8.3 acid- base properties of salt solutions

AcidBase Equilibrium 581NEL

8.38.3AcidBase Properties

of Salt SolutionsSalts are solids (at SATP) composed of cations and anions arranged in a crystalline lat-tice. When they dissolve in water, they dissociate into individual hydrated ions that mayor may not affect the pH of the solution.

Sodium chloride, NaCl(s), like all sodium salts, is a strong electrolyte. When dissolvedin water it dissociates quantitatively into aqueous sodium and chloride ions.

NaCl(s) Na(aq) Cl(aq)

When the pH of a sodium chloride solution is tested with a pH meter or acidbaseindicator, it shows a pH of 7 at SATP: neutral. Sodium chloride does not produce hydro-nium or hydroxide ions in solution, so it is classed as a neutral electrolyte.

Sodium carbonate, Na2CO3(s), is another highly soluble ionic compound. It dissoci-ates quantitatively in water to produce aqueous sodium and carbonate ions.

Na2CO3(s) 2 Na(aq) CO32(aq)

When tested with a pH meter or acidbase indicator, the sodium carbonate solutionproves to have a pH greater than 7: basic. However, sodium carbonate cannot contributehydronium or hydroxide ions to the solution directly. So, why is a sodium carbonatesolution basic while a sodium chloride solution is neutral? The reason must lie in the actionof aqueous carbonate ions, not sodium ions, because both salts release sodium ions insolution.

The basic character of carbonate solutions can be explained by the BrnstedLowrytheory. As in the case of aqueous ammonia, a BrnstedLowry base acts as a protonacceptor, and may remove a proton from water to form hydroxide ions in solution. Thefollowing equation describes how the carbonate ion acts as a BrnstedLowry base.

Empirical studies have shown that the carbonate ion is a relatively weak base with abase ionization constant, Kb, of 2.1 10

4.Ammonium chloride, NH4Cl(s), is a soluble salt that forms an acidic solution when dis-

solved in water. The dissociation of the ions in aqueous solution is described by theequation

NH4Cl(s) NH4(aq) Cl(aq)

The pH of an NaCl(aq)solution is neutral, so pH is unaffected by the presence of Cl(aq)

ions. Is it possible that the acidity of ammonium chloride is caused by the NH4(aq) ion?

We can hypothesize that a Brnsted-Lowry proton-transfer reaction occurs betweenammonium ions and water molecules:

NH4(aq) H2O(l) e H3O(aq) NH3(aq)

Laboratory tests show that the ammonium ion is indeed a weak acid with an acidionization constant, Ka, of 5.8 10

10.In general, since salts contain two different ions, the pH of an aqueous salt solution

may be affected by the anion, the cation, or both.

The pH of Salt Solutions (p. 627)Do soluble salts affect the pH ofaqueous solutions?

INVESTIGATION 8.3.1

CO3(aq) H2O(l)

H

base acid

OH(aq) HCO3(aq)2

582 Chapter 8 NEL

The reaction of an ion with water to produce an acidic or basic solution is calledhydrolysis (from the Greek hydro, meaning water, and lysis, meaning splitting).

In the case of sodium chloride, neither sodium nor chloride ions hydrolyze, so neitherion affects the acidbase properties of an aqueous solution; the solution remains neu-tral. Conversely, when ammonium carbonate dissolves, carbonate ions hydrolyze to pro-duce hydroxide ions (which may produce a basic solution) and ammonium ions hydrolyzeto produce hydronium ions (which may produce an acidic solution). You must alwaysconsider both ions when assessing the effect of a salt on the pH of an aqueous solution.

So, can we accurately predict if a salt will produce an acidic or a basic solution? In thefollowing analysis, we will develop models that will help you make such predictions.

Salts That Form Neutral SolutionsIn general, salts that consist of the cations of strong bases (like Na(aq) and K

(aq)) and

the anions of strong acids (like Cl(aq) and NO3(aq)) have no effect on the pH of an aqueous

solution. Examples include NaCl(aq), KCl(aq), NaI(aq), and NaNO3(aq) (Table 1).

hydrolysis a reaction of an ion withwater to produce an acidic or basicsolution (hydronium or hydroxideions)

Do not confuse the definition ofhydrolysis as it applies toacidbase chemistry with themeaning of the term presentedin Chapter 1. In acidbasechemistry it means a reaction ofan ion with water to produce anacidic or basic solution. InChapter 1 you used it to meanthe breaking of a covalent bondusing the elements of water.

