Reactions in aqueous solutions - Simone Damiano · 116 CHAPTER 4 Reactions in Aqueous Solution 4.1 | GENERAL PROPERTIES OF AQUEOUS SOLUTIONS A solution is a homogeneous mixture of

Reactions in aqueous solutions Precipitation Reactions

116 CHAPTER 4 Reactions in Aqueous Solution

4.1 | GENERAL PROPERTIES OFAQUEOUS SOLUTIONS

A solution is a homogeneous mixture of two or more substances. •(Section 1.2) Thesubstance present in the greatest quantity is usually called the solvent, and the othersubstances are called solutes; they are said to be dissolved in the solvent. When a smallamount of sodium chloride (NaCl) is dissolved in a large quantity of water, for example,water is the solvent and sodium chloride is the solute.

Electrolytic PropertiesAt a young age we learn not to bring electrical devices into the bathtub so as not to elec-trocute ourselves. That’s a useful lesson because most of the water you encounter indaily life is electrically conducting. Pure water, however, is a very poor conductor of elec-tricity. The conductivity of bathwater originates from the substances dissolved in thewater, not from the water itself.

Not all substances that dissolve in water make the resulting solution conducting.Imagine preparing two aqueous solutions—one by dissolving a teaspoon of table salt(sodium chloride) in a cup of water and the other by dissolving a teaspoon of tablesugar (sucrose) in a cup of water (! FIGURE 4.2). Both solutions are clear and color-less, but they possess very different electrical conductivities: the salt solution is a goodconductor of electricity, whereas the sugar solution is not.

In order for the bulb in the device of Figure 4.2 to light up, there must be a current(that is, a flow of electrically charged particles) between two electrodes immersed in thesolution. The conductivity of pure water is not sufficient to complete the electrical cir-cuit and light the bulb. The situation changes when ions are present in solution becausethe ions carry electrical charge from one electrode to the other, completing the circuit.Thus, the conductivity of NaCl solutions indicates the presence of ions. The lack of con-ductivity of sucrose solutions indicates the absence of ions. When NaCl dissolves inwater, the solution contains and ions, each surrounded by water molecules.When sucrose (C12H22O11) dissolves in water, the solution contains only neutral sucrosemolecules surrounded by water molecules.

Cl-Na+

CO2

!

!

!

!

"

"

"2

H2O

HCO3!

HCO3!

CaCO3 Ca2"

H"

H"

Acidic water reacts with CaCO3, dissolving limestone and forming a cenote

2Atmospheric CO2 dissolves in groundwater, forming H+

and HCO3–

1

" FIGURE 4.1 Cenote formation.

Chemical reactions that occur in water are responsible for creation of cenotes. When carbon dioxide, CO2, dissolves in water, the resulting solution is slightly acidic and reacts with CaCO3 in the limestone:

Aqueous solutions

cenotes in Yucatan - Mexico

CaCO3(s) + H2O(l) + CO2 (aq) ⟶ Ca(HCO3)2 (aq)

A solution in which water is the dissolving medium is called an aqueous solution


4.1 | GENERAL PROPERTIES OFAQUEOUS SOLUTIONS

A solution is a homogeneous mixture of two or more substances. •(Section 1.2) Thesubstance present in the greatest quantity is usually called the solvent, and the othersubstances are called solutes; they are said to be dissolved in the solvent. When a smallamount of sodium chloride (NaCl) is dissolved in a large quantity of water, for example,water is the solvent and sodium chloride is the solute.

Electrolytic PropertiesAt a young age we learn not to bring electrical devices into the bathtub so as not to elec-trocute ourselves. That’s a useful lesson because most of the water you encounter indaily life is electrically conducting. Pure water, however, is a very poor conductor of elec-tricity. The conductivity of bathwater originates from the substances dissolved in thewater, not from the water itself.

Not all substances that dissolve in water make the resulting solution conducting.Imagine preparing two aqueous solutions—one by dissolving a teaspoon of table salt(sodium chloride) in a cup of water and the other by dissolving a teaspoon of tablesugar (sucrose) in a cup of water (! FIGURE 4.2). Both solutions are clear and color-less, but they possess very different electrical conductivities: the salt solution is a goodconductor of electricity, whereas the sugar solution is not.

In order for the bulb in the device of Figure 4.2 to light up, there must be a current(that is, a flow of electrically charged particles) between two electrodes immersed in thesolution. The conductivity of pure water is not sufficient to complete the electrical cir-cuit and light the bulb. The situation changes when ions are present in solution becausethe ions carry electrical charge from one electrode to the other, completing the circuit.Thus, the conductivity of NaCl solutions indicates the presence of ions. The lack of con-ductivity of sucrose solutions indicates the absence of ions. When NaCl dissolves inwater, the solution contains and ions, each surrounded by water molecules.When sucrose (C12H22O11) dissolves in water, the solution contains only neutral sucrosemolecules surrounded by water molecules.

