11.1 Describing Chemical Reactions 11t1lara.weebly.com/uploads/1/6/3/2/1632178/ch11bookpdf.pdf ·...

Chemical Reactions 321

Print• Guided Reading and Study Workbook,

Section 11.1• Core Teaching Resources,

Section 11.1 Review• Small-Scale Chemistry Laboratory

Manual, Lab 14• Transparencies, T113–T117

Technology• Interactive Textbook with ChemASAP,

Animation 12; Simulation 11; Problem-Solving 11.2, 11.4, 11.6; Assessment 11.1

• Go Online, Section 11.1

11.1

FOCUSObjectives11.1.1 Describe how to write a word

equation.11.1.2 Describe how to write a skele-

ton equation11.1.3 Describe the steps for writing a

balanced chemical equation.

Guide for Reading

Build VocabularyParaphrase Write the following chemi-cal equation on the board and have stu-dents provide synonymous words or phrases for each of the vocabulary words as they describe the equation:

Reading StrategySequence As students read this sec-tion, have them write in their own words the sequence of steps that need to be taken to write and balance a chemical equation.

INSTRUCT

Have students study the photograph and read the text. Explain that airships during the 1930s often contained hydrogen because it is the lightest ele-ment. However, when hydrogen is ignited in air, a vigorous reaction occurs. Have students describe the results as shown in the photograph. Write the word equation for the reac-tion of hydrogen with oxygen to pro-duce water. Ask, Do you think there is a simpler way to write chemical equations? (Yes, describe the reaction using chemical symbols of the elements and chemical formulas of compounds.)

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MnO22H2O2 2H2O + O2

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Answers to...Figure 11.1 The separate ingredi-ents of the bread are now combined into one product—the loaf of bread.

Connecting to Your World

Section 11.1 Describing Chemical Reactions 321

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11.1 Describing Chemical Reactions

On May 6, 1937, the huge air-ship Hindenburg was heading for its landing site in Lakehurst, New Jersey,

after an uneventful trans-Atlantic crossing. Suddenly, to the horror of observers on the ground, the airship

erupted into a fireball. Within a short time, 210,000 cubic meters of the airship’s lifting

gas, hydrogen, had burned and the airship was destroyed. The chemical reaction

that occurred can be described as “hydrogen combines with oxygen to

produce water.” In this section, you will learn to represent this chemical reaction

by a chemical equation.

Guide for Reading

Key Concepts• How do you write a word

equation?• How do you write a skeleton

equation?• What are the steps in writing a

balanced chemical equation?

Vocabularychemical equation

skeleton equation

catalyst

coefficients

balanced equation

Reading StrategyRelating Text and Visuals As you read this lesson, look carefully at the illustrations of equations. In your notebook, explain how the illustrations demonstrate the difference between a balanced and unbalanced chemical equation.

Writing Chemical EquationsEvery minute of the day chemical reactions take place—both inside youand around you. Not all are as dramatic as the explosion of the Hinden-burg, but many are more complex. After a meal, a series of chemical reac-tions take place as your body digests food. Similarly, plants use sunlight todrive the photosynthetic processes needed to produce plant growth.Although the chemical reactions involved in photosynthesis and digestionare different, both chemical reactions are necessary to sustain life.

In a chemical reaction, one or more substances (the reactants) changeinto one or more new substances (the products). Figure 11.1a shows theingredients for making leavened bread—flour, salt, yeast, and water. Theseare the reactants in a chemical reaction that takes place when the ingredi-ents are mixed together and heated in the oven. Figure 11.1b shows theproduct—a loaf of bread. The four ingredients (reactants) were changed tobread (the product). Like all chemical reactions, baking bread involveschanging substances. Chemists use a chemical equation—a quick, short-hand notation—to convey as much information as possible about whathappens in a chemical reaction.

Figure 11.1 Chemical changes occur when bread is baked. Flour, salt, yeast, and water are the ingredients for making leavened bread. The reactants (ingredients) undergo chemical changes to form the product (baked bread). Observing What evidence shows that chemical changes have occurred?

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Section Resources

322 Chapter 11

Section 11.1 (continued)

Writing Chemical Equations

DiscussTo assess students’ prior knowledge about chemical compounds, ask, What are the differences between molecu-lar compounds and ionic com-pounds? (Molecular compounds are composed of nonmetallic elements. Ionic compounds are composed of a metal cation and an anion.) How are chemi-cal compounds represented? (By chemical formulas that show the kinds and numbers of atoms in each molecule or formula unit.) What is the differ-ence between a monatomic ion and a polyatomic ion? (A monatomic ion is formed when an atom gains or loses one or more electrons. A polyatomic ion is a tightly bound group of atoms that behaves as a unit and carries a charge.)

Use VisualsFigure 11.1 Have students study the photographs. Ask, What are some indications that a chemical reaction has taken place? (increase in volume and change in color) Explain that bakers use yeast to make bread dough rise. Yeasts are unicellular organisms that are able to extract energy from sugar in the absence of oxygen, a process known as fermentation. Point out that yeasts produce proteins that catalyze or facilitate the breakdown of carbohy-drates in the dough, producing CO2 and ethanol. The bubbles of trapped CO2 cause the dough to rise. As the dough bakes, the ethanol evaporates.

Download a worksheet on Chemi-cal Equations for students to com-plete, and find additional teacher support from NSTA SciLinks.

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English LearnersAs they read through the chapter, Spanish-speaking students with limited English profi-ciency may find it helpful to consult the chapter summary and key terms in the Span-ish supplement in the Teacher’s Resource Package.

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Differentiated Instruction

322 Chapter 11

Figure 11.2 Three common chemical reactions are shown.

When methane gas burns, it combines with oxygen to form carbon dioxide and water.

Iron turns to red-brown rust (iron(III) oxide) in the presence of oxygen in the air. Hydrogen peroxide decomposes to water and oxygen when used as an antiseptic.

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Word Equations How do you describe what happens in a chemical reac-tion? Recall from Chapter 2 the shorthand method for writing a descriptionof a chemical reaction. In this method, the reactants were written on theleft and the products on the right. An arrow separated them. You read thearrow as yields, gives, or reacts to produce.

Reactants ¡ products

How could you describe the rusting of iron shown in Figure 11.2b? Youcould say: “Iron reacts with oxygen to produce iron(III) oxide (rust).” That’sa perfectly good description, but it might be quicker and easier to identifythe reactants and product by means of a word equation.

Iron � oxygen ¡ iron(III) oxide

To write a word equation, write the names of the reactants to the leftof the arrow separated by plus signs; write the names of the products tothe right of the arrow, also separated by plus signs. Notice that no plussign is needed on the product side of this equation because iron(III) oxideis the only product.

Have you ever poured the antiseptic hydrogen peroxide on an open cut?Bubbles of oxygen gas form rapidly, as shown in Figure 11.2c. The produc-tion of a new substance, a gas, is evidence of a chemical change. Two newsubstances are produced in this reaction, oxygen gas and liquid water. Youcould describe this reaction by saying, “Hydrogen peroxide decomposes toform water and oxygen gas.” You could also write a word equation.

Hydrogen peroxide ¡ water � oxygen

When you light a burner on your stove, methane gas bursts into flamesand produces the energy needed to heat your soup. Methane is the majorcomponent of natural gas, a common fuel for heating homes and cookingfood. The burning of methane, as shown in Figure 11.2a, is a chemical reac-tion. How would you write the word equation for this reaction? Burning asubstance typically requires oxygen, so methane and oxygen are the reac-tants. The products are water and carbon dioxide. Thus the word equationis this:

Methane � oxygen ¡ carbon dioxide � water

Checkpoint What does the arrow (¡¡¡¡) in a word equation mean?

For: Links on Chemical Equations

Visit: www.SciLinks.orgWeb Code: cdn-1111

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Use VisualsTable 11.1 Use an overhead projector to display Table 11.1. Make sure stu-dents understand the meaning and role of each symbol. Point out that the phase symbols provide important clues about reactions. Note that the items placed above the yield arrow represent conditions that must be met before the reaction can take place at a reasonable pace.

Use VisualsFigure 11.3 Point out that although hydrogen peroxide contains the same kinds of elements as water, it has very dif-ferent properties. Hydrogen peroxide is much less stable than water. Have stu-dents write the balanced chemical equa-tion for the reaction described in the caption. Students should indicate the MnO2 catalyst above the reaction arrow. Write the equation on the board and explain that this is an example of a decomposition reaction—a single com-pound is broken down into two or more products. Explain that although hydro-gen peroxide is an unstable compound that readily decomposes to produce water and oxygen, it does so slowly at room temperature. Therefore, hydrogen peroxide must come in contact with a catalyst in order for the decomposition to proceed rapidly, as shown in b.

RelateTell students that frequently in nature the products of one reaction become the reactants of a subsequent reaction. An example is the combustion of octane (C8H18), the primary ingredient of gasoline, which produces carbon dioxide and water vapor. The carbon dioxide can combine with atmospheric water vapor to produce carbonic acid (H2CO3), which is a component of acid rain. Ask students if they can think of other such processes.

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Answers to...

Checkpoint

The arrow means yields, gives, or reacts to produce.

Hydrogen PeroxideChallenge students to propose a hypothesis about why our bodies produce an enzyme that helps catalyze the decomposition of hydrogen peroxide. (to prevent the accumula-tion in the body of hydrogen peroxide, a toxic by-product of reactions involving oxygen.) Stu-

dents may be interested to learn that hydro-gen peroxide is sometimes used to restore the clarity of old paintings. Lead-based paints darken with time (PbS). Hydrogen per-oxide converts PbS to PbSO4.

Facts and Figures


withChemASAP

Chemical Equations Word equations adequately describe chemicalreactions, but they are cumbersome. It’s easier to use the formulas for thereactants and products to write chemical equations. A chemical equation isa representation of a chemical reaction; the formulas of the reactants (onthe left) are connected by an arrow with the formulas of the products (onthe right). Here is a chemical equation for rusting:

Fe � O2 ¡ Fe2O3

Equations that show just the formulas of the reactants and productsare called skeleton equations. A skeleton equation is a chemical equationthat does not indicate the relative amounts of the reactants and products.The first step in writing a complete chemical equation is to write the skele-ton equation. Write the formulas of the reactants to the left of theyields sign (arrow) and the formulas of the products to the right.

To add more information to the equation, you can indicate the physi-cal states of substances by putting a symbol after each formula. Use (s) for asolid, (l) for a liquid, (g) for a gas, and (aq) for a substance in aqueous solu-tion (a substance dissolved in water). Here is the equation for rusting withsymbols for the physical states added:

Fe(s) � O2(g) ¡ Fe2O3(s)

In many chemical reactions, a catalyst is added to the reaction mixture.A catalyst is a substance that speeds up the reaction but is not used up inthe reaction. A catalyst is neither a reactant nor a product, so its formula iswritten above the arrow in a chemical equation. For example, Figure 11.3shows that the compound manganese(IV) oxide (MnO2(s)) catalyzes thedecomposition of an aqueous solution of hydrogen peroxide (H2O2(aq)) toproduce water and oxygen.

Many of the symbols commonly used in writing chemical equationsare listed in Table 11.1.

H2O21aq 2 ¡MnO2 H2O1l 2 + O21g2

Figure 11.3 Hydrogen peroxide decomposes to form water and oxygen gas. Bubbles of oxygen appear slowly as decomposition proceeds.

With the addition of the catalyst manganese(IV) oxide (MnO2), decomposition speeds up. The white “smoke” is condensed water vapor.

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Animation 12 Relate chemical symbols and formulas to the information they communicate.

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Table 11.1

Symbols Used in Chemical Equations

Symbol Explanation

� Used to separate two reactants or two products

¡ “Yields,” separates reactants from products

∆ Used in place of ¡ for reversible reactions

(s) Designates a reactant or product in the solid state; placed after the formula

(l) Designates a reactant or product in the liquid state; placed after the formula

(g) Designates a reactant or product in the gaseous state; placed after the formula

(aq) Designates an aqueous solution; the substance is dis-solved in water; placed after the formula

Indicates that heat is supplied to the reaction

A formula written above or below the yield sign indicates its use as a catalyst (in this example, platinum).