LEARNING TIP

Table 2 Composition and Hydrolysis of Acidic Salts

Salt Cation (acid) of weak base Hydrolysis reaction

NH4Cl NH4(aq) NH3 NH4(aq) H2O(l) e H3O(aq) NH3(aq)N2H5Br N2H5(aq) N2H4 N2H5(aq) H2O(l) e H3O(aq) N2H4(aq)

Table 1 Composition of Neutral Salts

Salt Cation from strong base Anion from strong acid

NaCl NaOH HCl

KCl KOH HCl

NaI NaOH HI

NaNO3 NaOH HNO3

Certain metal cations also hydrolyze and act as acids. An aluminum nitrate solution,Al(NO3)3(aq), is acidic. However, other solutions, such as sodium nitrate, NaNO3(aq),and calcium nitrate, Ca(NO3)2(aq), are neutral. Why are some metal salts acidic andsome neutral? We could propose a hypothesis that the aluminum ion is responsible forthe acidity of the solution. Further tests with aluminum solutions indeed establish thatAl3 behaves as an acid in water (increases [H(aq)]). The research indicates that Group1 and 2 metal ions (except for Be2) do not produce acidic solutions, but that highlycharged small ions do form acidic solutions. (See Table 3 and Appendix C9.)

Ions such as Al3(aq), Fe3

(aq), and Sn2

(aq) have large (positive) charge densities a largeamount of charge in a small volume. These cations produce hydronium ions indirectlyby a slightly different reaction than the ammonium ion example above. When a highlycharged metal ion such as Al3 dissolves in water, it becomes hydrated with six water mol-ecules (waters of hydration) according to the following equation:

Al3(aq) 6 H2O(l) e Al(H2O)63(aq)

Table 3 Ka for Some Metal Ionsat SATP

Metal cation* KaZr4(aq) 2.1

Sn2(aq) 2.0 102

Fe3(aq) 1.5 103

Cr3(aq) 1.0 104

Al3(aq) 9.8 106

Be2(aq) 3.2 107

Fe2(aq) 1.8 107

Pb2(aq) 1.6 108

Cu2(aq) 1.0 108

* The aqueous metal ion is ahydrated complex ion (e.g.,Cu(H2O)42(aq)). Aqueous ionsof transition metals are usu-ally written in a simplifiedform, without showing thenumber of water moleculespresent in the actual hydratedcomplex ion, as shown inTable 3.

Salts That Form Acidic SolutionsCations such as the ammonium ion, NH4

(aq), and hydrazinium ion, N2H5

(aq), act as

BrnstedLowry acids. They hydrolyze to form hydronium ions in solution. Consequently,solutions of NH4Cl(aq) and N2H5Cl(aq) are acidic.

In general, cations that are conjugate acids of weak molecular bases act as weak acidsthat have a tendency to lower the pH of a solution (Table 2).


This means that six water molecules bond to the ion with relatively weak electrostaticforces of attraction. The high-charge density of the Al3(aq) ion increases the polarity of theOH bonds in the H2O molecules. That is, the H2O molecules in Al(H2O)6

3(aq) are

more likely to transfer a proton to the solvent (H2O(l)) than are H2O(l) molecules inpure water. Although we might imagine that several (or all) of the water molecules inAl(H2O)6

3(aq) could transfer a proton, experiments show that only one H2O will. Thus,

Al(H2O)63(aq) behaves as a weak monoprotic acid in aqueous solution, according to the

following equation:

Notice that this weak acid and its acid ionization constant appear in Appendix C9.No cation with low charge density acts as an acid in this way. This includes all of the

singly charged ions of Group 1 metals, Li, Na, K, Rb, and Cs. Since they cannothydrolyze, they do not affect the pH of aqueous solutions. With the notable exceptionof Be2, none of the Group 2 cations acts as an acid either.

Salts That Form Basic SolutionsA sodium chloride solution, NaCl(aq), is neutral, but a sodium acetate solution,NaC2H3O2(aq), is basic. Why? Both solutions contain aqueous sodium ions that do nothydrolyze in solution, so the change in pH must be due to the effect of the acetate ion,C2H3O2

(aq). It is reasonable to suspect hydrolysis to be the cause. In this case, hydrolysis

caused by the acetate ion produces hydroxide ions and increases the pH of the solution.The following two equations show the dissociation of sodium acetate followed by thehydrolysis of the resulting acetate ion:

NaC2H3O2(s) Na(aq) C2H3O2(aq) (dissociation)

C2H3O2(aq) H2O(l) e HC2H3O2(aq) OH(aq) (hydrolysis)

Section 8.3

Al3+Al3+

Al(H2O)3+6 (aq) Al(H2O)5(OH)

2+(aq) H

+(aq)

++

+

Al(H2O)6(aq) H2O(l)

H(aq)

H3O(aq) Al(H2O)5(OH)(aq)3 2

acid base conjugatebase

conjugateacid

584 Chapter 8 NEL

Earlier, we explained that Group 1 anions like Cl(aq) cannot hydrolyze. So, why is it that theacetate ion is able to hydrolyze, but the chloride ion cannot? Earlier in this chapter, youlearned that the stronger the acid, the weaker its conjugate base. Extremely strong acidssuch as HCl(aq) and HNO3(aq) possess extremely weak conjugate bases, Cl

(aq) and NO3

(aq).

