CHAPTER 7: REACTIONS INAQUEOUS SOLUTIONS

INTRODUCTION

Water is a good solvent in the chemical laboratory because many compounds are soluble inwater. There are many chemical reactions which occur in water and you will study some of themin this chapter. To predict what will happen when two compounds that are dissolved in water aremixed, you will need to learn some rules. These rules will help you predict what kinds ofcompounds are soluble in water, and what kinds of products you can make.

Water is also an important part of our environment, and many common reactions take place inwater. For example, the rusting of iron takes place in water. For these reasons, a whole chapteris devoted to what happens when substances are added to water.

GOALS FOR THIS CHAPTER

1. Know the four common forces that will often cause a reaction to happen. (Section 7.1)2. Know what happens when an ionic compound is dissolved in water. (Section 7.2)3. Be able to decide what products will form when two water-soluble ionic compounds are

mixed. (Section 7.2)4. Learn the rules that help predict which ionic salts are soluble in water, and which are not.

(Section 7.2)5. Be able to write the molecular equation, the complete ionic equation, and the net ionic

equation for the reactions of two ionic compounds dissolved in water. (Section 7.3)6. Be able to recognize reactions between strong acids that contain H+ ion and strong bases

that contain OH- ion and that always produce water as a product. (Section 7.4)

QUICK DEFINITIONS

Precipitate

Strong electrolyte

Soluble

Insoluble

The solid that forms during some chemical reactions, calledprecipitation reactions. (Section 7.2)

A substance which breaks apart completely into ions when it isdissolved in water. (Section 7.2)

A property of substances that dissolve completely in water.(Section 7.2)

A property of substances that do not dissolve in water, or so little ofwhich dissolves that the substance does not seem to disappear.(Section 7.2)

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Slightly soluble The same as insoluble. (Section 7.2)

Molecular equation A chemical equation that shows the reactants and products ascomplete molecules. (Section 7.3)

Complete ionic equation A chemical equation that shows the reactants and products that arestrong electrolytes as though they were ions in solution. (Section7.3)

Net ionic equation A chemical equation that shows only ions which react to formproducts, and the products which are formed. (Section 7.3)

Spectator ions Ions that are present during a chemical reaction, but which do notparticipate in the reaction. (Section 7.3)

Acid Defined by Arrhenius as a substance that produces hydrogen ionswhen dissolved in water. (Section 7.4)

Strong acids Acids that completely break apart into ions when dissolved in water.(Section 7.4)

Base Defined by Arrhenius as a substance that produces hydroxide ionswhen dissolved in water. (Section 7.4)

Strong bases Bases that completely break apart into ions when dissolved in water.(Section 7.4)

Salt An ionic compound that dissolves in water. (Section 7.4)

PRETEST

1. What ions are produced when the compounds below are dissolved in water?

2. Predict whether or not each of the compounds below is soluble in water.

a. Pb(N03)2

b. CaSc. LiCl

Pretest 117

3. Write the molecular equation for the word description of the following reaction. Anaqueous solution of potassium carbonate is mixed with an aqueous solution of Zn(N03)2'

A solid product is formed.

4. What is the complete ionic equation for the reaction below?

3 NaOH(aq) + FeCI3(aq) --..... Fe(OH)is) + 3 NaCI(aq)

5. Complete and balance the equation below and identify the spectator ions.

CaCI2(aq) + NazS(aq) --.....

6. Write the net ionic equation for the reaction below.

3 HzS(aq) + 2 AICI3(aq) --..... A12Sls) + 6 HCI(aq)

7. What salts in aqueous solutions could you mix together to produce the solids below?

a. FeCr04

b. CUzS

8. Are the substances below strong acids, strong bases, or neither?

a. KN03

b. KOH

9. What is the complete ionic equation for the reaction of aqueous solutions of sodiumhydroxide and sulfuric acid?

10. What is the net ionic equation for the reaction of aqueous solutions of sodium hydroxideand sulfuric acid?

PRETEST ANSWERS

1.

2.

a.

b.

a.b.c.

2 K+and Cr ° 2­2 7

Fe3+ and 3 cr

water soluble (rule 2)not soluble (rule 6)water soluble (rule 3)

118 7 Reactions in Aqueous Solutions

5. The spectator ions are Na+and Cr.

CaClz(aq) + NazS(aq) ....... CaS(s) + 2 NaCI(aq)

7. There are other correct answers besides the ones given below. If your answer is different,use the solubility rules to help you decide whether or not your answer is correct.

a. Fe(N03)z(aq) and NazCrOiaq)

b. CuClz(aq) and KzS(aq)

8. a.b.

KN03 is a salt. In water it produces K+ and N03- ions.

KOH is a strong base. In water it produces K+ and OH- ions.