Cl-Na+

CO2

!

!

!

!

"

"

"2

H2O

HCO3!

HCO3!

CaCO3 Ca2"

H"

H"

Acidic water reacts with CaCO3, dissolving limestone and forming a cenote

2Atmospheric CO2 dissolves in groundwater, forming H+

and HCO3–

1

" FIGURE 4.1 Cenote formation.

A solution is a homogeneous mixture of two or more substances. The substance present in the greatest quantity is usually called the solvent, and the other substances are called solutes, they are said to be dissolved in the solvent.

Pure water is a very poor conductor of electricity. The conductivity of bathwater originates from the substances dissolved in the water, not from the water itself. Some substances (e.g NaCl) that dissolve in water make the resulting solution conducting.

Aqueous solutions

Electrolytic Properties Two aqueous solutions, 1. NaCl in water 2. Table sugar (sucrose) in water.

Both solutions are colorless, but they have different conductivities: the salt solution is a good conductor of electricity, whereas the sugar solution is not.

Current is a flow of electrically charged particles. In a solution the ions carry electrical charges. Thus, the conductivity of NaCl solutions indicates the presence of ions. The lack of conductivity of sucrose solutions indicates the absence of ions. When NaCl dissolves in water, the solution contains Na+ and Cl- ions, each surrounded by water molecules. When sucrose (C12H22O11) dissolves in water, the solution contains only neutral sucrose molecules surrounded by water molecules. A substance whose aqueous solutions contain ions is called an electrolyte. A substance that does not form ions in solution is called a non electrolyte.

SECTION 4.1 General Properties of Aqueous Solutions 117

Pure water,H2O(l)

does not conduct electricity

Sucrose solution,C12H22O11(aq)Nonelectrolyte


Sodium chloride solution,NaCl(aq)Electrolyte

conducts electricity

Not lit Not lit Lit ! FIGURE 4.2Electricalconductivities ofwater and twoaqueous solutions.

A substance (such as NaCl) whose aqueous solutions contain ions is called anelectrolyte. A substance (such as C12H22O11) that does not form ions in solution iscalled a nonelectrolyte. The different classifications of NaCl and C12H22O11 ariselargely because NaCl is ionic, whereas C12H22O11 is molecular.

Ionic Compounds in WaterRecall from Figure 2.21 that solid NaCl consists of an orderly arrangement of and

ions. When NaCl dissolves in water, each ion separates from the solid structure anddisperses throughout the solution [" FIGURE 4.3(a)]. The ionic solid dissociates into itscomponent ions as it dissolves.

Cl-Na+

(a) Ionic compounds like sodium chloride, NaCl, form ions when they dissolve

Ionic compounddissolves in water

Na!Cl"

Methanol

(b) Molecular substances like methanol, CH3OH, dissolve without forming ions

H2O molecules separate Na+ and Cl–

ions from solid NaCl

1

H2O molecules surround Na+ and Cl– ions

2

Na+ and Cl– ions dispersethroughout the solution

3

# FIGURE 4.3 Dissolution in water. (a) When an ionic compound, such as sodium chloride, NaCl, dissolves in water, H2O moleculesseparate, surround, and uniformly disperse the ions into the liquid. (b) Molecular substances that dissolve in water, such as methanol, CH3OH,usually do so without forming ions. We can think of this as a simple mixing of two molecular species. In both (a) and (b) the water molecules havebeen moved apart so that the solute particles can be seen clearly.

G O F I G U R EWhich solution, NaCl(aq) or CH3OH(aq), conducts electricity?

Ionic Compounds in Water When NaCl dissolves in water, each ion separates from the solid structure and disperses in the solution. The ionic solid dissociates into its component ions.


Pure water,H2O(l)










Cl-Na+



Na!Cl"

Methanol




1


2


3




Water is a very effective solvent for ionic compounds. Although H2O is an electri-cally neutral molecule, the O atom is rich in electrons and has a partial negative charge,denoted by . Each H atom has a partial positive charge, denoted by . Cations areattracted by the negative end of H2O, and anions are attracted by the positive end.

As an ionic compound dissolves, the ions become surrounded by H2O molecules, asshown in Figure 4.3(a). The ions are said to be solvated. In chemical equations, we de-note solvated ions by writing them as and , where aq is an abbreviationfor “aqueous.” •(Section 3.1) Solvation helps stabilize the ions in solution and pre-vents cations and anions from recombining. Furthermore, because the ions and theirshells of surrounding water molecules are free to move about, the ions become dis-persed uniformly throughout the solution.