These bases attract protons (H ions) so poorly that they are unable to hydrolyze to any sig-nificant degree. If they did attract protons strongly, HCl and HNO3 would not be strong acidsand would not ionize so readily in aqueous solution. In general, the conjugate bases ofstrong acids (e.g., Cl(aq), Br

(aq), I

(aq), NO

3(aq)) do not affect the pH of an aqueous solution.

Conversely, the acetate ion, C2H3O2(aq), is the conjugate base of acetic acid, HC2H3O2(aq).

Acetic acid is a weak acid because the acetate ion portion of the molecule possesses arelatively high affinity for the ionizable proton (H ion). This relatively strong attrac-tion for protons makes the acetate ion able to hydrolyze to form OH(aq) ions in aqueoussolution. A solution of sodium acetate is basic because the sodium ions cannot act as acids,but the acetate ions do act as weak bases according to the hydrolysis equation above.Remember that, in general, the conjugate base of a weak acid is also weak. The Ka ofacetic acid is 1.8 105 (weak), and the Kb of its conjugate base, the acetate ion, is 5.6 1010 (weak). However, when the acetate ion is introduced as the anion of a saltcontaining a nonhydrolyzing cation like Na(aq) or K

(aq), the acetate ion forms the major

acidbase equilibrium in solution. (The only other reaction is the autoionization ofwater.) This results in a net production of OH(aq) and a basic solution.

When determining whether an anion will affect the pH of a solution, it helps toremember that

the anion of a strong acid is too weak a base to affect the pH of an aqueous solu-tion (e.g., Cl(aq) and NO3

(aq)), and

the anion of a weak acid, although it is also weak, is a strong enough base (whenacting alone) to affect the pH of an aqueous solution. It will tend to increase thepH of the solution and make it more basic.

In general, a salt made of a nonhydrolyzing cation (like Na(aq) or K(aq)), and an anion that

is the conjugate base of a weak acid, will form a basic aqueous solution (pH 7).

Salts That Act as Acids and BasesSome salts contain the cation of a weak base and the anion of a weak acid both ionscan hydrolyze. To obtain a precise estimate of the pH of such a solution, we would haveto solve two hydrolysis equilibria simultaneously. This is a difficult mathematical problem.However, we can predict whether the solution will be acidic, basic, or (approximately)neutral without performing any calculations.

Consider the ions produced when ammonium cyanide, NH4CN(s), dissolves in water.

NH4CN(s) NH4(aq) CN(aq) (dissociation)

The cation, NH4(aq), is the conjugate acid of the weak molecular base, NH3(aq), so it

will hydrolyze to produce hydronium ions in solution. Its tendency is to lower the pH.The anion, CN(aq), is the conjugate base of the weak acid, HCN(aq), so will hydrolyze toproduce hydroxide ions in solution. Its tendency is to increase the pH. What is the neteffect? That depends on the relative strengths of the two ions, as judged by the Ka valuefor the acid and the Kb value for the base. If these values are equal, then the salt willhave no net effect on the pH of the solution. If the Ka of the acid is larger than the Kb ofthe base, then the solution will be more acidic. Conversely, a higher Kb would result ina basic solution. In the case of ammonium cyanide,


Section 8.3

The following steps will help you predict whether the ions of a salt have an effect on thepH of an aqueous solution.

Step 1 Determine if the cation is the conjugate acid of a weak base or a cationwith high charge density. If so, it will make the solution more acidic. Ifnot, it will not affect the pH of the solution.

Step 2 Determine if the anion is the conjugate base of a weak acid. If so, it willmake the solution more basic. If not, it will not affect the pH of thesolution.

Step 3 If the salt has a cation and an anion that can both hydrolyze, comparethe Ka and Kb values of the cation and anion. If Ka Kb, then the solu-tion will become more acidic. If Ka Kb, the solution will becomemore basic. If Ka Kb, the solution will be neutral.

Predicting the AcidBase Behaviour of a SaltSUMMARY

(a) Predict whether a 0.10 mol/L solution of NaNO2(aq) will be acidic, basic, orneutral.

(b) Calculate the pH of a 0.10 mol/L solution of NaNO2(aq) .

(a) First, write the dissociation equation for NaNO2(s).

