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18.4 Solubility Equilibrium >18.4 Solubility Equilibrium >

1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Chapter 18Reaction Rates and Equilibrium

18.1 Rates of Reaction18.2 The Progress of Chemical Reactions18.3 Reversible Reactions and Equilibrium

18.4 Solubility Equilibrium

18.5 Free Energy and Entropy



How is it possible to ingest a poison without being harmed?

CHEMISTRY & YOUCHEMISTRY & YOU

Barium sulfate, a poison, can absorb X-rays, so tissues coated with the liquid will appear as light areas in the X-ray images.



The Solubility Product ConstantThe Solubility Product Constant

The Solubility Product Constant

What is the relationship between the solubility product constant and the solubility of a compound?



• 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.


• 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.



This table provides some general rules for the solubility of ionic compounds in water.

Interpret DataInterpret Data

Solubility of Ionic Compounds in WaterCompounds Solubility Exceptions

Salts of Group 1A metals and ammonia

Soluble Some lithium compounds

Ethanoates, nitrates, chlorates, and perchlorates

Soluble Few exceptions

Sulfates Soluble Compounds of Pb, Ag, Hg, Ba, Sr, and Ca

Chlorides, bromides, and iodides

Soluble 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



Most insoluble ionic compounds will actually dissolve to some extent in water.

• These compounds are said to be slightly soluble in water.





When the “insoluble” compound silver chloride is mixed with water, a very small amount of silver chloride dissolves in the water.

An equilibrium is established between the solid and the dissolved ions in the saturated solution.

AgCl(s) Ag+(aq) + Cl–(aq)




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]




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




The smaller the numerical value of the solubility product constant, the lower the solubility of the compound.




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 Ionic compound Ksp Ionic compound Ksp

Halides

AgCl

AgBr

AgI

PbCl2PbBr2

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 10–16Sulfides

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



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)

Sample Problem 18.6Sample Problem 18.6

Finding the Ion Concentrations in a Saturated Solution



KNOWNS

UNKNOWN[Pb2+] = ? M

[CrO42–] = ? M

Analyze List the knowns and the unknowns.1


Write the expression for Ksp. Then modify it so that there is a single unknown.

Ksp = 1.8 10–14

PbCrO4(s) Pb2+(aq) + CrO42–(aq)



• Start with the general expression for the solubility product constant.

Calculate Solve for the unknowns.2


The exponent for each ion is 1.Ksp = [A]a [B]b

Ksp = [Pb2+] [CrO42–] = 1.8 10–14

• Use the chemical equation to write the correct expression for Ksp for the reaction.



Substitute [Pb2+] for [CrO42–]

in the expression to get an equation with one unknown.



At equilibrium, [Pb2+] = [CrO4

2–].

Ksp = [Pb2+] [Pb2+] = [Pb2+]2 = 1.8 10–14



Solve for [Pb2+].



[Pb2+] = 1.8 10–14

[Pb2+] = [CrO42–] = 1.3 10–7 M



• 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.

Evaluate Does the result make sense?3




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



The Common Ion EffectThe Common Ion Effect

The Common Ion Effect

How can you predict whether precipitation will occur when two solutions are mixed?




In a saturated solution of lead(II) chromate, an equilibrium is established between the solid lead(II) chromate and its ions in solution.

What would happen if you added some lead nitrate to this solution?






• 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.




The yellow solid in the test tube, which is PbCrO4, cannot dissolve because the solution is saturated with Pb2+ and CrO4

2– ions.

When some Pb(NO3)2 is added, the excess lead ions combine with the chromate ions in solution to form additional solid PbCrO4.




• 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.

In this example, the lead ion is a common ion.



The Ksp of BaSO4 is 1.1 10–10. Why can a patient ingest the toxic BaSO4 without being harmed?




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.



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 0.020 mol of AgNO3 is added?


Finding Equilibrium Ion Concentrations in the Presence of a Common Ion



KNOWNS UNKNOWN[Br–] = ? M

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 + 0.020 be the equilibrium concentration of silver ion.

Ksp = 5.0 10–13

moles of AgNO3 added = 0.020 mol

AgBr(s) Ag+(aq) + Br–(aq)



• Write the expression for Ksp.

Calculate Solve for the unknown.2


Ksp = [Ag+] [Br–]

Ksp = [Ag+] x

• Substitute x for [Br–] in the solubility product expression.



• Rearrange the equation to solve for x.


Based on the small value of Ksp, you can assume that x will be very small compared to 0.020. Thus, [Ag+] ≈ 0.020 M.

• Substitute the values for Ksp and [Ag+] in the expression and solve.

Ksp x = [Ag+]

(5.0 10–13)x = 0.020

[Br–] = 2.5 10–11 M

Calculate Solve for the unknown.2



• 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.

Evaluate Does the result make sense?3





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.



Does a precipitate form when 1 L of 0.034 M Na2SO4 is mixed with 1 L of 0.0012 M Ba(NO3)2? The Ksp for BaSO4 is 1.1 10–10.



Does a precipitate form when 1 L of 0.034 M Na2SO4 is mixed with 1 L of 0.0012 M Ba(NO3)2? The Ksp for BaSO4 is 1.1 10–10.

[Ba2+] [SO42–] = (0.0006 M) (0.014 M) = 8.4 10–6

The result is larger than the Ksp value for BaSO4, so a

precipitate will form.



Key Concepts and Key Concepts and Key EquationKey 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



Glossary TermsGlossary 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



END OF 18.4END OF 18.4

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