Transcript Compounds in Aqueous Solutions

Compounds in
Aqueous Solutions
Total Ionic Equations
• Once you write the molecular equation (synthesis,
decomposition, etc.), you should check for reactants
and products that are soluble or insoluble.
• We usually assume the reaction is in water
• We can use a solubility table to tell us what
compounds dissolve in water.
• If the compound is soluble (does dissolve in water),
then splits the compound into its component ions
• If the compound is insoluble (does NOT dissolve in
water), then it remains as a compound
General Solubility Guidelines
Total Ionic Equations
Molecular Equation:
K2CrO4 + Pb(NO3)2 
PbCrO4 + 2 KNO3
Soluble
Insoluble
Soluble
Soluble
Total Ionic Equation:
2 K+ + CrO4 -2 + Pb+2 + 2 NO3- 
PbCrO4 (s) + 2 K+ + 2 NO3-
Net Ionic Equations
• These are the same as total ionic equations,
but you should cancel out ions that appear on
BOTH sides of the equation
Total Ionic Equation:
2 K+ + CrO4 -2 + Pb+2 + 2 NO3- 
PbCrO4 (s) + 2 K+ + 2 NO3Net Ionic Equation:
CrO4 -2 + Pb+2  PbCrO4 (s)
Net Ionic Equations
• Try this one! Write the molecular, total ionic, and net ionic
equations for this reaction: Silver nitrate reacts with Lead
(II) Chloride in hot water.
AgNO3 + PbCl2 
Molecular:
2 AgNO3 + PbCl2  2 AgCl + Pb(NO3)2
Total Ionic:
2 Ag+ + 2 NO3- + Pb+2 + 2 Cl-  2 AgCl (s) + Pb+2 + 2 NO3Net Ionic:
Ag+ + Cl-  AgCl (s)
Chapter 13
Ionization
• Ions are formed from solute molecules by the
action of the solvent in a process called
ionization.
• When a molecular compound dissolves and
ionizes in a polar solvent, ions are formed where
none existed in the undissolved compound.
• Hydrogen chloride, HCl, is a molecular compound
that ionizes in aqueous solution.
• HCl contains a highly polar bond.
H2O
HCl 
 H (aq) + Cl– (aq)
Chapter 13
The Hydronium Ion
• Some molecular compounds ionize in an aqueous
solution to release H+.
• The H+ ion attracts other molecules or ions so
strongly that it does not normally exist alone.
H2O
HCl 
 H3O (aq) + Cl– (aq)
• The H3O+ ion is known as the hydronium ion.
Chapter 13
Strong and Weak Electrolytes
• Electrolytes are substances that yield ions and conduct an
electric current in solution.
• The strength with which substances conduct an electric
current is related to their ability to form ions in solution.
• Strong and weak electrolytes differ in the degree of
ionization or dissociation.
Models for Strong and Weak
Electrolytes and Nonelectrolytes
Chapter 13
Strong Electrolytes
• A strong electrolyte is any compound whose
dilute aqueous solutions conduct electricity well;
this is due to the presence of all or almost all of
the dissolved compound in the form of ions.
• To whatever extent they dissolve in water, they
yield only ions.
• HCl, HBr, HI
• All soluble ionic compounds
Chapter 13
Weak Electrolytes
• A weak electrolyte is any compound whose dilute
aqueous solutions conduct electricity poorly; this
is due to the presence of a small amount of the
dissolved compound in the form of ions.
• Some molecular compounds form aqueous
solutions that contain not only dissolved ions but
also some dissolved molecules that are not
ionized.

–


HF 
H
O
(
aq
)
+
F
(aq )
 3
[HF] >> [H+] and [F–]
lowering
Vapor Pressure Reduction
Pure Solvent
Solution
Vapor Pressure
of concentrated
solution is lower
because solute
particles interfere
with vaporization
Also results in boiling
point elevation:
ΔTb = Kbm
Solvent Particles travel from Right to Left
Less concentrated to more concentrated
Boiling Point Elevation
The change in boiling point is proportional to the molality of
the solution:
Tb = Kb m i
where:
Kb = molal boiling point elevation constant, a property of the solvent.
m = molality of the solvent
i = Van’t Hoff constant
Tb is added to the normal boiling point of the solvent.
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Freezing Point Depression
• The change in freezing point can be found similarly:
Tf = Kf m i
where:
Kf = molal freezing point depression constant, a property of the solvent.
m = molality of the solvent
i = Van’t Hoff constant
• Tf is subtracted from the normal boiling point of the
solvent.
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Boiling Point Elevation and Freezing
Point Depression
Nonvolatile solutesolvent interactions also
cause solutions to have
higher boiling points
and lower freezing
points than the pure
solvent.
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Colligative Properties of Electrolytes
Since these properties depend on the number of particles
dissolved, solutions of electrolytes (which dissociate in
solution) should show greater changes than those of
nonelectrolytes.
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Colligative Properties of Electrolytes
However, a 1M solution of NaCl does not show
twice the change in freezing point that a 1M
solution of methanol does.
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van’t Hoff Factor
• One mole of NaCl in water does
not really give rise to two moles
of ions.
• Some Na+ and Cl- reassociate for
a short time, so the true
concentration of particles is
somewhat less than two times
the concentration of NaCl.
• Reassociation is more likely at
higher concentration.
© 2009, Prentice-Hall, Inc.
van’t Hoff Factor
© 2009, Prentice-Hall, Inc.
Vapor Pressure Reduction/Boiling Point Elevation
ΔTb = Kbm
Freezing Point Depression
ΔTf = Kfm