A MODEL FORMULA Chemicals react in very precise ways.In this activity, you will model achemical reaction and predict howchemicals react.
Procedure
1. You will receive severalmarshmallow models.The models are marshmallowsstuck together with toothpicks.Each of these models is a model A.
2. Your teacher will show you modelB and model C. Take apart one ormore model A’s to make copies ofmodel B and model C.
3. Do you have any pieces left over?If so, use them to make moremodel B’s and/or model C’s. Doyou need more parts to completemodel B or model C? If so, takeapart another model A.
4. If needed, repeat step 3 until youdo not have any parts left over.
Analysis
5. How many model A’s did you useto make copies of model B andmodel C?
6. How many model B’s did youmake? How many model C’s didyou make?
7. Suppose you needed to make sixmodel B’s. How many model A’swould you need? How manymodel C’s could you make withthe leftover parts?
Reaction to the RescueA car slams into a wall at 97 km/h (60 mph) during a crashtest. Although both dummies are wearing seat belts, onesuffers a crushing blow to the head as it strikes the dash-board. The other suffers only minor bruises thanks to an airbag. Air bags can inflate rapidly due to a chemical reactionthat produces gas at a very fast rate. In this chapter youwill learn how to identify and describe a chemical reaction.You will also learn about factors that affect the rate of areaction.
Reaction to the RescueA car slams into a wall at 97 km/h (60 mi/h) during a crash test. Although both dummies arewearing seat belts, one suffers a crushing blow tothe head as it strikes the dashboard. The other suffers only minor bruises thanks to an air bag. Air bags can inflate rapidly because of a chemicalreaction that produces gas at a very fast rate. Inthis chapter, you will learn how to identify anddescribe a chemical reaction. You will also learnabout factors that affect the rate of a reaction.
When you mix water with baking powder,substances in baking powder react to form bubbles of carbon dioxide gas. When you bake a muffin, the bubbles give thetasty treat its spongelike texture.
Forming New SubstancesEach fall, a beautiful change takes place. Leaves changecolor. You see bright reds, oranges, and yellows that hadbeen hidden by green all year. What causes this change?
To answer this question, you need to know what causes leavesto be green in the first place. Leaves are green because theycontain a green substance called chlorophyll (KLAWR uh FIL).During the spring and summer, the leaves containa lot of chlorophyll. But in the fall, whentemperatures drop and there are fewer hoursof sunlight, the chlorophyll goes through achemical change. The chlorophyll changesinto new substances that have no color. Whenthis happens, the red, orange, and yellowcolors that were always present in the leavescan now be seen.
Chemical ReactionsThe chemical change that happens as chlorophyll breaks downinto new substances is one example of a chemical reaction. A chemical reaction is the process by which one or more substances change to produce one or more different substances.These new substances have different chemical and physicalproperties from the original substances. Many of the changesyou see every day are chemical reactions. For example, strikinga match, taking a photograph, and using a battery all involvechemical reactions. Check out Figure 1 to see two more exam-ples of chemical reactions.
112 Chapter 5
Terms to Learn
chemical reaction
What You’ll Do
● Name the clues that showthat a chemical reaction might be taking place.
● Demonstrate that substancesmay react chemically to formnew substances.
A reaction between gasoline and oxygenforms carbon dioxide, water, and other substances. The formation of the new substances causes the pistons to move, which makes the car run.
Clues to Chemical ReactionsHow can you tell when a chemical reaction is taking place?There are several clues to look for. Look at Figure 2. A changein color can be one clue. The formation of gas bubbles or asolid also may show that a reaction is happening. A solid formedin a solution as a result of a chemical reaction is called a pre-cipitate (pree SIP uh TAYT). Also, during many chemical reac-tions, energy is taken in or given off as light, thermal, orelectrical energy. The more of these clues you see, the morelikely it is that a chemical reaction is taking place.
Chemical Reactions 113
Gas FormationBubbles of carbon dioxide form when hydrochloric acid is placed on a piece of limestone.
Figure 2 Some Clues to Chemical Reactions
Solid FormationHere you see potassium chromatesolution being added to a silver nitrate solution. The dark red solid is a precipitate of silver chromate.
Energy ChangeThe chemical reaction that takesplace in a fire gives off light andthermal energy. During some otherreactions, energy is taken in.