10. H+(aq) + Ol-l'(aq) ....... Hz0(l)

CHAPTER REVIEW

7.1 PREDICTING WHETHER A REACTION WILL OCCUR

When You Mix Two Substances Together, How Can You Tell Whether They Will React?

When you mix two substances together, if anyone of four common results or processes canhappen, then a reaction will probably occur. The four results that indicate a reaction will takeplace are the formation of a solid, the formation of water, the formation of a gas, and the transferof electrons. For example, when AgN03 is mixed with NazS04' solid AgZS04 could form. Areaction is likely to occur.

7.2 REACTIONS IN WIDCR A SOLID FORMS

How Can You Tell Whether a Solid Will Form When Two Solutions Are Mixed?

When we dissolve a water-soluble ionic compound in water, the compound breaks apart intoindividual ions. The ions are free to move around in the water solution.

Chapter Review 119

-::::. ::::>

oJ -'Na+ sd- Na+ Pb2+ NO;4

Na+ SO~-NO; Pb2+

Na+ NO; NO;

In a solution of N~S04' Na+ ions only come into contact with sot ions. In a solution ofPb(N03)z, Pbz+ ions only come into contact with N03- ions. After the solutions are mixed, eachkind of ion can come in contact with three other kinds of ions. Some of the new ioncombinations react to form a white solid.

Na+ + sot + Pbz+ + N03- -... white solid

What combinations of ions are possible? Na+ ions could combine with sot ions to formN~S04' Pbz+ ions could combine with N03- ions to form Pb(N03)z. Two other combinationsare possible. Na+ ions could combine with N03- ions to form NaN03 and Pbz+ ions couldcombine with sot ions to form PbS04. We already know that NaZS04 and Pb(N03)z do notform solids in water, because our original solutions contained these ions, and neither of themproduced a solid when NazS04 or Pb(N03)z were dissolved in water. If a solid forms, it must bemade of Na+and N03- ions or of Pbz+ and SO4z- ions.

How do we know which combination of ions results in the white solid? To determine this, youmust learn the rules of solubility presented in Table 7.1 of your text and reproduced here.

Simple Rules for Solubility ofIonic Compounds (Salts) in Water

1. Most nitrate (NO; ) salts are soluble.

2. Most salts of Na" , K+, and NHl are soluble.

3. Most chloride salts are soluble. Notable exceptions are AgCl, PbClz, and Hg, Clz.

4. Most sulfate salts are soluble. Notable exceptions are BaS04, PbS04, and CaS04.5. Most hydroxide salts are only slightly soluble. The important soluble hydroxides are

NaOH, KOH, and Ca(OHh.

6. Most sulfide (Sz-), carbonate (C03z- , and phosphate (P043-) salts are only slightlysoluble.

Using rules 1 and 2 of this table, NaN03 will be soluble in water. So, the white solid cannot beNaN03. Rule 4 says that most sulfate salts are soluble, with the exception of PbS04' The whitesolid must be PbS04•

120 7 Reactions in Aqueous Solutions

What Is the Formula of the Solid?

The following steps will help you determine the formula of the solid that forms during achemical reaction. First, write all possible formulas for ion pairs, remembering that an anionmust pair with a cation. Second, eliminate those ion pair combinations that were present togetherin your original solutions. These ion pairs are soluble in water; they do not form solids. Third,use the solubility rules to eliminate the remaining ion pairs, and thus identify the solid.

Example:

Aqueous solutions of CaCl2 and N~C03 are mixed. A white solid forms. What is theinsoluble product of the reaction? The possible ion pairs are Ca2

+ + 2Cr, Ca2+ + CO/,

Na++ cr and 2Na+ + C032-. Two of these ion pairs were present in the original aqueous

solutions, so they cannot be solid products. The ion pairs which remain are Ca2+ + C032­

and Na+ + Cr. The solubility rules tell us that NaCl will be soluble in water (rules 1 and 3)

and that CaC03will not be soluble in water (rule 6). The solid product is CaC03.

7.3 DESCRIBING REACTIONS IN AQUEOUS SOLUTIONS

What Are the Different Ways You Can Write Chemical Equations?

We can write the molecular formula for each reactant and product compound, ignoring the factthat the reactants and products may consist of individual ions. Use the symbol (aq) after eachmolecular formula to indicate those compounds in aqueous solution that are ions. The symbol (s)tells you which of the molecular formulas is the solid product. This way of writing chemicalequations is called the molecular equation. An example is

Molecular formulas are not very accurate portrayals of reactions because soluble ioniccompounds actually exist as individual ions when they are in aqueous solution.

Another way to write chemical equations is to write each ion in the reactants separately, and eachion in the product separately. Insoluble ionic products are shown with their molecular formulas.Use the symbol (aq) for aqueous ions, and (s) to indicate the solid product. For example,

This way of writing chemical equations is called the complete ionic equation. Complete ionicequations are accurate representations of what kinds of particles are present in solution beforeand after the reaction.

