More Equilibria in Aqueous Solutions: Slightly Soluble Salts and Complex Ions

1
More Equilibria in Aqueous SolutionsSlightly
Soluble Salts and Complex Ions
Chapter Sixteen
2
The Solubility Product Constant, Ksp

• Many important ionic compounds are only slightly
  soluble in water (we used to call them
  insoluble Chapter 4).
• An equation can represent the equilibrium between
  the compound and the ions present in a saturated
  aqueous solution

• Solubility product constant, Ksp the equilibrium
  constant expression for the dissolving of a
  slightly soluble solid.

Ksp Ba2 SO42
3
(No Transcript)
4

• Example 16.1
• Write a solubility product constant expression
  for equilibrium in a saturated aqueous solution
  of the slightly soluble salts (a) iron(III)
  phosphate, FePO4, and (b) chromium(III)
  hydroxide, Cr(OH)3.

5
Ksp and Molar Solubility

• Ksp is an equilibrium constant
• Molar solubility is the number of moles of
  compound that will dissolve per liter of
  solution.
• Molar solubility is related to the value of Ksp,
  but molar solubility and Ksp are not the same
  thing.
• In fact, smaller Ksp doesnt always mean lower
  molar solubility.
• Solubility depends on both Ksp and the form of
  the equilibrium constant expression.

6

• Example 16.2
• At 20 C, a saturated aqueous solution of silver
  carbonate contains 32 mg of Ag2CO3 per liter of
  solution. Calculate Ksp for Ag2CO3 at 20 C. The
  balanced equation is
• Ag2CO3(s) 2 Ag(aq) CO32(aq)
  Ksp ?
• Example 16.3
• From the Ksp value for silver sulfate, calculate
  its molar solubility at 25 C.
• Ag2SO4(s) 2 Ag(aq) SO42(aq)
• Ksp 1.4 x 105 at 25 C

7

• Example 16.4 A Conceptual Example
• Without doing detailed calculations, but using
  data from Table 16.1, establish the order of
  increasing solubility of these silver halides in
  water AgCl, AgBr, AgI.

8
The Common Ion Effectin Solubility Equilibria

• The common ion effect affects solubility
  equilibria as it does other aqueous equilibria.
• The solubility of a slightly soluble ionic
  compound is lowered when a second solute that
  furnishes a common ion is added to the solution.

9
Common Ion Effect Illustrated
The added sulfate ion reduces the solubility of
Ag2SO4.
Na2SO4(aq)
Saturated Ag2SO4(aq)
Ag2SO4 precipitates
10
Common Ion Effect Illustrated
When Na2SO4(aq) is added to the saturated
solution of Ag2SO4
Ag attains a new, lower equilibrium
concentration as Ag reacts with SO42 to produce
Ag2SO4.
11

• Example 16.5
• Calculate the molar solubility of Ag2SO4 in 1.00
  M Na2SO4(aq).

12
Solubility and Activities

• Ions that are not common to the precipitate can
  also affect solubility.
• CaF2 is more soluble in 0.010 M Na2SO4 than it is
  in water.
• Increased solubility occurs because of interionic
  attractions.
• Each Ca2 and F is surrounded by ions of
  opposite charge, which impede the reaction of
  Ca2 with F.
• The effective concentrations, or activities, of
  Ca2 and F are lower than their actual
  concentrations.

13
Will Precipitation Occur? Is It Complete?

• Qip is the ion product reaction quotient and is
  based on initial conditions of the reaction.

Qip and Qc new look, same great taste!

• Qip can then be compared to Ksp.
• Precipitation should occur if Qip gt Ksp.
• Precipitation cannot occur if Qip lt Ksp.
• A solution is just saturated if Qip Ksp.
• In applying the precipitation criteria, the
  effect of dilution when solutions are mixed must
  be considered.

14

• Example 16.6
• If 1.00 mg of Na2CrO4 is added to 225 mL of
  0.00015 M AgNO3, will a precipitate form?
• Ag2CrO4(s) 2 Ag(aq) CrO42(aq)
  Ksp 1.1 x 1012

15

• Example 16.7 A Conceptual Example
• Pictured here is the result of adding a few drops
  of concentrated KI(aq) to a dilute solution of
  Pb(NO3)2. What is the solid that first appears?
  Explain why it then disappears.

