Chapter 6: Chemical Reactions and Equations - Glencoe/McGraw-Hill

6CHAPTER

188

Chemical Reactions and EquationsChemical Reactions and Equations6

CHAPTER

6.1 Chemical Equations

MiniLab 6.1 Energy Change

6.2 Types of ReactionsMiniLab 6.2 A Simple ExchangeChemLab Exploring Chemical

Changes

6.3 Nature of ReactionsMiniLab 6.3 Starch-Iodine Clock

Reaction

Chapter PreviewSections What a Cool Chemical Reaction!

You probably do not think about chemicalreactions while watching fireworks, but youare seeing chemistry in action. The beautiful

colors are a result of metal salts such as bariumchloride and strontium chloride undergoingchemical reactions.

What a Cool Chemical Reaction!

189

Observing a Chemical ReactionReactants are consumed in a chemical reaction as prod-ucts are produced. What evidence can you observe that a reaction takes place?

Safety Precautions

Materials

• 10-mL graduated cylinder• 100-mL beaker• stirring rod• 0.01M potassium permanganate (KMnO4)• 0.01M sodium hydrogen sulfite (NaHSO3)

Procedure

1. Measure 5.0 mL of 0.01M potassium permanganatesolution (KMnO4) and pour it into a 100-mL beaker.

2. Add 5.0 mL of 0.01M sodium hydrogen sulfite solu-tion (NaHSO3) to the potassium permanganate solu-tion while stirring. Record your observations.

3. Slowly add additional 5.0-mL portions of the NaHSO3

solution until the KMnO4 solution turns colorless.Record your observations.

4. Record the total volume of the NaHSO3 solution you used to cause the beaker’s contents to becomecolorless.

Analysis

What evidence do you have that a reaction occurred?Would anything more have happened if you continuedto add NaHSO3 solution to the beaker? Explain.

Scan through Chapter 6. Use theheadings you see to create a chapteroutline. Make notes on your outlineas you read the chapter.

Reading Chemistry

Review the following conceptsbefore studying this chapter.Chapter 1: what a compound is; themeaning of a formula Chapter 3: what periodicity is; howperiodicity applies to elementsChapter 4: reasons why atoms combineChapter 5: how to write chemicalformulas

What I Already Know

Preview this chapter’s content andactivities at chemistryca.com

Start-up ActivitiesStart-up Activities

http://chemistryca.com

6.1

What do you rememberabout last summer’sFourth of July? Amazing

bursts of color from fireworksshot over a lake? Mouthwateringaromas coming from a barbecuegrill? Do the processes that resultin those colors and smells haveanything in common?

You learned in Chapter 1 that substances undergo both physical andchemical changes. A physical change does not change the substance itself,but a chemical change does. Did the chemicals making up the fireworks, char-coal, and barbecued food undergo chemical changes?

Recognizing Chemical ReactionsWhen a substance undergoes a chemical change, it takes part in a

chemical reaction. After it reacts, it no longer has the same chemical iden-tity. While it may seem amazing that a substance can undergo a changeand become part of a different substance, chemical reactions occuraround you all the time. Chemical reactions can be used to heat a home,power a car, manufacture fabrics for clothing, make medicines, and pro-duce paints and dyes in your favorite colors. Reactions also provide energyfor walking, running, working, and thinking.

Many important clues indicate when chemical reactions occur. None ofthem alone proves that such a change occurs because some physicalchanges involve one or more of these signs. Examine the photographs inFigure 6.1 to see what clues to look for.

ChemicalEquations

SECTION

Objectives✓ Relate chemicalchanges and macro-scopic properties.

✓ Demonstrate howchemical equationsdescribe chemicalreactions.

✓ Illustrate how tobalance chemical reac-tions by changingcoefficients.

Review VocabularyAllotrope: any of twoor more molecules ofa single element thathave different crys-talline or molecularstructures.

New Vocabularyreactantproductcoefficient

SECTION PREVIEW

Figure 6.1Signs of Chemical ReactionWhen substances undergo chemicalchanges, observable differences usuallyoccur. If you know what signs to look for, you can determine whether or not a chemical reaction has taken place.

190 Chapter 6 Chemical Reactions and Equations

Color changes often accompa-ny chemical changes. If youplace brownish-red iodine solu-tion on a freshly cut potato, itreacts with white starch to pro-duce a blue compound. �

6.1 Chemical Equations 191

� Precipitation of a solid from a solution canresult from a chemical change. Using soapwith hard water produces a precipitatecalled soap scum because the soap reactschemically with ions in the water.

Energy changes occur during allchemical changes. Heat or light canbe absorbed or released during achemical reaction, such as whenwood or another fuel rapidly com-bines with oxygen during burning. �

� Gas release sometimes occursas a result of a chemicalchange. Automobile exhaustcontains gases produced bythe combustion of gasoline.

� Odor changes can indicate that asubstance has undergone a chemi-cal change. When food spoils,odors change as a result of chemi-cal changes within the food.


Writing Chemical EquationsIn order to completely understand a chemical reaction, you must be

able to describe any changes that take place. Part of that descriptioninvolves recognizing what substances react and what substances form. Asubstance that undergoes a reaction is called a reactant. When reactantsundergo a chemical change, each new substance formed is called a prod-uct. For example, a familiar chemical reaction involves the reactionbetween iron and oxygen (the reactants) that produces rust, which isiron(III) oxide (the product). The simplest reactions involve a single reac-tant or a single product, but some reactions involve many reactants andmany products. Examine the chemical reactions shown in Figure 6.2.

Complete the DescriptionSeveral possible observations help determine when a chemical reaction

has taken place. But these observations don’t completely describe whathappens between reactants to form products. Have you ever seen whathappens when baking soda and vinegar are mixed together? They reactquickly, as you can tell by the bubbles that seem to explode out of the mix-ture, as shown in Figure 6.3. In describing this reaction, you could say thatbaking soda and vinegar turn into bubbles. But does that completelyexplain what happens? What are the bubbles made of? Do all of the atomsin vinegar and baking soda form bubbles? The reaction involves more thanwhat can be determined by observation alone. Just as you can write a sen-tence to tell others what happened on your way to school today, chemistsrepresent the changes taking place in a reaction by writing equations.

product:pro (L) forwardducere (L) to lead

Formation of aproduct helps pulla reaction forward.

Figure 6.2 Parts of a Chemical Reaction

� Compounds in woodchemically combine withoxygen when they burnto form water and car-bon dioxide.

Energy is needed by ourbodies to perform dailyactivities. This energy isprovided when glucosecombines with oxygen incells. This reaction formsthe same two substancesproduced when compoundsin wood combine with oxy-gen. What are the reactantsin these reactions? Whatare the products? �


Word EquationsThe simplest way to represent a reaction is by using words to describe

all the reactants and products, with an arrow placed between them to rep-resent change, as shown in Figure 6.3. As you can see in this word equa-tion, reactants are placed to the left of the arrow, and products are placedto the right. Plus signs are used to separate reactants and also to separateproducts.

Vinegar and baking soda are common names. The compound in vine-gar that is involved in the reaction is acetic acid, and baking soda is sodi-um hydrogen carbonate. These scientific names can also be used in aword equation.

acetic acid � sodium hydrogen carbonate ˇsodium acetate � water � carbon dioxide

Chemical EquationsWord equations describe reactants and products, but they are long and

awkward and do not adequately identify the substances involved. Wordequations can be converted into chemical equations by substituting chem-ical formulas for the names of compounds and elements. Recall fromChapter 5 that these formulas can be written by using the oxidation num-bers of the elements and the charges of the polyatomic ions. For example,the equation for the reaction of vinegar and baking soda can be writtenusing the chemical formulas of the reactants and products.

HC2H3O2 � NaHCO3 ˇ NaC2H3O2 � H2O � CO2

By examining a chemical equation, you can determine exactly what ele-ments make up the substances that react and form.

Figure 6.3A Sample Word EquationVinegar and baking soda react vigorously, forming a bubblyproduct. This reaction was formerly used in fire extinguishersbecause the bubbles produced contain carbon dioxide, which iseffective in putting out fires. This reaction can be described by

the following word equation.

vinegar � baking soda ˇsodium acetate � water � carbon dioxide


Whitening WhitesWhy might eating this meal be worrisome?

There isn’t a paper napkin in sight—only whitelinen ones! Not to worry. You’ll be able to removemost evidence of sloppy etiquette with a six percent aqueous solution of sodium hypo-chlorite.

Household bleach Perhaps the most popular type of household bleach is an aque-ous solution of sodium hypochlorite,NaClO. Sodium hypochlorite is madeby reacting chlorine gas with an aque-ous solution of sodium hydroxide, asshown in the following equation.

Cl2(g) � 2NaOH(aq) ˇNaClO(aq) � NaCl(aq) � H2O(l)

In an aqueous solution, NaClO doesnot exist as a complete unit, but assodium ions, Na�, and hypochloriteions, ClO�. The ingredient responsiblefor the bleaching action in this type ofbleach is the hypochlorite ion. Many other solidand liquid bleaches contain hydrogen peroxide,H2O2, instead of sodium hypochlorite. In thesebleaches, the active substance in solution is theperhydroxyl ion, HOO�. What do the ClO� andHOO� ions have in common?

Bleaching reactions As you can see, each of thesepolyatomic ions carries a single negative charge. Ifeach could react with a hydrogen ion, the follow-ing reactions would happen.