D¡ heat¡Pt¡

324 Chapter 11


Special NeedsHave students, working in pairs, take turns writing equations for each other to balance. Two pairs of students may enjoy getting together for a “doubles match” in which each pair works as a team to balance equations that the opposing team creates.

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CONCEPTUAL PROBLEM 11.1

Answers1. When solid sodium is dropped in

water, hydrogen gas and aqueous sodium hydroxide are produced.

2. S(s) + O2(g) → SO2(g)

Balancing Chemical EquationsDiscussReview with students the law of con-servation of mass. Emphasize that mass, or matter, cannot be created or destroyed. Explain that it is because of this law that a chemical equation is not complete until it is balanced.

Students often think that they can bal-ance an equation by changing the sub-scripts in one or more of the formulas. Use the following example to show why this approach is incorrect. H2(g) + O2(g) → H2O(l) could be balanced by changing the formula of the product to H2O2. But H2O2 (hydrogen peroxide) is not the same substance as water. Therefore, the equation would describe a different reaction. To help students overcome this misconcep-tion, have them draw a box around the formulas for the reactants and prod-ucts before they start to balance the equation. Tell them that the boxes are “off-limits” or “out of bounds.” They cannot change any number that appears inside a box.

RelateGlass is often etched to provide a design. Glass contains calcium silicate, CaSiO3. When hydrofluoric acid, HF, is placed on the glass, it reacts with the calcium silicate, etching the glass by producing aqueous calcium fluoride, silicon tetrafluoride, and water. Have students write a balanced chemical equation for this reaction. (CaSiO3 + 6HF → CaF2 + SiF4 + 3H2O)

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324 Chapter 11

Practice Problems


Writing a Skeleton EquationHydrochloric acid and solid sodium hydrogen carbonate areshown before being placed in the beaker to react. The productsformed are aqueous sodium chloride, water, and carbon diox-ide gas. Write a skeleton equation for this chemical reaction.

Analyze Identify the relevant concepts.

Write the correct formula for each substance in the reaction. Separate the reactants from the products by means of an arrow. Indicate the state of each substance.

Solve Apply concepts to this situation.

solid sodium hydrogen carbonate: NaHCO3(s)hydrochloric acid: HCl(aq)aqueous sodium chloride: NaCl(aq)water: H2O(l)carbon dioxide gas: CO2(g)NaHCO3(s) � HCl(aq) ¡ NaCl(aq)� H2O(l) � CO2(g)

1. Write a sentence that describes this chemical reaction.Na(s) � H2O(l) ¡ NaOH(aq) � H2(g)

2. Sulfur burns in oxygen to form sulfur dioxide. Write a skeleton equation for this chemical reaction. Include appropriate symbols from Table 11.1.

Balancing Chemical EquationsHow would you write a word equation for the manufacture of bicycles?Simplify your task by limiting yourself to four major components: frames,wheels, handlebars, and pedals. Your word equation for making a bicyclecould read like this.

Your word equation shows the reactants (the kinds of parts) and theproduct (a bicycle). But if you were responsible for ordering parts to make abicycle, this word equation would be inadequate because it does not indi-cate the quantity of each part needed to make one bicycle.

A standard bicycle is composed of one frame (F), two wheels (W), onehandlebar (H), and two pedals (P). Using these symbols, the formula for abicycle would be FW2HP2. The skeleton equation for bicycle assemblywould be this:

F � W � H � P ¡ FW2HP2

This is an unbalanced equation. An unbalanced equation does notindicate the quantity of the reactants needed to make the product. A com-plete description of the reaction must include not only the kinds of partsinvolved but also the quantities of parts required.

Frame + wheel + handlebar + pedal ¡ bicycle(reactants) (product)

Practice Problems

withChemASAP

Problem-Solving 11.2Solve Problem 2 with the help of an interactive guided tutorial.


TEACHER DemoTEACHER Demo

An Example of Chemical ChangePurpose Students observe a dramatic example of chemical change.

Materials 150-mL Pyrex beaker, table sugar, hood, concentrated sulfuric acid

Procedure Fill a 150-mL Pyrex beaker to the 30-mL mark with table sugar. Place the beaker in a hood, and add 30 mL of concentrated sulfuric acid, which will dehydrate the sugar.

Safety and Disposal The hood will remove any sulfur dioxide and carbon monoxide, which are possible by-prod-ucts of the reaction. Wear goggles, a face shield, a lab apron, and protective gloves. Use a plexiglas shield to sepa-rate the students from the reaction. As a further precaution, have them wear goggles. While wearing protective gear, place the beaker and contents into a 500-mL beaker half full of water. Swirl the small beaker in the water and then neutralize using a base with a molarity of less than 1M. Decant the liquid and fill the large beaker halfway with water once again. Let stand over-night in a protected location, test for pH, and neutralize. Repeat if necessary. Wrap the carbon in newspaper before placing in a wastebasket.

Expected Outcome The carbon left behind will expand to form a porous, foam cylinder as the water vapor escapes.

DiscussTell students that it is not necessary for coefficients to be the same on both sides of the equation in order for the number of atoms of each type to bal-ance. Use the diagram of the formation of CO2 on the student page to illustrate this concept. The atoms balance, but the coefficients total 2 on the left side of the equation and 1 on the right side. If the sums of the coefficients on both sides of a balanced chemical equation are the same number, it is coincidental.

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Checkpoint

In any physical or chemical change, mass is con-served.

Gifted and TalentedPoint out to students that not all chemical equations are easily balanced. Methods other than those they currently use must be used to balance some equations, such as those they will encounter in Chapter 21.

Challenge students to balance the following equation.HCl + HNO3 → HOCl + NO + H2O (3HCl + 2HNO3 → 3HOCl + 2NO + H2O )

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This is a balanced equation for making a bicycle. It tells you that oneframe, two wheels, one handlebar, and two pedals produce one bicycle. Tobalance the equation, the number 2 was placed before wheels and pedals.The number 1 is understood to be in front of frame, handlebar, and bicycle.These numbers are called coefficients—small whole numbers that areplaced in front of the formulas in an equation in order to balance it. In thisbalanced equation, the number of each bicycle part on the reactant side isthe same as the number of those parts on the product side. A chemicalreaction is also described by a balanced equation in which each side of theequation has the same number of atoms of each element and mass is con-served. Real bicycles are being assembled in Figure 11.4.

Recall that John Dalton’s atomic theory states that as reactants areconverted to products, the bonds holding the atoms together are brokenand new bonds are formed. The atoms themselves are neither creatednor destroyed; they are merely rearranged. This part of Dalton’s theoryexplains the law of conservation of mass: In any chemical change, mass isconserved. The atoms in the products are the same atoms that were in thereactants—they are just rearranged.

Representing a chemical reaction by a balanced chemical equation is atwo-step process. To write a balanced chemical equation, first writethe skeleton equation. Then use coefficients to balance the equation sothat it obeys the law of conservation of mass. In every balanced equation,each side of the equation has the same number of atoms of each element.

Sometimes a skeleton equation may already be balanced. For example,carbon burns in the presence of oxygen to produce carbon dioxide.

This equation is balanced. One carbon atom and two oxygen atoms are oneach side of the equation. You do not need to change the coefficients; theyare all understood to be 1.

Checkpoint What is the law of conservation of mass?

Figure 11.4 If a bicycle factory runs out of any part needed for a bicycle, production must stop.

F � 2W � H � 2P FW2HP2

CO2(g)O2(g)C(s) �

� ⎯⎯→

⎯⎯→

Reactants

1 carbon atom, 2 oxygen atomsProduct

1 carbon atom, 2 oxygen atoms


326 Chapter 11


Quick LABQuick LAB

Removing Silver Tarnish

Objective After completing this activity, students will be able to:

• describe and write a balanced chemi-cal equation for a single-replacement reaction.

Skills Focus Observing, Inferring, Communicating Results

Prep Time 10 minutesClass Time 20 minutes

Expected Outcome Aluminum replaces the silver in the tarnish on the fork or spoon.

Analyze and Conclude1. After the reaction, the fork looks sil-very, and the tarnish is gone.2. The aluminum becomes darkened.3. Al2S34. 3Ag2S(s) + 2Al(s) → 6Ag(s) + Al2S3(s)

For EnrichmentHave students repeat the Quick Lab using a piece of magnesium instead of alumi-num foil. Have them compare the results and the speed of the reaction. Students will find that the reaction occurs more quickly with magnesium. After students learn about single-replacement reac-tions on page 333, have them explain why the reaction proceeded more rapidly with magnesium. (Magnesium is more active than aluminum.)

RelateTell students that one way of purifying air in space vehicles is to react lithium peroxide with carbon dioxide, produc-ing lithium carbonate and oxygen. Ask, What is the balanced chemical equa-tion for this reaction? (2Li2O2 + 2CO2 → 2Li2CO3 + O2)

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Special NeedsPolystyrene models can help students under-stand the meaning of subscripts and coeffi-cients when balancing chemical reactions. They can also help illustrate how atoms are rearranged during a chemical reaction. Use a knife or razor blade to cut polystyrene balls of various sizes in half. Paint the hemispheres

different colors to stand for various elements. Glue a small, flat magnet to the flat side of each of the hemispheres. Using the spheres, show on a metal board how formulas of reac-tants become formulas of products in vari-ous types of reactions.

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326 Chapter 11

Quick LABQuick LAB

Removing Silver Tarnish

Materials

• aluminum foil, 20 cm �20 cm

• large beaker or glass pan

• tarnished silver fork or spoon

• sodium hydrogen carbonate

• plastic tablespoon

• hot water

Procedure1. Fill the beaker about three-quarters full

of hot water and add 2 tablespoons of sodium hydrogen carbonate (NaHCO3).

2. Crush the aluminum foil into a loose ball and place it in the beaker.

3. Write a brief description of the tar-nished silver fork, then place it in the beaker so that it is touching the alumi-num ball.

4. Allow the beaker to stand undisturbed for 30 minutes.

5. Remove the fork and aluminum ball and rinse them with water.

Analyze and Conclude1. Compare the silver fork with your

observations before placing the fork in the water. What changes do you observe?

2. Did a chemical reaction occur? How do you know?

3. The tarnish on the silver fork is silver sulfide (Ag2S). Silver becomes tarnished when it is exposed to air, egg yolk, or rubber bands. Each of these substances contains sulfur. Look carefully for a pale yellow precipitate of aluminum sulfide on the bottom of the beaker. Write the formula for aluminum sulfide.

4. The unbalanced equation for the reac-tion is

Ag2S(s) � Al(s) ¡Al2S3(s) � Ag(s)

Balance the equation.

What about the equation for the reaction of hydrogen gas and oxygengas? This is the reaction that occurred in the Hindenburg disaster, whichyou read about in Connecting to Your World. When hydrogen and oxygenare mixed, a spark will initiate a rapid reaction. The product of the reactionis water. This is the equation for the burning of hydrogen:

The formulas for all the reactants and product are correct, but this equationis not balanced. Count the atoms on both sides of the equation. Two oxygenatoms are on the reactant (left) side of the equation and only one oxygenatom is on the product (right) side. As written, the equation does not obeythe law of conservation of mass and so it does not describe what really hap-pens. What can you do to balance it? A few guidelines for writing and bal-ancing equations will help.

H2(g)Hydrogen

O2(g)Oxygen

H2O(l )Water

�

�

⎯⎯→

⎯⎯→

Reactants

2 hydrogen atoms2 oxygen atoms

Product

2 hydrogen atoms1 oxygen atom

withChemASAP

Simulation 11 Sharpen your skills by balancing chemical equations.



Answers3. a. 2AgNO3 + H2S → Ag2S + 2HNO3

b. 3Zn(OH)2 + 2H3PO4 → Zn3(PO4)2 + 6H2O

4. a. H2 + S → H2Sb. 2FeCl3 + 3Ca(OH)2 →

2Fe(OH)3 + 3CaCl2

Practice Problems Plus1. Rewrite the following word equa-tion as a balanced chemical equa-tion: aluminum sulfate + calcium hydroxide →→→→ aluminum hydroxide + calcium sulfate. (Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4)2. Rewrite the following word equa-tion as a balanced chemical equa-tion: phosphoric acid + sodium hydroxide →→→→ sodium phosphate + water. (H3PO4 + 3NaOH → Na3PO4 + 3H2O)


Balance a Chemical EquationPurpose Students observe a chemical reaction and write skeleton and balanced chemical equations for the reaction.

