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CHAPTER 14
Ions in Aqueous Solutions and
Colligative Properties
Dissociation Equations
NaCl(s)  Na+(aq) + Cl-(aq)
AgNO3(s)  Ag+(aq) + NO3-(aq)
MgCl2(s)  Mg2+(aq) + 2 Cl-(aq)
Na2SO4(s)  2 Na+(aq) + SO42-(aq)
AlCl3(s)  Al3+(aq) + 3 Cl-(aq)
Dissolution of sodium Chloride
Double replacement forming a precipitate…
Double replacement (ionic) equation
Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
Complete ionic equation shows compounds as aqueous ions
Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) +2 I-(aq)  PbI2(s) + 2K+(aq) + 2 NO3-(aq)
Net ionic equation eliminates the spectator ions
Pb2+(aq) + 2 I-(aq)  PbI2(s)
Solubility of Ionic Compounds
The ability of a solution to conduct an electric
current can be measured with a simple device.
The ammeter measures the flow of electrons (current)
through the circuit.
If the ammeter measures a current, and the bulb
glows, then the solution conducts.
If the ammeter fails to measure a current, and the
bulb does not glow, the solution is non-conducting.
Definition of Electrolytes and
Nonelectrolytes
An electrolyte is:
A substance whose aqueous solution conducts
an electric current.
A nonelectrolyte is:
A substance whose aqueous solution does not
conduct an electric current.
Try to classify the following substances as
electrolytes or nonelectrolytes…
Electrolytes?
1.Pure water
2.Tap water
3.Sugar solution
4.Sodium chloride solution
5.Hydrochloric acid solution
6.Lactic acid solution
7.Ethyl alcohol solution
8.Pure sodium chloride
ELECTROLYTES:
NONELECTROLYTES:
Tap water (weak)
Pure water
NaCl solution
Sugar solution
HCl solution
Ethanol solution
Lactate solution (weak)
Pure NaCl
But why do some compounds conduct electricity in
solution while others do not…?
Answers to Electrolytes
Ions tend to stay in solution where they can
conduct a current rather than re-forming a
solid.
The reason for this is
the polar nature of
the water molecule…
Positive ions associate with the negative
end of the water dipole (oxygen).
Negative ions associate with the positive
end of the water dipole (hydrogen).
Some covalent compounds IONIZE in solution
Covalent acids form ions in solution, with the
help of the water molecules.
For instance, hydrogen chloride molecules,
which are polar, give up their hydrogens to
water, forming chloride ions (Cl-) and
hydronium ions (H3O+).
Ionization of HCl makes it a strong
electrolyte
Strong acids such as HCl are completely
ionized in solution.
Other examples of strong acids include:
Sulfuric acid, H2SO4
Nitric acid, HNO3
Hydriodic acid, HI
Perchloric acid, HClO4
Weak acids such as lactic
acid usually ionize less than
5% of the time.
Many of these weaker acids
are “organic” acids
that contain a “carboxyl”
group.
The carboxyl group does not easily give up its
hydrogen.
Because of the carboxyl group, organic acids are
sometimes called “carboxylic acids”.
Other organic acids and their sources include:
Citric acid – citrus fruit
Malic acid – apples
Butyric acid – rancid butter
Amino acids – protein
Nucleic acids – DNA and RNA
Ascorbic acid – Vitamin C
This is an enormous group of compounds, these
are only a few examples.
However, most covalent compounds do not ionize
at all in solution.
Sugar (sucrose – C12H22O11),
and ethanol (ethyl alcohol – C2H5OH) do not
ionize - That is why they are nonelectrolytes!
Colligative Properties
Colligative properties are those that depend
on the concentration of particles in a
solution, not upon the identity of those
properties.
 Boiling Point Elevation
 Freezing Point Depression
 Osmotic Pressure
Freezing Point Depression
Each mole of solute particles lowers the
freezing point of 1 kilogram of water by
1.86 degrees Celsius.
Kf = 1.86 C  kilogram/mol
Boiling Point Elevation
Each mole of solute particles raises the
boiling point of 1 kilogram of water by
0.51 degrees Celsius.
Kb = 0.51 C  kilogram/mol
Freezing Point Depression and Boiling
Point Elevation Constants
The van’t Hoff Factor,
i
Electrolytes may have two, three or more
times the effect on boiling point and
freezing point, depending on its dissociation.
T = i  K  m
Dissociation Equations
i = 2
NaCl(s)  Na+(aq) + Cl-(aq)
AgNO3(s)  Ag+(aq) + NO3-(aq)
i = 2
i = 3
MgCl2(s)  Mg2+(aq) + 2 Cl-(aq)
Na2SO4(s)  2 Na+(aq) + SO42-(aq)
i = 3
AlCl3(s)  Al3+(aq) + 3 Cl-(aq)
i = 4
Preventing
icing of roads
using CaCl2
Ideal vs. Real van’t Hoff Factor
The ideal van’t Hoff Factor is only achieved
in VERY DILUTE solution.