1. Chapter 18 Reaction Rates and Equilibrium 18.4 Solubility Equilibrium
18.1 Rates of Reaction
18.2 The Progress of Chemical
Reactions
18.3 Reversible Reactions
and Equilibrium
18.4 Solubility Equilibrium
18.5 Free Energy and Entropy
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2. How is it possible to ingest a poison without being harmed?
CHEMISTRY & YOU
How is it possible to ingest a poison without being harmed?
Barium sulfate, a poison, can absorb X-rays, so tissues coated with the liquid will appear as light areas in the X-ray images.
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3. The Solubility Product Constant
What is the relationship between the solubility product constant and the solubility of a compound?
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4. The Solubility Product Constant
Most ionic compounds containing alkali metals are soluble in water.
For example, more than 35 g of sodium chloride will dissolve in only 100 g of water.
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5. The Solubility Product Constant
Most ionic compounds containing alkali metals are soluble in water.
For example, more than 35 g of sodium chloride will dissolve in only 100 g of water.
By contrast, some ionic compounds are insoluble in water.
For example, compounds that contain phosphate, sulfite, or carbonate ions tend not to dissolve in water.
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6. Solubility of Ionic Compounds in Water
Interpret Data
This table provides some general rules for the solubility of ionic compounds in water.
Solubility of Ionic Compounds in Water
Compounds
Solubility
Exceptions
Salts of Group 1A metals and ammonia
Soluble
Some lithium compounds
Ethanoates, nitrates, chlorates, and perchlorates
Few exceptions
Sulfates
Compounds of Pb, Ag, Hg, Ba, Sr, and Ca
Chlorides, bromides, and iodides
Compounds of Ag and some compounds of Hg and Pb
Sulfides and hydroxides
Most are insoluble
Alkali metal sulfides and hydroxides are soluble. Compounds of Ba, Sr, and Ca are slightly soluble.
Carbonates, phosphates, and sulfites
Insoluble
Compounds of the alkali metals and of ammonium ions
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7. The Solubility Product Constant
Most insoluble ionic compounds will actually dissolve to some extent in water.
These compounds are said to be slightly soluble in water.
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8. The Solubility Product Constant
When the “insoluble” compound silver chloride is mixed with water, a very small amount of silver chloride dissolves in the water.
AgCl(s) Ag+(aq) + Cl–(aq)
An equilibrium is established between the solid and the dissolved ions in the saturated solution.
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9. The Solubility Product Constant
When the “insoluble” compound silver chloride is mixed with water, a very small amount of silver chloride dissolves in the water.
AgCl(s) Ag+(aq) + Cl–(aq)
You can write an equilibrium-constant expression for this process.
Keq =
[Ag+]  [Cl–]
[AgCl]
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10. The Solubility Product Constant
To compare the solubility of salts, it is useful to have a constant that reflects only the concentrations of the dissolved ions.
This constant is called the solubility product constant (Ksp).
It is equal to the product of the concentrations of the ions each raised to a power equal to the coefficient of the ion in the dissociation equation.
Ksp = [A]a  [B]b
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11. The Solubility Product Constant
The smaller the numerical value of the solubility product constant, the lower the solubility of the compound.
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12. The Solubility Product Constant
The table below lists the Ksp values for some ionic compounds that are slightly soluble in water.
Solubility Product Constants (Ksp) at 25oC
Ionic compound
Ksp
Halides
AgCl
AgBr
AgI
PbCl2
PbBr2
PbI2
PbF2
CaF2
1.8  10–10
5.0  10–13
8.3  10–17
1.7  10–5
2.1  10–6
7.9  10–9
3.6  10–8
3.9  10–11
Sulfates
PbSO4
BaSO4
CaSO4
6.3  10–7
1.1  10–10
2.4  10–5
Hydroxides
Al(OH)3
Zn(OH)2
Ca(OH)2
Mg(OH)2
Fe(OH)2
3.0  10–34
3.0  10–16
6.5  10–6
7.1  10–12
7.9  10–16
Sulfides
NiS
CuS
Ag2S
ZnS
FeS
CdS
PbS
4.0  10–20
8.0  10–37
8.0  10–51
3.0  10–23
8.0  10–19
1.0  10–27
3.0  10–28
Carbonates
CaCO3
SrCO3
ZnCO3
Ag2CO3
BaCO3
4.5  10–9
9.3  10–10
1.0  10–10
8.15  10–12
5.0  10–9
Chromates
PbCrO4
Ag2CrO4
1.8  10–14
1.2  10–12
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13. Finding the Ion Concentrations in a Saturated Solution
Sample Problem 18.6
Finding the Ion Concentrations in a Saturated Solution
What is the concentration of lead ions and chromate ions in a saturated solution of lead(II) chromate at 25oC?