Materials 100-mL graduated cylinder, 0.1M copper(II) chloride solution, 400-mL beaker, metric ruler, scissors, alumi-num foil

Safety CuCl2 is an irritant.

Procedure Add 200 mL of 0.1M cop-per(II) chloride solution to a 400-mL beaker. Cut a 5-cm square piece of alu-minum foil. Crumple the piece of alu-minum foil into a loose ball and place it in the copper solution. Ask students to note any evidence of chemical change. Write a sentence on the board that describes the chemical reaction. Have students write a skeleton equation for the reaction in their notebooks. Then have students balance the equation. Decant the liquid and flush down the drain with excess water. Dry the copper residue and use it again. Expected Outcome The solution will begin to produce gas and a red-brown precipitate of copper will fall to the bottom of the beaker. The skeleton equation is CuCl2(aq) + Al(s) → AlCl3(aq) + Cu(s). The balanced chemi-cal equation is 3CuCl2(aq) + 2Al(s) → 2AlCl3(aq) + 3Cu(s).

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English LearnersHave students name different elements in their native languages and in English. Some periodic table Internet sites list the names of elements in several languages other than English. Then have them list the symbols for several elements in their native languages.

Point out that the names might differ, but chemical symbols are the same in any lan-guage. Balancing equations uses only univer-sal symbols and numbers and is not language dependent.

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2H2(g)Hydrogen

2H2O(l )Water

O2(g)Oxygen

¡

¡

�

�

Reactants


Products


Rules for Writing and Balancing Equations1. Determine the correct formulas for all the

reactants and products.

2. Write the skeleton equation by placing theformulas for the reactants on the left andthe formulas for the products on the rightwith a yields sign (¡) in between. If twoor more reactants or products are involved,separate their formulas with plus signs.

3. Determine the number of atoms of eachelement in the reactants and products.Count a polyatomic ion as a single unit if itappears unchanged on both sides of theequation.

4. Balance the elements one at a time byusing coefficients. When no coefficient iswritten, it is assumed to be 1. Begin bybalancing elements that appear only onceon each side of the equation. Never bal-ance an equation by changing the sub-scripts in a chemical formula. Eachsubstance has only one correct formula.

5. Check each atom or polyatomic ion to besure they are equal on both sides of theequation.

6. Make sure all the coefficients are in thelowest possible ratio.


Writing a Balanced Chemical EquationHydrogen and oxygen react to form water. Thereaction releases enough energy to launch a rocket.Write a balanced equation for the reaction.


Apply the rules for balancing equations to theskeleton equation describing the reaction.


Write correct formulas to give the skeletonequation.

H2(g) � O2(g) ¡ H2O(l)

Count the number of each kind of atom. Hydrogen is balanced but oxygen is not. If you put the coefficient 2 in front of H2O, the oxygen will be balanced.

H2(g) � O2(g) ¡ 2H2O(l)

Now twice as many hydrogen atoms are in the product as are in the reactants. To correct this, put the coefficient 2 in front of H2. Four hydrogen atoms and two oxygen atoms are on each side of the chemical equation. The equation is now balanced.

Practice Problems

3. Balance each equation.a. AgNO3 � H2S ¡ Ag2S � HNO3

b. Zn(OH)2 � H3PO4 ¡ Zn3(PO4)2 � H2O

4. Rewrite these word equations as balanced chemi-cal equations.a. hydrogen � sulfur ¡ hydrogen sulfideb. iron(III) chloride � calcium hydroxide ¡

iron(III) hydroxide � calcium chloridewithChemASAP

Problem-Solving 11.4 Solve Problem 4 with the help of an interactive guided tutorial.


328 Chapter 11


Special NeedsProvide students with molecular model kits or Styrofoam balls of different sizes and col-ors. Using the balls to represent different types of atoms, have students model balanc-ing a simple equation. Provide students with several additional unbalanced chemical

equations, and have them balance the equa-tions using the models. Caution students to keep “atoms” in compounds together so that they use multiples of the entire unit and not multiples of just one atom in the unit.

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Answers5. a. FeCl3 + 3NaOH →

Fe(OH)3 + 3NaClb. CS2 + 3Cl2 → CCl4 + S2Cl2

6. Ca(OH)2 + H2SO4 → CaSO4 + 2H2O

DiscussEmphasize to students that in order to write a correct balanced chemical equation, the formulas for all reactants and products must be written cor-rectly. Write an unbalanced chemical equation on the board, with all the for-mulas written correctly. Rewrite the equation with one formula written incorrectly. Identify the incorrect for-mula so that students consistently rec-ognize that it is not correct. Have students balance the two equations and note the differences. Often an equation with an incorrect formula cannot be balanced.

RelatePoint out that balanced chemical equations can help manufacturers who use chemical processes to know how much of each reactant should be pur-chased and how much of each product will be made. For example, a welder might use oxygen and acetylene (C2H2) in a torch. Have students write a balanced chemical equation for the reaction that occurs in the torch. Water and carbon dioxide are produced. (2C2H2 + 5O2 → 4CO2 + 2H2O ) Ask, From this balanced equation, what would be an effective acetylene: oxygen ratio in the torch? (2:5)

Have students research chemistry-related careers in the library or on the Internet. Students can then con-struct a table that describes the nature of the work, educational and training requirements, employ-ment outlook, working conditions, and other necessary information.

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328 Chapter 11

Practice Problems

The reaction of copper metal with an aqueous solution of silver nitrateis described by this skeleton equation. How can it be balanced?

Cu(s) � AgNO3(aq) ¡ Cu(NO3)2(aq) � Ag(s)The nitrate ion appears unchanged on both sides of the equation, so thision can be balanced as a unit. Do this by placing a 2 in front of AgNO3.

Cu(s) � 2AgNO3(aq) ¡ Cu(NO3)2(aq) � Ag(s)But now the atoms of silver are not balanced. You must add a 2 in front of Agon the product side to balance the atoms of silver.

Cu(s) � 2AgNO3(aq) ¡ Cu(NO3)2(aq) � 2Ag(s)The equation is now balanced because the same number of each kind ofatom are on both sides of the equation.


Balancing a Chemical EquationAluminum is a good choice for outdoor furniture because itreacts with oxygen in the air to form a thin protective coat ofaluminum oxide. Balance the equation for this reaction.

Al(s) � O2(g) ¡ Al2O3(s)


Apply the rules for balancing equations.


First balance the aluminum by placing thecoefficient 2 in front of Al(s).

2Al(s ) � O2(g) ¡ Al2O3(s)How can the odd number of oxygen atoms in the product (right side) balance the even number of oxygen atoms on the left? Any whole-number coefficient placed in front of the O2 will give an even number of oxygen atoms on the left. This is because the coeffi-cient is always being multiplied by the sub-script 2. The solution is to multiply the formula with the odd number of oxygen atoms by 2.

2Al(s) � O2(g) ¡ 2Al2O3(s)

Now six oxygen atoms are on the right. Bal-ance the oxygens on the left by placing a 3 in front of O2. Then rebalance the aluminum by changing the coefficient of Al(s) from 2 to 4.

4Al(s) � 3O2(g) ¡ 2Al2O3(s)Suppose the equation for the formation of aluminum oxide was written this way.

8Al(s) � 6O2(g) ¡ 4Al2O3(s)Because this equation obeys the law of con-servation of mass, it is correct. However, equa-tions are normally written with coefficients in their lowest possible ratio. Each of the coeffi-cients can be divided by 2 to give the previous equation, which has the lowest whole-number ratio of coefficients.

Practice Problems

5. Balance each equation.a. FeCl3 � NaOH ¡ Fe(OH)3 � NaClb. CS2 � Cl2 ¡ CCl4 � S2Cl2

6. Write and balance this equation.calcium hydroxide � sulfuric acid ¡

calcium sulfate � waterwithChemASAP




Hazardous Materials SpecialistMaterials are classified as hazardous when the chemical reactions they undergo are potentially dangerous to humans or the environment. Being a hazardous materials specialist is an ever-changing career because the list of materials that are considered haz-ardous is constantly changing.

ASSESSEvaluate UnderstandingTo evaluate students’ understanding of how to write and interpret chemical equations, write some word equations on the board and have students pro-duce skeleton equations. Write some unbalanced chemical equations on the board; have students balance them and describe them in words. Write some equations on the board and begin to balance them by changing subscripts. When students object, ask why this approach is incorrect.

ReteachReview with students the key steps in writing balanced equations. Have them make a flow chart to describe the best way to subdivide the task.

Student answers will vary but should reflect an understanding of the term biodegradable as applied to materials that can and will decompose through natural organic processes.

with ChemASAP

If your class subscribes to the Interactive Textbook, use it to review key concepts in Section 11.1.

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Section 11.1 Assessment7. Write the names of the reactants sepa-

rated by a plus sign followed by an arrow followed by the names of the products separated by a plus sign.

8. Write the formulas of the reactants to the left of the yields sign and the formulas of the products to the right.

9. Write the skeleton equation with the cor-rect formulas of the reactants on the left and the correct formulas of the products on the right. Then balance the equation

using coefficients so that it obeys the law of conservation of mass.

10. a. b. NaHCO3 → Na2CO3 + CO2 + H2O

11. a. 2Fe(s) + 3Cl2(g) → 2FeCl3(s) b. Al2(CO3)3(s) → Al2O3(s) + 3CO2(g)c. Mg(s) + 2AgNO3(aq) → 2Ag(s) + Mg(NO3)2(aq)

12. a. 2SO2 + O2 → 2SO3 b. Fe2O3 + 3H2 → 2Fe + 3H2O c. 4P + 5O2 → P4O10d. 2Al + N2 → 2AlN

heatCuS + O2 Cu + SO2


Hazardous Materials SpecialistHazardous materials specialists do the vital work of keeping the environment safe from harmful substances. Backed by regulations devised primarily by the Environ-mental Protection Agency (EPA), these specialists are responsible for the safe handling, treatment,

storage, and transportation of hazardous materials. They work at all levels of government and are employed by industries and uni-versities where dangerous materi-als are generated or used. Part of their jobs may be to educate those who need to know, including the public, about the rules and reg-ulations applied to hazardous materials.

Hazardous materials specialists often work in the field inspecting, testing, overseeing cleanup work, and determining whether storage facilities are in compliance with regulations. First on the scene when a chemical spill occurs, these specialists must be able to identify the spilled chemical sub-stance and know how to take

emergency action to neutralize its effect and thus protect the public. Then they must devise a cleanup procedure and oversee its implementation.

Entry into the field usually requires a degree from a four-year college with a major in chemistry or other physical science. Other areas of concentration might be industrial hygiene, environmental health, or engineering.

For: Careers in ChemistryVisit: PHSchool.comWeb Code: cdb-1111

7. Key Concept How do you write a word equation?

8. Key Concept How do you write a skeleton equation?

9. Key Concept Describe the steps in writing a balanced chemical equation.

10. Write skeleton equations for these reactions. a. Heating copper(II) sulfide in the presence of

diatomic oxygen produces pure copper and sulfur dioxide gas.

b. When heated, baking soda (sodium hydrogen car-bonate) decomposes to form the products sodium carbonate, carbon dioxide, and water.

11. Write and balance equations for the following reactions.

a. Iron metal and chlorine gas react to form solid iron(III) chloride.

b. Solid aluminum carbonate decomposes to form solid aluminum oxide and carbon dioxide gas.

11.1 Section Assessment

c. Solid magnesium reacts with aqueous silver ni-trate to form solid silver and aqueous magne-sium nitrate.

12. Balance the following equations. a. SO2 � O2 ¡ SO3

b. Fe2O3 � H2 ¡ Fe � H2O c. P � O2 ¡ P4O10

d. Al � N2 ¡ AlN

Paragraph Some products are marketed as biode-gradable. What does biodegradable mean? Identify three biodegradable products. How do these prod-ucts benefit the environment?

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Assessment 11.1 Test yourself on the concepts in Section 11.1.

330 Chapter 11


Section 11.2• Core Teaching Resources,

Section 11.2 Review• Laboratory Manual, Labs 14, 15• Small-Scale Chemistry Laboratory

Manual, Lab 15• Transparencies, T118–T120

• Lab Practical 11–1


Simulation 12; Problem-Solving 11.14, 11.15, 11.17, 11.18, 11.21; Assessment 11.2

• Go Online, Section 11.2

11.2

FOCUSObjectives11.2.1 Describe the five general types

of reactions.11.2.2 Predict the products of the five

general types of reactions.