We can usually predict the nature of the ions in a solution of an ionic compoundfrom the chemical name of the substance. Sodium sulfate (Na2SO4), for example, disso-ciates into sodium ions and sulfate ions . You must remember theformulas and charges of common ions (Tables 2.4 and 2.5) to understand the forms inwhich ionic compounds exist in aqueous solution.

G I V E I T S O M E T H O U G H TWhat dissolved species are present in a solution of a. KCN,b. NaClO4?

Molecular Compounds in WaterWhen a molecular compound dissolves in water, the solution usually consists of intactmolecules dispersed throughout the solution. Consequently, most molecular com-pounds are nonelectrolytes. As we have seen, table sugar (sucrose) is a nonelectrolyte. Asanother example, a solution of methanol (CH3OH) in water consists entirely of CH3OHmolecules dispersed in the water [Figure 4.3(b)].

A few molecular substances have aqueous solutions that contain ions. Acids are themost important of these solutions. For example, when HCl(g) dissolves in water to formhydrochloric acid, HCl(aq), it ionizes; that is, it dissociates into and ions.

Strong and Weak ElectrolytesElectrolytes differ in the extent to which they conduct electricity. Strong electrolytesare those solutes that exist in solution completely or nearly completely as ions. Essen-tially all water-soluble ionic compounds (such as NaCl) and a few molecular com-pounds (such as HCl) are strong electrolytes. Weak electrolytes are those solutes thatexist in solution mostly in the form of neutral molecules with only a small fraction inthe form of ions. For example, in a solution of acetic acid (CH3COOH) most of thesolute is present as CH3COOH(aq) molecules. Only a small fraction (about 1%) of theCH3COOH has dissociated into and ions.*

We must be careful not to confuse the extent to which an electrolyte dissolves (itssolubility) with whether it is strong or weak. For example, CH3COOH is extremely sol-uble in water but is a weak electrolyte. Ca(OH)2, on the other hand, is not very solublein water, but the amount that does dissolve dissociates almost completely. Thus,Ca(OH)2 is a strong electrolyte.

When a weak electrolyte, such as acetic acid, ionizes in solution, we write the reac-tion in the form

[4.2]

The half-arrows pointing in opposite directions mean that the reaction is significantin both directions. At any given moment some CH3COOH molecules are ionizing to

CH3COOH(aq2 ∆ CH3COO-1aq2 + H+1aq2

CH3COO-1aq2H+(aq)

Cl-1aq2H+(aq)

1SO42-21Na+2

Cl-1aq2Na+(aq)

d+d-

*The chemical formula of acetic acid is sometimes written HC2H3O2 so that the formula looks like that ofother common acids such as HCl. The formula CH3COOH conforms to the molecular structure of acetic acid,with the acidic H on the O atom at the end of the formula.

d!

d!

d"

As an ionic compound dissolves, the ions become surrounded by H2O molecules (solvation). In chemical equations, solvated ions are noted as Na+(aq) and Cl-(aq), whereas aq is an abbreviation for “aqueous”.

Solvation stabilizes the ions in solution and prevents cations and anions from recombining.

Water is polar and is an effective solvent for ionic compounds. The O atom is rich in electrons and has a partial negative charge, denoted by δ-. Each H atom has a partial positive charge (δ+).

Cations are attracted by the negative end of H2O, anions are attracted by the positive end.

Molecular Compounds in Water When a molecular compound dissolves in water, the molecules remain intact and dispersed throughout the solution. Consequently, most molecular compounds are non-electrolytes.

Molecular substances like methanol, CH3OH, dissolve without forming ions.

A few molecular substances have aqueous solutions that contain ions. Acids are the most important of these solutions. For example, when HCl(g) dissolves in water to form hydrochloric acid, HCl(aq), it ionizes; that is, it dissociates into H+ (aq) and Cl- (aq) ions.


Pure water,H2O(l)










Cl-Na+



Na!Cl"

Methanol




1


2


3



Strong and Weak ElectrolytesStrong electrolytes are those solutes that exist in solution completely or nearly completely as ions. Essentially all water-soluble ionic compounds (such as NaCl) and a few molecular compounds (such as HCl) are strong electrolytes.

Weak electrolytes are those solutes that exist in solution mostly in the form of neutral molecules.

For example, in a solution of acetic acid (CH3COOH) most of the solute is present as CH3COOH (aq) molecules. Only a small fraction of the CH3COOH is dissociated into H+ ions (aq) and CH3COO- (aq) ions.