NaNO2(s) Na(aq) NO2(aq)

Next, examine the cation. Na(aq) is a Group 1 metal cation so will have no effect on pH.Now, examine the anion, NO2(aq). It is the conjugate base of the weak acid, HNO2(aq),so will act as a base and increase the pH.

A 0.1 mol/L solution of NaNO2(aq) will be basic.

(b) Since Na(aq) cannot hydrolyze, the pH of a 0.10 mol/L NaNO2(aq) solution is deter-mined solely by the reaction of NO2(aq) with water.

Write the chemical equation for the hydrolysis reaction between the base, NO2(aq),and water, then write the corresponding Kb expression.

NO2(aq) H2O(l) e OH(aq) HNO2(aq)

Kb[OH(aq)][HNO2(aq)]

[NO2(aq)]

Predicting the Acidic or Basic Nature of Solutions SAMPLE problem

NH4(aq) H2O(l) NH3(aq) H3O(aq) Ka 5.8 1010 mol/L

CN(aq) H2O(l) HCN(aq) OH(aq) Kb 1.6 105 mol/L

Since the Kb of the cyanide ion (1.6 105 mol/L) is much larger than the Ka of the

ammonium ion (5.8 1010 mol/L), an aqueous solution of ammonium cyanide willbe basic.

586 Chapter 8 NEL

Calculate the value of Kb for NO2(aq) from the value of Ka for HNO2(aq) (which you can findin Appendix C9).

KaKb Kw

Kb K

Kw

a

17..02

1100

1

4

4

Kb 1.4 1011

Now, construct an ICE table to show the changes in concentration that occur as thereaction reaches equilibrium. Note that, in the reaction, one mole of HNO2(aq) and onemole of OH(aq) are produced for every mole of NO2(aq) that reacts. Let x represent thechanges in concentration that occur as the reaction establishes equilibrium.

[OH(a

[q

N)]

O

[H

2(

N

aq

O

)]2(aq)]

Kb

[OH(aq)] [HNO2(aq)] x

[NO2(aq)] 0.10 x 0.10

(Since the value of Kb is so small, we will make the assumption that (0.10 x ) 0.10.This simplification is warranted by the hundred rule. Check it yourself.)

0

x

.1

2

0 1.4 1011

x 1.4 1012

x 1.2 106

As usual, we now justify our simplifying assumption, using the 5% rule.

1.2

0.11006

100% 1.2 103 %

Since 1.2 103 % 5%, our assumption is justified. Therefore,

x 1.2 106

[OH(aq)] 1.2 106 mol/L

Now, we can calculate pOH and pH.

pOH log [OH(aq)]

log(1.2 106 mol/L)

pOH 5.92

pH pOH 14.00

14.00 pOH

14.00 5.92

pH 8.08

The pH of a 0.10 mol/L NaNO2(aq) solution is 8.08.

Table 4 ICE Table for the Hydrolysis of NO2(aq)

NO2(aq) H2O(l) e OH(aq) HNO2(aq)Initial concentration (mol/L) 0.10 0 0

Changes in concentration (mol/L) x x x

Equilibrium concentration (mol/L) 0.10 x x x


Section 8.3

Example

(a) Predict whether a 0.20 mol/L solution of ammonium chloride, NH4Cl(aq), will be acidic,basic, or neutral.

(b) Calculate the pH of a 0.20 mol/L solution of NH4Cl(aq).

Solution

(a) First, write the dissociation equation for NH4Cl(aq).

NH4Cl(aq) NH4(aq) Cl(aq)

Since Cl(aq) is the conjugate base of a strong acid, it will not affect the pH of the solu-tion. NH4(aq) is the conjugate acid of the weak base, NH3(aq), so it will hydrolyzeaccording to the following equation:

NH4(aq) H2O(l) e H3O(aq) NH3(aq)

A solution of NH4Cl(aq) will be acidic.

(b) NH4(aq) H2O(l) e H3O(aq) NH3(aq)

Ka 5.8 1010 (from Appendix C9)

[H3O

[

(

Naq

H)]

4

[(

N

aq

H

)]3(aq)]

Ka

Ka

0.20

x2

x 5.8 1010

Predicting whether 0.20 x 0.20...

[HA

K

]

a

initial

5.8 0.2

10010

3.4 108

Since 3.4 108 100, we assume that 0.20 x 0.20.

0x.2

2

0 5.8 1010

x2 2.9 109

x 5.4 105

Since x [H3O(aq)],

[H3O(aq)] 5.4 105

pH log [H3O(aq)]

log(5.4 105)

pH 4.

The pH of a 0.20 mol/L NH4Cl(aq) solution is 4.