Guide for Reading

Build VocabularyGraphic Organizers Have students draw concept maps entitled “Types of chemical reactions.” Have them include all the vocabulary terms in the concept map. Ask them to explain how the name of each reaction tells what occurs in the reaction.

Reading StrategyVisualize As students read the sec-tion, have them visualize the process that occurs in each type of chemical reaction. Suggest that they sketch their visualizations.

INSTRUCT

Have students study the photograph and read the text. Explain that an important product of combustion reactions is energy, which can be in the form of heat and/or light. Where else can you observe combustion reac-tions on a daily basis? (Examples include natural gas stoves and heaters, butane lighters, acetylene torches, and automobile engines.)

Download a worksheet on Reaction Types for students to complete, and find additional teacher support from NSTA SciLinks.

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330 Chapter 11

11.2 Types of Chemical Reactions

Often charcoal briquettes provide the heat for barbeque grills through the burning of carbon. Have you ever felt the heat and smelled the smoke coming from a burning charcoal grill? The heat and smoke are the products of a combustion reaction. Combustion is one of the five general types of chemical reactions. In this chapter, you will learn that if you can recognize a reaction as being a particular type, you may be able to predict the products of the reaction.

Guide for Reading

Key Concepts• What are the five general types

of reactions?• How can you predict the

products of the five general types of reactions?

Vocabularycombination reaction

decomposition reaction

single-replacement reaction

activity series

double-replacement reaction

combustion reaction

Reading StrategyOutlining As you read, make an outline of the most important ideas in this section. Use the red headings as the main topics and the blue headings as subtopics. Add a sentence or a note after each heading to provide key infor-mation about each topic.

Classifying ReactionsThe five general types of reaction are combination, decomposition,

single-replacement, double-replacement, and combustion. Not all chemicalreactions fit uniquely into only one category. Occasionally, a reaction mayfit equally well into two categories. Nevertheless, recognizing a reaction asa particular type is useful. Patterns of chemical behavior will becomeapparent and allow you to predict the products of reactions.

Combination Reactions The first type of reaction is the combination,or synthesis, reaction. A combination reaction is a chemical change inwhich two or more substances react to form a single new substance. Asshown in Figure 11.5, magnesium metal and oxygen gas combine to formthe compound magnesium oxide.

2Mg(s) � O2(g) ¡ 2MgO(s)

Notice that in this reaction, as in all combination reactions, the prod-uct is a single substance (MgO), which is a compound. The reactants in thiscombination reaction (Mg and O2) are two elements. This is often the case,but two compounds may also combine to form a single substance.

When a Group A metal and a nonmetal react, the product is a com-pound consisting of the metal cation and the nonmetal anion.

2K(s) � Cl2(g) ¡ 2KCl(s)

When two nonmetals react in a combination reaction, more than oneproduct is often possible.

S(s) � O2(g) ¡ SO2(g) sulfur dioxide2S(s) � 3O2(g) ¡ 2SO3(g) sulfur trioxide

More than one product may also result from the combination reactionof a transition metal and a nonmetal.

Fe(s) � S(s) ¡ FeS(s) iron(II) sulfide2Fe(s) � 3S(s) ¡ Fe2S3(s) iron(III) sulfide

For: Links on Reaction Types

Visit: www.SciLinks.orgWeb Code: cdn-1112

Section Resources


Answers to...Figure 11.5 The reaction produces bright, white light.

Gifted and TalentedCombination reactions are sometimes referred to as synthesis reactions, but synthe-sis reactions and combination reactions are no longer considered to be synonymous. Have students research the meaning of syn-thesis. Have them investigate the synthesis of

polymers and the process of photosynthesis. Point out that synthesis involves making a final product from other substances, but not all synthesis processes—such as those in pho-tosynthesis—are true combination reactions.

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Section 11.2 Types of Chemical Reactions 331


Writing Equations for Combination ReactionsCopper and sulfur, shown in the photo, are the reac-tants in a combination reaction. Complete the equa-tion for the reaction.

Cu(s) � S(s) ¡ (two reactions possible)


Two reactions are possible because copper isa transition metal and has more than onecommon ionic charge (Cu� and Cu2�). Deter-mine the formulas for the two products. Bal-ance the two possible equations.


Write the skeleton equation first, then applythe rules for balancing equations.For copper(II):

Cu(s) � S(s) ¡ CuS(s) (balanced)For copper(I):

Cu(s) � S(s) ¡ Cu2S(s)2Cu(s) � S(s) ¡ Cu2S(s) (balanced)

Practice Problems

13. Complete and balance this equation for a combi-nation reaction.

Be � O2 ¡14. Write and balance the equation for the formation

of magnesium nitride(Mg3N2) from its elements.

2Mg(s)Magnesium

Mg

2MgO(s)Magnesium oxide

O2(g)Oxygen

O2 O2� Mg2�

�

�

�

¡

¡⎯⎯→

withChemASAP


Figure 11.5 When ignited, magnesium ribbon reacts with oxygen in the surrounding air to form magnesium oxide, a white solid. This is a combination reaction. Observing Why do you think this reaction was once used in flashbulbs for photography?

Classifying ReactionsCONCEPTUAL PROBLEM 11.4

Answers13. 2Be + O2 → 2BeO14. 3Mg + N2 → Mg3N2

Practice Problems PlusWrite and balance an equation for the formation of aluminum chloride (AlCl3) from its elements. (2Al + 3Cl2 → 2AlCl3)

DiscussWrite a number of equations for com-bination reactions on the board and have students practice balancing them. Point out that many combina-tion reactions release large amounts of energy. Rewrite some of the equations with the product on the reactant side and the reactants on the product side. Ask, Is it possible to reverse the pro-cess? (yes) What is the name of the reverse process? (a decomposition reaction) Why is energy usually needed for a decomposition reac-tion to occur? (Decomposition involves the breaking of bonds, which requires energy.)

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332 Chapter 11



Answers15. 2HI → H2 + I216. HBr

Use VisualsConceptual Problem 11.5 Have stu-dents study the photograph in the problem. Explain that the explosive properties of dynamite are due to the rapid production of large amounts of gases. A related reaction is the decom-position of trinitrotoluene (TNT). Write the equation for the decomposition of TNT on the board: 2C7H5N3O6(s) → 3N2(g) + 7CO(g) + 5H2O(g) + 7C(s). Point out to students that for every 2 mol of TNT that decompose, 15 mol of hot, expanding gases are produced.

DiscussHelp students create a list of criteria for identifying decomposition and combi-nation reactions. For example, decom-position reactions are characterized by one molecule on the reactant side and smaller molecules or elements on the product side. Students should supplement their lists later, when they study single-replacement, double-replacement, and combustion reactions.

DiscussEmphasize to students that decompo-sition reactions consist of one sub-stance forming two or more different substances. Students tend to limit decomposition reactions to the decomposition of a compound into its component elements. A compound can also break down into an element and a compound or two or more com-pounds. Provide students with the fol-lowing chemical equations as examples of decomposition reactions that result in products other than elements: 2H2O2 → 2H2O + O2 and H2CO3 → H2O + CO2.

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Practice Problems


Writing the Equation for a Decomposition ReactionDecomposition reactions that produce gases and heat aresometimes explosive, as the photo shows. Write a balancedequation for the following decomposition reaction.

H2O1l 2¬¡electricity


Water, a binary compound, breaks down intoits elements. Balance the equation, remem-bering that hydrogen and oxygen are bothdiatomic molecules.


Write the skeleton equation, then apply therules for balancing equations.

2H2O1l 2¬¡electricity2H21g2 + O21g2 1balanced2

H2O1l 2¬¡electricityH21g2 + O21g2

Practice Problems

15. Complete and balance this decompositionreaction.

HI ¡16. Write the formula for the binary compound that

decomposes to the products H2 and Br2.

heat—→

heat—→

�

� O2(g)Oxygen

2Hg(l)Mercury

2HgO(s)Mercury(ll) oxide

Hg2+

O2– Hg O2

Figure 11.6 When orange-colored mercury(II) oxide is heated, it decomposes into its constituent elements: liquid mercury and gaseous oxygen.Comparing and Contrasting How are the reactions pictured in Figures 11.5 and 11.6 similar? How are they different?

withChemASAP


Decomposition Reactions When mercury(II) oxide is heated, it decom-poses or breaks down into two simpler compounds, as shown in Figure 11.6.

A decomposition reaction is a chemical change in which a single com-pound breaks down into two or more simpler products. Decompositionreactions involve only one reactant and two or more products. The prod-ucts can be any combination of elements and compounds. It is usually dif-ficult to predict the products of decomposition reactions. However, when asimple binary compound such as HgO breaks down, you know that theproducts must be the constituent elements Hg and O2. Most decomposi-tion reactions require energy in the form of heat, light, or electricity.

2HgO 1s2 ¡ 2Hg 1l 2 + O2 1g2


Use VisualsTable 11.2 With Table 11.2 displayed on an overhead projector, review the definition of a single-replacement reaction on the board. Explain that sin-gle-replacement reactions can be com-pared to partners cutting in on each other at a dance. A person who is alone approaches a couple and cuts in. The person replaces one member of the couple, who is now left alone. In a chemical reaction, however, only cer-tain substances can replace other sub-stances in a given compound. The activity series of metals tells us which metals can replace other metals in a given compound. Review the reactions on this page. Show students how to use Table 11.2 to predict whether a reaction will occur.


Single-Replacement ReactionsPurpose Students observe single-replacement reactions.

Materials magnesium ribbon, 10-mL graduated cylinder, 1M HCl, sodium, 2 beakers, tongs, water

Safety HCI(aq) is corrosive and can cause severe burns. The piece of sodium should be no larger than a match head. Wear plastic gloves and use tongs to avoid contact between sodium and your skin.

Procedure Place a 2-cm piece of mag-nesium ribbon in 10 mL of 1M HCl. Using tongs, place a small piece of sodium in 250 mL of cold water.Expected Outcomes In both reac-tions, the metal replaces hydrogen in a compound. Hydrogen gas is released. The reaction using sodium is much more dramatic and rapid than the reac-tion involving magnesium.

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Answers to...Figure 11.6 Both involve two ele-ments and one compound. In Figure 11.5, a compound is being formed from the elements; in Figure 11.6, a compound is being decomposed to its elements.Figure 11.7 to prevent their reac-tion with water vapor and oxygen in the air

Gifted and TalentedEncourage students to devise general state-ments to represent each of the first four types of chemical reactions discussed in this section. For example: X + Y → XY, XY → X + Y, X + CD → XD + C, AB + CD → AD + CB.

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K

2K(s)Potassium

H2O

2H2O(l)Water

� —→

—→

OH–

H2

2KOH(aq)Potassium hydroxide

� H2(g)Hydrogen

K+

Single-Replacement Reactions Dropping a small piece of potassiuminto a beaker of water creates the vigorous reaction shown in Figure 11.7.The reaction produces hydrogen gas and a large quantity of heat. Thereleased hydrogen gas can ignite explosively.

2K(s) � 2H2O(l) ¡ 2KOH(aq) � H2(g)

Similar but less spectacular reactions can occur. For example, if youdrop a piece of zinc into a solution of copper nitrate, this reaction occurs:

Zn(s) � Cu(NO3)2(aq) ¡ Cu(s) � Zn(NO3)2(aq)

These equations describe two examples of single-replacement reac-tions. A single-replacement reaction is a chemical change in which one ele-ment replaces a second element in a compound. You can identify a single-replacement reaction by noting that both the reactants and the productsconsist of an element and a compound. In the equation above, zinc andcopper change places. The reacting element Zn replaces copper in thereactant compound Cu(NO3)2. The products are the element Cu and thecompound Zn(NO3)2.

Whether one metal will displace another metal from a compounddepends upon the relative reactivities of the two metals. The activity seriesof metals, given in Table 11.2, lists metals in order of decreasing reactivity.A reactive metal will replace any metal listed below it in the activity series.Thus iron will displace copper from a copper compound in solution, butiron does not similarly displace zinc or calcium.

