chapter · 2020-03-16 · general chemistry ii. 2 733. ... 2 = 0.45 m metal hydroxides are basic...
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General Chemistry II
SOLUBILITY AND PRECIPITATIONEQUILIBRIA
16.1 The Nature of Solubility Equilibria
16.2 Ionic Equilibria between Solids and Solutions
16.3 Precipitation and the Solubility Product
16.4 The Effects of pH on Solubility
16.5 Complex Ions and Solubility
16.6 Selective Precipitation of Ions
16CHAPTER
General Chemistry II
General Chemistry II 2
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General Chemistry II
General Features of Solubility Equilibria Saturation
~ Dissolution-precipitation equilibrium
Fig. 16.1 Deposit of K2PtCl4 from the saturatedaqueous solution as the water evaporates.
16.1 THE NATURE OF SOLUBILITY EQUILIBRIA734
Recrystallization ~ Purification of solids
Solvent of crystallization
2 Li+(aq) + SO42-(aq) + H2O(l) → Li2SO4H2O(s)
~ different chemical formula & mass
Supersaturation ~ Slow equilibrium
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The solubility of Ionic Solids
Fig. 16.3 Temperature dependence of solubility.
Solubility at 25°C,
AgClO4 ; 5570 g/L, AgCl; 0.0018 g/L
Temperature dependence
- Mostly endothermic
→ Solubility increases with T
- CaSO4 exothermic
→ Solubility decreases with T
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Classification (at 25 °C)
Soluble > 10 g/L,
Slightly soluble 0.1~10 g/L,
Insoluble < 0.1 g/L
General Chemistry II
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General Chemistry II
Highly soluble salt: Nonideal solution, CsCl(s) Cs+(aq) + Cl-(aq)
Fig. 16.5 The dissolution of the ionic solid CsCl in water
16.2 IONIC EQUILIBRIA BETWEEN SOLIDS AND SOLUTIONS
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General Chemistry II
Solubility and Ksp
Solubility product:
Ksp = [Ag+][Cl-] = 1.6×10-10 at 25 °C
AgCl(s) Ag+(aq) + Cl-(aq)
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Solubility (S) of AgCl at 25°C calculated from Ksp
Ksp = [Ag+][Cl-] = S2 = 1.6×10-10
S = 1.26×10-5 M
Gram solubility = (1.26×10-5 mol/L) × (143.3 g/mol)
= 1.8×10-3 g/L
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General Chemistry II
CaF2(s) Ca2+(aq) + 2 F-(aq)
Ksp = [Ca2+][F-]2 = 3.9×10-11 at 25°C
[Ca2+] = S, [F-] = 2S
Ksp = [Ca2+][F-]2 = S (2S)2 = 4S3 → S = 2.1 ×10-4 M
Gram solubility = (2.1×10-4 mol/L) × (78.1 g/mol) = 0.017 g/L
EXAMPLE 16.1 Calculation of [Ca2+] and [F-] in a saturated solution
of CaF2 at 25°C: Ksp → Solubility
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General Chemistry II
Ksp = [Ag+]2[CrO42-] = 2.7 ×10-12
Gram solubility: 0.029 g/L
Molar solubility: 0.029 g/L = 8.74 ×10-5 mol/L = S
[Ag+]= 2S, [CrO42-] = S
Ksp = [Ag+]2[CrO42-] = 4S3 = 2.7 ×10-12
→ 42 % greater than the tabulated value, 1.9 ×10-12
Solubility (0.029 g/L) → Ksp
Ag2CrO4(s) 2 Ag+(aq) + CrO42-(aq)
EXAMPLE 16.2
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General Chemistry II
Fig. 16.6 A plot of precipitation and dissolution equilibrium for AgCl in water.The slope of the path toward equilibrium represented by red or blue arrow is 1.
