University College - Chemistry Chem 112A Lecture Notes - Lecture 12: Alkali Metal, Solubility Table, Solubility Equilibrium
13 Dec 2016
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Department
Professor
15 February 2016
Lecture 12: Intro to Solubility Equilibria
Equation of the Day: Ksp = [A+][X-]
I. Solubility Equilibria
A. Basics
Equilibrium between a solid and its ions: AX(s) ↔ A+(aq) + X-(aq)
1. At equilibrium the rate of dissolution = the rate of precipitation
2. A solution is said to be saturated at equilibrium
3. The equilibrium constant is called the solubility product (Ksp)
a. For AX, Ksp = (aA+)(ax-)/(aAX) = [A+][X-]
• Using activities, a unit-less quantity
b. For AX2, AX2 ↔ A2+ + 2X-, Ksp = [A2+][X-]2
4. Thinking about LeChatelier’s principle
a. Adding water would dissolve more solid
b. Taking away water would precipitate more AX
c. The amount of solid doesn’t matter or effect the concentration in solution
5. The solubility (s) of a substance in a solvent is the greatest amount (in g or mol) that
will dissolve at equilibrium in a liter of solvent at a particular temperature
a. For example, s(AgClO4) = 5570 g/L in water at 25°C
• But s(AgCl) = 0.01 g/L in water at 25°C
b. If the solubility of a salt, s < 0.1 g/L it is “insoluble”
c. If the solubility of a salt, 0.1 < s < 10 g/L it is “slightly soluble”
d. If the solubility of a salt, s > 10.0 g/L it is “soluble”
e. Note: these ranges are arbitrary but reasonable
f. At high solubility, ions start to cluster or interact and start to act non ideal, at
lower solubility there are fewer interactions and Ksp is more representative for
insoluble and slightly soluble species (“sparingly soluble”)
6. Sample solubility table:
a. Most salts where A+ (the cation) is an alkali metal (N+, K+ etc.) or NH4+, are
soluble
• But there are still always exceptions
B. The Common Ion Effect
1. Sample solubility calculation: PbI2(s) ↔ Pb2+(aq) + 2I-(aq), Ksp = 1.4 x 10-8 at 25°C
Anion
Soluble
Slightly Soluble
Insoluble
NO3-
All
ClO4-
Most
KClO4
Cl-
Most
PbCl2
AgCl, Hg2Cl2
SO42-
Most
CaSO4, Ag2SO4, Hg2SO4
SrSO4, BaSO4, PdSO4
OH-
NaOH, KOH, Ba(OH)2
Sr(OH)2, Ca(OH)2
Most
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