Chapter 19
PRECIPITATION
REACTIONS
WOLPA/AP CHEMISTRY/CDO
Solubility of Ionic Solids
• Depends on the balance of two forces:
– Attraction between H2O molecules and ions of
solid.
– Force of attraction between oppositely
charged ions within solid.
WOLPA/AP CHEMISTRY/CDO
Solubility Rules
• Use in predicting results of precipitation
reactions. MEMORIZE THE SOLUBILITY
RULES!!!!!
– Determine ions present and possible
products.
– Use solubility rules to determine if any are
insoluble.
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Example 1
• Ba(NO3)2(aq) + Na2CO3(aq)
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Solubility Rules
• 1. Salts containing Group I elements are soluble (Li+, Na+,
K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing
the ammonium ion (NH4+) are also soluble.
•
2. Salts containing nitrate ion (NO3-) are generally soluble.
•
3. Salts containing Cl-, Br-, I- are generally soluble.
Important exceptions to this rule are halide salts of Ag+, Pb2+,
and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble.
•
WOLPA/AP CHEMISTRY/CDO
Solubility Rules Continued
• 4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are
common soluble salts of silver; virtually anything else is insoluble.
•
5. Most sulfate salts are soluble. Important exceptions to this rule
include BaSO4, PbSO4, Ag2SO4 and SrSO4.
• 6. Most hydroxide salts are only slightly soluble. Hydroxide salts
of Group I elements are soluble. Hydroxide salts of Group II
elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of
transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3,
Co(OH)2 are not soluble.
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Solubility Rules Continued
• 7. Most sulfides of transition metals are highly insoluble. Thus,
CdS, FeS, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth,
and lead sulfides are also insoluble.
•
8. Carbonates are frequently insoluble. Group II carbonates (Ca,
Sr, and Ba) are insoluble. Some other insoluble carbonates include
FeCO3 and PbCO3.
•
9. Chromates are frequently insoluble. Examples: PbCrO4,
BaCrO4
•
10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2,
Ag2PO4
• 11. Fluorides are frequently insoluble. Examples: BaF2, MgF2
PbF2.
WOLPA/AP CHEMISTRY/CDO
Example 1 Continued
• Ba(NO3)2(aq) + Na2CO3(aq)
WOLPA/AP CHEMISTRY/CDO
Stoichiometry
• Mole Relations
– Coefficients in the net ionic equation can be
used in the usual way to relate the moles of
reactants and products.
– Moles of ions can be deduced from solute
concentrations.
WOLPA/AP CHEMISTRY/CDO
Example 2
• What is the molar concentration of Ba2+
and F- in a solution containing 0.0075 M
BaF2
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Precipitation Titrations
• Used to determine the concentration of
species in solution or in a solid mixture.
• Indicator shows, usually by color change,
when the species being analyzed for has
been consumed
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General Principles
• Involves formation of a precipitate
• Must determine the volume of a
standardized titrant needed to just
precipitate all of the ion.
• Need an indicator or electrode to
determine when the precipitation is
complete
WOLPA/AP CHEMISTRY/CDO
Solubility Equilibria
• Solubility Product Constant, Ksp
– Precipitation reactions like all reactions, reach
a position of equilibrium.
– Expression for Ksp
MaXb <----------> aM+b + bX-a
Ksp = [M+b]a [X-a]b
WOLPA/AP CHEMISTRY/CDO
Solubility Product Principle
• In any water solution in equilibrium with a
slightly soluble ionic compound, the
product of the concentrations of its ions,
each raised to a power equal to its
coefficient in the solubility equation is a
constant. This constant, Ksp, has a fixed
value at a given temperature, independent
of the concentrations of the individual ions.
WOLPA/AP CHEMISTRY/CDO
Two Ion Compound
• AgCl
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Three Ion Compound
• PbCl2
WOLPA/AP CHEMISTRY/CDO
Four Ion Compound
• Al(OH)3
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Calculation of Ksp
• Calculated from measured solubility
– AgCl
Ksp = (s) (s) = s2
– PbCl2
Ksp = (s) (2s)2 = 4s3
– Al(OH)3
Ksp = (s) (3s)3 = 27s4
•
WOLPA/AP CHEMISTRY/CDO
Example 3
• At 20 oC, a saturated aqueous solution of
silver acetate, AgC2H3O2, contains 1.0 g
dissolved in 100.0 mL of solution.
Calculate the Ksp for AgC2H3O2.