A halogen can also replace another halogen from a compound. Theactivity of the halogens decreases as you go down Group 7A of the periodictable—fluorine, chlorine, bromine, and iodine. Bromine is more activethan iodine, so this reaction occurs:

Br2(aq) � NaI(aq) ¡ NaBr(aq) � I2(aq)

But bromine is less active than chlorine, so this reaction does not occur:

Br2(aq) � NaCl(aq) ¡ No reaction

Table 11.2

Activity Series of Metals

Figure 11.7 The alkali metal potassium displaces hydrogen from water and forms a solution of potassium hydroxide in a single-replacement reaction. The heat of the reaction is often sufficient to ignite the hydrogen. Inferring Why are alkali metals stored under mineral oil or kerosene?

Name Symbol

Dec

reas

ing

rea

ctiv

ity

Lithium Li

Potassium K

Calcium Ca

Sodium Na

Magnesium Mg

Aluminum Al

Zinc Zn

Iron Fe

Lead Pb

(Hydrogen) (H)*

*Metals from Li to Na will replace H from acids and water; from Mg to Pb they will replace H from acids only.

Copper Cu

Mercury Hg

Silver Ag

334 Chapter 11



Answers17. a. Fe(s) + Pb(NO3)2(aq) →

Fe(NO3)2(aq) + Pb(s)b. Cl2(g) + 2NaI(aq) →

2NaCl(aq) + I2(aq)c. Ca(s) + 2H2O(l) →

Ca(OH)2(aq) + H2(g)

Practice Problems PlusComplete the equation for this sin-gle-replacement reaction that takes place in aqueous solution. Balance the equation. If a reaction does not occur , write “no reaction.” (Use the activity series.)K(s) + H2SO4(aq) → (2K(s) + H2SO4(aq) → K2SO4(aq) + H2(g))

DiscussExplain to students that a double-replacement reaction always involves two ionic compounds in aqueous solu-tion. In addition, one of the products must be a precipitate, a gas, or a molecular compound.


A Double-Replacement ReactionPurpose Students observe double-replacement reactions.

Materials equimolar solutions of BaCl2,Na2SO4, Na3PO4, CaCl2, Pb(NO3)2,and KI

Procedure Mix equimolar solutions of the following ionic compounds: BaCl2 and Na2SO4; Na3PO4 and CaCl2; and Pb(NO3)2 and KI. Ask, What evidence of a double-replacement reaction do you observe? (a precipitate) Write the formula of each precipitate formed in the reactions.Expected Outcome Precipitates of BaSO4, Ca3(PO4)2, and PbI2 form.

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Gifted and TalentedWhen hydrochloric acid (HCl) is dropped on calcium carbonate (CaCO3) a double-replace-ment reaction occurs. Ask, What are the products of this reaction? (CaCl2 and H2CO3) Neither of these products is a gas, but bubbles

of gas are released during this reaction. Ask students to infer the identity of the gas. (CO2) Ask, What type of reaction could produce this gas? (The decomposition reaction H2CO3 → H2O + CO2 releases carbon dioxide gas.)

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Practice Problems


Writing Equations for Single-Replacement ReactionsThe photo shows the reaction between Zn(s) and H2SO4(aq). Write abalanced chemical equation for each single-replacement reaction.The reactions take place in aqueous solution.a. Zn(s) � H2SO4(aq) ¡b. Cl2(aq) � NaBr(aq) ¡


a. According to the activity series of metals, zinc displaces hydrogen from an acid and takes its place. Balance the equation, remembering that elemental hydrogen is diatomic.

b. Chlorine is more reactive than bromine and displaces bromine from its compounds. Bal-ance the equation. Bromine is diatomic.


Write the skeleton equation first, then applythe rules for balancing equations.a. Zn(s) � H2SO4(aq) ¡ ZnSO4(aq) � H2(g)

(balanced)b. Cl2(aq) � NaBr(aq) ¡ NaCl(aq) � Br2(aq)

Cl2(aq) � 2NaBr(aq) ¡ 2NaCl(aq) �

Br2(aq) (balanced)

Practice Problem

17. Complete the equations for these single-replacement reactions in aqueous solution. Bal-ance each equation. Write “no reaction” if a reaction does not occur. Use the activity series.a. Fe(s) � Pb(NO3)2(aq) ¡b. Cl2(aq) � NaI(aq) ¡c. Ca(s) � H2O(l) ¡

Double-Replacement Reactions Sometimes, when two solutions ofionic compounds are mixed, nothing happens. At other times, the ions inthe two solutions react. Figure 11.8 shows that mixing aqueous solutions ofpotassium carbonate and barium chloride results in a chemical reaction. Awhite precipitate of solid barium carbonate is formed. Potassium chloride,the other product of the reaction, remains in solution. This is an example ofa double-replacement reaction, which is a chemical change involving anexchange of positive ions between two compounds. Double-replacementreactions are also referred to as double-displacement reactions. They gen-erally take place in aqueous solution and often produce a precipitate, a gas,or a molecular compound such as water. For a double-replacement reac-tion to occur, one of the following is usually true.

1. One of the products is only slightly soluble and precipitates from solu-tion. For example, the reaction of aqueous solutions of sodium sulfide andcadmium nitrate produces a yellow precipitate of cadmium sulfide.

Na2S(aq) � Cd(NO3)2(aq) ¡ CdS(s) � 2NaNO3(aq)

2. One of the products is a gas. Poisonous hydrogen cyanide gas is pro-duced when aqueous sodium cyanide is mixed with sulfuric acid.

2NaCN(aq) � H2SO4(aq) ¡ 2HCN(g) � Na2SO4(aq)

3. One product is a molecular compound such as water. Combiningsolutions of calcium hydroxide and hydrochloric acid produces water.

Ca(OH)2(aq) � 2HCl(aq) ¡ CaCl2(aq) � 2H2O(l)

withChemASAP





Answers18. a. 3NaOH(aq) + Fe(NO3)3(aq) →

Fe(OH)3(s) + 3NaNO3(aq) b. 3Ba(NO3)2(aq) + 2H3PO4(aq) →

Ba3(PO4)2(s) + 6HNO3(aq)19. a. 3KOH(aq) + H3PO4(aq) →

K3PO4(aq) + 3H2O(l)b. 3H2SO4(aq) + 2Al(OH)3(aq) →

Al2(SO4)3(aq) + 6H2O(l)

Practice Problems PlusWrite the products for this double-replacement reaction. Then balance the equation. Pb(NO3)2(aq) + NaI(aq) →→→→ (lead iodide is a precipitate)(Pb(NO3)2(aq) + 2NaI(aq) → PbI2(s) + 2NaNO3(aq))


Law of Conservation of MassPurpose Demonstrate the law of con-servation of mass using a double-replacement reaction.

Materials 50 mL 1.0M K2CrO4, 250-mL Erlenmeyer flask, 5 mL dilute AgNO3 solution, small test tube, stopper, 50-mL graduated cylinder

Safety Avoid skin contact with silver nitrate.

Procedure Place 50 mL of 1.0M potas-sium chromate (K2CrO4) in a 250-mL Erlenmeyer flask. Place about 5 mL of dilute silver nitrate solution (AgNO3) in a small test tube that will fit inside the flask. Stopper the flask and determine the mass of the system. Invert the flask sufficiently so that the two solutions mix. Ask students to note any evidence of chemical changes. Measure the mass of the flask after the reaction. Flush the products down the drain with excess water.Expected Outcomes A bright red precipitate forms. The mass has not changed.

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Launching the Space ShuttleDecomposition and displacement reactions supply the energy needed for launching the space shuttle into space. Have students research what fuels are used to propel the space shuttle orbiter. Have volunteers describe the reactions that occur and how the products provide the thrust necessary to launch vehicles.

Facts and Figures



Writing Equations for Double-Replacement ReactionsWrite a balanced chemical equation for each double-replacement reaction.a. CaBr2(aq) � AgNO3(aq) ¡ (A precipitate of silver bromide is formed.)b. FeS(s) � HCl(aq) ¡ (Hydrogen sulfide gas (H2S) is formed.)


a. The driving force behind the reaction is the formation of a precipitate, which is shown in the photo. Write correct formulas of the products using ionic charges. Then balance the equation.

b. A gas is formed. Use ionic charges to write the correct formula of the other product. Then balance the equation.


For each reaction, write the skeleton equa-tion first, then apply the rules for balancingequations.a.

b.

CaBr21aq 2 + AgNO31aq 2 ¡ AAgBr1s 2 + Ca(NO3)21aq 2

CaBr21aq 2 + 2AgNO31aq 2 ¡2AgBr1s 2 + Ca(NO3)2 1aq 2 1balanced2

FeS1s 2 + HCl1aq 2 ¡ H2S1g 2 + FeCl21aq 2FeS1s 2 + 2HCl1aq 2 ¡

H2S1g 2 + FeCl21aq 2 1balanced2Practice Problems

Ba2� Ba2�Cl�

Cl�CO32�

CO32

K�

K�

K2CO3(aq)Potassium carbonate

�

� BaCl2(aq)Barium chloride

2KCl(aq)Potassium chloride

� BaCO3(s)Barium carbonate

18. Write the products of these double-replacement reactions. Then balance each equation.a. NaOH(aq) � Fe(NO3)3(aq) ¡

(Iron(III) hydroxide is a precipitate.)b. Ba(NO3)2(aq) � H3PO4(aq) ¡

(Barium phosphate is a precipitate.)

19. Write a balanced equation for each reaction.a. KOH(aq) � H3PO4(aq) ¡b. H2SO4(aq) � Al(OH)3(aq) ¡

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Figure 11.8 Aqueous solutions of potassium carbonate and barium chloride react in a double-replacement reaction to form the white precipitate barium carbonate. Potassium chloride, the other product of the reaction, remains in solution.

336 Chapter 11


DiscussPoint out to students that an impor-tant product of most combustion reac-tions is energy, which is usually in the form of heat or light. Emphasize that one of the reactants must be oxygen. Relate this fact to everyday experience by recalling how removing oxygen from a combustion reaction—for example, snuffing out a candle—causes the reaction to stop.

RelateAsk students to infer why it is important that combustion reactions, such as those used to heat a home or run an automo-bile, take place in properly ventilated areas. (Without proper ventilation and enough available oxygen, the combustion may be incomplete, and poisonous carbon monoxide may be produced.)

Use VisualsFigure 11.9 Have students study the figure. Remind them that the complete combustion of a hydrocarbon, such as methane, always produces water, car-bon dioxide, heat, and light. Use a dis-posable lighter to show students the combustion of butane. Ask students to write the balanced equation for the combustion of butane (C4H8). Have them use the diagram and chemical equation on this page as an aid.

DiscussExplain that some combustion reac-tions are also combination reactions in which an element or a compound combines with oxygen to form a single product plus energy. For example: 2Mg(s) + O2(g) → 2MgO(s), and 4Fe(s) + 3O2(g) → 2Fe2O3(s).

Download a worksheet on Combus-tion for students to complete, and find additional teacher support from NSTA SciLinks.

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English LearnersMake sure that students understand clearly the five terms that describe general types of chem-ical reactions: combination, decomposition, single-replacement, double-replacement, and

combustion. Help them to spell and pro-nounce each term as well as to define it. Encourage them to ask questions about any term they do not fully understand.

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336 Chapter 11

Combustion Reactions The flames of a campfire or a gas grill are evi-dence that a combustion reaction is taking place. A combustion reaction isa chemical change in which an element or a compound reacts with oxygen,often producing energy in the form of heat and light. A combustion reac-tion always involves oxygen as a reactant. Often the other reactant is ahydrocarbon, which is a compound composed of hydrogen and carbon.The complete combustion of a hydrocarbon produces carbon dioxide andwater. But if the supply of oxygen is limited during a reaction, the combus-tion will not be complete. Elemental carbon (soot) and toxic carbon mon-oxide gas may be additional products. The complete combustion of ahydrocarbon releases a large amount of energy as heat. That’s why hydro-carbons such as methane (CH4), propane (C3H8), and butane (C4H10) areimportant fuels. The combustion reaction for methane is shown in Figure11.9. Gasoline is a mixture of hydrocarbons that can be approximately rep-resented by the formula C8H18. The complete combustion of gasoline in acar engine is shown by this equation.