When a weak electrolyte, such as acetic acid, ionizes in solution, we write the reaction as

CH3COOH (aq) ⇆ CH3COO- (aq) + H+ (aq)

The arrows pointing in opposite directions mean that the reaction is at the chemical equilibrium.

At any given moment some CH3COOH molecules are ionizing to form H+ and CH3COO- ions but those are recombining to form CH3COOH.


PbI2(s) ! 2 KI(aq)2 KI(aq) ! Pb(NO3)2(aq)ProductsReactants

Pb2!

Pb2!(aq) and I–(aq) combineto form precipitate

K!

NO3"

I"

! FIGURE 4.4 A precipitation reaction.

Precipitation reactions occur when pairs of oppositely charged ions attract eachother so strongly that they form an insoluble ionic solid. To predict whether certaincombinations of ions form insoluble compounds, we must consider some guidelinesconcerning the solubilities of common ionic compounds.

Solubility Guidelines for Ionic CompoundsThe solubility of a substance at a given temperature is the amount of the substance thatcan be dissolved in a given quantity of solvent at the given temperature. In our discus-sions, any substance with a solubility less than will be referred to as insoluble.In those cases the attraction between the oppositely charged ions in the solid is too greatfor the water molecules to separate the ions to any significant extent; the substance re-mains largely undissolved.

Unfortunately, there are no rules based on simple physical properties such as ioniccharge to guide us in predicting whether a particular ionic compound will be soluble.Experimental observations, however, have led to guidelines for predicting solubility forionic compounds. For example, experiments show that all common ionic compoundsthat contain the nitrate anion, , are soluble in water. " TABLE 4.1 summarizes thesolubility guidelines for common ionic compounds. The table is organized according tothe anion in the compound, but it also reveals many important facts about cations. Notethat all common ionic compounds of the alkali metal ions (group 1A of the periodic table)and of the ammonium ion are soluble in water.1NH4

+2NO3

-

0.01 mol/L

G O F I G U R EWhich ions remain in solution after PbI2 precipitation is complete?

PRECIPITATION REACTIONS Reactions that result in the formation of an insoluble solid are called precipitation reactions. A precipitate is formed by a reaction in solution.

Pb(NO3)2 (aq) + KI (aq) ⟶ PbI2 (s) + 2 KNO3 (aq)

The figure shows two clear solutions being mixed. One solution contains potassium iodide, KI, dissolved in water and the other contains lead nitrate, Pb(NO3)2, dissolved in water. The reaction between these two solutes produces a water-insoluble yellow solid.The precipitate is lead iodide (PbI2), a compound that has a very low solubility. The other product of this reaction, potassium nitrate (KNO3), remains in solution.

PRECIPITATION REACTIONS Solubility Guidelines for Ionic Compounds

Precipitation reactions occur when pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid.

The solubility of a substance is the amount of the substance that can be dissolved in a given quantity of solvent at the given temperature. In the case of an insoluble substance the attraction between the ions in the ionic solid is too great for the water molecules to separate the ions.

All common ionic compounds of the alkali metal ions (group 1A of the PTE) and of the ammonium ion (NH4+) are soluble in water.

PRECIPITATION REACTIONS Solubility Guidelines for Ionic CompoundsExperimental observations, however, have led to guidelines for predicting solubility for ionic compounds. E.g. all common ionic compounds that contain the nitrate anion, NO3-, are soluble in water.

Solubility guidelines for common ionic compoundsSoluble Ionic Compounds Important ExceptionsCompounds containing NO3- None

CH3COO- None Cl- Compounds of Ag+, Hg22+, and Pb2+ Br- Compounds of Ag+, Hg22+, and Pb2+

I- Compounds of Ag+, Hg22+, and Pb2+

SO42- Compounds of Sr2+, Ba2+, Hg22+, and Pb2+

Insoluble Ionic Compounds Important Exceptions Compounds containing S2- Compounds of NH4+, the alkali metal cations, Ca2+, Sr2+,

and Ba2+CO32- Compounds of NH4+ and the alkali metal cations PO43- Compounds of NH4+ and the alkali metal cations OH- Compounds of NH4+, the alkali metal cations, Ca2+, Sr2+,

and Ba2+

PRECIPITATION REACTIONS Solubility Guidelines for Ionic CompoundsTo predict whether a precipitate, we must

1.note the ions present in the reactants2.consider the possible cation-anion combinations,3.use the “Solubility guidelines for common ionic compounds” table to determine if any of these combinations is insoluble.

E.g. Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed?