ClO� � H�� ˇ HCl � O

HOO� � H�� ˇ H2O � O

Because the compounds HCl and H2O are morestable than the hypochlorite and perhydroxyl ions,these reactions occur. The reactions result in ableaching action because the released oxygen

reacts with molecules of materials that cause thestain. The molecules of the compounds that causecolor in a stain are structured in a way that givesthem the physical property of producing color. Inthe reaction between these compounds and atomicoxygen, the compound or compounds formedhave different structures. These structures do nothave physical properties of producing color. So ableach bleaches by rendering a colored compoundcolorless.

Chemistry

Exploring Further

1. Applying Liquid bleaches containing sodiumhypochlorite are often sold in opaque, plasticcontainers because sunlight causes the com-pound to decompose to produce oxygen gasand sodium chloride. Write the balanced chem-ical equation for this reaction.

2. Inferring Why do you think bleaches contain-ing sodium hypochlorite tend to damage finerfabrics more than bleaches containing hydrogenperoxide?

To find out more about bleaching, visit theChemistry Web site at chemistryca.com



It may also be important to know the physical state of each reactantand product. How can we indicate that the bubbles we see during thisreaction are CO2? Symbols in parentheses are put after formulas to indi-cate the state of the substance. Solids, liquids, gases, and water (aqueous)solutions are indicated by the symbols (s), (l), (g), and (aq). The followingequation shows these symbols added to the equation for the reaction ofvinegar and baking soda.

HC2H3O2(aq) � NaHCO3(s) ˇ NaC2H3O2(aq) � H2O(l) � CO2(g)

Now the equation tells us that mixing an aqueous solution of aceticacid (vinegar) with solid sodium hydrogen carbonate (baking soda)results in the formation of an aqueous solution of sodium acetate, liquidwater, and carbon dioxide gas. If you had examined this equation beforeyou mixed vinegar and baking soda, you could have predicted that bub-bles would form.

Energy and Chemical EquationsNoticeable amounts of energy are often released or absorbed during a

chemical reaction. Some reactions absorb energy. If energy is absorbed,the reaction is known as an endothermic reaction. Figure 6.4 shows anexample of an endothermic reaction. For a reaction that absorbs energy,the word energy is sometimes written along with the reactants in thechemical equation. For example, the equation for the reaction in whichwater breaks down into hydrogen and oxygen gases shows that energymust be added to the reaction.

2H2O(l) � energy ˇ 2H2(g) � O2(g)

Figure 6.4 An Endothermic Reaction

� The reaction of ammonium chlorideand barium hydroxide octahydrate isendothermic.

Noble gases are rela-tively unreactive, butthey are not totallyinert. The first com-pound containing anoble gas element wassynthesized by NeilBartlett in 1962. Hemade xenon hexafluo-ride using the followingreaction.Xe(g) � 3F2(g) ˇ

XeF6(s)Stable compounds ofkrypton and radonhave also been synthe-sized. Helium, neon,and argon form noknown stable com-pounds, but they havebeen observed to formcompounds for shortperiods of time.

If these reactants are mixed at room tem-perature, which is about 20°C, the temper-ature in the mixture drops as energy isabsorbed by the reaction. �

All reactions that occur in a Bunsen burner or gas grill or those used topower an automobile release energy. The How It Works feature showsanother example of such a reaction. As you will recall from Chapter 1,reactions that release heat energy are called exothermic reactions. Whenwriting a chemical equation for a reaction that produces energy, the wordenergy is sometimes written along with the products. For example, theequation for the reaction that occurs when you light methane in a Bunsenburner shows that energy is released. Some of this energy is in the form oflight.

CH4(g) � 2O2(g) ˇ CO2(g) � 2H2O(g) � energy

You may have noticed that the word energy is not always written in anequation. It is used only if it is important to know whether energy isreleased or absorbed. For the burning of methane, energy would be writ-ten in the equation because the release of heat is an important part ofburning a fuel. Energy would also be written in the equation thatdescribes the reaction when water is broken down into hydrogen and oxy-gen because the reaction would not occur without the addition of energy.In many reactions, such as the formation of rust, energy might be releasedor absorbed but it is not included in the equation because it is not impor-tant to know about the energy involved in that particular reaction.

Energy ChangeAll chemical reactions involve an energy change. This change may be

so slight that it can be detected only with sensitive instruments, or itmay be quite noticeable.

Procedure1. Place 25 g of iron powder and

1 g of NaCl in a resealable plastic bag.

2. Add 30 g of vermiculite tothe bag, seal the zipper,and shake the bag to mixthe contents.

3. Add 5 mL of water to thebag, reseal the zipper, andgently squeeze and shakethe contents to mix them.

4. Hold the bag betweenyour hands and note anychanges in temperature.

Analysis1. What did you observe?

What type of reactionproduces this kind ofchange?

1


2. Using the photo, what practicalapplication can you think of forthis reaction?


1. Light sticks are plastic rods thatcontain two solutions of chemi-cals. Enclosed inside a thinglass ampoule is one solu-tion that is an oxidizingagent, and surround-ing it in the rod is asecond solutionthat contains afluorescentdye.

Emergency Light SticksThe light given off by an emergency light stick is energy

released from a chemical reaction. Light sticks are a goodsource of light when no electricity is available. Reac-tions in which light is given off are called chemi-luminescent reactions. Different chemicals used inthese reactions give off different colors, so lightsticks come in many colors. The light from alight stick is only temporary; when thereactants are used up, light is no longerproduced.

1. Explain why thechemical reaction inthe light stick is notexothermic, eventhough it producesenergy.

2. What are someadvantages of lightsticks over conven-tional light sources?

Thinking Critically4. When the electrons drop back

down to their original energy lev-els, the extra energy is given off aslight. This light is sometimesreferred to as cool light because nonoticeable heat is given off in thereaction.

2. When the light stick is bent, theglass ampoule is broken and thetwo solutions mix. The reactionbegins. Energy is given off whenthe two solutions react.

3. The energy raises the energy level ofthe electrons of the dye molecules.


Balancing Chemical EquationsWhat do you think happens to the atoms in reactants when they are

converted into products? Some products, such as the CO2 produced frombaking powder when a cake is baked, seem to disappear into the air. Whatreally happens to them?

The Law of Conservation of MassRecall from Chapter 2 that you can test a reaction to determine whether

the same amount of matter is contained in the products and the reactants.That type of experiment was first carried out by the French scientistAntoine Lavoisier (1743-1794), Figure 6.5. His results indicated that themass of the products is always the same as the mass of the reactants thatreact to form them. The law of conservation of mass summarizes thesefindings. Matter is neither created nor destroyed during a chemical reaction.

Conservation of AtomsRemember that atoms don’t change in a chemical reaction; they just

rearrange. The number and kinds of atoms present in the reactants of achemical reaction are the same as those present in the products. Whenstated this way, it becomes the law of conservation of atoms. For a chemi-

cal equation to accurately representa reaction, the same number ofeach kind of atom must be on theleft side of the arrow as are on theright side. If an equation followsthe law of conservation of atoms, itis said to be balanced.

How can you count atoms in anequation? The easiest way to learnis to practice—first with a simplereaction and then with some thatare more complex. For example,consider the equation that repre-sents breaking down carbonic acidinto water and carbon dioxide.

H2CO3(aq) ˇ H2O(l) � CO2(g)

Because a subscript after thesymbol for an element representshow many atoms of that elementare found in a compound, you cansee that there are two hydrogen,one carbon, and three oxygenatoms on each side of the arrow.All of the atoms in the reactants arethe same as those found in theproducts.

Figure 6.5 Lavoisier’s ContributionOne of the experiments thatLavoisier used to discoverthe law of conservation ofmass was the decompositionof the red oxide of mercuryto form mercury metal andoxygen gas. He weighed theamount of HgO that decom-posed and found it to be thesame as the total weight ofHg and O2

produced.


Why can’t subscripts be changed when balancing anequation? Changing a subscript changes the identity ofthat substance. Look at the equation in Sample Problem 1.Changing the subscript of the oxygen in water to 2 changeswater, H2O, to hydrogen peroxide, H2O2, a different com-pound. Changing a coefficient simply means that you arechanging the amount of that substance compared to theother substances in the reaction. Changing the coefficientof water to 2 means there are two molecules of water,2H2O. The identity of water stays the same.

Figure 6.6 How many atoms?Examine the balanced equa-tion that shows what happenswhen carbon reacts with oxy-gen to form carbon dioxide(left). If a piece of coal con-tains 10 billion C atoms, howmany molecules of O2 will itreact with? How many mole-cules of CO2 will be formed?

C � O2 CO2

1 carbonatom

1 oxygenmolecule

1 molecule ofcarbon dioxide

Examine the equation for the formation of sodium carbonate andwater from the reaction between sodium hydroxide and carbon dioxide.

NaOH(aq) � CO2(g) ˇ Na2CO3(s) � H2O(l)

Do both sides of the equation have the same number of each type ofatom? No. One carbon atom is on each side of the arrow, but the sodium,oxygen, and hydrogen atoms are not balanced. The equation, as written,does not truly represent the reaction because it does not show conserva-tion of atoms.

A Balancing ActTo indicate more than one unit taking part or being formed in a reac-

tion, a number called a coefficient is placed in front of it to indicate howmany units are involved. Look at the previous equation with a coefficientof 2 in front of the sodium hydroxide formula.

2NaOH(aq) � CO2(g) ˇ Na2CO3(s) � H2O(l)

Is the equation balanced now? Two sodium atoms are on each side.How many oxygen atoms are on each side? You should be able to findfour on each side. How about hydrogen atoms? Now two are on each side.Because one carbon atom is still on each side, the entire equation is bal-anced; it now represents what happens when sodium hydroxide and car-bon dioxide react.