Example 16.8 If 0.100 L of 0.0015 M MgCl2 and
0.200 L of 0.025 M NaF are mixed, should a
precipitate of MgF2 form? MgF2(s)
Mg2(aq) 2 F(aq) Ksp 3.7 x 108
16
To Determine Whether Precipitation Is Complete

• A slightly soluble solid does not precipitate
  totally from solution
• but we generally consider precipitation to be
  complete if about 99.9 of the target ion is
  precipitated (0.1 or less left in solution).
• Three conditions generally favor completeness of
  precipitation

1. A very small value of Ksp.
2. A high initial concentration of the target ion.
3. A concentration of common ion that greatly
   exceeds that of the target ion.

17

• Example 16.9
• To a solution with Ca2 0.0050 M, we add
  sufficient solid ammonium oxalate, (NH4)2C2O4(s),
  to make the initial C2O42 0.0051 M. Will
  precipitation of Ca2 as CaC2O4(s) be complete?

CaC2O4(s) Ca2(aq) C2O42(aq)
Ksp 2.7 x 109
18
Selective Precipitation
AgNO3 added to a mixture containing Cl and I
19

• Example 16.10
• An aqueous solution that is 2.00 M in AgNO3 is
  slowly added from a buret to an aqueous solution
  that is 0.0100 M in Cl and also 0.0100 M in I.
• Which ion, Cl or I, is the first to precipitate
  from solution?
• When the second ion begins to precipitate, what
  is the remaining concentration of the first ion?
• Is separation of the two ions by selective
  precipitation feasible?

AgCl(s) Ag(aq) Cl(aq)
Ksp 1.8 x 1010
AgI(s) Ag(aq) I(aq)
Ksp 8.5 x 1017
20
Effect of pH on Solubility

• If the anion of a precipitate is that of a weak
  acid, the precipitate will dissolve somewhat when
  the pH is lowered

Added H reacts with, and removes, F
LeChâteliers principle says more F forms.

• If, however, the anion of the precipitate is that
  of a strong acid, lowering the pH will have no
  effect on the precipitate.

H does not consume Cl acid does not affect
the equilibrium.
21

• Example 16.11
• What is the molar solubility of Mg(OH)2(s) in a
  buffer solution having OH 1.0 x 105 M, that
  is, pH 9.00?

Mg(OH)2(s) Mg2(aq) 2 OH(aq)
Ksp 1.8 x 1011
Example 16.12 A Conceptual Example Without
doing detailed calculations, determine in which
of the following solutions Mg(OH)2(s) is most
soluble (a) 1.00 M NH3 (b) 1.00 M NH3 /1.00 M
NH4 (c) 1.00 M NH4Cl.
22
Equilibria Involving Complex Ions
Silver chloride becomes more soluble, not less
soluble, in high concentrations of chloride ion.
23
Complex Ion Formation

• A complex ion consists of a central metal atom or
  ion, with other groups called ligands bonded to
  it.
• The metal ion acts as a Lewis acid (accepts
  electron pairs).
• Ligands act as Lewis bases (donate electron
  pairs).
• The equilibrium involving a complex ion, the
  metal ion, and the ligands may be described
  through a formation constant, Kf

AgCl2 Kf 1.2 x 108
AgCl2
24
Complex Ion Formation
Concentrated NH3 added to a solution of pale-blue
Cu2
forms deep-blue Cu(NH3)42.
25
(No Transcript)
26
Complex Ion Formationand Solubilities
But if the concentration of NH3 is made high
enough
the AgCl forms the soluble Ag(NH3)2 ion.
AgCl is insoluble in water.
27

• Example 16.13
• Calculate the concentration of free silver ion,
  Ag, in an aqueous solution prepared as 0.10 M
  AgNO3 and 3.0 M NH3.

Example 16.14 If 1.00 g KBr is added to 1.00 L of
the solution described in Example 16.13, should
any AgBr(s) precipitate from the solution?
Ksp 5.0 x 1013
28
Example 16.15 What is the molar solubility of
AgBr(s) in 3.0 M NH3?
AgBr(s) 2 NH3(aq) Ag(NH3)2(aq)
Br(aq)
Kc 8.0 x 106
Example 16.16 A Conceptual Example Figure
16.10 shows that a precipitate forms when
HNO3(aq) is added to the solution in the beaker
on the right in Figure 16.9. Write the
equation(s) to show what happens.
29
Complex Ions in AcidBase Reactions

• Water molecules are commonly found as ligands in
  complex ions (H2O is a Lewis base).

Na(H2O)4
Al(H2O)63
Fe(H2O)63

• The electron-withdrawing power of a small, highly
  charged metal ion can weaken an OH bond in one
  of the ligand water molecules.
• The weakened OH bond can then give up its proton
  to another water molecule in the solution.
• The complex ion acts as an acid.