Color ChangeDon’t accidentally spill chlorine bleach on your jeans! The bleach reacts with the blue dye on the cloth, which causes the cloth’s color to change.
Breaking and Making BondsIn a chemical reaction, bonds are broken and new bonds areformed. When chemical bonds in the starting substances arebroken, new bonds can form between different atoms to makethe new substances. Look at the model in Figure 3 to under-stand how this process happens.
New Substances Form in a Chemical ReactionYou’ve read that a color change is one clue to a chemical reaction. A color change may indicate that a new substancehas formed. For example, a color change takes place whenchlorine gas reacts with hydrogen gas. Chlorine gas is green-ish yellow in color. Hydrogen is a flammable, colorless gas.The two substances combine to form hydrogen chloride, anonflammable, colorless gas. Hydrogen chloride has differentchemical and physical properties than the substances that combined to form it. So, hydrogen chloride is a different sub-stance than hydrogen and chlorine.
The elements hydrogen and chlorine are diatomic.Diatomic molecules are made of two atoms bondedtogether. For these molecules to react, the bondsjoining the atoms must break.
Molecules of the new substance, hydrogen chloride,are formed as new bonds are made betweenhydrogen atoms and chlorine atoms.
TOPIC: Chemical ReactionssciLINKS NUMBER: HSTP330
What is a chemical reaction?
List four clues that can help you demonstrate that achemical reaction is taking place.
In a chemical reaction, what must happen to thechemical bonds of the reacting substances in order tomake a new substance?
Applying Concepts Is a chemical reaction taking placewhen the metal of a car body rusts? Explain your answer.
Hydrogen
ChlorineHydrogen chloride
Figure 3 Reaction of Hydrogen and Chlorine
Breaking bonds Making bonds
Reaction Ready
1. Place a piece ofchalk in a plasticcup.
2. Add 1 tsp of vin-egar to the cup.Record your obser-vations in yourScienceLog.
3. What evidence do you seethat a chemical reaction istaking place?
Chemical Formulas and EquationsThink about how many words you can create using the26 letters of the alphabet. Hundreds of thousands ofwords are possible!
In the same way that letters are used to form words, chemicalsymbols are used to represent substances. You can think ofchemical formulas as “words” made from chemical symbols.
Chemical FormulasAll substances are formed from about 100 elements. Each el-ement has its own chemical symbol. A chemical formula is ashorthand notation that uses chemical symbols and numbersto represent a substance. A chemical formula shows how manyof each kind of atom are present in a molecule.
Look at Figure 4. The chemical formula for water is H2O. Thisformula tells you that one water molecule is made of twoatoms of hydrogen and one atom of oxygen. The small 2 inthe formula is a subscript. A subscript is a number written belowand to the right of a chemical symbol in a formula. When asymbol, such as the O for oxygen in water’s formula, has nosubscript, only one atom of that element is present. Figure 4also explains two more chemical formulas.
Chemical Reactions 115
Terms to Learn
chemical formulachemical equationreactantproductlaw of conservation of mass
What You’ll Do
● Interpret and write simplechemical formulas and equations that express what happens in a chemical reaction.
● Explain how a balancedequation shows the law of conservation of mass.
Water molecules are made up of3 atoms—2 atoms of hydrogenbonded to 1 atom of oxygen.
Oxygen is a diatomic element.Each molecule of oxygen gas has 2 atoms of oxygen bondedtogether.
Glucose molecules have 6 atomsof carbon, 12 atoms of hydrogen,and 6 atoms of oxygen. (Glucoseis the sugar made by plants during photosynthesis.)
Water Oxygen Glucose
Figure 4 Chemical Formulas of Different Substances
Writing Formulas for Covalent CompoundsYou can often write a chemical formula if you know the nameof the substance. Covalent compounds are usually made oftwo nonmetals. The names of covalent compounds use pre-fixes to tell you how many atoms of each element are in theformula. Each prefix represents a number. The prefixes areshown in the table at left. Figure 5 shows two examples ofhow to write a chemical formula of a covalent compound.