The balanced equation tells us that when sodium hydroxide and carbondioxide react, two units of sodium hydroxide react with each molecule ofcarbon dioxide to form one unit of sodium carbonate andone molecule of water. Look at a different balanced equa-tion in Figure 6.6.


Write word and chemical equations for the reaction of hydrogenand oxygen gases to form gaseous water and release energy. This reac-tion powers the main stage of the space shuttle.

Analyze • To write a word equation for the reaction, write the names of the reac-tants, draw an arrow, then write the name of any product. If there ismore than one reactant or product, plus signs should separate them.

hydrogen � oxygen ˇ water � energy

Set Up • To write the chemical equation, use chemical formulas to replace thenames of the reactants and products in the word equation you wrote.Then add symbols to represent the physical state of each compound.Remember that hydrogen and oxygen occur as diatomic gases.

H2(g) � O2(g) ˇ H2O(g) � energy

Solve • To balance the atoms on each side of thearrow, count the number of atoms ofeach type on each side of the arrow. Onthe left are two hydrogen atoms and twooxygen atoms. On the right are also twohydrogen atoms but only one oxygenatom. Change the coefficient of the water to 2 so that the number ofoxygen atoms will be balanced. Because that puts four hydrogen atomson the right side of the arrow, the coefficient of hydrogen gas alsomust be changed to 2.

2H2(g) � O2(g) ˇ 2H2O(g) � energy

Check • Make a final check of all atoms to make sure they are balanced.

Writing a Simple EquationSAMPLE PROBLEM 1

Write word and chemical equations for the reaction that takes placewhen an aqueous solution of magnesium chloride is added to a silvernitrate solution. Aqueous magnesium nitrate and solid silver chlorideform.

Analyze • To write a word equation for the reaction, write the names of the reac-tants, draw an arrow, then write the name of any product. If there ismore than one reactant or product, plus signs should separate them.

magnesium chloride � silver nitrate ˇmagnesium nitrate � silver chloride

Set Up • To write the chemical equation, use chemical formulas to replace thenames of the reactants and products in the word equation you wrote.Remember to use the oxidation number of an element and the chargeon a polyatomic ion to write a correct formula. Then add symbols torepresent the physical state of each compound.

MgCl2(aq) � AgNO3(aq) ˇ Mg(NO3)2(aq) � AgCl(s)

Writing an EquationSAMPLE PROBLEM 2

Be sure to change only coefficients, notsubscripts, when balancing equations.

Prob lem -So l v i ngH I N T


Write word equations and chemical equations for the following reactions.1. Magnesium metal and water combine to form solid magnesium

hydroxide and hydrogen gas.

2. An aqueous solution of hydrogen peroxide (dihydrogen dioxide)and solid lead(II) sulfide combine to form solid lead(II) sulfate andliquid water.

3. When energy is added to solid manganese(II) sulfate heptahydratecrystals, they break down to form liquid water and solid man-ganese(II) sulfate monohydrate.

4. Solid potassium reacts with liquid water to produce aqueous potas-sium hydroxide and hydrogen gas.

PRACTICE PROBLEMS

Solve • To balance the atoms on each side of the arrow, count the number ofatoms of each type on each side of the arrow. On the left are one mag-nesium atom and two chlorine atoms. On the right, there is also onemagnesium atom but only one chlorine atom. Change the coefficientof the AgCl to 2 so that the number of chlorine atoms will be bal-anced. Because that puts two silver atoms on the right side of thearrow, the coefficient of AgNO3 also must be changed to 2. This bal-ances the oxygen and nitrogen atoms, as well.

MgCl2(aq) � 2AgNO3(aq) ˇ Mg(NO3)2(aq) � 2AgCl(s)

Check • Make a final check of all atoms on both sides of the equation to makesure they are balanced.

Understanding Concepts1. Explain how you can tell whether a chemical

reaction has taken place.

2. Write balanced chemical equations for the reac-tions described.

a) sodium metal � chlorine gas ˇsodium chloride crystals

b) propane gas � oxygen ˇcarbon dioxide � water vapor � energy

c) zinc metal � hydrochloric acid ˇzinc chloride solution � hydrogen

3. Why is it important to balance a chemical equation?

Thinking Critically4. Applying Concepts Use the law of conserva-

tion of mass to determine the following:

a) The number of grams of CO2 that formfrom 4.00 g C and 10.67 g O2.

C � O2 ˇ CO2

b) The number of grams of water formed if7.75 g H2CO3 forms 5.50 g CO2.

H2CO3 ˇ H2O � CO2

Applying Chemistry5. Catalytic Converter In the catalytic converter

of a car, a reaction occurs when nitrogenmonoxide gas, NO, reacts with hydrogen gas.Ammonia gas and water vapor are formed.Write a balanced equation for this reaction.

SECTION REVIEW

For more practice with solvingproblems, see SupplementalPractice Problems,Appendix B.

chemistryca.com/self_check_quiz

http://chemistryca.com/self_check_quiz

6.2

Chemistry has a lot in commonwith cooking. What do you haveto do to become a good cook? A

lot of study and probably even morepractice are necessary. The same istrue of chemistry. If you combinethe right amounts of the right reac-tants in the correct order under certainconditions, you will get the right products.

As you study chemistry, you will begin torecognize what types of reactants and conditions lead to certain products. Inthis section, you will learn to recognize five major classes of reactions. Just asan experienced chef can tell a sauté from a soufflé, you will soon be able todistinguish between combustion and decomposition.

Why Reactions Are ClassifiedWhy are reactions grouped into classes? Think about why scientists

classify plants and animals. If you were hiking in the Rocky Mountains forthe first time, you might see animals you have never seen before, such asthe ones in Figure 6.7. How would you decide whether or not to be cau-tious around them? You would probably decide what familiar animalsthey most resemble. If the animal looks like a cat, it is most likely a preda-tor that you should be careful around. If it looks like a goat, you willprobably realize that it is more likely to run from you than eat you.

SECTION

Objectives✓ Distinguish amongthe five major types ofchemical reactions.

✓ Classify a reactionas belonging to one offive major types.

Review VocabularyReactant: a substancethat undergoes a reaction.

New Vocabularysynthesisdecompositionsingle displacementdouble displacementcombustion

SECTION PREVIEW

Figure 6.7ClassifyingAround which animal would you be most cautious?Did you mentally classify the animals into categories ofsome type before answering?

202

Types of Reactions

6.2 Types of Reactions 203

Major Classes of ReactionsJust as there are thousands of species of animals, there are many differ-

ent types of chemical reactions. Five types are common. If you can classifya reaction into one of the five major categories by recognizing patternsthat occur, you already know a lot about the reaction.

In one type of reaction, two substances—either elements or com-pounds—combine to form a compound. Whenever two or more sub-stances combine to form a single product, the reaction is called a synthesisreaction.

BIOLOGY CONNECTION

Connecting to Chemistry

1. Applying Identifyby name the prod-ucts of the reactionbetween carbondioxide and lithiumhydroxide.

2. Comparing andContrasting Howdoes the chemical

removal of carbondioxide from theatmosphere of theOrbiter comparewith the geochemi-cal removal of car-bon dioxide fromEarth’s atmosphere?

Air in SpaceThe concentration of carbon dioxide in

Earth’s atmosphere is regulated through acomplex interplay of human, biological, andgeological mechanisms. In a space vehicle,these mechanisms aren’t available. If not con-trolled, carbon dioxide from the respirationof astronauts could become toxic to them. Sohow is air quality maintained within a spacevehicle?

A 66-m3 atmosphere The volume of thecrew compartment of the space shuttleOrbiter is about 66 m3. The air is maintainedat a pressure of 100 kPa, which is similar toEarth’s atmospheric pressure at sea level.During a flight, the composition of the air inthe crew compartment is maintained at 79percent nitrogen and 21 percent oxygen,which is almost identical to that of Earth’satmosphere. Oxygen is carried in the Orbiteras a liquid stored in two cryogenic tanks inthe mid-fuselage. The gaseous oxygen fromthe tanks passes through pressurizing andheating nozzles and moves into the crewcompartment. A five-member crew will nor-mally consume about 4 kg of oxygen everyday. Nitrogen is supplied from two systems,each made of two storage tanks, also in themid-fuselage of the Orbiter. The atmosphereof the compartment is recycled about everyseven minutes.

Filtering the airDuring recycling,odors are removedby filters containingactivated charcoalgranules, whichabsorb from the airthe chemical sub-stances that cause theodor. Carbon dioxidegas is removed fromthe air by reacting itwith solid lithiumhydroxide.

CO2(g) � 2LiOH(s) ˇ Li2CO3(s) � H2O(g)

The lithium hydroxide is stored in canistersthat are changed every 12 hours. The usedcanisters are then stored for disposal whenthe Orbiter returns to Earth.


An example of a synthesis reaction involving elements as reactants isshown in Figure 6.8. A synthesis reaction also occurs when two com-pounds combine, such as when rainwater combines with carbon dioxidein the air to form carbonic acid, or when an element and a compoundcombine, as when carbon monoxide combines with oxygen to form car-bon dioxide.

In a decomposition reaction, a compound breaks down into two ormore simpler substances. The compound may break down into individualelements, such as when mercury(II) oxide decomposes into mercury andoxygen. The products may be an element and a compound, such as whenhydrogen peroxide decomposes into water and oxygen, or the compoundmay break down into simpler compounds, as shown in Figure 6.9.

Figure 6.9A Decomposition ReactionWhen ammonium nitrate is heatedto a high temperature, it explosive-ly breaks down into dinitrogenmonoxide and water. The decom-position reaction taking place isrepresented by a balanced equa-tion that shows one reactant andmore than one product.