(Ksp = 1.8  10–14)
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14. Analyze List the knowns and the unknowns.
Sample Problem 18.6
Analyze List the knowns and the unknowns. 1
Write the expression for Ksp. Then modify it so that there is a single unknown.
KNOWNS
Ksp = 1.8  10–14
PbCrO4(s) Pb2+(aq) + CrO42–(aq)
UNKNOWN
[Pb2+] = ? M
[CrO42–] = ? M
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15. Calculate Solve for the unknowns.
Sample Problem 18.6
Calculate Solve for the unknowns. 2
Start with the general expression for the solubility product constant.
The exponent for each ion is 1.
Ksp = [A]a  [B]b
Use the chemical equation to write the correct expression for Ksp for the reaction.
Ksp = [Pb2+]  [CrO42–] = 1.8  10–14
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16. Calculate Solve for the unknowns.
Sample Problem 18.6
Calculate Solve for the unknowns. 2
Substitute [Pb2+] for [CrO42–] in the expression to get an equation with one unknown.
At equilibrium, [Pb2+] = [CrO42–].
Ksp = [Pb2+]  [Pb2+] = [Pb2+]2 = 1.8  10–14
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17. Calculate Solve for the unknowns.
Sample Problem 18.6
Calculate Solve for the unknowns. 2
Solve for [Pb2+].
[Pb2+] =
1.8  10–14
[Pb2+] = [CrO42–] = 1.3  10–7 M
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18. Evaluate Does the result make sense?
Sample Problem 18.6
Evaluate Does the result make sense? 3
Calculate [Pb2+]  [CrO42–] to evaluate the answers.
The result is 1.7  10–14, which is close to the value of Ksp.
The result varies slightly from the actual value because the answers were rounded to two significant figures.
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19. Which of the following compounds is least soluble at 25oC?
A. PbF2
B. ZnS
C. SrCO3
D. CaSO4
Ksp at 25oC
Ionic compound
Ksp
CaSO4
2.4  10–5
PbF2
3.6  10–8
SrCO3
9.3  10–10
ZnS
3.0  10–23
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20. Which of the following compounds is least soluble at 25oC?
A. PbF2
B. ZnS
C. SrCO3
D. CaSO4
Ksp at 25oC
Ionic compound
Ksp
CaSO4
2.4  10–5
PbF2
3.6  10–8
SrCO3
9.3  10–10
ZnS
3.0  10–23
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21. The Common Ion Effect
The Common Ion Effect
How can you predict whether precipitation will occur when two solutions are mixed?
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22. PbCrO4(s) Pb2+(aq) + CrO42–(aq)
The Common Ion Effect
In a saturated solution of lead(II) chromate, an equilibrium is established between the solid lead(II) chromate and its ions in solution.
PbCrO4(s) Pb2+(aq) + CrO42–(aq)
What would happen if you added some lead nitrate to this solution?
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23. PbCrO4(s) Pb2+(aq) + CrO42–(aq)
The Common Ion Effect
PbCrO4(s) Pb2+(aq) + CrO42–(aq)
Lead(II) nitrite is soluble in water, so adding Pb(NO3)2 causes the concentration of lead ion to increase.
The addition of lead ions is a stress on the equilibrium.
Applying Le Châtelier’s principle, the stress can be relieved if the reaction shifts to the left.
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24. The Common Ion Effect
The yellow solid in the test tube, which is PbCrO4, cannot dissolve because the solution is saturated with Pb2+ and CrO42– ions.
When some Pb(NO3)2 is added, the excess lead ions combine with the chromate ions in solution to form additional solid PbCrO4.
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25. In this example, the lead ion is a common ion.
The Common Ion Effect
In this example, the lead ion is a common ion.