Writing Formulas for Ionic CompoundsIf the name of a compound has the name of a metal and anonmetal, the compound is probably ionic. To write the for-mula for an ionic compound, you must make sure the com-pound’s overall charge is zero. In other words, the formula musthave subscripts that cause the charges of the ions to cancelout. Figure 6 shows two examples of how to write a chemicalformula from the name of an ionic compound.
One sodium ion and one chloride ion have an overallcharge of (1�) � (1�) � 0.
A magnesium ion has a 2� charge.
A chloride ion has a 1� charge.
One magnesium ion and twochloride ions have an overallcharge of (2�) � 2(1�) � 0.
Sodium chloride Magnesium chloride
Figure 6 The formula of an ionic compound is written by usingenough of each ion so that the overall charge is zero.
Counting AtomsSome formulas have parentheses.When counting atoms, multiplyeverything inside the parenthesesby the subscript. For example,Ca(NO3)2 has1 calcium atom 2 nitrogen atoms (2 � 1)6 oxygen atoms (2 � 3)
Now It’s Your TurnFind the number of atoms of eachelement in the formulas Mg(OH)2and Al2(SO4)3.
mono- 1 hexa- 6
di- 2 hepta- 7
tri- 3 octa- 8
tetra- 4 nona- 9
penta- 5 deca- 10
Figure 5 The formulas of these covalent compounds can be written using the prefixes in their names.
Prefixes Used in Chemical Names
Chemical EquationsThink about a piece of music, such as the one in Figure 7.Someone writing music must communicate to the musicianwhat notes to play, how long to play each note, and how eachone should be played. He or she does not use words to describethe piece. Instead, musical symbols are used to communicatein a way that can be understood by anyone who can read music.
Equations Describe ReactionsIn the same way, chemists around the world must communi-cate about reactions clearly. Descriptions using sentences wouldneed translations into other languages. Instead of using longsentences, chemists can use chemical equations to describereactions. A chemical equation uses chemical formulas as a short-hand description of a chemical reaction. A chemical equationis short and understood by anyone who understands chemicalformulas. Each element’s chemical symbol is understood aroundthe world. So, a chemical equation does not need translation.
Reactants Yield ProductsWhen carbon burns, it reacts with oxygen to form carbondioxide. How would a chemist describe this reaction? Look atFigure 8. The starting materials in a chemical reaction are reactants (ree AK tuhnts). The substances formed from a reac-tion are products. In this example, carbon and oxygen arereactants. Carbon dioxide is the product.
Figure 7 The symbols on thismusic are understood around theworld—just like chemical symbols.
Figure 8 The Parts of a Chemical Equation
The formulas of the reactantsare written before the arrow.
The formulas of the productsare written after the arrow.
A plus sign separates the for-mulas of two or more reactantsor products from one another.
Charcoal is used to cook food on a barbecue.When carbon in charcoal reacts with oxygen inthe air, the primary product is carbon dioxide.
The arrow is called the yieldssign. It separates the formulasof the reactants from the for-mulas of the products.
Figure 9 Examples of Similar Symbols and Formulas
Accuracy Is ImportantThe symbol or formula for each substance in the equationmust be written correctly. For a compound, decide if it is cova-lent or ionic. Then write the correct formula. For an element,use the proper chemical symbol. Be sure to use a subscript of2 for the diatomic elements. (The seven diatomic elements arehydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, andiodine.) An equation with the wrong chemical symbol orformula will not correctly describe the reaction. In fact, evena simple mistake can make a huge difference, as in Figure 9.
An Equation Must Be BalancedIn a chemical reaction, every atom in the reactants becomespart of the products. Atoms are never lost or gained in a chemi-cal reaction. When writing a chemical equation, you must balance the equation so that the number of atoms of each element in the reactants equals the number of atoms of thoseelements in the products.
Self-CheckWhy is it important to use correct chemical formulas in chemicalequations? (See page 698 to check your answer.)
Hydrogen gas, H2, is an impor-tant fuel that may help reduceair pollution. Water is the onlyproduct formed as hydrogenburns. That’s why there is littleair pollution from a car thatuses hydrogen as fuel.
The chemical formula for thecompound carbon dioxide isCO2. Carbon dioxide is a colorless, odorless gas.
The chemical formula for thecompound carbon monoxide isCO. Carbon monoxide is a colorless, odorless, poisonous gas.
The chemical symbol for the element cobalt is Co. Cobalt is a hard, bluish gray metal.