[H3O(aq)][NH3(aq)]

[NH4(aq)]

Table 5 ICE Table for the Hydrolysis of NH4(aq)

NH4(aq) H2O(l) e H3O(aq) NH3(aq)Initial concentration (mol/L) 0.20 0 0

Changes in concentration (mol/L) x x x

Equilibrium concentration (mol/L) 0.20 x x x

588 Chapter 8 NEL

Hydrolysis of Amphoteric IonsRemember that all polyatomic ions whose chemical formulas begin with hydrogen, H (e.g.,HCO3

(aq) and HSO4

(aq)) are amphoteric. As mentioned in Section 8.1, the term

amphiprotic may also be used to describe such entities because they can either donateor accept a hydrogen ion (proton).

NaHCO3(s)dissolves in water, forming a conducting solution containing sodium andhydrogen carbonate ions.

NaHCO3(s) Na(aq) HCO3(aq)

Because it is amphoteric, the HCO3(aq) ion may hydrolyze as an acid or a base, according

to the following equilibrium equations:

HCO3(aq) H2O(l) e H3O+(aq) CO32(aq) (acid hydrolysis)

HCO3(aq) H2O(l) e OH(aq) H2CO3(aq) (base hydrolysis)

Testing a sodium hydrogen carbonate solution with litmus paper reveals that it isbasic. Therefore, we assume that the base hydrolysis equilibrium dominates in solution.

Similarly, the acidic character of a sodium hydrogen sulfate, NaHSO4(s) solution isexplained by the dissociation and subsequent acid hydrolysis of the hydrogen sulfate ion.

NaHSO4(s) Na(aq) HSO4(aq) (dissociation)

HSO4(aq) H2O(l) e H3O+(aq) SO42(aq) (acid hydrolysis )

In both cases, the hydrolyzing ion may act as an acid or a base, but one equilibriumpredominates and gives rise to the overall acidic or basic character of the aqueous solu-tion. How do we predict which one will win out? The following discussion describesthe theory.

Picture a laboratory setting in which a student tests the pH of a sodium dihydrogenborate, NaH2BO3(aq), solution. The solution has a pH greater than 7. What reactionoccurs to make the solution basic?

PracticeUnderstanding Concepts

1. Predict whether the following solutions are acidic, basic, or neutral. Provide expla-nations to support your predictions.

(a) ammonium phosphate, (NH4)3PO4(aq) (fertilizer)

(b) ammonium sulfate, (NH4)2SO4(aq) (fertilizer)

(c) magnesium oxide, MgO(aq) (milk of magnesia)

2. Predict whether a solution of sodium sulfite, Na2SO3(aq) (photographic developer)will be acidic, basic, or neutral.

3. Calculate the pH of a 0.30 mol/L ammonium nitrate (fertilizer) solution.

4. Calculate the pH of 0.25 mol/L NH4Br(aq).5. Predict whether a solution of NH4C2H3O2(aq) is acidic, basic, or neutral. Explain.

Making Connections

6. What kind of fertilizers would be appropriate for acid-loving plants like evergreens?

Answers

3. 4.88

4. 4.92


Section 8.3

Predict whether an aqueous solution of baking soda, NaHCO3(s), is acidic, basic,or neutral.

First, write the dissociation equation.

NaHCO3(s) Na(aq) HCO3(aq) (dissociation)

Next, find the acid ion dissociation constant (from a reference table), as if the hydrogencarbonate ion were to act as an acid:

HCO3(aq) H2O(l) e CO32(aq) H3O(aq) Ka 4.7 1011 (hydrolysis as an acid)

Next, find the base ion dissociation constant, as if the hydrogen carbonate ion were toact as a base (use either a table of Kb values, or calculate from Ka):

HCO3(aq) H2O(l) e H2CO3(aq) OH(aq) Kb 2.7 108 (hydrolysis as a base)

According to the relative values of Ka and Kb, the baking soda solution should be basicbecause the Kb is larger than the Ka. The hydrogen carbonate ion is a stronger base than itis an acid.

ExamplePredict whether a solution of NaHBO3(aq) is acidic, basic, or neutral.

SolutionNaH2BO3(s) Na(aq) H2BO3(aq) (dissociation)

H2BO3(aq) H2O(l) e HBO32(aq) H3O(aq) Ka 5.8 1010 (hydrolysis as an acid)

H2BO3(aq) H2O(l) e H3BO3(aq) OH(aq) Kb 1.7 105 (hydrolysis as a base)

Since Kb Ka, a solution of NaH2BO3(aq) is basic.