2C8H18(l) � 25O2(g) ¡ 16CO2(g) � 18H2O(l)

The reactions between oxygen and some elements other than carbonare also examples of combustion reactions. For example, both magnesiumand sulfur will burn in the presence of oxygen. As you look at these com-bustion equations, notice that the reactions could also be classified ascombination reactions.

2Mg(s) � O2(g) ¡ 2MgO(s)

S(s) � O2(g) ¡ SO2(g)

Checkpoint What are the products of the combustion of a hydrocarbon?

CH4(g )Methane

2O2(g )Oxygen

� CO2(g ) 2H2O(g )Carbon dioxide Water

�¡

� ¡

Figure 11.9 Methane gas reacts with oxygen from the surrounding air in a combustion reaction to produce carbon dioxide and water.Inferring What else is produced in this reaction?

For: Links on CombustionVisit: www.SciLinks.orgWeb Code: cdn-1114



Practice Problems Answers20. a. 2HCOOH + O2 → 2CO2 + 2H2O

b. C7H16 + 11O2 → 7CO2 + 8H2O21. C6H12O6 + 6O2 → 6CO2 + 6H2O

Practice Problems PlusWrite a balanced equation for the complete combustion of ethanol (C2H5OH). (C2H5OH + 3O2 → 2CO2 + 3H2O)

Predicting the Products of a Chemical Reaction


Combustion of IronPurpose Students observe the com-bustion of iron.

Materials superfine steel wool, plastic sandwich bag, balance, ring stand, util-ity clamp, matches

Safety Steel wool burns readily.

Procedure Place a superfine steel wool pad in a plastic sandwich bag, measure the mass, and write the value on the board. Take the steel wool out of the bag and unfold it to full length. Clamp it to a ring stand with a utility clamp. Set the steel wool on fire. Ask students to predict what effect the burning will have on the mass of the sample. As soon as the steel wool stops burning and is cool, place the remains in the same plastic sandwich bag and mea-sure the mass again. Write the new value on the board next to the original mass. Discuss why the sample gained mass.Expected Outcomes Two elements, Fe(s) and O2(g), combined in a com-bustion reaction to form a binary com-pound, Fe2O3. The sample gained mass because initially only the mass of the iron was determined. During the reac-tion, oxygen from the air combined with the iron.

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Answers to...Figure 11.9 energy in the form of heat and light

Checkpoint

carbon dioxide and water, plus energy

Gifted and TalentedHave students choose a particular chemical compound, such as NaOH, H2SO4, or CH3OH, and research how it is produced commer-cially. A report or poster should be pre-sented describing in as much detail as possible the reactions that are necessary to produce the compound.

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Writing Equations for Combustion ReactionsAn alcohol lamp often uses ethanol as its fuel. Write balanced equations for the complete combustion of these compounds.a. benzene (C6H6(l)) b. ethanol (CH3CH2OH(l))


Oxygen is the other reactant in these com-bustion reactions. The products are CO2 and H2O. Write the skeleton equation for each reaction, then balance the equation.


For each reaction, write the skeleton equation, then apply the rules for balancing equations.a. C6H6(l) � O2(g) ¡ CO2(g) � H2O(g)

2C6H6(l) � 15O2(g) ¡ 12CO2(g) � 6H2O(g)(balanced)

b. CH3CH2OH(l) � O2(g) ¡ CO2(g) � H2O(g)CH3CH2OH(l) � 3O2(g) ¡ 2CO2(g) �

3H2O(g) (balanced)

Practice Problems

20. Write a balanced equation for the complete com-bustion of each compound.a. formic acid (HCOOH)b. heptane (C7H16)

21. Write a balanced equation for the complete com-bustion of glucose (C6H12O6).

Predicting the Products of a Chemical ReactionNow that you have learned about some of the basic reaction types, you canpredict the products of many reactions. The number of elements and/or compounds reacting is a good indicator of possible reaction type andthus possible products. For example, in a combination reaction, two ormore reactants (elements or compounds) combine to form a single prod-uct. In a decomposition reaction, a single compound is the reactant; two ormore substances are the products. An element and a compound are thereactants in a single-replacement reaction. A different element and a newcompound are the products. In a double-replacement reaction, two ioniccompounds are the reactants; two new compounds are the products. Thereactants in a combustion reaction are oxygen and usually a hydrocarbon.The products of most combustion reactions are carbon dioxide and water. withChemASAP


Simulation 12 Practice classifying reactions according to reaction type.

338 Chapter 11


Facts and Figures


A Combination ReactionPurpose Students observe a combina-tion reaction.

Materials magnesium ribbon, large crucible, lab burner, cobalt blue glass or exposed photographic film, crucible tongs, matches or igniter

Safety Students should not look directly at burning magnesium. Have them observe through pieces of cobalt blue glass or exposed photographic film. Another option is to conduct the reaction inside a large, metal can.

Procedure Explain to students that in a combination reaction, the reactants combine to make one new product. Measure a 5- to 7-cm strip of magne-sium ribbon. Light the burner. Hold one end of the magnesium ribbon with a pair of crucible tongs. Ignite the magnesium and hold it above the cru-cible. Ask students to note any evi-dence of chemical change. Have students note the condition of the resi-due compared to the original magne-sium. The product may be disposed of in the trash.Expected Outcomes Metallic magne-sium and oxygen gas from the air form a white powder, magnesium oxide. Other evidence of chemical change includes release of energy as heat and light. Tell students that small amounts of magnesium nitride also form.

For EnrichmentExtend the demonstration on this page. Ask, What are the reactants in this combination reaction? (Mg and O2) What is the product of this reac-tion? (MgO) Does this reaction obey the law of conservation of mass? Explain. (Yes, the sum of the masses of magnesium and oxygen is equal to the mass of the magnesium oxide formed.) Have students write the balanced equation for the reaction. (2Mg(s) + O2(g) → 2MgO(s)) Challenge students to write the balanced equation for the formation of the small amount of mag-nesium nitride that forms during this reaction. (3Mg(s) + N2(g) → Mg3N2(s)).

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Magnesium from Sea WaterMagnesium metal is an important compo-nent of alloys used to make consumer mate-rials. The main commercial source of Mg(s) is seawater. The Mg2+ ion is the third most abundant dissolved ion in the oceans. A

process for isolating magnesium from sea-water depends on the fact that because of a double-replacement reaction, Mg2+ will pre-cipitate when OH− is added. Students can research the commercial process.

338 Chapter 11

Figure 11.10 The five types of chemical reactions discussed in this chapter are summarized here.

2Mg(s) O2( g) 2MgO(s)� —→

2HgO(s) 2Hg(l) O2(g)�—→

2K(s) 2H2O(l) 2KOH(aq) H2(g)� �—→

Combination Reaction

General Equation: R � S ¡ RS

Reactants: Generally two elements, or two compounds (where at least one compound is a molecular compound)Probable Products: A single compoundExample: Burning magnesium in air

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Decomposition Reaction

General Equation: RS ¡ R � S

Reactants: Generally a single binary com-pound or a compound with a polyatomic ionProbable Products: Two elements (for a binary compound), or two or more elements and/or compounds (for a compound with a polyatomic ion)Example: Heating mercury(ll) oxide

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Reactants: An element and a compound In a single-replacement reaction, an element replaces another element from a compound in aqueous solution. For a single-replacement reaction to occur, the element that is dis-placed must be less active than the element that is doing the displacing.Probable Products: A different element and a new compoundExample: Potassium in water

Single-Replacement Reaction

General Equation: T � RS ¡ TS � R

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ASSESSEvaluate UnderstandingAsk students to give an example of each type of reaction discussed in this section. Write several chemical equa-tions on the board and ask students to classify them. In addition, write only the products of combination, decom-position, and displacement reactions, and challenge students to fill in the reactants. Have students refer to Table 11.2 to answer the following questions. What would happen if a piece of iron was placed in a solution of lead(II) nitrate? (Iron will displace lead.) What would happen if a piece of alumi-num was placed in a solution of cal-cium chloride? (Nothing will happen because Al will not displace Ca.)

ReteachHelp students develop a branched flowchart of chemical reactions similar to those used in qualitative analysis. Start by asking if there is a single reac-tant. If so, the reaction is a decomposi-tion. If not, proceed to the next step. Ask if oxygen is one of the reactants. If so, the reaction is either combination or combustion. If not, proceed to the next step. Continue through the five general types of reactions.

Connecting Concepts

Paragraphs should include the infor-mation that hydrogen atoms and oxygen atoms tend to share elec-trons to become stable. Units formed when atoms share electrons are molecular compounds.

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Section 11.2 Assessment22. combination, decomposition, single-

replacement, double-replacement, and combustion

23. the number of elements and/or com-pounds reacting

24. a. combustion; 2C3H6 + 9O2 → 6CO2 + 6H2O b. decomposition; 2Al(OH)3 → Al2O3 + 3H2O c. combination; 4Li + O2 → 2Li2O d. single-replacement; Zn + 2AgNO3 → 2Ag + Zn(NO3)2

25. a. double replacement; two different compounds b. decomposition; two or

more elements and/or compoundsc. combination; a compound d. com-bustion; carbon dioxide and water

26. a. CaI2 + Hg(NO3)2 → HgI2 + Ca(NO3)2b. 2Al + 3Cl2 → 2AlCl3 c. no reaction d. 2C2H2 + 5O2 → 4CO2 + 2H2O

e. 27. gas, precipitate, and molecular com-

pound

MgCl2 Mg + Cl2electricity



22. Key Concept What are the five types of chem-ical reactions?

23. Key Concept What are the keys to predicting the products of the five general types of reactions?

24. Classify each reaction and balance the equations. a. C3H6 � O2 ¡ CO2 � H2O b. Al(OH)3 ¡ Al2O3 � H2O c. Li � O2 ¡ Li2O d. Zn � AgNO3 ¡ Ag � Zn(NO3)2

25. Which of the five general types of reaction would most likely occur, given each set of reactants? What are the probable products?

a. an aqueous solution of two ionic compounds b. a single compound c. two elements d. oxygen and a compound of carbon and hydrogen

26. Complete and balance an equation for each reaction.

a. CaI2 � Hg(NO3)2 ¡ (HgI2 precipitates.)

b. Al � Cl2 ¡ c. Ag � HCl ¡ d. C2H2 � O2 ¡ e. MgCl2 ¡27. What are the three types of products that result

from double-replacement reactions?

Molecular Compounds Hydrogen peroxide is an antiseptic that undergoes a decomposition reaction in the presence of living cells. Refer to Section 8.1 and write a paragraph giving evidence that hydro-gen peroxide is a molecular compound.

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K2CO3(aq) BaCl2(aq) 2KCl(aq) BaCO3(s)� �

CH4(g) 2O2(g) CO2(g) 2H2O(g)� —→ �

Double-Replacement Reaction

General Equation: R� S� � T� U� ¡ R� U� � T� S��

Reactants: Two ionic compoundsIn a double-replacement reaction, two ionic compounds react by exchanging cations to form two different compounds. Probable Products: Two new compounds Double-replacement reactions are driven by the formation of a precipitate, a gaseous product, or water.Example: Reaction of aqueous solutions of barium chloride and potassium carbonate

Combustion Reaction

General Equation: Cx Hy � (x � y/4) O2 ¡ xCO2 � (y/2)H2O

Reactants: Oxygen and a compound of C, H, (O)When oxygen reacts with an element or compound, combustion may occur.Probable Products: CO2 and H2OWith incomplete combustion, C and CO may also be products.Example: The combustion of methane gas in air

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340 Chapter 11

Fire Extinguisher LabelsFire extinguishers indicate on their labels what types of fires they are designed to extinguish. Some extinguishers might be labeled “AB,” for example, indicating that they are effective on more than one type of fire. Many fire extinguishers now are labeled with pictures that show the type of fire. For

example, a type A extinguisher might show a picture of burning wood. The extinguisher might also show what type of fire the extin-guisher should not be used on by showing a picture of that type of fire in a circle with a black line through it.

DiscussAfter students have read the article, ask, What three things are necessary for a fire to burn? (fuel, oxygen, and energy to initiate combustion) Why is it not safe to use a single kind of fire extinguisher on all fires? (A fire extin-guisher that controls one type of fire may actually enhance other types of combus-tion reactions. For example, water is not sprayed on burning magnesium because the intense heat can decompose water, producing flammable hydrogen and oxygen gases.)

CLASS ActivityCLASS

Classifying FiresPurpose Students classify fires accord-ing to type.