CO2 CO Co
Reactants
How to Balance an EquationWriting a balanced equation calls for the use of coefficients(KOH uh FISH uhnts). A coefficient is a number placed in frontof a chemical symbol or formula. When counting atoms, youmultiply a coefficient by the subscript of each of the elementsthat follows the coefficient in the formula. This means that2CO2 stands for two carbon dioxide molecules. Together thetwo carbon dioxide molecules have two carbon atoms and fouroxygen atoms.
Coefficients are used to balance equations. The reason isthat the subscripts in chemical formulas cannot be changed.Changing a subscript changes a formula so that it no longerstands for the correct substance. Study Figure 10 to see how touse coefficients to balance a simple equation—the formationof water. Then you can practice balancing equations by doingthe MathBreak at right.
Count the atoms of each element in thereactants and in the products. You can seethat there are fewer oxygen atoms in theproducts than in the reactants.
To balance the oxygen atoms, place thecoefficient 2 in front of the formula forwater. This gives you two oxygen atoms inboth the reactants and the products. Butnow there are too few hydrogen atoms in the reactants.
To balance the hydrogen atoms, place thecoefficient 2 in front of hydrogen’s formula.To be sure your answer is correct, alwaysdouble-check your work!
Balancing ActWhen balancing an equation,place coefficients in front of anentire chemical formula. Neverplace them in the middle of aformula. For example:
F2 + 2KCl 2KF + Cl2
Now It’s Your TurnBalance the following equations:
HCl + Na2S H2S + NaCl
Al + Cl2 AlCl3
2
3
H � 2
H2 � O2 H2O
H2 � O2 2H2O
Reactants Products
Follow these steps to write a balancedequation for H2 � O2 H2O.
Mass Is Conserved—It’s a Law!The practice of balancing equations comes from the work ofa French chemist, Antoine Lavoisier (lah vwah ZYAY). In the1700s, Lavoisier carefully measured and compared the massesof the substances in chemical reactions. He found that the
total mass of the reactants was thesame as the total mass of theproducts. Lavoisier’s work led tothe law of conservation of mass.This law states that mass isneither created nor destroyed inordinary chemical and physicalchanges. This law means that achemical equation must show thesame number and kind of atomon both sides of the arrow. Youcan see how this law works inFigure 11 and in the QuickLabbelow.
TOPIC: Chemical FormulassciLINKS NUMBER: HSTP335
TOPIC: Chemical EquationssciLINKS NUMBER: HSTP340
120 Chapter 5
Mass Conservation
1. Place 1 tsp of baking soda into a sealableplastic bag.
2. Place 1 tsp of vinegar into a plastic filmcanister. Close the lid.
3. Place the canister into the bag. Squeeze theair out of the bag. Seal it tightly.
4. Use a balance to find the mass of the bagand its contents.
5. Keeping the bag closed, open the canister inthe bag. Mix the vinegar with the baking soda.
6. When the reaction has stopped, measure the mass of the bag and its contents.
7. Compare the mass of the materials beforeand after the reaction took place.
Explain why chemical formulas and chemical equationsare important.
Write the chemical formulas for carbon tetrachlorideand calcium bromide.
Explain how a balanced chemical equation illustratesthat mass is never lost or gained in a chemical reaction.
Applying Concepts Write a balanced chemical equationto express what happens when methane, CH4, and oxy-gen gas react to form water and carbon dioxide.
Figure 11 Magnesium in theflashbulb of a camera reacts withoxygen. The mass is the samebefore and after the reaction.
● Identify and demonstratethat loss of energy happensduring exothermic reactions.
● Identify and demonstratethat gain of energy happensduring endothermic reactions.
● Explain activation energy and energy diagrams.
● Describe the factors thataffect the rate of reaction.
Energy and Rates of Chemical ReactionsWhat’s the difference between eating a meal and runninga mile? You could say that one gives you energy, whilethe other uses up energy.
Chemical reactions can be described in the same way. Somereactions give off energy. Other reactions absorb energy. In thissection, you will learn how to classify a chemical reaction interms of the reaction’s energy. You will also learn how to changethe rate at which a chemical reaction happens.