Predicting the Acidity of Amphoteric Ions SAMPLE problem


7. Make a list of all amphoteric ions from your acidbase table, Appendix C9.

8. Predict whether the following solutions are acidic, basic, or neutral.(a) NaHSO4(aq) (b) Na2HPO4(aq)

The following equations describe the dissociation of NaH2BO3(s) in water, and thehydrolysis of the H2BO3

(aq) ions in solution.

NaH2BO3(s) Na(aq) H2BO3(aq) (dissociation)

H2BO3(aq) H2O(l) e H3O(aq) HBO32(aq) (acid hydrolysis)

H2BO3(aq) H2O(l) e OH(aq) H3BO3(aq) (base hydrolysis)

Since the solution is basic when tested with an acidbase indicator, H2BO3(aq) must

hydrolyze primarily as a base. Is there a way to predict the acidbase character of a solu-tion without testing it directly in the laboratory? Remember that when a salt containingboth a hydrolyzing anion and a hydrolyzing cation, such as NH4CN(aq), is dissolved inwater, we predict the acidbase characteristics of the solution by comparing Ka and Kbvalues. The following sample problem will provide you with a similar model for predicting hydrolysis in solutions of amphoteric ions.

590 Chapter 8 NEL

Hydrolysis of Metal and Nonmetal OxidesWhen calcium oxide, CaO(s), is dissolved in water it produces a basic solution. However,an aqueous carbon dioxide solution, CO2(aq), is acidic. Can we explain this evidenceusing the hydrolysis theory? First, we must realize that calcium oxide and carbon dioxideboth have low solubility in water. Therefore, we show the pure substance reacting withwater rather than dissolving in water.

CaO(s) H2O(l) Ca2(aq) 2 OH(aq)

As you know, most metal oxides have low solubility in water, but the accepted theoryis that the solid state oxide ions are converted quantitatively into aqueous hydroxideions by the reaction with water to form a basic solution, according to the following equa-tion.

O2(s) H2O(l) 2 OH(aq)

The O2(s) ions do not exist in aqueous solution; they are quantitatively converted toOH(aq) ions from the solid state (in CaO(s)). However, the equation may be used toexplain the basic nature of the solutions of metal oxides.

Now, let us try to explain the acidic character of carbon dioxide in water. A possibleexplanation is the two-step process presented below.

CO2(g) H2O(l) e H2CO3(aq)H2CO3(aq) H2O(l) e H3O(aq) HCO3(aq)

CO2(g) 2 H2O(l) e H3O(aq) HCO3(aq) (net equation)

Chemists have done numerous tests on metal oxides and nonmetal oxides to determinethe acidic and basic character of the solutions formed. Their evidence led to the fol-lowing generalizations.

Metal oxides react with water to produce basic solutions.

Nonmetal oxides react with water to produce acidic solu-tions.

Predict, using the hydrolysis concept, whether solutions of the following oxideswill be acidic, neutral, or basic. Write an appropriate hydrolysis equation in eachcase.(a) magnesium oxide, MgO(s)(b) sulfur dioxide, SO2(g)

(a) Magnesium oxide, MgO(s), is a metal oxide, so will react with water to form a basicsolution, according to the following hydrolysis equation:

MgO(s) H2O(l) e Mg2(aq) 2 OH(aq)

The hydroxide ions are produced according to the following equation:

O(s)2 H2O(l) e 2 OH

(aq)

(b) Sulfur dioxide, SO2(g), is a nonmetal oxide, so will react with water to form an acidicsolution, according to the following hydrolysis equation:

SO2(g) 2 H2O(l) e H3O(aq) HSO

3(aq)

Predicting the Acidity of Solutions of OxidesSAMPLE problem


The acidic properties of nonmetal oxides are responsible for many natural processessuch as the the weathering of minerals, the absorption of nutrients by the roots of plants,and the chemistry of tooth decay. For example, limestone, CaCO3(s), dissolves in waterthat is made acidic by the dissolution of CO2(g) from the atmosphere, according to thefollowing two-step process:

CO2(g) H2O(l) e H+(aq) HCO3(aq) (acidification of rain or ground water)

H(aq) CaCO3(s) e Ca2+(aq) HCO3(aq) (dissolution of CaCO3(s) in acidic solution)

Limestone caves and the stalagmites and stalactites they contain are formed by theaction of acidic ground water on limestone deposits in Earths crust.

A cave is formed when the above reactions proceed to the right and calcium carbonatein underground limestone deposits is dissolved. Stalactites are formed in the cave whenthe aqueous solution on the ceiling evaporates. As it evaporates, carbon dioxide escapes.This shifts the equilibrium to the left, causing solid calcium carbonate to precipitate.This precipitation causes stalactites to form on the ceilings and stalagmites to formwhere drops of the solution drip onto the cave floor. Stalactite and stalagmite formationis very slow; the growth rate is, on average, about one millimetre per century.