Materials research materials, graph paper

Procedure Have students find out what the letters A, B, C, and D refer to in classifying fires. Have them contact their local fire department to obtain statistics on how many class A, B, C, and D fires have occurred in their area during the past year or six months. As a class, have students graph the data and discuss any conclusions that can be drawn from the data.

Expected Outcomes Class A fires are those where ordinary combustibles, such as wood or plastic, are burning. Class B fires involve flammable liquids, such as gasoline. Class C fires are elec-trical. Class D fires are metal fires.

RelateDiscuss with students the proper use of a fire extinguisher. Tell them to remember PASS. Pull the pin. Aim the nozzle at the base of the fire. Squeeze the handle. Sweep the contents of the extinguisher from side to side until the fire is out. Then, they should shut off the fire extinguisher and watch to see that the fire does not rekindle.

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340 Chapter 11

Combating Combustion

A fire has three requirements: oxygen, fuel, and a temperature high

enough to initiate and sustain combustion. Firefighters put out fires

by eliminating one or more of these requirements. When water is

sprayed on a typical building fire, it stops the fire by lowering the

temperature of the burning material and soaking it with

noncombustible water. Steam from the vaporizing water also tends

to displace air from around the flames and deny them oxygen. To

improve the ability of water to saturate the fuel, for example,

upholstered furniture and rugs, a substance called a surfactant is

added to the water to form a penetrating foam. Inferring How can it

help to roll on the ground if your clothes are on fire?

Water Water is the most important tool for firefighters. Water-based foams are more effective and environmentally friendly, but they are also more expensive.

Facts and Figures


RelateFire extinguishers can be quite heavy. Purchasers of fire extinguishers should be sure that they can handle and operate an extinguisher effectively. Extinguish-ers must be recharged after each use, even if they are not entirely empty. Peri-odically, they should be checked to make sure they are operable and effective.

Discuss with students the require-ments of a gas that might be used to put out a fire. The gas cannot itself burn or support the combustion of another material. It must be heavier than air so that it will settle on a fire, depriving it of oxygen. Discuss some specific gases in terms of these factors. For example, nitrogen and helium are relatively nonreactive, but they are not heavy enough. Hydrogen and meth-ane will burn. Oxygen supports the burning of the fuel. Carbon dioxide will not burn or support combustion and is heavier than air. Ask, Is a piece of paper combustible? Is it flammable? (Paper is combustible but not flamma-ble.) Explain that these two terms are often used synonymously, but com-bustible means that the material will burn, and flammable means a material may easily burst into flame.

CLASS ActivityCLASS

Model a Fire ExtinguisherPurpose Students observe the effect of carbon dioxide on a flame.

Materials calcium carbonate, dilute hydrochloric acid, 3 beakers, candle, matches

Safety Exert caution when using open flames or acid.

Procedure Place some calcium car-bonate in a beaker. Add hydrochloric acid, and allow several minutes for the reaction to produce collectable amounts of carbon dioxide. Collect the carbon dioxide in a beaker. This gas will stay in the beaker because it is heavier than air. Light a candle, and place it in the third beaker. Pour the carbon diox-ide over the flame, and observe what happens. Expected Outcomes The carbon diox-ide extinguishes the flame.

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Gifted and TalentedHave students check the Internet to learn more about careers in fire science. Prepare a class list of each career, its description, and its requirements. Careers might include fire-fighter, arson investigator, or fire extin-guisher manufacturer.

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Technology and Society 341

Forest fires Firefighters combat forest fires from the air by spreading substances that coat the surfaces of the trees to prevent burning. They can also cut the fire off from its fuel by using bulldozers to cut a clear path through the trees or by setting a controlled blaze.

Grease fires Water sprayed on

a grease fire can spread the flames. A

carbon-dioxide fire extinguisher produces a

cloud of heavier-than-air CO2that blankets the fire and cuts off the oxygen supply.

Electrical fires Chemicals such as sodium hydrogen

carbonate and ammonium dihydrogen phosphate,

blown from a dry-chemical extinguisher, cover an electrical

fire and cut off oxygen. This type of extinguisher also works on burning metals such as magnesium and sodium.

Answers to...Inferring Rolling on the ground deprives the burning clothes of oxygen.

342 Chapter 11


Section 11.3• Core Teaching Resources, Section 11.3

Review, Interpreting Graphics• Laboratory Manual, Labs 16, 17, 18• Small-Scale Chemistry Laboratory

Manual, Lab 16, 17• Transparencies, T121• Lab Practical, 11-2, 11-3, 11-4, 11-5


Problem-Solving 11.28, Assessment 11.3• Virtual Chemistry Lab, Lab 12, 13

11.3

FOCUSObjectives11.3.1 Describe the information

found in a net ionic equation.11.3.2 Predict the formation of a pre-

cipitate in a double-replace-ment reaction.

Guide for Reading

Build VocabularyList the Parts You Know Help stu-dents appreciate the meaning of “net ionic equation” by exploring the mean-ing of phrases such as net income and net worth.

Reading StrategyAnticipation Guide Ask, In a double-replacement reaction, two ions form a solid precipitate, a molecular com-pound, or a gas. What happens to the other ions? (They remain dissolved in the solution.) What would you need to know to predict whether a precipitate forms in a double-replacement reac-tion? (The solubility of all compounds that could form.)

INSTRUCT

Point out that the formation of stalac-tites and stalagmites is a precipitation reaction. Ask, How do limestone cav-erns form? (Calcium carbonate reacts with carbon dioxide dissolved in water to form soluble calcium hydrogen carbon-ate. Carbon dioxide then converts cal-cium hydrogen carbonate back to calcium carbonate, which precipitates and forms stalactites and stalagmites.)

Net Ionic EquationsDiscussAsk, What type of reaction is a pre-cipitation reaction? (double-replace-ment) What evidence shows that a double-replacement reaction has occurred? (formation of a precipitate, a gas, or a molecular compound)

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342 Chapter 11

11.3 Reactions in Aqueous Solution

The beauty of a limestone cav-ern is the result of chemical reactions involving water. Limestone caverns form as calcium carbonate reacts with carbon dioxide dissolved in water and forms solu-ble calcium hydrogen carbonate. Addi-tional carbon dioxide then converts the calcium hydrogen carbonate back into calcium carbonate. The calcium carbonate precipitates and forms dramatic stalactites and stalag-mites. In this section, you will learn to predict the formation of precipitates and write equations to describe the reactions that produce them.

Guide for Reading

Key Concepts• What does a net ionic equation

show?• How can you predict the

formation of a precipitate in a double-replacement reaction?

Vocabularycomplete ionic equation

spectator ion

net ionic equation

Reading StrategyComparing and ContrastingWhen you compare and contrast things, you examine how they are alike and different. As you read, list the ways that complete ionic equations and net ionic equations are the same and how they are different.

Net Ionic EquationsYour world is water-based. More than 70% of Earth’s surface is covered bywater, and about 66% of the adult human body is water. It is not surprising,then, that many important chemical reactions take place in water—that is,in aqueous solution.

The reaction of aqueous solutions of silver nitrate with sodium chlo-ride to form solid silver chloride and aqueous sodium nitrate is a double-replacement reaction. The reaction is shown in Figure 11.11.

AgNO3(aq) � NaCl(aq) ¡ AgCl(s) � NaNO3(aq)

This is the way you have been writing equations involving aqueoussolutions of ionic compounds. However, the equation does not show thatlike most ionic compounds, the reactants and one of the products dissoci-ate, or separate, into cations and anions when they dissolve in water. Forexample, when sodium chloride dissolves in water, it separates intosodium ions (Na�(aq)) and chloride ions (Cl�(aq)). Similarly, silver nitratedissociates into silver ions (Ag�(aq)) and nitrate ions (NO3

�(aq)). You canuse these ions to write a complete ionic equation, an equation that showsdissolved ionic compounds as dissociated free ions.

Ag�(aq) � NO3�(aq) � Na�(aq) � Cl�(aq) ¡

AgCl(s) � Na�(aq) � NO3�(aq)

Notice that the nitrate ion and the sodium ion appear unchanged onboth sides of the equation. The equation can be simplified by eliminatingthese ions because they don’t participate in the reaction.

AgCl1 s 2 + Na+1aq 2 + NO3-1aq 2

Ag+1aq2 + NO3-1aq2 + Na+1aq2 + Cl-1aq2 ¡

Figure 11.11 A precipitate of silver chloride forms when aqueous solutions of silver nitrate and sodium chloride are mixed. Inferring Whichions do not participate in the reaction?

Section Resources


Answers to...Figure 11.11 Na+ and NO3

–

Less Proficient ReadersInitiate a discussion with students about pre-cipitation reactions that occur in everyday life. Examples include kidney stones, soap scum in bathtubs and showers, and lime-stone deposits. Explain the value of being able to write net ionic equations as a direct

and simple way to describe chemical change. Relate the concept of net ionic equations to net income. Spectator ions can be compared to two people who go to a dance on a dou-ble date and then watch while their partners pair up to dance.

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Section 11.3 Reactions in Aqueous Solution 343

An ion that appears on both sides of an equation and is not directlyinvolved in the reaction is called a spectator ion. When you rewrite anequation leaving out the spectator ions, you have the net ionic equation.The net ionic equation is an equation for a reaction in solution that showsonly those particles that are directly involved in the chemical change.

Ag �(aq) � Cl�(aq) ¡ AgCl(s)

In writing balanced net ionic equations, you must make sure that theionic charge is balanced. For the previous reaction, the net ionic charge oneach side of the equation is zero and is therefore balanced. But considerthe skeleton equation for the reaction of lead with silver nitrate.

Pb(s) � AgNO3(aq) ¡ Ag(s) � Pb(NO3)2(aq)

The nitrate ion is the spectator ion in this reaction. The net ionic equa-tion is this.

Pb(s) � Ag�(aq) ¡ Ag(s) � Pb2�(aq) (unbalanced)

Why is this equation unbalanced? Notice that a single unit of positivecharge is on the reactant side of the equation. Two units of positive chargeare on the product side. Placing the coefficient 2 in front of Ag�(aq) bal-ances the charge. A coefficient of 2 in front of Ag(s) rebalances the atoms.

Pb(s) � 2Ag�(aq) ¡ 2Ag(s) � Pb2�(aq) (balanced)

A net ionic equation shows only those particles involved in the reac-tion and is balanced with respect to both mass and charge.


Writing and Balancing Net Ionic EquationsIn the photograph, aqueous solutions of iron(III) chloride and potas-sium hydroxide are mixed. A precipitate of iron(III) hydroxide forms.Identify the spectator ions and write a balanced net ionic equationfor the reaction.


Write the complete ionic equation for the reac-tion, showing any soluble ionic compounds as individual ions. Eliminate aqueous ions that appear as both reactants and products. Bal-ance the net ionic equation.


Fe3�(aq) � 3Cl�(aq) � 3K�(aq) � 3OH�(aq)¡ Fe(OH)3(s) � 3K�(aq) � 3Cl�(aq)

The spectator ions are K� and Cl�. The bal-anced net ionic equation is

Fe3�(aq) � 3OH�(aq) ¡ Fe(OH)3(s)

Practice Problems

28. Write the balanced net ionic equation for this reaction.

Ca2�(aq) � OH�(aq) � H�(aq) � PO43�(aq) ¡

Ca2�(aq) � PO43�(aq) � H2O(l)

29. Write the complete ionic equation and net ionic equation for the reaction of aqueous calcium hydroxide with phosphoric acid. The products are calcium phosphate and water.

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Word OriginsSpectator comes from the Latin verb spectare, meaning “to watch.” Thus a spectator ion can be thought of as only watching a reaction, not par-ticipating. During a football game, what analogy can you draw to the people in the seats and the football players on the field?



Answers28. OH–(aq) + H+(aq) → H2O(l)29. complete ionic equation:

3Ca2+(aq) + 6OH−(aq) + 6H+(aq) + 2PO4

3−(aq) → Ca3(PO4)2(s) + 6H2O(l)

net ionic equation: same as com-plete ionic equation

Practice Problems PlusWrite a balanced net ionic equation for the reaction between sulfuric acid (H2SO4) and sodium hydroxide (NaOH). (H+(aq) + OH− (aq) → H2O(l))

Word OriginsThe spectators at a football game are present at the game, but they don’t participate in it.