Every Reaction Involves EnergyChemical energy is part of all chemical reactions. During areaction, as chemical bonds in the reactants break, energy isabsorbed. As new bonds form in the products, energy is givenoff. You can compare the chemical energy of the reactantswith the chemical energy of the products. This is how youdetermine whether energy is given off or absorbed in the overall reaction.
Energy Is Released in Exothermic ReactionsSometimes, the chemical energy of the reactants is greater thanthe chemical energy of the products. This kind of reaction givesoff energy. A chemical reaction in which energy is given off iscalled exothermic. Exo- means “go out” or “exit,” and -thermicmeans “heat” or “energy.” A chemical reaction can give offenergy in several different forms, as shown in Figure 12 on thenext page. The energy given off in an exothermic reaction isoften written as a product, as in this equation:
2Na + Cl2 2NaCl + energy
The Temperature Tells
1. Fill a plastic cup half fullwith hydrogen peroxidesolution.
2. Measure the temperatureof the solution with a thermometer.
3. Carefully add 20 cubes of raw potato to the cup.
4. Record your observations inyour ScienceLog.
5. When the reaction hasstopped, record the tem-perature of the solution.
6. What evidence did you see that an exothermicreaction took place? (Hint: Was energy lost orgained?)
Energy Is Absorbed in Endothermic ReactionsSometimes, the chemical energy of the reactants is less thanthe chemical energy of the products. In this kind of reaction,energy is absorbed. A chemical reaction in which energy isabsorbed is called endothermic. Endo- means “go in.” The energyabsorbed in an endothermic reaction is often written as a reactant, as in this equation:
2H2O � energy 2H2 � O2
Energy Is Conserved—It’s a Law!You learned that mass is never created or destroyed in chemical reactions. The same holds true for energy. The law of conservation of energy states that energy can be neithercreated nor destroyed. The energy given off in exothermicreactions was originally stored in the reactants. And the energyabsorbed in endothermic reactions is stored in the productsthat form. Imagine that you could measure all the energy ina reaction. You would find that the total amount of energy(of all types) is the same before and after the reaction.
Activation Energy Allows a Reaction to HappenA match can be used to light a campfire—but only if the matchis lit! A strike-anywhere match has all the reactants it needsto be able to burn, but the reactants will not ignite by them-selves. Energy is needed for the reaction to occur. Rubbing thetip of a match on a rough surface provides the energy neededfor the chemicals to react. The smallest amount of energyneeded for substances to react is called activation energy.
Where Does Activation Energy Come From?The friction of striking a match heats the substances on thematch, breaking bonds in the reactants. This allows the newbonds in the products to form. Chemical reactions need someenergy to occur. An electric spark in a car’s engine providesactivation energy for the burning of gasoline. Light can alsoprovide the activation energy for a reaction. You can betterunderstand activation energy by studying the diagrams inFigure 13. You can also use the diagrams to compare exother-mic reactions and endothermic reactions.
Chemical Reactions 123
Fresh Hydrogen Peroxide
Hydrogen peroxide solution is used as a disinfectant forsmall scrapes and cuts. This is because it decomposesinto oxygen gas and water. These products help cleanthe wound. Explain why hydrogen peroxide must bestored in a dark bottle to keep it fresh. (Hint: What typeof energy would be blocked by this type of bottle?)
Once begun, an exothermic reaction can continueto react. The energy given off as the product formscontinues to supply the activation energy neededfor the substances to react.
An endothermic reaction continues to absorb energy.Energy must be absorbed to provide the activationenergy needed for the substances to react.
Exothermic Reaction Endothermic Reaction
Ener
gy
Reaction progress
Reactants
Energy given off
Activation energy
Products
�
�
Ener
gy
Reactants
Energy absorbed
Activation energy
Products
Reaction progress
�
�
Figure 13 Energy Diagrams
Factors Affecting Rates of ReactionsYou can think of a reaction as happening only if the particlesof reactants collide with enough energy to break the rightbonds. So, how much energy the particles have and how oftenthey collide affect the rate of reaction. The rate of a reactionis a measure of how quickly the reaction takes place. Four factors that affect the rate of a reaction are temperature, concentration, surface area of the reactants, and the presenceof a catalyst or inhibitor.
