Ch 19' Solubility of Ionic Salts

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Ch 19. Solubility of Ionic Salts

• Brady Senese, 4th Ed.

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(No Transcript)
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Chapter 19 Solubility and Simultaneous
Equilibria

• None of the salts describe as insoluble in
  Chapter 5 are totally insoluble
• For example, if solid AgCl is added to water a
  small amount dissolves
• The ion product is the product of the molar
  concentrations of the dissolved ions of the solute

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Learning Check

• Write the reactions and mass action
  expressions for the dissolution of the following
  substances in water
• Ag2CO3
• (NH4)2SO4

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• The solubility product constants for a number of
  ionic compounds are given in Table 19.1 (and
  Appendix C)
• Ksp can be calculated by determining the molar
  solubility or the number of moles of salt
  dissolved in one liter of the saturated solution
• We can assume that the salt that dissolves is
  100 dissociated (see Facets of Chemistry 19.1
  for a discussion of the validity of this
  assumption)

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Learning Check

• What is the molar solubility of AgCl at 250C?
• What is the molar solubility of Ca3(PO4)2 at
  250C?

1.3 x 10-5 M
7.1 x 10-7 M
Ksp AgCl 1.8 x 10-10 Ca3(PO4)2 2.0 x 10-29
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Learning Check

• Given solubilities, we can find Ksp
• The solubility of an salt, A2B3, is found to be
  3.0 10-5 M. What is the value of Ksp?
• If the solubility of a salt, AB2, is found to be
  2.5 10-6 M, what is its Ksp?

2.6 10-21
6.3 10-17
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Common Ion Effect

• When two substances form the same ion, a
  competition arises
• In general, when the reactions are nearly equal
  in rate, the most efficient reaction occurs first
  and suppresses the less efficient
• If the reactions are of greatly different rates,
  the faster reaction occurs before the slower one
• In either case, the first reaction inhibits the
  second by shifting the second equilibrium to the
  left

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• Example The molar solubility of PbF2 in a 0.10 M
  Pb(NO3)2 solution at 25oC is 3.1x10-4 mol L-1.
  What is Ksp for PbF2?
• ANALYSIS Write the solubility equation, setup
  and evaluate the ICE table, and calculated the
  requested quantity.
• SOLUTION

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Learning Check Common Ion Effect

• What is the molar solubility of Ca3(PO4)2 in
  0.1M Na3PO4?
• Ksp2.0 10-29
• Calculate the molar solubility of BaSO4 in 0.1M
  BaCl2
• Ksp 1.1 10-10
• Note Make sure that the common ion is not
  capable of converting the solid into a complex
  ion! (i.e., subsequent reaction)

4.2 10-10 M
1.1 10-9 M
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Saturation (QspKsp)

• Solutions may be unsaturated (QspltKsp) or
  saturated (QspKsp)
• If Qsp exceeds Ksp, precipitation occurs of the
  excess solute
• Thus, the formation of a solid by a reaction is
  not guaranteed
• To form a solid, the solution must be
  supersaturated

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Learning Check

• Will the following form a precipitate?
• 20.00 mL of 0.1M CaCl2 20.00 mL 0.01M Na2CO4
• 10.00 mL of 0.1M Pb(NO3)2 10.00 mL of 0.001M
  CaCl2

Qsp0.0025
Qsp1.25 x 10-9
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Dissolution In A Reacting Solution

• Basic solids are enhanced by dissolution in acid
• Similarly, competing ions may assist the
  dissolution process in a double replacement
  reaction
• Complexation is also a possibility here
• The net reaction requires a net equilibrium
  constant, Knet
• When we sum reactions, we multiply their K
  values.

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• Most water-insoluble metal oxides dissolve in
  acid
• The H reacts with the O2- to produce H2O
• This releases the metal ion from the solid
• Recall that the oxide ion is too powerful a base
  to exist in aqueous solution
• Example Fe2O3(s)6H(aq)?2Fe3(aq)3H2O
• Water-insoluble metal oxides can form in solution
• This can occur in basic solution

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Metal Oxides Undergo A Reaction With Water

• We usually ignore the reaction of an ionic solid
  with water
• If the anion of the salt is very basic, a
  subsequent reaction of the anion with water
  occurs
• Such is the case of many metal oxides the Ksp
  value listed takes this subsequent reaction into
  account

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• Elimination of water is often involved
• The metal ion must be capable of reacting with
  the hydroxide ion to extract an oxide ion and
  leave water (or possibly H)
• Example 2Ag(aq)2OH-(aq)?Ag2O(s)H2O
• Sulfur is below oxygen in Group VIA
• As a result, metal sulfides are similar to metal
  oxides
• Apparently, S2- (like O2-) is too strong of a
  base to exist in water
• Sulfides dissolve by reacting with water
• Example Na2S(s)H2O?2Na(aq)HS-(aq)OH-(aq)

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Metal Sulfides Also Undergo A Reaction With Water

• Sulfide ion is also very basic-- in fact, it is
  not known to exist in aqueous solution
• Metal sulfides also undergo a subsequent reaction
  with water

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• Metal sulfides can form when metal ions are
  exposed to HS-
• Some metal ions are so reactive that they react
  with H2S directly
• These active ions include Cu2, Pb2, and Ni2
• A typical reaction is
• The large value of K indicates that the
  equilibrium lies far to the right and only the
  forward reaction is important
• The sulfides require closer investigation

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• The solubility of CuS in water is describe by
• Table 19.2 lists a number of these Ksp
• They vary greatly in magnitude, from 2x10-53 to
  3x10-11, a spread of a factor of 1042!