RelateIf any students have visited under-ground caves such as Carlsbad Cav-erns, have them describe what they saw. Explain that the caverns form when carbonic acid dissolves the cal-cium carbonate (calcite) in limestone. The icicle-like deposits on the roof of the cavern are called stalactites. The deposits on the floor are called stalag-mites. Both form when dissolved cal-cium carbonate precipitates from groundwater.

DiscussExplain to students that a net ionic equation differentiates between ions that react to form a solid precipitate, a gas, or water, and ions that simply remain in aqueous solution. It is impor-tant to note, however, that spectator ions are not completely unaffected by the reaction. They end up paired with different anions or cations than they were paired with when the reaction began.

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344 Chapter 11


Predicting the Formation of a PrecipitateDiscussOn the board, write the products only of several double-replacement reac-tions; challenge students to fill in the reactants. Examples include: → AgBr(s) + KNO3(aq), → BaSO4(s) + 2NaCl(aq), and → 2KCl(aq) + CaSO4(s). (KBr(aq) + AgNO3(aq), BaCl2(aq) + Na2SO4(aq), and CaCl2(aq) + K2SO4(aq)) Remind stu-dents that in a double-replacement reaction, the positive ions (cations) of one compound trade places with the positive ions of another compound.

ASSESSEvaluate UnderstandingTo evaluate students’ understanding of complete ionic equations, net ionic equations, and the formation of precipi-tates, write the reactants in some pre-cipitation reactions on the board. Include at least one example for which no reaction occurs. Have students write complete and net ionic equations.

ReteachReview with students the writing and balancing of complete and net ionic equations and the use of solubility rules to predict the outcome of dou-ble-replacement reactions. Stress that it is important to note the physical states of reactants and products in pre-cipitation reactions.

Elements Handbook

In the formation of limestone caves, calcium carbonate dissolves in car-bonic acid (H2CO3). Another reaction occurs when stalagmites form: Ca2+(aq) + 2HCO3

− (aq) → CaCO3(s) + CO2(g) + H2O(l)

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Section 11.3 Assessment30. A net ionic equation shows only those

particles involved in the reaction and is balanced with respect to both mass and charge.

31. Use the general rules for solubility of ionic compounds (Table 11.3).

32. a. Pb2+(aq) + SO42− (aq) → PbSO4(s)

b. Pb2+(aq) + 2Cl− (aq) → PbCl2(s)c. Fe3+(aq) + PO4

3−(aq) → FePO4(s)d. Co2+(aq) + S2−(aq) → CoS(s)

33. a. Ag+(aq) + Cl−(aq) → AgCl(s); spectator ions are H+(aq) and NO3

−(aq)b. Pb2+(aq) + 2Cl−(aq) → PbCl2(s); spec-tator ions are Li+(aq) and C2H3O2

−(aq)c. Cr3+ (aq) + PO4

3− (aq) → CrPO4 (s); spectator ions are Na+(aq) and Cl−(aq)

34. a. BaSO4(s) b. Al(OH)3(s) c. Ag2S(s)d. PbCl2(s) e. CaCO3(s)

35. a. yes; Ag2SO4(s) b. no c. yes; CaSO4(s)d. yes; PbCl2

344 Chapter 11

Table 11.3

Solubility Rules for Ionic Compounds

Handbook

Predicting the Formation of a PrecipitateYou have seen that mixing solutions of two ionic compounds can some-times result in the formation of an insoluble salt called a precipitate. Somecombinations of solutions produce precipitates, while others do not.Whether or not a precipitate forms depends upon the solubility of the newcompounds that form. You can predict the formation of a precipitateby using the general rules for solubility of ionic compounds. These rulesare shown in Table 11.3. Will a precipitate form when aqueous solutions ofNa2CO3(aq) and Ba(NO3)2(aq) are mixed?

2Na�(aq) � CO32�(aq) � Ba2�(aq) � 2NO3

�(aq) ¡ ?

When these four ions are mixed, the cations could change partners. Ifthey did, the two new compounds that would form are NaNO3 and BaCO3.These are the only new combinations of cation and anion possible. To findout if an exchange will occur, refer to Table 11.3, which gives guidelines fordetermining whether ion combinations are soluble. Recall that sodium isan alkali metal. Rows 1 and 2 tell you that sodium nitrate will not form aprecipitate because alkali metal salts and nitrate salts are soluble. Row 5indicates that carbonates in general are insoluble. Barium carbonate willprecipitate. In this reaction Na� and NO3

� are spectator ions. The net ionicequation for this reaction is as follows.

Ba2�(aq) � CO32�(aq) ¡ BaCO3(s)


30. Key Concept What is a net ionic equation?

31. Key Concept How can you predict the forma-tion of a precipitate in a double-replacement reaction?

32. Write a balanced net ionic equation for each reaction. a. Pb(NO3)2(aq) � H2SO4(aq) ¡ PbSO4(s) �

HNO3(aq) b. Pb(C2H3O2)2(aq) � HCl(aq) ¡ PbCl2(s) �

HC2H3O2(aq) c. Na3PO4(aq) � FeCl3(aq) ¡ NaCl(aq) �

FePO4(s) d. (NH4)2S(aq) � Co(NO3)2(aq) ¡ CoS(s) �

NH4NO3(aq)

33. Write a balanced net ionic equation for each reac-tion. Identify the spectator ions in each reaction.

a. HCl(aq) � AgNO3(aq) ¡ b. Pb(C2H3O2)2(aq) � LiCl(aq) ¡ c. Na3PO4(aq) � CrCl3(aq) ¡34. Identify the precipitate formed when solutions of

these ionic compounds are mixed. a. H2SO4 � BaCl2 ¡ b. Al2(SO4)3 � NH4OH ¡ c. AgNO3 � H2S ¡

d. CaCl2 � Pb(NO3)2 ¡ e. Ca(NO3)2 � Na2CO3 ¡35. Will a precipitate form when the following aque-

ous solutions of ionic compounds are mixed? a. AgNO3 and Na2SO4

b. NH4Cl and Ba(NO3)2

c. CaCl2 and K2SO4

d. Pb(NO3)2 and HCl

Limestone Caves Refer to page R13 in the Elements Handbook to learn more about the formation of limestone caves. What part does acid (hydrogen ion) play in the dissolving process? What is the reaction that deposits stalactites and stalagmites?

Compounds Solubility

Salts of alkali met-als and ammonia

Soluble

Nitrate salts and chlorate salts

Soluble

Sulfate salts, except com-pounds with Pb2�,Ag�, Hg2

2�, Ba2�,Sr2�, and Ca2�

Soluble

Chloride salts, except compound with Ag�, Pb2�,and Hg2

2�

Soluble

Carbonates, phos-phates, chromates, sulfides, and hydroxides

Most are insoluble

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Small-ScaleLAB

Small-ScaleLAB

Precipitation Reactions: Formation of SolidsObjective Students observe, identify, and write balanced equations for pre-cipitation reactions.

Prep Time 30 minutesClass Time 40 minutes

Teaching Tips• For disposal, flush all chemicals down

the drain with excess water.

Expected Outcome See below.

Analyze1. Na2CO3 + 2AgNO3 →

2NaNO3 + Ag2CO3(s)2Na3PO4 + 3Pb(NO3)2 →

6NaNO3 + Pb3(PO4)2(s)2. Sodium hydroxide and calcium chlo-

ride form sodium chloride and solid calcium hydroxide.

3. Mixings d, n, and o do not react.4. Na3PO4 + 3AgNO3 →

3NaNO3 + Ag3PO4(s)NaOH + AgNO3 → NaNO3 + AgOH(s)(actual products: NaNO3 + Ag2O(s) + H2O)NaCl + AgNO3 → NaNO3 + AgCl(s)Na2CO3 + Pb(NO3)2 →

2NaNO3 + PbCO3(s)2NaOH + Pb(NO3)2 →

2NaNO3 + Pb(OH)2(s)Na2SO4 + Pb(NO3)2 →

2NaNO3 + PbSO4(s)2NaCl + Pb(NO3)2 →

2NaNO3 + PbCl2(s)Na2CO3 + CaCl2 → 2NaCl + CaCO3(s)2Na3PO4 + 3CaCl2 →

6NaCl + Ca3(PO4)2(s)2NaOH + CaCl2 →

2NaCl + Ca(OH)2(s)5. 2Ag+ + CO3

2− → Ag2CO3(s)3Ag+ + PO4

3− → Ag3PO4(s)Ag+ + OH− → AgOH(s)Ag+ + Cl−→ AgCl(s)Pb2+ + CO3

2− → PbCO3(s)

Solution Preparation0.05M AgNO3 2.1 g in 250 mL

0.2M Pb(NO3)2 16.6 g in 250 mL0.5M CaCl2 13.9 g in 250 mL

1.0M Na2CO3 26.5 g in 250 mL0.1M Na3PO4 9.5 g Na3PO4•12H2O

in 250 mL0.5M NaOH 20.0 g in 1.0 L

0.2M Na2SO4 7.1 g in 250 mL1.0M NaCl 14.6 g in 250 mL

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Small-Scale Lab 345

Small-ScaleLAB

Small-ScaleLAB

Precipitation Reactions: Formation of Solids

PurposeTo observe, identify, and write balanced equations for precipitation reactions.

Materials

• pencil

• paper

• ruler

• reaction surface

• chemicals shown in the grid below

ProcedureCopy the grid on two sheets of paper. Make each square 2 cm on each side. Draw large black Xs on one of the grids. Place a reaction surface over the grid with black Xs and add the chemicals as shown. Use the other grid as a data table to record your observations for each solution.

AnalyzeUsing your experimental data, record your answers to the following in the space below your data table.

1. Translate the following word equations into balanced chemical equations and explain how the equations represent what happens in grid spaces a and g.

a. In grid space a, sodium carbonate reacts with silver nitrate to produce sodium nitrate and solid silver carbonate.

b. In grid space g, sodium phosphate reacts with lead(II) nitrate to produce sodium nitrate and solid lead(II) phosphate.

2. Write a word equation to represent what happens in grid space m.

3. What happens in grid space d? Which other mixings gave similar results? Is it necessary to write an equation when no reaction occurs? Explain.

4. Write balanced equations for the other precipitation reactions you observed.

5. Write balanced net ionic equations for the other pre-cipitation reactions you observed.

You’re The ChemistThe following small-scale activities allow you to develop your own procedures and analyze the results.

1. Explain It! Mix a solution of potassium iodide (KI) with silver nitrate. Then mix potassium iodide solution with lead(II) nitrate. Describe your results. Write balanced equations and net ionic equations for each reaction.

2. Design It! Table salt is mostly sodium chloride. Design and carry out an experiment to find out if table salt will form a precipitate with either lead(II) nitrate or silver nitrate. Interpret your results.

3. Design It! Design and carry out an experiment to show that iodized table salt contains potassium iodide.

a f k

b g l

c

d i n

e j o

Na2CO3

(CO32�)

Na3PO4

(PO43�)

NaOH

(OH�)

Na2SO4

(SO42�)

NaCl

(Cl�)

h m

g

h

AgNO3

(Ag�)

Pb(NO3)2

(Pb2�)

CaCl2(Ca2�)

3Pb2+ + 2PO43− → Pb3(PO4)2(s)

Pb2+ + 2OH− → Pb(OH)2(s)Pb2+ + SO4

2− → PbSO4(s)Pb2+ + 2Cl− → PbCl2(s)Ca2+ + CO3

2− → CaCO3(s)3Ca2+ + 2PO4

3− → Ca3(PO4)2(s)Ca2+ + 2OH− → Ca(OH)2(s)

For EnrichmentHave students design and conduct an experi-ment to determine which soaps and detergents form the least amount of precipitate when

added to “hard” water. (Soaps form precipitates; detergents do not.)

You’re The Chemist1. KI + AgNO3 → KNO3 + AgI(s)

Ag+ + I− → AgI(s)2KI + Pb(NO3)2 → 2KNO3 + PbI2(s)Pb2+ + 2I− → PbI2(s)

2. Add a drop of Pb(NO3)2 or AgNO3 to a few grains of salt. Look for white crystals.

3. Place a drop of Pb(NO3)2 on a small pile of dry table salt. Keep part of the pile dry. Look for yellow lead iodide.

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