Tooth decay is also caused by the dissolution of minerals in acidic solutions. Toothenamel is composed of the mineral hydroxyapatite, Ca5(PO4)3OH (Ksp 6.8 10

37).Acids (in saliva, from fruits and fruit juices, or formed when sugars are metabolized bybacteria in the mouth) react with hydroxyapatite, leading to erosion of tooth enamel andeventually tooth decay (cavities). Fluoride salts (as a source of fluoride ions) are oftenadded to toothpaste and to drinking water in treatment plants in some communities tohelp prevent tooth decay. The fluoride ions react with the Ca5(PO4)3OH in tooth enamelto form more decay-resistant fluorapatite Ca5(PO4)3F (Ksp 1.0 10

60).

Section 8.3

Growing Up or Growing Down?When viewing photos of stalactitesand stalagmites in a cave, you maynot be able to tell which way is up. Look at the tips of the formations. Stalactites almostalways have pointed tips, whereasstalagmites are usually rounded orflat. Is the photo right-side up orupside down?

DID YOU KNOW ??


9. For each of the following solutions of compounds, write an ionization or dissociationequation where appropriate, and then write a net equation showing reactions withwater to produce either hydronium or hydroxide ions (consistent with the evidence).(a) Na2O(s) in solution turns red litmus blue.(b) SO3(g) in solution turns blue litmus red.

ExampleUse hydrolysis concepts to predict whether an aqueous solution of copper(II) oxide,CuO(s), is acidic, basic, or neutral.

SolutionCopper (II) oxide, CuO(s), is a metal oxide, so will react with water to form a basic solution, according to the following equation:

CuO(s) H2O(l) e Cu2(aq) 2 OH(aq)O(s)2

H2O(l) e 2 OH(aq)

592 Chapter 8 NEL

The Lewis Model of Acids and BasesThe Arrhenius and BrnstedLowry models successfully explain much of the behav-iour of acids and bases. Nevertheless, both of these models contain limitations. Rememberthat the Arrhenius model could not adequately explain the basic properties of an aqueousammonia solution. In the early 1920s, G. N. Lewis expanded the BrnstedLowry modelto encompass a number of substances that would not normally be classified asBrnstedLowry acids or bases. According to the Lewis model, a Lewis acid is an elec-tron-pair acceptor and a Lewis base is an electron-pair donor. In order to act as a Lewisbase, a substance must possess a non-bonded pair of electrons in one of its orbitals.Conversely, in order to act as a Lewis acid, a substance must possess an empty valenceorbital that may accept (share) a pair of non-bonding valence electrons from a Lewis base.The following structural formula equation illustrates the reaction between a Lewis acid,H(aq), and a Lewis base, H2O(l), to form H3O

(aq).

AcidBase Characteristics of SaltsSUMMARY

Table 6

Type of Salt Examples Description pH

cation of strong base and NaCl(aq), KNO3(aq) does not hydrolyze neutralanion of strong acid NaI(aq) as an acid or as a base

cation of strong base and NaC2H3O2(aq) anion hydrolyzes as a base; basicanion of weak acid KF(aq) cation does not hydrolyze

cation is conjugate acid NH4NO3(aq) cation hydrolyzes as an acid; acidicof a weak base; anion does not hydrolyzeanion of a strong acid NH4Cl(aq)cation is conjugate acid NH4C2H3O2(aq) cation hydrolyzes as an acid; acidic if Ka Kbof a weak base; anion is NH4F(aq) anion hydrolyzes as a base basic if Ka Kbconjugate base of a weak neutral if Ka Kbacid

cation is highly AlCl3(aq) hydrated cation hydrolyzes acidiccharged metal ion; FeI3(aq) as an acid;anion of a strong acid anion does not hydrolyze

metal oxides CuO(s) solid state oxide ion reacts with basicwater to form OH(aq)

nonmetal oxides CO2(g) compound reacts with water acidicto form H3O(aq)

Lewis acid an electron-pairacceptor

Lewis base an electron-pair donor

Making Connections

10. Limestone caves are very popular tourist attractions. Some of the most popular cavesattract up to 500 000 people per year. Many speleologists (cave experts) believe thatcave formations such as stalactites and stalagmites are in danger of dissolving awayon account of the large numbers of visitors.(a) Explain how the presence of large numbers of humans may affect the chemical

stability of stalagmites and stalactites in caves.(b) Suggest possible solutions that do not include barring people from the caves.

11. Many communities are against the addition of fluoride to municipal drinking water.Conduct library and Internet research to learn more about this issue. List argumentsfor and against this practice and write a brief position paper.

