4(s)  2K+(aq) + SO42-(aq)
• note the charges always have
to be balanced and equal
Ionization
• The formation of ions from
polar molecules by the action
of a solvent
• i.e. HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq)
• Remember HCl is not an ionic
substance, but it is molecular
• HCl breaks down into H+ and Cl-,
but H+ is immediately hydrated
into H3O+
• i.e.
• HBr + H2O  H3O+(aq) + Br-(aq)