Both substances are soluble ionic compounds and strong electrolytes (see the “Solubility guidelines for common ionic compounds” table).Mixing the solutions first produces a solution containing Mg2+, NO3-, Na+, and OH- ions.

Will either cation interact with either anion to form an insoluble compound?

Knowing that Mg(NO3)2 and NaOH are both soluble in water, our only possibilities are Mg2+ with OH- and Na+ with NO3-. From the table we see that hydroxides are generally insoluble. Because Mg2+ is not an exception, Mg(OH)2 is insoluble and thus forms a precipitate. NaNO3, however, is soluble, so Na+ and NO3- remain in solution.

The balanced equation for the precipitation reaction is

Mg(NO3)2 (aq) + NaOH (aq) ⟶ Mg(OH)2 (s) + 2 NaNO3 (aq)

PRECIPITATION REACTIONS PRACTICE EXERCISE

• Classify these ionic compounds as soluble or insoluble in water: (a) sodium carbonate, Na2CO3

(b) lead sulfate, PbSO4

SOLUTION We are given the names and formulas of two ionic compounds and asked to predict whether they are soluble or insoluble in water.

We can use the “Solubility guidelines for common ionic compounds” table to answer the question. Thus, we need to focus on the anion in each compound because the table is organized by anions.

(a) According to the table, most carbonates are insoluble. But carbonates of the alkali metal cations (such as sodium ion) are an exception to this rule and are soluble. Thus, Na2CO3 is soluble in water.

(b) Although most sulfates are water soluble, the sulfate of Pb2+ is an exception. Thus, PbSO4 is insoluble in water.

• Classify the following compounds as soluble or insoluble in water: (a) cobalt(II) hydroxide(b) barium nitrate(c) ammonium phosphate.

PRECIPITATION REACTIONS (Exchange Reactions)

In the previous reaction

the reactant cations exchange anions (Mg2+ ends up with OH-, and Na+ ends up with NO3-).

Reactions in which cations and anions appear to exchange partners conform to the general equation


AX + BY ⟶ AY + BX


AgNO3 (aq) + KCl (aq) ⟶ AgCl (s) + KNO3 (aq)

Such reactions are called either exchange reactions and precipitation reactions conform to this pattern.


• (a) Predict the identity of the precipitate that forms when aqueous solutions of BaCl2 and K2SO4 are mixed. (b) Write the balanced chemical equation for the reaction.

SOLUTION We are given two ionic reactants and asked to predict the insoluble product that they form.

We need to write the ions present in the reactants and exchange the anions between the two cations. Once we have written the chemical formulas for these products, we can use the “Solubility guidelines for common ionic compounds” table to determine which is insoluble in water.

Knowing the products also allows us to write the equation for the reaction.

(a) The reactants contain Ba2+, Cl-, K+, and SO42- ions. Exchanging the anions gives us BaSO4 and KCl. According to the “Solubility guidelines for common ionic compounds” table, most compounds of SO42- are soluble but those of Ba2+ are not. Thus, BaSO4 is insoluble and will precipitate from solution. KCl is soluble.

(b) From part (a) we know the chemical formulas of the products, BaSO4 and KCl. The balanced equation is:

BaCl (aq) + K2SO4 (aq) ⟶ BaSO4 (s) + 2 KCl (aq)

BaCl (aq) + K2SO4 (aq) ⟶ BaSO4 (s) + KCl (aq)


• (a) What compound precipitates when aqueous solutions of Fe2(SO4)3 and LiOH are mixed? (b) Write a balanced equation for the reaction.

• (c) Will a precipitate form when solutions of Ba(NO3)2 and KOH are mixed?

Answers:

(a) Fe(OH)3, (b) Fe2(SO4)3(aq) + 6 LiOH (aq) ⟶ 2 Fe(OH)3 (s) + 3 Li2SO4 (aq)

(c) no (both possible products, Ba(OH)2 and KNO3, are water soluble)


• Using the “Solubility guidelines for common ionic compounds” table, predict whether each of the following compounds is soluble or insoluble in water:

A. MgBr2

B. PbI2C. (NH4)2CO3

D. Sr(OH)2

E. ZnSO4

• Write balanced equations for the reactions that occur in each of the following cases:Cr2(SO4)3(aq) + (NH4)CO3 (aq) ⟶ __________________Ba2(NO3)2(aq) +K2SO4 (aq) ⟶ __________________Fe(NO3)2(aq) + KOH (aq) ⟶ __________________

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Reactions in aqueous solutions - Simone Damiano · 116 CHAPTER 4 Reactions in Aqueous Solution 4.1 | GENERAL PROPERTIES OF AQUEOUS SOLUTIONS A solution is a homogeneous mixture of