GO www.science.nelson.com


In the above reaction, the H(aq) ion (proton) acts as the Lewis acid (electron pairacceptor) and the water molecule acts as a Lewis base (electron-pair donor).

In this case, the Lewis base is also a BrnstedLowry base, and the Lewis acid is alsoa BrnstedLowry acid. However, in the following example, this is not the case.

Note that the BrnstedLowry model also accounts for ammonia as a base, but doesnot characterize boron trifluoride as an acid.

The Lewis acidbase theory explains the reaction between BF3 (boron trifluoride)and NH3 (ammonia). Boron trifluoride is a trigonal planar molecule with sp

2 hybridorbitals. This arrangement leaves an empty 2p orbital on the boron atom that is able toaccommodate the pair of nonbonding electrons in the sp3 hybrid orbital of NH3. Acovalent bond forms between the boron and the nitrogen, forming the compoundBF3NH3.

The Lewis acidbase theory can also explain why small, highly positive ions such asAl3 form complex ions in water:

Al3(aq) + 6 H2O(l) e Al(H2O)63(aq)

This is a Lewis acidbase reaction. The water molecules each possess nonbondingpairs of electrons and so act as Lewis bases, and the Al3(aq) ion possesses empty 3s, 3p, and3d orbitals that may accommodate electron pairs. The electron configuration of theAl3 ion can be represented as Al3: [Ne] 3s0 3p0. The complex Al(H2O)6

3(aq) is formed

when an Al3 ion, acting as a Lewis acid, bonds with six water molecules, each acting asa Lewis base.

The Lewis model is a more general model of acids and bases that not only encom-passes the BrnstedLowry model, but extends it. BrnstedLowry acids and bases areacids and bases in the Lewis model, but the reverse is not always true.

Section 8.3

H+H

HO

H

HOH

+

H+ ion(Lewis acid)

water(Lewis base)

hydronium ion

B

F

F

N H

H

H

N H

H

H

BFF

F

F

boron trifluoride(Lewis acid)

ammonia(Lewis base)

boron trifluorideammonia complex

ExampleIdentify the Lewis acid and the Lewis base in the following reaction.

SO3(aq) H2O(l) e H2SO4(aq)

S

O

O O

H

HO O S O

H

H

O

O

sulfur trioxide(Lewis acid)

sulfuric acidwater(Lewis base)

SolutionSO3(aq) is the Lewis acid and the water is the Lewis base.

594 Chapter 8 NEL

Section 8.3 QuestionsUnderstanding Concepts

1. Predict whether the following solutions are acidic, basic, orneutral. Provide explanations to support your predictions.(a) table salt, NaCl(aq) (saline or brine) solution(b) aluminum chloride, AlCl3(aq) (antiperspirant)(c) Na2CO3(aq) (washing soda)

2. Predict whether a solution of ammonium carbonate (a com-ponent of baking powder) will be acidic, basic, or neutral.

3. What is the strongest possible acid in an aqueous solution,and what is the strongest possible base in an aqueoussolution?

4. Will an aqueous solution of BeCl2(aq) turn litmus red orblue? Explain.

5. Predict whether the following solutions are acidic, basic, orneutral.(a) a carbonated beverage containing CO2(aq) (pop)(b) strontium oxide, SrO(s)

Applying Inquiry Skills

6. Analyze the Evidence (Table 7) to determine the kind ofsolutions (acidic, basic, or neutral) formed when Period 3oxides are placed in water.

Experimental Design

Oxides of elements in Period 3 are tested in water, usinglitmus paper. To all the oxides, a strong acid (hydrochloric

acid) and a strong base (sodium hydroxide) are added todetermine if a neutralization reaction occurs.

Evidence

Extension

7. Nitrogen oxides such as nitrogen dioxide, NO2(g), are pro-duced in automobile engines and released into the atmos-phere via exhaust fumes. In the atmosphere, nitrogendioxide will dissolve in droplets of rain. Use hydrolysis con-cepts to predict what will happen to the pH of rain whenNO2(g) dissolves.


12. Identify the Lewis acid and the Lewis base in each of the following reactions.(a) H(aq) OH(aq) H2O(l)(b) H(aq) NH3(aq) e NH4(aq)

Table 7 Litmus and Neutralization Tests on Oxides

Oxide Litmus test HCl(aq) test NaOH(aq) test

Na2O(s) red to blue neutralizes no reaction

MgO(s) red to blue neutralizes no reaction

As2O3(s) (insoluble) neutralizes neutralizes

SiO2(s) (insoluble) no reaction neutralizes

P2O3(s) blue to red no reaction neutralizes

SO3(g) blue to red no reaction neutralizes

Cl2O(g) blue to red no reaction neutralizes

8.3 acid- base properties of salt solutions

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