TemperatureA higher temperature means a faster rate of reaction. At highertemperatures, particles of reactants move faster. This meansthey collide more often and with more energy. More particlesthen have the activation energy needed to react. And more of them can change into products faster. So, more particles react in a shorter time. You can see this effect by looking at Figure 14 and by doing the QuickLab at left.
ConcentrationGenerally, a higher concentration of reactants means a fasterrate of reaction, as shown in Figure 14. Concentration is ameasure of the amount of one substance dissolved in another.When the concentration is high, there are more reactant particles in a given volume. This means that there is a smallerdistance between particles. The particles run into each othermore often. So, more particles react each second.
1. Fill a plastic cuphalf full with warmwater. Fill a secondplastic cup half fullwith cold water.
2. Place �14� effervescenttablet in each of the twocups of water at the sametime.
3. Record your observationsin your ScienceLog.
4. In which cup did the reac-tion happen at a fasterrate? Explain.
The light stick on the right glows brighter than theone on the left. The higher temperature causes afaster rate of reaction.
The reaction on the right produces bubbles ofhydrogen gas at a faster rate, because the concentration of hydrochloric acid used is higher.
Temperature Concentration
Figure 14 Two Factors That Affect Rates of Reactions
Surface AreaWhen the surface area, or the amount of exposed surface, ofsolid reactants is greater, the rate of a reaction is faster. Grindinga solid into a powder exposes more particles of the reactantto other reactant particles. The particles collide more often.So, the rate of the reaction is faster. You can see this effect inthe QuickLab at right.
Catalysts and InhibitorsSome chemical reactions would be too slow to be useful with-out a catalyst (KAT uh LIST). A catalyst is a substance thatspeeds up a reaction without being permanently changed. Acatalyst lowers the activation energy of a reaction. This allowsthe reaction to happen more rapidly. Most reactions in yourbody are sped up using catalysts called enzymes. Catalysts areeven found in car parts, such as the one shown in Figure 15.
An inhibitor (in HIB it uhr) is a substance that slows downor stops a chemical reaction. Preservatives added to foods areinhibitors. They slow down reactions in the bacteria or fungithat can spoil food. Many poisons are also inhibitors.
List four ways to make therate of a reaction faster.
Comparing Concepts Compareexothermic and endothermicreactions.
Interpreting Graphics Howcan you tell that this energydiagram shows an endother-mic reaction?
I’m Crushed!
1. Fill two plastic cupshalf full with room-temperature water.
2. Fold a sheet ofpaper around �14�
effervescent tablet.Carefully crush the tablet.
3. Get a second �14� efferves-cent tablet. Carefully pourthe crushed tablet into onecup. At the same time,place the uncrushed tabletin the second cup.
4. Record your observationsin your ScienceLog.
5. In which cup did the reac-tion happen at a fasterrate? Explain.
Ener
gy
Reaction progress
�
�
TOPIC: Exothermic and EndothermicReactions
sciLINKS NUMBER: HSTP345
Figure 15 This catalytic converter contains platinumand palladium. These two catalysts speed up the rate of reactions that make a car’s exhaust less polluting.
126 Chapter 5
Putting Elements Together You have learned that the products of a chemical reaction have differentchemical and physical properties thanthe reactants. In this activity, you willobserve the reaction between copperand oxygen to form copper(II) oxide.
• metric balance
• evaporating dish
• weighing paper
• copper powder
• ring stand and ring
• wire gauze
• Bunsen burner orportable burner
• spark igniter
• tongs
• computer (optional)
Procedure
1 Copy the table shown on the next page into yourScienceLog, or create a similar one on a computer.
2 Use the metric balance to measure the mass (to the nearest 0.1 g) of the empty evaporatingdish. Record this mass in the table.
3 Using weighing paper, measure approximately 10 g of copper powder. Record the mass (to the nearest 0.1 g) in the table.Caution: Wear goggles, an apron, and protectivegloves when working with copper powder.
4 Use the weighing paper to place the copper powder in the evaporating dish. Spread the powder over the bottom and the sides as much as possible. Throw away the weighing paper.
5 Set up the ring stand and ring. Place the wiregauze on top of the ring. Carefully place theevaporating dish on the wire gauze.
6 Place the Bunsen burner under the ring and wiregauze. Use the spark igniter to light the Bunsenburner. Heat the evaporating dish for 10 minutes.Caution: Use extreme care when working near anopen flame.