NH4NO3(s) ˇ N2O(g) � 2H2O(g)

Figure 6.8A Synthesis ReactionWhen iron rusts, iron metaland oxygen gas combine toform one new substance,iron(III) oxide. The balancedequation for this synthesisreaction shows that there ismore than one reactant butonly one product.

4Fe(s) � 3O2(g) ˇ 2Fe2O3(s)

synthesis:syn (Gk) togethertithenai (Gk) toplace

A synthesis reac-tion involves plac-ing elements orcompoundstogether to make acompound.

In a single-displacement reaction, one element takes the place of anoth-er in a compound, as shown in Figure 6.10. The element can replace thefirst part of a compound, or it can replace the last part of a compound.

A Simple ExchangeSingle-displacement reactions occur when one element replaces

another in a compound. However, such a reaction does not automati-cally occur just because an element and a compound are mixed togeth-er. Whether or not such a reaction occurs depends on how reactive theelement is compared to the element it is to displace.

Procedure

1. Pour 0.1M AgNO3 solutioninto a test tube until it is halffull. Clean a piece of copperwire or copper foil with steelwool.

2. Drop the copper foil or copperwire into the solution.

3. Keeping the test tube absolutelystill, observe what happens overa half-hour period of time.

2


Figure 6.10Single Displacement

� If an iron nail is placed into an aqueous solution of copper(II)sulfate, the iron displaces the copper ions in solution, andcopper metal forms on the nail.

Fe(s) � CuSO4(aq) ˇ FeSO4(aq) � Cu(s)

When the chlorine gas in the flaskon the left is bubbled through an aqueous solution of sodiumbromide, the chlorine replaces the bromine in the compound. The reddish-brown bromine can be seen in the solution. �

Cl2(g) � 2NaBr(aq) ˇ2NaCl(aq) � Br2(l)

Analysis1. What changes do you

observe in the wire? Inthe solution?

2. Write a balanced equa-tion for the reaction.

3. Does copper displacesilver in silver nitrate?Does silver displace copper in copper(II) nitrate? How do you know?


Exploring Chemical ChangesMost reactions can be classified into five major

types. As you carry out this experiment, you’llobserve examples of each of these types. In doingso, you will also learn to recognize many of thephysical changes that accompany reactions.

ProblemWhat are some of the physical changes that

indicate that a reaction has occurred?

Objectives

•Observe physical changes that take place duringchemical reactions.

•Compare changes that take place during differ-ent types of chemical reactions.

Materials125-mL flasks (4) large test tube and one-balance hole stopper with glasshot plate tube and rubber tubingwatch glass attachedspatula ring standstirring rod test-tube clamplab burner 0.1M CuSO4

file granular copper, Cunew penny powdered sulfur, S250-mL flask CaCO3, finely groundice saturated Ca(OH)2

tongs solution, limewater100-mL graduated 6M HClcylinder 0.5M Na2CO3

0.5M CuCl2

Safety Precautions

Wear an apron and goggles. Use care whenhandling hot objects. Dispose of the reaction mix-ture and products as instructed by your teacher.

For each of the following reactions, record in adata table all changes that you observe.

Synthesis Reaction1. Place 50 mL 0.1M CuSO4 in a 125-mL flask.

2. Place 1.6 g granular copper and 0.8 g pow-dered sulfur on a watch glass and mix togetherthoroughly with a spatula.

3. Heat the flask on a hot plate set at high untilthe solution begins to boil.

4. Stir the Cu/S mixture into the boiling CuSO4

solution.

5. Continue boiling until a black solid forms.

Decomposition Reaction1. Place 100 mL of saturated Ca(OH)2 solution

(limewater) in the 250-mL flask.

2. Add finely ground CaCO3 to a large test tubeuntil it is one-fourth full. Stopper the tubewith the stopper/glass tube/rubber tubingassembly, and clamp the tube to the ring stand.

3. Light a laboratory burner, and begin to heat thetest tube. Submerge the end of the rubber tub-ing into the limewater so that any gas producedin the tube will bubble through the limewater.

4. Continue heating the CaCO3 until you observea change in the limewater. The presence ofCO2 causes limewater to become cloudy.

Single-Displacement Reaction1. Place 30 mL 6M HCl in a 125-mL flask.

SMALLSCALESMALLSCALE


2. Using a file, cut six 0.2-cm notches evenlyspaced around the perimeter of a new penny.

3. Place the penny in the flask of acid and leave itin a fume hood overnight.

Double-Displacement Reaction1. Add 25 mL 0.5M Na2CO3 and 25 mL 0.5M

CuCl2 to a 125-mL flask.

2. Swirl the flask gently until you observe the for-mation of a precipitate.

Combustion Reaction1. Light a laboratory burner and adjust the air

and gas supplies until the flame is blue.Observe what happens.

2. Using tongs, hold a flask or beaker with ice init about 10 cm over the flame for approximate-ly one minute. Move the flask away from theflame and observe the bottom of the flask.

1. Making Inferences Which observations notedduring each of the reactions indicated that areaction had occurred?

2. Comparing andContrastingWhat did allof the reac-tions havein com-mon?

3. MakingInferencesWrite thename and for-mula of the

a) black solid formed in the synthesis reaction.

b) gaseous product of the decomposition reac-tion.

c) solid product of the decomposition reac-tion.

d) pale blue precipitate in the double-displacement reaction.

e) liquid product of the combustion reaction.

4. Observing and Inferring Explain how thepenny changed during the single-displacementreaction. What would happen if a pre-1983penny, which is solid copper, were used?

5. Relating Concepts Is energy a reactant orproduct of the combustion reaction?

1. Were there any physical changes that oftenoccur during a reaction that you did notobserve while doing this ChemLab? If so, whatwere they?

2. Write balanced chemical equations for all ofthe reactions carried out.

3. Why do you think pennies are no longer madefrom only copper metal?

Reaction Observations

Synthesis

Decomposition

Single displacement

Double displacement

Combustion

In double-displacement reactions, the positive por-tions of two ionic compounds are interchanged. For adouble-displacement reaction to take place, at leastone of the products must be a precipitate or water. Anexample of a double-displacement reaction is shownin Figure 6.11.

Reaction Type General EquationSynthesis element/compound � element/compound ˇ compound

Examples: 2Na(s) � Cl2(g) ˇ 2NaCl(s)CaO(s) � SiO2(l) ˇ CaSiO3(l)

Decomposition compound ˇ two or more elements/compounds Examples: PCl5(s) ˇ PCl3(s) � Cl2(g)

2Ag2O(s) ˇ 4Ag(s) � O2(g)

Single displacement *element a � compound bc ˇ element b � compound acExample: 2Al(s) � Fe2O3(s) ˇ 2Fe(s) � Al2O3(s)element d � compound bc ˇ element c � compound bdExample: Cl2(aq) � 2KBr(aq) ˇ 2KCl(aq) � Br2(aq)

Double displacement compound ac � compound bd ˇ compound ad � compound bcExamples: PbCl2(s) � Li2SO4(aq) ˇ PbSO4(s) � 2LiCl(aq)

BaCl2(aq) � H2SO4(aq) ˇ 2HCl(aq) � BaSO4(s)

Combustion element/compound � oxygen ˇ oxide(s)Examples: CH4(g) � 2O2(g) ˇ CO2(g) � 2H2O(g)

C6H12O6(s) � 6O2(g) ˇ 6CO2(g) � 6H2O(l)

*The letters a, b, c, and d each represent different elements or parts of compounds. For example, in compound ac, arepresents the positive part of the compound, and c represents the negative part.

Table 6.1 Types of Reactions

Figure 6.11Double DisplacementWhen clear aqueous solutions of lead(II) nitrate andpotassium iodide are mixed, a double-displacementreaction takes place and a yellow solid appears in themixture. This solid is lead(II) iodide, and it precipitatesout because it is insoluble in water, unlike the tworeactants and the other product.

Pb(NO3)2(aq) � 2KI(aq) ˇ PbI2(s) � 2KNO3(aq)


The fifth common type of reac-tion is a combustion reaction. Acombustion reaction is one inwhich a substance rapidly combineswith oxygen to form one or moreoxides, as shown in Figure 6.12.

Although there are exceptions,many thousands of specific reac-tions fit into these five classes. One example is in the Chemistry and Tech-nology feature on pages 216-217. Table 6.1 summarizes important infor-mation about each of these reaction types.

combustion:comburere (L) toburn up

When wood burns,it reacts with oxy-gen in a combus-tion reaction.

Figure 6.12CombustionWhen welding is done with an acety-lene torch, acetylene combines withoxygen to form carbon dioxide andwater. This combustion reaction isexothermic, and enough energy isreleased to melt metal.

2C2H2(g) � 5O2(g) ˇ4CO2(g) � 2H2O(g) � energy


SECTION REVIEWFor more practice

with solving problems,see Supplemental Practice Problems, Appendix B.

Understanding Concepts1. Explain why classifying reactions can be useful.

2. Using different symbols to represent differentatoms, draw pictures to represent an example ofeach of the following kinds of reactions.

a) synthesisb) decompositionc) single displacementd) double displacemente) combustion

3. Classify each of the following reactions.

a) N2O4(g) ˇ 2NO2(g)

b) 2Fe(s) � O2(g) ˇ 2FeO(s)

c) 2Al(s) � 3Cl2(g) ˇ 2AlCl3(s)

d) BaCl2(aq) � Na2SO4(aq) ˇBaSO4(s) � 2NaCl(aq)

e) Mg(s) � CuSO4(aq) ˇ Cu(s) � MgSO4(aq)

Thinking Critically4. Making Inferences When a candle burns, wax

undergoes a combustion reaction. Would a can-dle burn longer in the open or when coveredwith an inverted glass jar? Explain.