A common ion is an ion that is found in both ionic compounds in a solution.
The lowering of the solubility of an ionic compound as a result of the addition of a common ion is called the common ion effect.
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26. CHEMISTRY & YOU
The Ksp of BaSO4 is 1.1  10–10. Why can a patient ingest the toxic BaSO4 without being harmed?
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27. CHEMISTRY & YOU
The Ksp of BaSO4 is 1.1  10–10. Why can a patient ingest the toxic BaSO4 without being harmed?
Barium sulfate is not very soluble. Therefore, very little of it can dissolve in bodily fluids and be absorbed as a toxin.
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28. Finding Equilibrium Ion Concentrations in the Presence of a Common Ion
Sample Problem 18.7
Finding Equilibrium Ion Concentrations in the Presence of a Common Ion
Small amounts of silver bromide can be added to the lenses used for eyeglasses. The silver bromide causes the lenses to darken in the presence of large amounts of UV light. The Ksp of silver bromide is 5.0  10–13. What is the concentration of bromide ion in a 1.00-L saturated solution of AgBr to which mol of AgNO3 is added?
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29. Analyze List the knowns and the unknown.
Sample Problem 18.7
Analyze List the knowns and the unknown. 1
Use one unknown to express both [Ag+] and [Br–]. Let x be the equilibrium concentration of bromide ion and x be the equilibrium concentration of silver ion.
KNOWNS
UNKNOWN
[Br–] = ? M
Ksp = 5.0  10–13
moles of AgNO3 added = mol
AgBr(s) Ag+(aq) + Br–(aq)
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30. Calculate Solve for the unknown.
Sample Problem 18.7
Calculate Solve for the unknown. 2
Write the expression for Ksp.
Ksp = [Ag+]  [Br–]
Substitute x for [Br–] in the solubility product expression.
Ksp = [Ag+]  x
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31. Calculate Solve for the unknown.
Sample Problem 18.7
Calculate Solve for the unknown. 2
Rearrange the equation to solve for x.
Ksp
x =
[Ag+]
Substitute the values for Ksp and [Ag+] in the expression and solve.
Based on the small value of Ksp, you can assume that x will be very small compared to Thus, [Ag+] ≈ M.
(5.0  10–13)
x =
0.020
[Br–] = 2.5  10–11 M
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32. Evaluate Does the result make sense?
Sample Problem 18.7
Evaluate Does the result make sense? 3
The concentration of Br– in a saturated solution of AgBr is 7.0  10–7 M (the square root of the Ksp).
It makes sense that the addition of AgNO3 would lower the concentration of Br– because the presence of a common ion, Ag+, causes AgBr to precipitate from solution.
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33. The Common Ion Effect
A precipitate will form if the product of the concentrations of two ions in the mixture is greater than the Ksp value for the compound formed from the ions.
As solutions of barium nitrate and sodium sulfate are mixed, a precipitate of BaSO4 is formed.
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34. Does a precipitate form when 1 L of 0
Does a precipitate form when 1 L of M Na2SO4 is mixed with 1 L of M Ba(NO3)2? The Ksp for BaSO4 is 1.1  10–10.
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35. Does a precipitate form when 1 L of 0
Does a precipitate form when 1 L of M Na2SO4 is mixed with 1 L of M Ba(NO3)2? The Ksp for BaSO4 is 1.1  10–10.
[Ba2+]  [SO42–] = ( M)  (0.014 M) = 8.4  10–6
The result is larger than the Ksp value for BaSO4, so a precipitate will form.
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36. Key Concepts and Key Equation
The smaller the value of the solubility product constant, the lower the solubility of the compound.
A precipitate will form if the product of the concentrations of two ions in the mixture is greater than the Ksp value of the compound formed from the ions.
Ksp = [A]a × [B]b
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37. Glossary Terms
solubility product constant: an equilibrium constant applied to the solubility of electrolytes; it is equal to the product of the concentrations of the ions each raised to a power equal to the coefficient of the ion in the dissociation equation
common ion: an ion that is common to both salts in a solution; in a solution of silver nitrate and silver chloride, Ag+ would be a common ion
common ion effect: a decrease in the solubility of an ionic compound caused by the addition of a common ion
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38. END OF 18.4
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