30
Ionization of a Complex Ion
The highly-charged iron(III) ion withdraws
electron density from the OH bonds.
Fe(H2O)63 H2O Fe(H2O)5OH2
H3O
Ka 1 x 107
31
Amphoteric Species

• Certain metal hydroxides, insoluble in water, are
  amphoteric they will react with both strong
  acids and strong bases.
• Al(OH)3, Zn(OH)2, and Cr(OH)3 are amphoteric.

32
Qualitative Inorganic Analysis

• Acidbase chemistry, precipitation reactions,
  oxidationreduction, and complex ion formation
  all apply to an area of analytical chemistry
  called classical qualitative inorganic analysis.
• Qualitative signifies that the interest is in
  determining what is present.
• Quantitative analyses are those that determine
  how much of a particular substance or species is
  present.
• Although classical qualitative analysis is not
  used as widely today as are instrumental methods,
  it is still a good vehicle for applying all the
  basic concepts of equilibria in aqueous solutions.

33
Qualitative Analysis Outline
In acid, H2S produces very little S2, so only
the most-insoluble sulfides precipitate.
In base, there is more S2, and the
less-insoluble sulfides also precipitate.
Some hydroxides also precipitate here.
34
Cation Group 1

• If aqueous HCl is added to an unknown solution of
  cations, and a precipitate forms, then the
  unknown contains one or more of these cations
  Pb2, Hg22, or Ag.
• These are the only ions to form insoluble
  chlorides.
• Any precipitate is separated from the mixture and
  further tests are performed to determine which of
  the three Group 1 cations are present.
• The supernatant liquid is also saved for further
  analysis (it contains the rest of the cations).
• If there is no precipitate, then Group 1 ions
  must be absent from the mixture.

35
Cation Group 1 (contd)Analyzing for Pb2

• Precipitated PbCl2 is slightly soluble in hot
  water.
• The precipitate is washed with hot water, then
  aqueous K2CrO4 is added to the washings.
• If Pb2 is present, a precipitate of yellow lead
  chromate forms, which is less soluble than PbCl2.
• (If all of the precipitate dissolves in the hot
  water, what does that mean?)

36
Cation Group 1 (contd)Analyzing for Ag and
Hg22

• Next, any undissolved precipitate is treated with
  aqueous ammonia.
• If AgCl is present, it will dissolve, forming
  Ag(NH3)2 (the dissolution may not be visually
  apparent).
• If Hg22 is present, the precipitate will turn
  dark gray/ black, due to a disproportionation
  reaction that forms Hg metal and HgNH2Cl.
• The supernatant liquid (which contains the Ag,
  if present) is then treated with aqueous nitric
  acid.
• If a precipitate reforms, then Ag was present in
  the solution.

37
Group 1 Cation Precipitates
PbCl2 precipitates when HCl is added.
The presence of lead is confirmed by adding
chromate ion yellow PbCrO4 precipitates.
Hg2Cl2 reacts with NH3 to form black Hg metal and
HgNH2Cl.
38
Hydrogen Sulfide in theQualitative Analysis
Scheme

• Once the Group 1 cations have been precipitated,
  hydrogen sulfide is used as the next reagent in
  the qualitative analysis scheme.
• H2S is a weak diprotic acid there is very little
  ionization of the HS ion and it is the
  precipitating agent.
• Hydrogen sulfide has the familiar rotten egg odor
  that is very noticeable around volcanic areas.
• Because of its toxicity, H2S is generally
  produced only in small quantities and directly in
  the solution where it is to be used.

39
Cation Groups 2, 3, 4, and 5

• The concentration of HS is so low in a strongly
  acidic solution, that only the most insoluble
  sulfides precipitate.
• These include the eight metal sulfides of Group
  2.
• Five of the Group 3 cations form sulfides that
  are soluble in acidic solution but insoluble in
  alkaline NH3/NH4.
• The other three Group 3 cations form insoluble
  hydroxides in the alkaline solution.
• The cations of Groups 4 and 5 are soluble.
• Group 4 ions are precipitated as carbonates.
• Group 5 does not precipitate these must be
  determined by flame test.

40

• Cumulative Example
• A solid mixture containing 1.00 g of ammonium
  chloride and 2.00 g of barium hydroxide is heated
  to expel ammonia. The liberated NH3 is then
  dissolved in 0.500 L of water containing 225 ppm
  Ca2 as calcium chloride. Will a precipitate form
  in this water?