Applying Chemistry5. Decomposition When a fungus breaks down

the wood in a fallen tree, the biological processis called decomposition. What does that processhave in common with the chemical decomposi-tion reaction type?



6.3

You know that changes constant-ly occur, but some changes arenot permanent. For example,

liquid water freezes into ice, but thenice melts and becomes liquid wateragain. In other words, the freezingprocess is reversed. Can chemicalreactions be reversed? Can the prod-uct of a reaction become a reactant?

Reversible ReactionsMany reactions can change direc-

tion. These reactions are called reversiblereactions. Some reactions, such as the oneshown in Figure 6.13, can be reversed basedon the energy flow.

Not all chemical changes are reversible. Caves form, paint hardens, andfuel burns. These chemical changes result in new products, and the reac-tions are said to go to completion because at least one of the reactants iscompletely used up and the reaction stops. The reactions can’t bereversed. But what happens when a reaction reverses?

SECTION

Objectives✓ Demonstrate fac-tors that influence the direction of a reaction.

✓ Classify factors thatinfluence the rate of areaction.

Review VocabularySynthesis: reaction inwhich two or moresubstances combine toform a single product.

New Vocabularyequilibriumdynamic equilibriumsolubleinsolubleactivation energyconcentrationlimiting reactantcatalystenzymeinhibitor

SECTION PREVIEW


Figure 6.13Reversible RechargeWhen an automobile battery releasesenergy when the car isn’t running,the reaction below moves to theright. If you leave the lights on andneed to recharge the battery with ajump start, the reaction moves to the left while the car engine runs.

Pb(s) � PbO2(s) � 2H2SO4(aq) Ø2PbSO4(s) � 2H2O(l) � energy

Nature of Reactions

6.3 Nature of Reactions 211

Picture what happens in a subway station when the door of a trainopens for the first time that day. Passengers rush in, streaming across theplatform and through the doors. No passengers get out because this is thefirst train of the day. When the train stops at the next station, more pas-sengers rush in. A few probably leave the train at this point, as well. Witheach successive stop, more passengers will be getting on, but more andmore will also be getting off. Look at Figure 6.14. To describe a similarsituation in chemical reactions in which a reaction automatically reversesand there is no net (overall) change, we use the term equilibrium.

EquilibriumWhen no net change occurs in the amount of reactants and products, a

system is said to be in equilibrium. In most situations, chemical reactionsexist in equilibrium when products and reactants form at the same rate.Such a system, in which opposite actions are taking place at the same rate,is said to be in dynamic equilibrium.

Reactions at equilibrium have reactants and products changing places,much like the passengers getting on and off a subway train. In equilibri-um, reactants are never fully used up because they are constantly beingformed from products. Eventually, reactants and products form at thesame rate.

One example of a reversible reaction that reaches equilibrium occurswhen lime, CaO, which is used to make soils less acidic, is formed bydecomposing limestone, CaCO3.

CaCO3(s) Ø CaO(s) � CO2(g)

Notice that the single arrow in the equation has been replaced withdouble arrows. Because an arrow shows what direction the reaction isgoing, the double arrow indicates that the reaction can go in either direc-tion. In this case, CaCO3 decomposes into CaO and CO2. But, as thoseproducts form, CO2 and CaO combine to form CaCO3. The rate, orspeed, of each reaction can be determined by how quickly a reactant dis-appears. Eventually, the rates of the two reactions are equal, and an equi-librium exists.

Figure 6.14No Net ChangeFor this train, the initialmovement is all in onedirection. Eventually, thenumber of passengers onthe train will not be chang-ing much because the num-ber getting off the train willbe about equal to the num-ber getting on.

On the JobMs. Sutliff, what do you do on the job?

I take care of flowers, shrubs, and trees.The company for which I work maintainsthe grounds of business properties, such as

real estate offices and banks, as well as privatehomes. My job involves working with chemicalssuch as herbicides, fungicides, and pesticides.

Are those chemicals hazardous?

They can be if you don’t treat them prop-erly. I have a spraying license that I earnedthrough the Agricultural Service Exten-

sion, so I have learned all the safety precautions.Mixing the chemicals to get them ready for use ismore hazardous than actually spraying them. Formixing, I wear a plastic-coated jacket and pants,heavy boots, rubber gloves, and a face mask.

Do you and your coworkers know what todo in case of a mishap?

We carry eyewash and first-aid kits in all ofour trucks. We’re trained to react quickly ifwe get a chemical on our clothes. In that

case, we must remove the affected clothing andwash up quickly. When some chemicals come incontact with your skin, they are absorbed into thebody and stored in fat cells. The cumulative effectcould be harmful. Because I’m very careful and fol-low directions exactly, I haven’t had any problems.

Why are chemicals necessary in yourwork?

On large properties, it’s a matter of econo-my and time—two hours of hand weedingand cultivating versus a few minutes of

spraying. However, chemical use is just part of ourprogram. Before we spray chemicals, we cultivate,do some hand weeding, and fertilize and aeratebeds of plants. Healthy plants are more likely tohave a fighting chance against weeds and insects.

Meet Caroline Sutliff, Plant-care Specialist

An old gardening adage goes like this: A weed is just aflower that is growing where you don’t want it. Landscapegardener Caroline Sutliff works to eradicate those hardy,invasive plants, such as Taraxacum officinale (betterknown as dandelions). In this interview, she tells abouthow she wages her weed battles with garden tools andchemicals.


Do you use the information you learnedin high school chemistry classes?

Yes. It’s important to be familiar with theproperties of chemicals—to know whatcan be safely mixed and what can’t. The

container used for mixing is important chemically,too. For example, stainless steel containers canerode and change the makeup of chemicals so theywon’t have the same effects. Heavy plastic is usedinstead.

Early InfluencesAs a child, were you interested in plants?

I wasn’t interested in planting thingsbecause I wanted immediate results. Plantsdon’t work that way. Now I have more

patience and love to tend to the flowers around myown home.

Did you plan to enter the plant-care fieldlater on?

No. My intentions were to get a degree inpsychology and then go to law school.While I was looking for a job as a parale-

gal, I began working in plant care and discovered Ilove working outside. Now I can’t imagine beinginside all day at a desk job.

Personal InsightsWould you recommend that studentsinvestigate a plant-care job like yours?

Only if they like hard, physical work. Myworkdays last ten or 12 hours, and I’m outin all kinds of weather. I certainly don’t

need to go to a gym to stay in shape! It’s satisfyingfor me to see that more women are entering thisfield.

How do you see the field of plant carechanging in the years ahead?

I think people will become even moreaware of the value of plants, particularlytrees. Trees can do so much to improve a

neighborhood. They clean the air, shade the side-walks, and beautify an area. Here in Iowa City, anorganization called Project Green helps plant treesin public places. I hope to get involved with thatgroup as a volunteer.


The following careers are also associated withplant care.

Horticulturist Master’s degree, research, andfieldworkLandscape Architect Bachelor’s degree, oftenfollowed by a licensing examinationSoil Conservation Technician Two-year collegeprogram

CAREER CONNECTION


Did you ever have to make up your mind?Saying that a reaction has reached equilibrium does not mean that

equal amounts of reactants and products are present. Equilibrium justmeans that no net change is taking place—the amounts of reactants andproducts are not changing. Often, a reaction at equilibrium contains dif-fering amounts of reactants and products.

Consider the following reaction in which phosphorus pentachloridedecomposes into phosphorus trichloride and chlorine.

PCl5(g) Ø PCl3(g) � Cl2(g)

Actual measurements of the amounts of reactant and products showthat the system is in equilibrium. But the measurements also show thatthere is more PCl5 than PCl3 and Cl2 present, even though the rates of thereactions are equal. The reversible reaction will favor the direction thatproduces the most stable products, which are those that are least likely tochange. In this example, PCl5 is less likely to decompose than the twoproducts, PCl3 and Cl2, are likely to combine. Reversible reactions at equi-librium are in a balance that favors stability.

Changing DirectionIf a reaction reaches equilibrium, how can you obtain large quantities

of a product? Won’t the product constantly become a reactant? Keep inmind that reactions in equilibrium are stable. The French scientist HenriLouis Le Chatelier proposed in 1884 that disturbing an equilibrium willmake a system readjust to reduce the disturbance and regain equilibrium,Figure 6.15. This principle regarding changes in equilibrium is called Le Chatelier’s principle.

Chemical engineers can apply this tendency of reactions to stay at equilib-rium to find ways to increase the yield of products in a reaction. For exam-ple, if products are removed from a reaction at equilibrium, more reactantswill go on to form products so that balance is regained. If this removal con-

tinues, most of the reactants can be converted into prod-ucts. For example, recall the reaction in which limestonedecomposes into lime and carbon dioxide.

CaCO3(s) Ø CaO(s) � CO2(g)

If the carbon dioxide is removed as it is produced,the reaction will favor the formation of more carbondioxide to reestablish equilibrium. Thus, the reactionwill shift in the direction that also produces more limeas a product.

Figure 6.15A System in StressBefore the dog started to drink, the water in this containerreached a stable equilibrium level. As the dog drinks, thislevel is stressed and water comes from the bottle into thebowl to reestablish the equilibrium level.

soluble:solvere (L) capableof being dis-solved in a liquid

Because sugar canbe dissolved inwater, it is water-soluble.

When one product is a gas and other products and reactants are not, asin the previous example and in Figure 6.16, it is easy to see how the gascan be removed from the reaction. How can products that are not isolatedgases be removed from a reaction? They usually can’t be picked out man-ually because of the mix of reactants and products.