Chapter Review 121

Another way to write chemical equations is to show on the reactant side only those ions that willform a product when mixed together, and on the product side only the molecular formula of theproduct or products. All the other ions are left out of the equation. This is called the net ionicequation. For example

The net ionic equation is useful because it allows you to see easily what is reacting and what isproduced. But don't forget that the reactant solution can't contain just Ag+ ions, or just P04-

3

ions. Each reactant solution must contain both cations and anions. Any time there is a cationpresent, an anion must accompany it; and any time there is an anion, there must also be a cation.

7.4 REACTIONS THAT FORM WATER: ACIDS AND BASES

What Kinds ofIonic Compounds Form Water When Mixed Together?

Some ionic compounds, when dissolved in water, break completely apart into ions where thecation is always the hydrogen ion (H+). The anion can be one of several. These ioniccompounds are the strong acids. They are called strong acids because they completely breakapart into hydrogen ions, and an anion. We will discuss the properties and definitions of acids inmore detail in Chapter 17. For now, you should learn the formulas of the most common strongacids, hydrochloric acid (HCI), sulfuric acid (H2S04) and nitric acid (HN03)'

Another kind of ionic compound, when dissolved in water, completely breaks apart into a cationand the hydroxide ion (OH"). The cation can be one of several. These ionic compounds are thestrong bases. They are strong bases because they break completely apart into cations andhydroxide ions when they are dissolved in water. The most common strong bases are NaOH andKOH. More information about the strong bases can be found in Chapter 17.

A solution containing the strong acid HN03 dissolved in water contains H+ ions and N03- ions.Another solution containing the strong base NaOH dissolved in water contains Na+ions and OH­ions. What happens when the two solutions are mixed together? New combinations of ions willcome in contact with each other and mix. What are all the possible ion combinations? Keep inmind that an anion must pair with a cation. H+ ions can pair with N03- ions, and Na+ ions canpair with OH- ions. These are the ion pairs present in our original solutions. They arecompletely soluble in water, and do not form any new products. The other possible ion pairswhich can form when the ions mix in the new solution are NaN03 and HOH <H2O). Na+ andN03- ions mix in solution, but don't form a solid product. They form a soluble salt. Accordingto the rules of solubility in Table 7.1, compounds composed of Na+ ions and N0:3- ions areusually soluble in water. But when H+ ions and OH- ions mix together, a new product is formed,water. A reaction has occured between the H+ion and the OH- ions. There are no longer W ionsand OH- ions floating in solution; instead, the product water has formed.

122 7 Reactions in Aqueous Solutions

All of the strong bases contain OR- ions. All of the strong acids contain R+ ions. Wheneverstrong acids and strong bases are mixed together, the product is always water. The cation whichaccompanied the OR- ion and the anion which accompanied the ~ ion remain in solution as asoluble salt. Water and a salt are always the products when a strong acid and a strong base aremixed together.

LEARNING REVIEW

1. Which one of the following does not tend to drive a reaction to produce products?

a. formation of a gasb. transfer of electronsc. color changed. formation of watere. formation of a solid

2. Write the formulas for the ions which are formed when these ionic compounds aredissolved in water. Row many of each kind of ion are produced for each moleculedissolved?

a. (NH4)2S04b. KN03

c. ·N~Cr207

d.e.f.

MgCl2Li3P0 4

AI(N03)3

3. When predicting a product for the reaction between two ionic compounds, we can alwayseliminate some of the ion pairs as possible products. Give a reason for eliminating each ofthe pairs as a product of the reaction below.

a.b.c.

Ag+,Na+Na+,N03­NO - PO 3-

3'· 3

4. Use the solubility rules to predict the water solubility of each of the following compounds.

a.b.c.d.

N~S

PbCl2~S04

(NH4) 2Cr04

e.f.g.h.

Pb(OH)2

Ca(N°3)2B~(P04)2

ZnC~

Learning Review 123

5. For each word description, write the balanced molecular equation, and identify the productof the reaction.

a. Aqueous solutions of sodium sulfate and lead(II) nitrate are mixed. One of theproducts is a white solid.

b. Aqueous solutions of potassium hydroxide and nickel(ll) chloride are mixed. Oneof the products is a green solid.

c. Aqueous solutions of potassium sulfide and zinc nitrate are mixed. A pale yellowsolid is produced.

d. Aqueous solutions of silver nitrate and ammonium phosphate are mixed. A whitesolid is produced.