WOLPA/AP CHEMISTRY/CDO
Determination of Solubility
• In pure water
– Ksp = s2
s = (Ksp)1/2
– Ksp = 4s3
s = (Ksp/4)1/3
– Ksp = 27s4 s = (Ksp/27)1/4
WOLPA/AP CHEMISTRY/CDO
Example 4
• Estimate the solubility of lead (II) bromide
in (a) moles per liter and (b) grams per liter
of pure water. Ksp = 6.3 x 10-6
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Uses of Ksp
• Calculation of concentration of one ion,
knowing that of the other
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Example 5
• You have a solution that has a lead (II)
concentration of 0.0012 M. What is the
maximum concentration of chloride ions
that would be present? Ksp = 1.7 x 10-5
WOLPA/AP CHEMISTRY/CDO
Uses of Ksp
• Determination of whether a precipitate will
form
• Compare original concentration product, P,
to Ksp
– if P < Ksp, no precipitate will form
– if P > Ksp, precipitate forms until P becomes
equal to Ksp
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Example 6
• You have 100.0 m of a solution that has a
lead (II) concentration of 0.0012 M. Does
PbCl2 precipitate when 1.20 g of solid
NaCl is added?
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Determination of Solubility
• In solution containing a common ion
– Solubility is much less than in pure water
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Example 7
• Calculate the solubility of silver carbonate,
Ag2CO3, in moles per liter, in pure water.
Compare this with the molar solublity of
Ag2CO3 in 225 mL of water to which 0.15 g
of Na2CO3 has been added.
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Simultaneous Equilibria
• Two or more reactions occur at the same
time is a solution, all of them being
described as equilibrium processes.
• The equilibrium for the overall reaction is
the product of the equilibrium constants for
the summed reactions.
That is Knet = K1 x K2
WOLPA/AP CHEMISTRY/CDO
Solubility and pH
• Any salt containing an anion that is the
conjugate base of a weak acid dissolves in
water to a greater extent than that given
by Ksp because the ions undergo a
hydrolysis reaction.
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Example 8
PbS
• PbS(s) <----------> Pb2+(aq) + S2-(aq)
Ksp = 8.4 x 10-28
• S2-(aq) + H2O(l) <----------> HS-(aq) + OH-(aq)
Kb = 1 x 105
• Overall
• PbS(s) + H2O(l) <---> Pb2+(aq) + HS-(aq) +
OH-(aq) Knet = 8.4 x 10-23
WOLPA/AP CHEMISTRY/CDO
• In general, the solubility of a salt
containing the conjugate base of a weak
acid is increased by addition of a stronger
acid to the solution. In contrast, the salts
are not soluble in strong acid if the anion is
the conjugate base of a strong acid.
WOLPA/AP CHEMISTRY/CDO
• CaCO3 <--------> Ca2+(aq)+ CO32-(aq)
K = Ksp = 3.8 x 10-9
• CO32-(aq) + H2O(l) <--------> HCO3-(aq) +
OH-(aq)
K = Kb = 2.1 x 10-4
• OH-(aq) + H3O+(aq) <--------> 2H2O(l)
K = 1/Kw = 1 x 1014
WOLPA/AP CHEMISTRY/CDO
• NET
• CaCO3(s) + H3O+(aq) <------> Ca2+(aq) +
HCO3-(aq) + H2O(l)
• Knet = (Ksp) (Kb)/(Kw) = 79.8
WOLPA/AP CHEMISTRY/CDO
Solubility and Complex Ions
• Examples of complex ions: AgCl2-, Ag(S2O3)23-, Ag(CN)2• The solubility of certain “insoluble “ compounds can be
increased when a complex ion is formed. Complex ions
usually refer to cations in which surrounding water
molecules have been replaced by some other electron
pair donor. The equilibrium constant will equal the
solubility constant times the formation constant for the
complex ions. (Note - Chapter 23 in your textbook
covers complex ions - their formation and nomenclature)
WOLPA/AP CHEMISTRY/CDO
Solubility and Complex ions
• AgCl(s) + 2NH3  Ag(NH3)2+ + Cl• K = (Ksp) (Kf)
• = (1.8 x 10-10) (1.6 x 107) = 0.00288 =
0.0029
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Example 9
• Solid gold (I) chloride AuCl, is dissolved
when excess cyanide ions, CN-, are added
to give a water soluble complex ion.
• AuCl(s) + 2CN- (aq) Au(CN)-(aq) + Cl• Show that this equation is the sum of two
other equations and calculate the Knet.
WOLPA/AP CHEMISTRY/CDO
Solubility, Ion Separations, and
Qualitative Analysis
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