A product that does not dissolve in water can be removed if all otherproducts and the reactants dissolve in water. A compound is soluble in aliquid if it dissolves in it; it is insoluble if it does not. An insoluble prod-uct will form a solid precipitate that sinks to the bottom of a liquid solu-tion, as shown in Figure 6.17. The precipitate cannot react easily becauseit is somewhat isolated from the other substances in the reaction.

Adding Reactants or EnergyAdding more reactants has basically the same effect as removing prod-

ucts. For the reaction to reestablish equilibrium amounts of reactants andproducts, more products must be made if more reactants are added.

Adding or removing energy, usually in the form of heat, can also influ-ence the direction of a reaction. Because energy is a part of any reaction,it can be thought of as a reactant or a product. Just as adding more reac-tants to a reaction pushes it to the right, so does adding more energy toan endothermic reaction. For example, the equation for a reaction thatproduces aluminum metal from bauxite, an aluminum ore, shows thatenergy must be added for products to form.

3C � 2Al2O3(s) � energy Ø 4Al(l) � 3CO2(g)

If more energy is added, the reaction goes to the right, formingmore aluminum and carbon dioxide.

For an exothermic reaction, adding more energy pushesthe reaction to the left. In the Haber process for producingammonia from hydrogen and nitrogen, for example, ener-gy is produced.

N2(g) � 3H2(g) Ø 2NH3(g) � energy

Adding energy favors the formation of nitrogen andhydrogen. Thus, temperature must be carefully controlledin the Haber process so that the desired product,ammonia, is produced in large quantities.

Figure 6.16Removing a GasWhen hydrogen peroxide,H2O2, is poured onto awound, the hydrogen perox-ide decomposes to formwater and oxygen. Thegaseous oxygen bubblesaway, preventing the re-formation of H2O2.

2H2O2(aq) ˇ 2H2O(l) � O2(g)

Notice that only a forwardarrow is used because theremoval of oxygen drives thereaction forward.

Figure 6.17Forming a PrecipitateWhen potassium hydroxide is added toan aqueous solution of calcium chlo-ride, calcium hydroxide and potas-sium chloride form. Calciumhydroxide is nearly insoluble inwater, so it precipitates out as asolid.

215

&TECHNOLOGYC H E M I S T R Y

Mining the AirDiatomic nitrogen makes up about 79 percent of

Earth’s atmosphere. Nitrogen is an essential elementfor life, yet only a few organisms can use atmosphericnitrogen directly. A few species of soil bacteria canproduce ammonia, NH3, from atmospheric nitrogen.Other species of bacteria can then convert theammonia into nitrite and nitrate ions, which can beabsorbed and used by plants.

The Haber ProcessNot all usable nitrogen compounds are pro-

duced naturally from atmospheric nitrogen.Ammonia can also be synthesized. The process ofsynthesizing large amounts of ammonia fromnitrogen and hydrogen gases was invented by FritzHaber, a German research chemist, and his Eng-lish research assistant, Robert LeRossignol. Theprocess was first demonstrated in 1909. Haber wasgranted the German patent for the process.

The Haber process involves a synthesis reac-tion. The two reactants are H2(g) and N2(g), andthe products are NH3(g) and heat. The processproduces high yields of ammonia by manipulat-ing three factors that influence the reaction—pressure, temperature, and catalytic action.

PressureIn the ammonia synthesis reaction, four mole-

cules of reactant, H2 and N2, produce two mole-cules of product, NH3. According to Le Chate-lier’s principle, if pressure on the reaction or thesystem is increased, the forward reaction willspeed up to lessen the stress because two mole-cules exert less pressure than four molecules.Increased pressure will also cause the reactants tocollide more often, thus increasing the reactionrate. Haber’s apparatus used a total pressure of2 � 105 kPa, which was the highest pressure hecould achieve in his laboratory.

TemperatureTwo factors influence the temperature at which

the process is carried out. A low temperaturefavors the forward reaction because this reducesthe stress of the heat generated by the reaction.But high temperature increases the rate of reac-tion because of more collisions between the reac-tants. Haber carried out the process at a tempera-ture of about 600°C.


Fritz Haber

Ammonia storage tanks

CatalystA catalyst is used to decrease the activation

energy and thus increase the rate at which equi-librium is reached. Haber used osmium and ura-nium as catalysts in his apparatus.

The ProcessHaber’s process incorporated several opera-

tions that increased the yield of ammonia. Thereactant gases entering the chamber were warmedby heat produced by the reaction. The reactant-product mixture was allowed to cool slowly afterreacting over the catalyst. Ammonia was removedfrom the process by liquefaction, while unreactednitrogen and hydrogen were recycled into theprocess.

The Haber process was gaining the attention ofBASF, a leading German chemical manufacturer.Carl Bosch, a German industrial chemist employedby BASF, was given the task of making Haber’sprocess commercially sound. He and his colleaguesdesigned, constructed, and tested new reactionchambers; improved pressurizing pumps; and usedinexpensive catalysts. By 1913, Bosch had built an

operating ammonia plant in Oppau, Germany. Theplant used nitrogen obtained from liquefied air andhydrogen obtained from methane and water.Throughout World War I, the Oppau plant and alater-built plant supplied Germany with ammoniafor the production of explosives. Today, the Haber-Bosch process is the main source for producingcommercial ammonia. Much of this ammonia isused in the production of agricultural fertilizers.

1. Applying How would slow cooling of thereactant-product mixture affect the equilibri-um of the reaction?

2. Acquiring Information How is ammoniaused in the production of many agriculturalfertilizers?

DISCUSSING THE TECHNOLOGY


HydrogenNitrogen

Compressor

Catalystchamber

Returnpump

Cooler

Liquid ammonia


Reaction RateYou have learned that many reactions reach equilibrium. At that time,

reactants and products are being formed and broken down at the samerate. How fast is this rate? If you pour a solution of potassium iodide intoa solution of lead nitrate, a yellow precipitate of lead iodide seems to forminstantly. If you strike a match, you can see that it takes a little time beforethe match burns completely. Many reactions proceed even more slowly. Tounderstand why reactions occur at different rates, look at what happenswhen reactions take place.

Activation EnergyFor a reaction to occur between two substances, particles of those sub-

stances must collide. Not only do they have to collide, they have to hiteach other with enough force to cause a change to take place. The amountof energy the particles must have when they collide is called the activa-tion energy of the reaction.

Do exothermic reactions require activation energy? Consider the highlyexothermic reaction between hydrogen and oxygen. Once it starts, thisreaction produces enough energy to power the main stage of the spaceshuttle. However, hydrogen and oxygen molecules can exist in the samecontainer for years without reacting. A spark can provide the activationenergy needed to start the reaction. Once started, the reaction producesenough energy to keep itself going. Activation energy differs from reactionto reaction. Some reactions have such high activation energies that thereactions do not take place under normal conditions.

Speed of ReactionWhether riding to school or completing math problems, the rate, or

speed, at which the process takes place can be important. Look at anexample of finding a rate in Figure 6.18. The rate at which a chemicalreaction takes place is also important.

To determine how fast a reaction is taking place, you can measure howquickly one of the reactants disappears or how quickly one of the products

Figure 6.18Reaction RateIf ten green chameleons turn red perminute, the rate of this change can beexpressed as ten green chameleons dis-appearing per minute or ten redchameleons appearing per minute.


appears. Either measurement will give an amount of substancechanged per unit time, which is the rate of reaction.

Why is reaction rate important? The rate of a reactionis important to a chemical engineer designing a process toget a good yield of product. The faster the rate, the moreproduct that can be made in a fixed amount of time. Rateof reaction is also important to food processors who hopeto slow down reactions that cause food to spoil. Can the rate of a reaction be changed? Four major factors affectthis rate.

Effect of TemperatureOne factor that affects rate of reaction is temperature, Figure 6.19.

Most reactions go faster at higher temperatures. Baking a cake speeds upthe reactions that change the liquid batter into a spongy product. Lower-ing the temperature slows down most reactions. Photographic film andbatteries stay useful longer if they are kept cool because the lower temper-ature slows the reactions that can ruin these products.

It’s a Matter of ConcentrationChanging the amounts of reactants present can also alter reaction rate.

The amount of substance present in a certain volume is called the concen-tration of the substance. Raising the concentration of a reactant will speedup a reaction because there are more particles per volume. More particlesresult in more collisions, so the reaction rate increases, Figure 6.20. Inmost cases, concentration is increased by adding more reactant. If a fire isburning slowly, fanning the flames increases the amount of oxygen avail-able and the fire burns faster. If a gas is involved, concentration may beincreased by increasing pressure. Increasing pressure does not increase thenumber of particles, but it brings the particles closer together so collisionsare more frequent. The Haber process, for example, uses high pressures toincrease the rate of reaction of hydrogenand nitrogen to form ammonia. Lower-ing concentration decreases the rate ofreaction. Many valuable historic docu-ments are stored in sealed cases withmost of the air removed to decrease thenumber of particles that might reactwith the paper.

Figure 6.19Effect of Temperature onReaction RateAdding heat to reactantshelps break bonds andincreases the speed at whichmolecules and atoms are mov-ing. The faster they move, themore likely it is that they willcollide and react. Removingheat slows down reactions.That’s why freezing foodmight help keep it from spoil-ing as quickly.

Figure 6.20Concentration and Reaction RateAdding more bumper boats increases thechance that a collision will occur. Takingsome away decreases the odds of two boatsmeeting. When more particles are added toa reaction mixture, the chance that they willcollide and react is increased.

