Aqueous Equilibria, Part 2
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AP Chemistry Lecture Outline
The Common-Ion Effect
Suppose we have a weak acid and a soluble salt of that acid.
CH3COOH
CH3COOH
NaCH3COO
CH3COO– + H+
Since NaCH3COO is strong, adding it to the solution…
By Le Chatelier…
The result is that…
This illustrates the common-ion effect:
“The dissociation of a weak electrolyte decreases when a strong electrolyte
– that has an ion in common with the weak electrolyte – is added to the solution.”
EX. Find the pH of a solution containing 0.085 M HNO2 (Ka = 4.5 x 10–4) and 0.10 M KNO2.
Buffered Solutions (“buffers”)
--- contain a weak conjugate acid-base pair, i.e.,
-- contain an acidic species and a basic species that do NOT consume each other
through neutralization
e.g.,
For a CH3COOH-and-CH3COO– buffer, you could use…
For an NH4+-and-NH3 buffer, you could use…
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buffer capacity: the amount of acid or base the buffer can “neutralize” before the pH begins
to change appreciably
--
EX. Find the pH of a buffer that is 0.12 M lactic acid, HC3H5O3 (Ka = 1.4 x 10–4) and 0.10 M
sodium lactate.
Alternately, you could use the Henderson-Hasselbalch equation: pH  pK a  log
base
acid 
--
Addition of Strong Acids or Bases to Buffers
-- Reactions between strong acids/bases and weak bases/acids proceed to completion.
-- We assume the strong acid/base is completely consumed.
-- When adding a “strong” to buffered solutions…
(1)
(2)
2
EX. A buffered solution of pH 4.74 contains 0.30 mol CH3COOH (Ka = 1.8 x 10–5) and
0.30 mol NaCH3COO. Calculate the pH after 0.020 mol NaOH is added. Ignore volume
changes.
Acid-Base Titrations
equivalence point:
Strong Acid – Strong Base Titrations
pH curve for HCl
titrated with NaOH
Any indicator whose color
change begins and ends along
the vertical line is okay.
-- phenolphthalein (pH 8.3-10.0)
pH
base =
acid =
7
-- methyl red (pH 4.2-6.0)
base =
acid =
mL of NaOH added
3
EX. Find pH when 24.90 mL of 0.10 M HNO3 are mixed with 25.00 mL of 0.10 M KOH.
pH curve for CH3COOH
titrated with NaOH
Weak Acid – Strong Base Titrations
The equivalence point is when, say,
50.0 mL of 0.10 M NaOH have been
pH
added to 50.0 mL of 0.10 M CH3COOH,
but pH is > 7 at that point because...
7
mL of NaOH added
pH curve for H2CO3
Titration curves for polyprotic acids
pH
(e.g., H2CO3) look something like
-- they have...
mL of base added
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EX. Calculate the pH when 10.0 mL of 0.050 M NaOH are added to 40.0 mL of 0.0250 M
benzoic acid (C6H5COOH, Ka = 6.3 x 10–5).
EX. Calculate the pH at the equivalence point when 40.0 mL of 0.0250 M C6H5COOH
(Ka = 6.3 x 10–5) are titrated with 0.050 M NaOH.
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Solubility Equilibria
-- involve the dissolution or precipitation of ionic compounds
Consider a saturated solution of barium sulfate:
For this case, the solubility-product constant is equal to:
-- Ksp is the equilibrium constant between
undissolved and dissolved ionic solute
in a saturated aqueous solution
EX. Write the solubility-product constant expression for calcium fluoride.
EX. Copper (II) azide has Ksp = 6.3 x 10–10. Find the solubility of Cu(N3)2 in water, in g/L.
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Factors That Affect Solubility
1. For solids, as temperature increases, solubility...
2. common-ion effect
Use Le Chatelier’s principle. For example, with...
Ca2+(aq) + 2 F–(aq)
CaF2(s)
3. pH and solubility
Compounds with anions exhibiting basic properties
(e.g., Mg(OH)2 / OH–, CaCO3 / CO32–,
CaF2 / F–)
_______________ in solubility as solution becomes more acidic.
4. presence of complex ions: metal ions and the Lewis bases bonded to them
e.g., AgCl(s) + 2 NH3(aq)
Ag(NH3)2+(aq) + Cl–(aq)
In general, the solubility of metal salts ______________ in the presence of suitable Lewis
bases (e.g., NH3, CN–, OH–) if the metal forms a complex ion with the bases.
5. amphoterism
Many metal hydroxides and oxides are amphoteric. They are insoluble at pH ~ 7, but will
dissolve in strongly acidic or strongly basic solutions. (Recall that many metal ions act
like acids in solution.) As an example...
Al(H2O)63+(aq) + OH–(aq)
Al(H2O)5(OH)2+(aq) + H2O(l)
If we continue adding more and more OH–(aq), we’ll get...
Al(H2O)5(OH)2+(aq) + OH–(aq)
Al(H2O)4(OH)2+(aq) + H2O(l)
Al(H2O)4(OH)2+(aq) + OH–(aq)
Al(H2O)3(OH)3(s)
+ H2O(l)
+ OH–(aq)
Al(H2O)2(OH)4–(aq) + H2O(l)
Al(H2O)2(OH)4–(aq) + OH–(aq)
Al(H2O)(OH)52–(aq) + H2O(l)
Al(H2O)(OH)52–(aq) + OH–(aq)
Al(OH)63–(aq)
Al(H2O)3(OH)3(s)
Often, the H2O is left out.
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+ H2O(l)
Precipitation and Separation of Ions
For
BaSO4(s)
Ba2+(aq) + SO42–(aq)...
Ksp = [ Ba2+ ] [SO42– ]
We could reach equilibrium from the left...
or from the right...
At any given time, the ion product Q = [ Ba2+ ] [SO42– ]
If Q > Ksp...
If Q < Ksp...
If Q = Ksp...
selective precipitation: using the different solubilities of ions to separate them
Consider a solution with Ag+ and Cu2+.
-- Add HCl.
The Ksp of AgCl = 1.8 x 10–10, but CuCl2 is soluble (i.e., Cu2+ doesn’t precipitate).
-- Then...
EX.
Will a precipitate form from mixing 0.10 L of 8.0 x 10–3 M Pb(NO3)2(aq)
and 0.40 L of 5.0 x 10–3 M Na2SO4(aq)?
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Qualitative Analysis for Metallic Elements
Steps:
(1)
(2)
aqueous solution
containing “god-knowswhat” metal ions
Add HCl.
insoluble chlorides (AgCl,
Hg2Cl2, PbCl2) ppt out.
Filter and test for a specific
ion.
Bubble H2S(g) in.
acid-insoluble sulfides
(CuS, Bi2S3, CdS, PbS,
HgS, As2S3, Sb2S3, SnS2)
ppt out. Filter and test for a
specific ion.
Add (NH4)2S.
base-insoluble sulfides and
hydroxides (Al(OH)3,
Fe(OH)3, Cr(OH)3, ZnS,
NiS, CoS, MnS) ppt out.
Filter and test for a specific
ion.
Add (NH4)2PO4.
insoluble phosphates
(Ba3(PO4)2, Ca3(PO4)2,
MgNH4PO4) ppt out. Filter
and test for a specific ion.
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