16.3 PRECIPITATION AND THE SOLUBILITY PRODUCT
Precipitation from Solution
Ksp = [Ag+][Cl-]
Q0 = [Ag+]0[Cl-]0
~ initial reaction quotient
Q0 > Ksp precipitation
Q0 < Ksp dissolution
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General Chemistry II
Cl- is the limiting reactant → complete precipitation first
Remaining [Ag+] = 0.0015 - 5.0 ×10-6 ≈ 0.0015 M
AgCl(s) Ag+(aq) + Cl-(aq)----------------------------------------------------------------------Initial 0.0015 0Change + y + y
--------------- ------Equilibrium 0.0015 + y y----------------------------------------------------------------------
Ksp = 1.60 ×10-10 = (0.0015 + y) y ≈ 0.0015 y
y = [Cl-] = 1.1 ×10-7 M, [Ag+] = 0.0015 M
[Ag+]0 = 0.0015 M, [Cl-]0 = 5.0×10-6 M
Equilibrium concentrations?
EXAMPLE 16.4
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The Common-Ion Effect ~ Solubility decreases in the presence of a common ion
AgCl NaCl or AgNO3
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EX. Solubility of AgCl(s) in 1.00 L of 0.100 M NaCl solution
[Ag+]NaCl = S, [Cl-]NaCl = 0.100 + S
Ksp = 1.60 × 10-10
= [Ag+] NaCl [Cl-] NaCl
= S (0.100 + S) ≈ 0.100 S
(S < Swater =1.3 ×10-5 << 0.100)
General Chemistry II
Fig. 16.7 Common-ion effect for the solubility of AgCl in AgNO3 solution and in NaCl solution.
2
5H O 3
90.1M NaCl
[Ag ] 1.3 10 8.1 10[Ag ] 1.6 10
+ −
+ −
×= = ×
×
[Ag+] NaCl = S = 1.60 ×10-9 M
[Cl-] NaCl = 0.100 M
( )2
5H O[Ag ] 1.3 10 M+ −= ×
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General Chemistry II
Fig. 16.8 Damage due to increased acidity from air pollution.On the east pier of Stanford White's Washington Square Arch is Herma A. MacNeil's Washington in War (1916)(Washington Square Park in the Greenwich Village neighborhood of Lower Manhattan in New York City)
CaCO3(s) + H3O+(aq) → Ca2+(aq) + HCO3-(aq) + H2O(l)
16.4 THE EFFECTS OF pH ON SOLUBILITY744
General Chemistry II
Solubility of Hydroxides
In pure water, [Zn2+] = S, [OH-] = 2S Ksp = S(2S)2
S = [Zn2+] = 2.2 ×10-6 M, [OH-] = 2S = 4.5 ×10-6 M, pH = 8.65
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EXAMPLE 16.6
Zn(OH)2(s) Zn2+(aq) + 2 OH-(aq)
Ksp = [Zn2+][OH-]2 = 4.5 ×10-17
In acidic solution, [OH-] decreases. → reaction goes to the right
Comparison of solubilities of Zn(OH)2(s) in pure water
and in a buffer with pH 6.00.
In a pH = 6.00 buffer, [OH-] = 1.0 ×10-8 M (fixed).
[Zn2+] = Ksp / [OH-]2 = 0.45 M
Metal hydroxides are basic → more soluble in acidic solution
General Chemistry II
Solubility of Salts of Bases
CaF2(s) Ca2+(aq) + 2 F-(aq), Ksp = 3.9 ×10-11
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- Solubility of CaF2(s) at low pH :
F-(aq) + H3O+(aq) HF(aq) + H2O(l), K = 2.9 ×103
→ more soluble in acidic solution (large K)
[H3O+] ↑ → [F-] ↓ → more CaF2(s) dissolves (Le Chatelier)
- Solubility of AgCl(s) at low pH :
AgCl(s) Ag+(aq) + Cl-(aq)
- Even in acidic solution,
Cl-(aq) + H3O+(aq) ← HCl(aq) + H2O(l)
→ negligible effect of pH on the solubility of AgCl
General Chemistry II
Problem Sets
For Chapter 16,
14, 22, 30, 34