Keep in mind that it doesn’t matter how much the concentration ofone reactant is increased if another reactant is depleted. Consider the situ-ation that exists in Figure 6.21. Sometimes, when two reactants are pres-ent in a reaction, more of one than the other is available for reacting. Theone that there is not enough of is called the limiting reactant. When it iscompletely used up, the reaction must stop. The reaction is limitedbecause the other reactant alone cannot form any product.

Starch-Iodine Clock ReactionIn a clock reaction, an observable change indicating

that a reaction has taken place occurs at some point intime after two or more different reactants are mixed.How soon this change occurs depends on the speed orrate of the reaction. Altering any factors that change reactionrate will change how quickly the observable change is seen.

Starch is an organic compound that reacts immediately withiodine to form a dark blue compound. The starch-iodine clock reactioninvolves timing the rate of formation of this blue compound after mix-ing two solutions. One solution reacts to produce iodine, and the othersolution contains starch.

Procedure

1. Use a wax pencil to label fivelarge test tubes with the num-bers 1 through 5, and placethem in a test-tube rack.

2. Place 10 mL of starch-contain-ing solution in each of the fivetest tubes.

3. Place tube 4 in an ice bath andtube 5 in a water bath at 35°C.Leave them there for at leastten minutes.

4. Add the following amounts ofiodine-producing solution tothe indicated test tubes, stircarefully with a clean stirringrod or a wire stirrer, andrecord the time it takes for theblue color to appear.

3

Figure 6.21Limiting ReactantsHow many s’mores can you make if you have six graham crackerhalves, three marshmallows, and two pieces of chocolate? Choco-late is the limiting reactant. When it is completely used up, onlytwo s’mores have been made, and the s’more making must stop.

a) 10 mL to tube 1b) 5 mL to tube 2c) 20 mL to tube 3d) 10 mL to tube 4e) 10 mL to tube 5

5. Summarize your results in atable.

Analysis1. What effect does changing the

amount, and therefore the con-centration, of a reactant haveon the rate of a reaction? Why?

2. What effect does lowering thetemperature have on the rate ofa reaction? Why?

3. What effect does raising thetemperature have on the rate ofa reaction? Why?



Stove in a SleeveThat Napoleon stated “An army marches on its

stomach” may be debated. However, few argueabout the meaning of the statement. One of thenecessities of military operations is supplyingtroops with meals, or rations. In the past, front-line troops often ate cold rations. Heating food byfire took too long, and the resulting smoke mighthave drawn enemy fire.What was needed was aportable, smokeless, self-contained method ofquickly heating rationswith readily availablematerials. The flamelessheater was developed tomeet these needs.

Flameless ration heatersThe flameless rationheater uses a heat-releas-ing chemical reaction sim-ilar to that of magnesiumand water in formingmagnesium hydroxide,Mg(OH)2, and hydrogengas. But pure magnesiumcan’t be used in theheaters. Magnesium reactswith oxygen in the air to form a coating ofmagnesium oxide, MgO, that prevents the magnesium from reacting with other materials.Other materials are added to the magnesium todissolve the MgO and promote the reactionbetween magnesium and water. A magnesium-iron alloy, composed of 95 percent magnesiumand five percent iron, and a small amount ofsodium chloride provide the desired effects.These materials are mixed with a powdered plasticto form a porous pad. Then the heat-producingreaction is started by adding water to the pad.

Hot MREs The heater is used to warm an individ-ual portion of rations called a ready-to-eat meal, orMRE (Meal, Ready-to-Eat). Each MRE comes in apouch inside a cardboard sleeve. The MRE pouchslips into a small, plastic bag containing the Mg-Fepad. After the water is added, the bag is placedinside the sleeve in which the MRE was packed.

The reaction transfersheat to the MRE pouch.The cardboard sleeve isan insulator to preventheat loss. The reactionproduces enough heat toraise the temperature ofan 8-ounce ration serv-ing by 60°C in about 12minutes and keep theration warm for about anhour. The heater andMRE can fit inside thepocket of a uniform, so itcan be carried easily bysoldiers. A hot meal-to-go might just be themorale booster a fieldsoldier needs.

Chemistry

Exploring Further

1. Classifying Is the chemical reaction in theheater classified as an endothermic or anexothermic reaction?

2. Applying What other applications mightflameless heating have?

What other chemicals produce heat? Visit theChemistry Web site at chemistryca.com



CatalystsAnother way to change the rate of a reaction is to add or take away a

catalyst. A catalyst is a substance that speeds up the rate of a reactionwithout being permanently changed or used up itself. Even though theyincrease the rate of reactions, catalysts do not change the position of equi-librium. Therefore, they do not affect how much product you can getfrom a reaction—just how fast a given amount of product will form.

How does a catalyst speed up a chemical reaction? You know that inchemical reactions, chemical bonds are broken and new ones are formed.The energy needed to break these bonds is the activation energy of thereaction. A catalyst speeds up the reaction by lowering the activation ener-gy. This process can be compared to kicking a football over a goalpost. Aplayer can kick the football over the goalpost at its regulation height, butif the height of the goalpost is lowered, the ball can be kicked over it usingless energy.

Figure 6.22Enzyme ActionGelatin made with freshpineapple will not hardenproperly, whereas gelatinmade with canned pine-apple will. Fresh pine-apple contains active pro-tease enzymes that breakdown protein molecules inthe gelatin. Canned pineapplehas been heated. Enzymes areheat-sensitive, so the pro-teases in the canned fruitare not active.

Many different compounds are able to act as catalysts. The most power-ful catalysts are those found in nature. They are needed to speed up thereactions necessary for a cell to function efficiently. These biological cata-lysts are called enzymes. Enzymes help your body use food for fuel, buildup your bones and muscles, and store extra energy as fat. Enzymes areinvolved in almost every process in a cell. For example, proteases areenzymes that break down proteins, as shown in Figure 6.22. Theseenzymes occur naturally in cells to help with recycling proteins so theirparts can be used over and over. Proteases are also used in many commonproducts, including contact lens-cleaning solution and meat tenderizer.

The importance of enzymes to our health can be seen when someonelacks a gene for a particular enzyme. For example, lactose intoleranceresults when an enzyme that breaks down lactose, the sugar in dairy prod-ucts, is not produced in a person’s digestive system, Figure 6.23. Whenlactose is not broken down, it accumulates in the intestine, causing bloat-ing and diarrhea.


Slowing Down ReactionsAdding a catalyst speeds up a reaction. Would you ever want to slow

down a reaction? Reactions that have undesirable products sometimeshave to be slowed down. Food undergoes reactions that cause it to spoil.Medications decompose, destroying or limiting their effectiveness. Canany substances slow down these reactions?

A substance that slows down a reaction is called an inhibitor. Just ascatalysts don’t make reactions occur, inhibitors don’t completely stopreactions. An inhibitor is placed in bottles of hydrogen peroxide to pre-vent it from decomposing too quickly into water and oxygen. If noinhibitor were added, the shelf life of hydrogen peroxide products wouldbe much shorter because the molecules would decompose at a faster rate.

Connecting IdeasNow you are familiar with the types of chemical reactions that occur

around you. You know the importance of equilibrium and are aware ofwhat factors can affect the rate of reaction. But why do some substancesreact with each other while others don’t? In the next chapter, you will findout how the structure of an atom affects the way it reacts with otheratoms.

SECTION REVIEWFor more practice

with solving problems,see Supplemental Practice Problems, Appendix B.

Understanding Concepts1. List and describe four factors that can influence

the rate of a reaction.

2. Explain whether an exothermic reaction that isat equilibrium will shift to the left or to theright to readjust after each of the following pro-cedures is followed.

a) Products are removed.b) More reactants are added.c) More heat is added.d) Heat is removed.

3. Explain the difference between an inhibitor anda catalyst.

Thinking Critically4. Analyzing Hydrogen gas can be produced by

the reaction of magnesium and hydrochloricacid, as shown by this equation.

Mg(s) � 2HCl(aq) ˇ MgCl2(aq) � H2(g)

In a particular reaction, 6 billion molecules ofHCl were mixed with 1 billion atoms of Mg.

a) Which reactant is limiting?b) How many molecules of H2 are formed

when the reaction is complete?

Applying Chemistry5. Catalytic Converters Catalytic converters on

cars use the metals rhodium and platinum ascatalysts to convert potentially dangerousexhaust gases to carbon dioxide, nitrogen, andwater. Why don’t cars need to have the rhodiumand platinum replaced after they are used?


See page 865 inAppendix F forPreventing a

Chemical Reaction

Lab



6.1 Chemical Equations■ Changes in color or odor, production or

absorption of heat or light, gas release, andformation of a precipitate are all observablemacroscopic changes that indicate that achemical reaction may have occurred.

■ Chemical equations are used to represent reac-tions. They are written using symbols and for-mulas for elements and compounds. Once thesymbols and formulas are written, the equationcan be balanced only by changing coefficients.

■ Examining a chemical equation can tell youhow elements and compounds change duringa reaction. It may also tell you whether a reac-tion is endothermic or exothermic.

■ Chemical equations must be balanced accord-ing to the law of conservation of mass, whichstates that matter cannot be created ordestroyed in a chemical reaction.

6.2 Types of Reactions■ Although thousands of individual chemical

reactions are known, most can be classifiedinto five major classes that are based on pat-terns of behavior of reactants and products.

■ The five general classes of reactions are syn-thesis, decomposition, single-displacement,double-displacement, and combustion.

6.3 Nature of Reactions■ Reversible reactions are those in which the

products can react to form the reactants.

■ Equilibrium occurs when no net change istaking place in a reaction.

■ The direction in which a reaction shifts canbe influenced by disturbing the equilibriumof reactants and products.