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Basic Salts Are More Soluble In Acids

• Subsequent reactions assist the solubility of
  solids
• If the anion of the salt is basic, it will react
  in acidic solution to dissolve more fully
• The net reaction of such dissolutions is called
  Kspa

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• Some of the metal sulfides, the acid-insoluble
  sulfides, have Ksp values so low that they dont
  dissolve in acid
• Cations in this group can be separated from other
  cations by bubbling H2S through sufficiently acid
  solutions of the several metal ions
• Both OH- and HS- would react (to form H2S and
  H2O) in acidic solutions
• This requires a modification to the ion product

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• Adding 2H to both sides of the equation
• A number of Kspa values are given in Table 19.2

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• The metal sulfides can be collected into two
  groups
• The acid-insoluble sulfides, which dissolve in
  neither acidic nor basic solution
• The acid-soluble or base-insoluble sulfides that
  dissolve in acidic, but not basic, solutions
• The difference in sulfide solubility can be used
  to selectively precipitate metal ions from
  solution
• Selective precipitation means causing one metal
  ion to precipitate while holding the other in
  solution

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• Example Over what range of pH is it possible to
  selectively precipitate Cu2 and Ni2 as their
  sulfides from a solution initially 0.010M in both
  ions? ( H2S0.1M in a saturated
  solution.)
• ANALYSIS From Table 19.2, Cu2 is an
  acid-insoluble sulfide (Kspa6x10-16) and Ni2 is
  a base-insoluble sulfide (Kspa4x101). Thus NiS
  will dissolve in acidic solution while CuS will
  not. The pH limits for the precipitation of both
  sulfides in saturated H2S must be calculated.

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• SOLUTION

(acid-insoluble)
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Thus, NiS will be soluble with a pH below 2.3
leaving CuS as a precipitate.
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• The pH dependence of metal carbonate solubility
  is similar to the pH dependence of metal sulfide
  solubility (see Example 19.10) and can provide a
  basis for selective precipitation. Consider the
  solubility of metal carbonates
• The amount of CO32- depends on H
• Thus, the CO32-
• increases as pH increases (because H
  decreases)
• decreases as pH decreases (because H
  increases)

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Learning Check

• What is the molar solubility of BaCO3 in 3M HCl?
• (Ksp BaCO35.0 x 10-9 H2CO3 Ka1 4.3 x 10-7
  Ka24.7 x 10-11)

1.50 M
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Fractional Precipitation

• Ions compete for the same precipitant .
• Each reaction will occur according to their
  individual saturation thresholds.

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Approach To Fractional Precipitation

• Set up a number line with precipitant
  concentration as the x axis
• A range of values can be determined at which one
  ion will precipitate, but not the other
• Write the reaction for each of the ions
• Determine the saturation concentration of
  precipitant (when Qsp Ksp) for the reaction

Saturated in A Saturated in B
Neither ppt lt A ppt only lt
ltBoth ppt
Concentration of common precipitant
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Learning Check

• What concentration of I- is needed to precipitate
  one ion but not the other in a mixture of 0.1M
  Pb2 and 0.1M Ag?

?
?
I-8.3x10-16 M
I-2.8x10-4M
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Learning Check

• What pH will prevent the precipitation of any
  metal ions in 0.1M H2S, 0.1M Cu2, and 0.1M Pb2?

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CuS pH -6.61
PbS pH -2.26
?
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Complexation Kinst

• complex ions are charged particles in which a
  metal ion is surrounded by anions or molecules
  called ligands, L
• Complex ions are soluble, hence complexation is a
  means of dissolving some solids
• Complexes are governed by the instability
  constant, Kinst
• When we reverse an equation, we invert K, thus
  Kform1/Kinst.

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Aqueous Metal Ions Are Complex Ions

• In the solvation of ionic compounds, ions are
  dissolved in water through ion-dipole
  interactions
• Water acts as a ligand, the Lewis base that forms
  a coordinate covalent bond with the metal

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• Copper(II) ion is a typical example
• The Lewis base that attaches itself to the metal
  ion is called a ligand (H2O is the ligand)
• The atom in the ligand that actually provides the
  electron pair is called the donor atom (O is the
  donor atom)
• The metal ion is called the acceptor (Cu2 is the
  acceptor)
• Compounds that contain complex ions are called
  coordination compounds and the complex itself is
  sometimes called a coordination complex

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• Formation or stability constants, Kform, can be
  used to describe complex ion formation
• For example, for copper(II) ions in solutions of
  ammonia
• The inverse of the formation constant is called
  the instability constant, Kinst, which are
  preferred by some

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• Table 19.3 lists both formation and instability
  constants for some ions
• Appendix C lists only formation constants
• Complex ion formation can affect the solubility
  of salts

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• The solubility of a slightly soluble salt
  increases when one of its ions can be changed
  into soluble complex ion
• Example Calculate the solubility of silver
  chloride in 0.10 M NH3.
• ANALYSIS The equations describing the solubility
  and complex ion formation must be combined into a
  single equation and solved.

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• SOLUTION

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• Setup the ICE table and solve for the molar
  solubility x

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Learning Check

• Calculate the Molar solubility of Ag2S in 2M NH3
• (Ag2S Ksp6.0 x 10-51 Ag(NH3)2
  Kinst6.3x10-8)
• What is the concentration of Cu2 available when
  10.0mL 0.1M Cu2 are combined with 10.0 mL of
  0.01M NH3?
• Kform1.1x1013

1.82x 10-12 M
0.0488 M
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Solubility Product (Ksp)

• The equilibrium that governs the dissolution of
  an ionic salt in water is termed the solubility
  product and corresponds to the reaction in which
  the ionic solid dissociates to form aqueous ions.
  
• For AgCl, this reaction would be
• AgCl(s) Ag(aq) Cl-(aq)
• Further, water is assumed to be present, shown by
  the (aq) designation, but is not included as a
  reactant.