6. For each of the balanced equations below, write the complete ionic equation.

a. 3CaClzCaq) + 2Na3POlaq) -... Ca3(P04)Z(s) + 6NaCI(aq)b. Cu(N03)z(aq) + KzS(aq) -... CuS(s) + 2KNOiaq)c. 2AgNOiaq) + KzSOiaq) .......... AgzSOis) + 2KN03(aq)

7. Complete and balance the equations below and identify the spectator ions.

a. Ca(N03)z(aq) + KzSOlaq) ..........b. (NH4)ZC03(aq) + CuClz(aq) ..........c. NaOH(aq) + Pb(N03)z(aq) -...d. NazS(aq) + Zn(N03)z(aq) -...e. CoClz(aq) + Ca(OH)z(aq) ..........

8. Write the net ionic equation for each reaction.

a. KZC03(aq) + CaClz(aq) .......... CaC03(s) + 2KCI(aq)b. Pb(N03)z(aq) + (NH4)zS(aq) 2NH4N03(aq) + PbS(s)c. 2LiCI(aq) + 2Hgz(N03)z(aq) HgzC!z(s) + 2LiN03(aq)d. 2NaOH(aq) + MgClz(aq) Mg(OH)z(s) + 2NaCI(aq)

9. What salts in aqueous solutions could you mix together to produce the solids below?

a.b.c.

Zn(OHh

B~(P°4)Z

PbClz

d.e.f.

CaS04

CoC03AgzS04

124 7 Reactions in Aqueous Solutions

10. Which of the substances below are strong acids, which are strong bases, and which areneither of these?

a.b.c.

d. HCIe. NaCI

f. ~S04

11. Write complete ionic equations for the reactions below.

a. Sodium hydroxide reacts with sulfuric acid.b. Hydrochloric acid reacts with potassium hydroxide.c. Nitric acid reacts with sodium hydroxide.

12. Write net ionic equations for each reaction in Problem 11.

13. Which of the reactions below are acid/base reactions?

a. ~SOiaq) + Pb(N03)(aq) -... PbSOis) + 2KN03(aq)

b. KOH(aq) + HN03(aq) -... KN03(aq) +~OO)c. ~SOiaq) + 2NaOH(aq) -... N~SOiaq)+ 2H20(l)

d. N~COlaq) + CoC~(aq) -... CoC03(s) + 2NaCI(aq)

ANSWERS TO LEARNING REVIEW

1. Only "c", color change, does not tend to make a reaction occur.

2. a. 2 NH + 1 SO 2- d. 1 Mg2+ 2 cr4 4b. 1 K+ 1 NO- e. 3 Li+ 1 PO 3-

3 4c. 2 Na+ 1 Cr2O/- f. 1 At3+ 3 NO-3

3. a.

b.c.

Both Ag+ and Na+ are cations. An anion and a cation are needed to form a neutralproduct.NaN03 is soluble in water, and so it exists in solution as Na+ ions and N03- ions.Both N03- and pot are anions. An anion and a cation are needed to form a neutralproduct.

4. a.b.c.d.

water soluble (rule 2)not soluble (rule 3)water soluble (rule 2)water soluble (rule 2)

e. not soluble (rule 5)f. water soluble (rule 1)g. not soluble (rule 6)h. water soluble (rule 3)

Answers to Learning Review 125

PRACTICE EXAM

1. When the materials below are mixed together, in which pair would a reaction not occur?

a. HN03 + NaOH ----b. Na:zS04 + CaCI2----c. Pb(N03}z + NH4CI ----d. HCI+KOH----e. KzS04 + NaN°3 ----

2. Which of the following is a strong electrolyte?.I

a. Zn(OH)2b. Hz°c. Ca(N03)2

d. PbCl2e. Ba3(P04)2

3. Which pair of ions will react to form a solid product when aqueous solutions of copper(II)sulfate and sodium nitrate are mixed?

a. sot and N03-

b. Cu2+ and 2N03-

c. 2Na+ and sotd. Cu2+ and 2Na+

e. No product would form when solutions containing these ions are mixed.

4. Which of the ionic compounds below is soluble in water?

a. CuC03

b. Na:zCr04

c. Pb(OH)2d. CaS04

e. HgS

128 7 Reactions in Aqueous Solutions

9. Which is neither a strong acid nor a strong base?

a. HN03

b. KOH

c. HzS0 4d. HCI

e. KCI

10. Which represents the correct net ionic equation for the reaction between a strong acid and astrong base?

a. HCI(aq) + NaOH(aq) -lIIo- NaCI(aq) + HzQ(l)b. HCI(aq) + NaOH(aq) -lIIo- Na++ OH- + lb0(l)c. W + OH- -lIIo- Hz0(l)d. H+ +cr~ HCI(aq)e. Na+ -+' Cl'~ NaCI(aq)

PRACTICE EXAM ANSWERS

1. e (7.2)2. c (7.2)3. e (7.2)4. b (7.2)5. a (7.2)6. b (7.3)7. a (7.3)8. c (7.3)9. e (7.4)10. c (7.4)

130 7 Reactions in Aqueous Solutions