■ How fast a reaction occurs can be influencedby temperature, concentration of reactants,and the presence of catalysts or inhibitors.

VocabularyFor each of the following terms, write a sentence that showsyour understanding of its meaning.

activation energy equilibriumcatalyst inhibitorcoefficient insolublecombustion limiting reactantconcentration productdecomposition reactantdouble displacement single displacementdynamic equilibrium solubleenzyme synthesis

UNDERSTANDING CONCEPTS1. On which side of the arrow are the products of

a reaction usually found? On which side arethe reactants found?

2. Use the equation below to answer the follow-ing questions.

2Sr(s) � O2(g) ˇ 2SrO(s)

a) What is the physical state of strontium?b) What is the coefficient of strontium oxide?c) What is the subscript of oxygen?d) How many reactants take part in this reaction?

3. Use word equations to describe the chemicalequations given.

a) AgNO3(aq) � NaBr(aq) ˇAgBr(s) � NaNO3(aq)

b) C5H12(l) � 8O2(g) ˇ 5CO2(g) � 6H2O(g)c) CoCO3(s) � energy ˇ CoO(s) � CO2(g)d) BaCO3(s) � C(s) � H2O(g) ˇ

2CO(g) � Ba(OH)2(s)

4. Explain why subscripts should never bechanged when balancing an equation.

REVIEWING MAIN IDEAS

CHAPTER 6 ASSESSMENT

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5. Balance the following equations.

a) Fe(s) � O2(g) ˇ Fe3O4(s)b) NH4NO3(s) ˇ N2O(g) � H2O(g)c) COCl2(g) � H2O(l) ˇ HCl(aq) � CO2(g)d) Sn(s) � NaOH(aq) ˇ Na2SnO2(aq) � H2(g)

6. Which of the five general types of reactions hasonly one reactant?

7. Hydrogen gas and iodine gas combine in areversible exothermic reaction to form hydro-gen iodide gas.

a) Write a word equation for this reaction.b) Write a balanced chemical equation for this

reaction.c) If more iodine gas is added after the reac-

tion reaches equilibrium, will the reactionbe shifted to the left or the right?

d) If heat is added after the reaction reachesequilibrium, will the reaction be shifted tothe left or the right?

APPLYING CONCEPTS8. List all the macroscopic changes that occur

while a cake is made that indicate that chemi-cal reactions have occurred.

9. One pollutant produced in automobile enginesis nitrogen dioxide, NO2. It changes to formnitrogen oxide, NO, and oxygen atoms whenexposed to sunlight. Of what reaction type isthis an example?

10. Catalytic converters help break down pollu-tants that result from gasoline combustion andare required in automobiles in the UnitedStates. Most catalytic converters contain a plat-inum-rhodium catalyst that coats the extensivesurface of a honeycomb structure in the con-verter. Why would having the catalyst spreadout over a large surface area be useful?

11. If you place a piece of a saltine cracker on yourtongue for a few minutes, it begins to tastesweet. Do you think a chemical or physicalchange leads to this taste change? Why?

Everyday Chemistry12. Explain why combining different cleaning prod-

ucts, such as those that contain ammonia andbleach, sometimes has deadly consequences.How could product manufacturers make theaverage person aware of hazards like this?

Chemistry and Technology13. Why are high temperature and pressure need-

ed for the Haber process?

Everyday Chemistry14. What purposes might there be for a unit similar

to an MRE that uses an endothermic reaction?

How It Works15. When do you think it might be important that

light sticks come in different colors?

Biology Connection16. Why is it necessary that the gases breathed on

the space shuttle are about the same composi-tion as air on Earth?

THINKING CRITICALLYRelating Cause and Effect17. Explain why it is best to store many chemicals

in tightly sealed bottles in dark locations atcool or moderate temperatures.

Applying Concepts18. ChemLab Why can’t you find a reaction in the

ChemLab that can be classified as two differentreaction types?

Forming a Hypothesis19. MiniLab 1 Does the reaction taking place in

MiniLab 1 go to completion, or does it reachan equilibrium? How do you know?

Making Predictions20. MiniLab 3 Predict the effect of each of the fol-

lowing on the speed of the starch-iodine clockreaction.

a) adding an inhibitor to the reactionb) adding a catalyst to the reactionc) raising the temperature of the reaction

Chapter 6 Assessment 225


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CUMULATIVE REVIEW21. Compare the charges, masses, and atomic loca-

tions of electrons, protons, and neutrons.(Chapter 2)

22. Compare the number of electrons in the outerenergy levels of metals, nonmetals, and metal-loids. (Chapter 3)

23. Write a balanced equation for the formation ofsodium chloride from sodium metal and chlo-rine gas. (Chapter 4)

24. Compare properties of ionic and covalentcompounds. (Chapter 5)

SKILL REVIEW25. Relating Cause and Effect Use what you have

learned about factors that influence rates ofchemical reactions to explain each of the fol-lowing statements.

a) Colors in the fabric of curtains in windowsexposed to direct sunlight often fade.

b) Meat is preserved longer when stored in afreezer rather than a refrigerator.

c) Taking one or two aspirin will not harmmost people, but taking a whole bottle atonce can be fatal.

d)BHA, or butylated hydroxyanisole, is an antioxidant often added to food,paints, plastics, and other products as a preservative.

26. Modeling Use toothpicks and Styrofoam ballsof different colors to balance the followingequations.

a) Cl2O(g) � H2O(l) ˇ HClO(aq)b) Fe2O3(s) � CO(g) ˇ Fe(s) � CO2(g)c) H2(g) � N2(g) ˇ NH3(g)d)ZnO(s) � HCl(aq) ˇ ZnCl2(aq) � H2O(l)

27. Interpreting Graphs The graph shown repre-sents the concentrations of two compounds, Aand B, as they take part in a reaction thatreaches equilibrium.

a)Which compound represents the reactant inthis reaction? The product?

b) How long did it take for the reaction toreach equilibrium?

c)Explain how the graph will change if moreproduct is added one minute after the reac-tion reaches equilibrium.

d)Summarize your answers in a report.

WRITING IN CHEMISTRY28. Write a short paper describing five chemical

reactions that commonly occur in your home,school, or neighborhood every day. Describewhat you can see, smell, feel, hear, and taste asa result of these reactions.

PROBLEM SOLVING29. Balance the following equations:

a) Xe(g) � F2(g) ˇ XeF6(s)b) Al(s) � H2SO4(aq) ˇ Al2(SO4)3(aq) � H2(g)c) CS2(1) � O2(g) ˇ CO2(g) � SO2(g)d)H2SO4(aq) � NaCN(s) ˇ

HCN(g) � Na2SO4(aq)e) KClO3(s) ˇ KCl(s) � O2(g)

30. Classify each reaction in question 29 as one ofthe five general reaction types.

1 2 3 4 5Time (minutes)

Con

cent

ratio

n(I

ncre

asin

gˇ

) A

B


1. When an iron nail is placed in water, a chemi-cal reaction causes the nail to rust. What is(are) the reactant(s) in this reaction?

a) ironb) waterc) iron and rustd) iron and water

2. Which of the following chemical equationsincludes the formation of water?

a) 2H20 → 2H2 � O2

b) HC2H3O2 � NaHCO3 → NaC2H3O2 �H2O + CO2

c) H2 � O2 → H2O2

d) H2O (aq) → H2O (s)

Use the table above to answer questions 3–4.

3. Which chemical reactions are endothermic?

a) a, b, and c c) all of the reactionsb) d and e d) none of the reactions

4. What is the amount of free energy associatedwith the conversion of liquid water into hydro-gen and oxygen gas?

a) �572 kJ/mol c) �458 kJ/molb) �572 kJ/mol d) �458 kJ/mol

5. What is the product of this synthesis reaction?

Cl2(g) � 2NO(g) → ?

a) NCl2 c) N2O2

b) 2NOCl d) 2ClO

6. What type of reaction is described by the fol-lowing equation?

Cs(s) � H2O(l) → CsOH(aq) � H2(g)

a) synthesisb) decompositionc) single displacementd) double displacement

7. Lightning striking water to create hydrogenand oxygen gas is an example of

a) synthesis. c) single displacement.b) decomposition. d) double displacement.

8. A system reaches chemical equilibrium when

a) no new product is formed by the forwardreaction.

b) the reverse reaction no longer occurs in thesystem.

c) the concentration of reactants in the systemequals the concentration of products.

d) the rate at which the forward reactionoccurs equal the rate of the reverse reaction.

9. Which of the following can be used to speedup a reaction?

a) increase temperatureb) increase concentrationc) add a catalystd) all of the above

10. How can a reaction be slowed down?

a) add an inhibitorb) increase the activation energyc) add more reactantsd) all of the above

Standardized Test Practice

Standardized Test Practice 227

Plan Your Work and Work Your Plan Planyour workload so that you do a little work each dayrather than a lot of work all at once. The key toretaining information is repeated revieew and prac-tice. You will retain more if you study one hour anight for five days in a row instead of cramming forfive hours on a Sunday night.

Test Taking Tip

chemistryca.com/standardized_test

Free Energy of Chemical Reactions

Chemical Reaction Free Energy (kJ/mol)

a) CH4(g) � 2O2(g) → CO2(g)� 2 H2O(l) �890

b) 2H2(g) � O2(g) → 2H2O(g) �458

c) 2H2(g) � O2(g) → 2H2O(l) �572

d) C2H4(g) � H2(g) → C2H6(g) �137

e) 6CO2 � 6H2O → C6H12O6

� 6O2 � 6H2O �470

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Documents

Chapter 6: Chemical Reactions and Equations - Glencoe/McGraw-Hill