7 Turn off the burner, and allow the evaporatingdish to cool for 10 minutes. Use tongs to removethe evaporating dish, and place it on the balanceto find the mass. Record the mass in the table.
8 Find the mass of the product of the reaction—copper(II) oxide—by subtractingthe mass of the evaporating dish from the combined mass of the evaporating dish and copper powder after heating.Record this mass in the table.
Analysis
9 What evidence of a chemical reaction didyou observe after the copper was heated?
10 Explain why there was a change in mass.
11 Why was powdered copper used rather than a small piece of copper? (Hint: How does surface area affect the rate of the reaction?)
12 Why was the copper heated? (Hint: Look in your book for the discussion of activation energy.)
13 The copper bottoms of cooking pots canturn black when used. How is that changesimilar to the results you got in this lab?
•Chemical reactions form newsubstances that have differ-ent properties than the starting substances.
•Clues that a chemical reac-tion is taking place includeformation of a gas or solid,a color change, and anenergy change.
Section 2
Vocabularychemical formula (p. 115)chemical equation (p. 117)reactant (p. 117)product (p. 117)law of conservation of mass
(p. 120)
Section Notes
•A chemical formula useschemical symbols and sub-scripts to describe the com-position of a compound.Subscripts are small num-bers written below and tothe right of a symbol in achemical formula.
•Chemical formulas can oftenbe written from the namesof covalent compounds andionic compounds.
•A chemical equation useschemical formulas, chemicalsymbols, and coefficients todescribe a reaction.
•Balancing an equationrequires that the numbers of atoms of each element in the reactants equal thenumber of atoms of thoseelements in the products.
•A balanced equation usescoefficients to illustrate thelaw of conservation of mass—mass is neither created nordestroyed during ordinaryphysical and chemicalchanges.
LabBook Finding a Balance (p. 656)
Section 3
Vocabularyexothermic (p. 121)endothermic (p. 122)law of conservation of energy
(p. 122)activation energy (p. 122)
Section Notes
•Energy is given off in exo-thermic reactions. The energygiven off can be written as a product in a chemicalequation.
•Energy is absorbed in endo-thermic reactions. The energyabsorbed can be written as a reactant in a chemicalequation.
•The law of conservation of energy states that energyis neither created nordestroyed.
•Activation energy is theenergy needed for a reaction to occur.
•Energy diagrams indicatewhether a reaction is exo-thermic or endothermic byshowing whether energy isgiven off or absorbed duringthe reaction.
•The rate of a chemical reac-tion is affected by tempera-ture, concentration, surfacearea, and the presence of acatalyst or inhibitor.
•Higher temperatures, higherconcentrations, greater sur-face areas, and the additionof a catalyst can make therate of a reaction faster.
LabBook Cata-what? Catalyst! (p. 657)Speed Control (p. 658)
8. Which is an example of the use of activation energy?a. plugging in an ironb. playing basketballc. holding a lit match to paperd. eating
9. Enzymes in your body act as catalysts.Thus, the role of enzymes is toa. speed up the rate of chemical reactions.b. slow down the rate of chemical reactions.c. help you breathe.d. inhibit chemical reactions.
Short Answer
10. Name two ways thatyou could speed upthe rate of a chemicalreaction. Name twoways that you couldslow down the rate ofa chemical reaction.
11. After carrying out a chemical reaction inlab, you notice that condensation on theoutside of the beaker formed ice crystals.Is the chemical reaction exothermic orendothermic? Explain.
12. Acetic acid, a compound found in vinegar,reacts with baking soda to form carbondioxide, water, and sodium acetate. Withoutwriting an equation, name the reactantsand the products of this reaction.
USING VOCABULARY
Complete the following sentences by choosingthe correct term from each pair of terms.
1. Adding a(n) ? will slow down a chemical reaction. (catalyst or inhibitor)
2. A chemical reaction that gives off energy is called an ? reaction. (exothermic orendothermic)
3. A shorthand notation that uses chemicalsymbols and numbers to represent a sub-stance is a ? . (chemical formula orchemical equation)
4. The 2 in the formula Ag2S is a ? . (subscript or coefficient)
5. The starting materials in a chemical reaction are ? . (reactants or products)
UNDERSTANDING CONCEPTS
Multiple Choice
6. Balancing a chemical equation so that thesame number of atoms of each element is found in both the reactants and theproducts is an illustration ofa. activation energy.b. the law of conservation of energy.c. the law of conservation of mass.d. an exothermic reaction.
7. What is the correct chemical formula forcalcium chloride?a. CaClb. CaCl2c. Ca2Cld. Ca2Cl2
13. Use the following terms to create a concept map: chemical reaction, chemical equation, chemical formulas, reactants, products, coefficients, andsubscripts.
CRITICAL THINKING ANDPROBLEM SOLVING
Write one or two sentences to answer the following questions:
14. Your friend is very worried by rumors he hasheard about a substance called dihydrogenmonoxide, which is found in the city’stap water. What could you say to yourfriend to calm his fears? (Hint: Write theformula of the substance.)
15. As long as proper safety precautions aretaken, why can explosives be transportedlong distances without exploding?
MATH IN SCIENCE
16. Calculate the number of atoms of eachelement shown in each of the following:a. CaSO4 c. Fe(NO3)2
b. 4NaOCl d. 2Al2(CO3)3
17. Write balanced equations for the following:a. Fe � O2 Fe2O3
b. Al � CuSO4 Al2(SO4)3 � Cuc. Ba(CN)2 � H2SO4 BaSO4 � HCN
18. Write balanced chemical equations fromeach of the following descriptions: a. Bromine reacts with sodium iodide to
form iodine and sodium bromide.b. Phosphorus reacts with oxygen gas to
form diphosphorus pentoxide.c. Lithium oxide decomposes to form
lithium and oxygen.
Take a minute to reviewyour answers to the Pre-Reading Questionsfound at the bottom
of page 110. Have your answers changed? If necessary, revise your answers based on whatyou have learned since you began this chapter.
ReadingCheck-up
INTERPRETING GRAPHICS
19. During an experiment,a scientist observes theevent shown in thephoto at right. Shehypothesizes that achemical reaction istaking place. Whatevidence supports her hypothesis?
20. Use the energy diagram below to answerthe questions that follow.
a. Which letter represents the energy ofthe products?
b. Which letter represents the activationenergy of the reaction?
c. Is energy given off or absorbed by thisreaction?
d. Is this reaction an exothermic orendothermic reaction? Explain.
Ener
gy
Reaction progress
A
D
B
C
�
�
131Chemical Reactions
4 A scientist carries out a reaction in a test tube. After the bubbling stops, shenotices that the test tube is very warm.What might she conclude about thereaction? F The reaction happened very quickly.G The reaction is endothermic.H The reaction is exothermic.J No reaction took place.
5 According to the following chemicalequation, how many reactants areneeded to form water and carbon dioxide?
H2CO3 H2O � CO2
A 1B 2C 3D 4
6 Look at the equations below. Which ofthe following statements is true?
CH4 � 2O2 CO2 � 2H2O �energy
N2 � O2 � energy 2NO
F Equation 1 is an endothermic reaction.G Equation 2 is an endothermic reaction.H Both equations 1 and 2 are exother-
mic reactions.J It is not possible to determine the
type of reaction.
Chapter 5
1 C6H12O6Glucose molecule
According to this chemical formula, allof the following elements are found in a glucose molecule EXCEPT A carbon.B nitrogen.C hydrogen.D oxygen.
2 Fe2O3Ferric oxide molecule
According to this chemical formula,how many atoms of oxygen are in a ferric oxide molecule? F 1G 2H 3J 4
3 A student collecting data in a laboratoryinvestigation measured liquid using agraduated cylinder. What is the volumeof water in this graduated cylinder? A 24.3 mLB 24.4 mLC 24.5 mLD 24.8 mL
1 Nina has 15 pens in her backpack. Thereare 3 red pens, 10 black pens, and 2 bluepens. If Ben selects a pen to borrow atrandom, what is the probability that the pen selected is red?
A �215�
B �15�
C �13�
D �23�
2 Which letter best represents the number2�
35� on the number line?
F PG QH RJ S
The key to an air bag’s success during acrash is the speed at which it inflates. Insidethe bag is a gas generator that containsthe compounds sodium azide, potassiumnitrate, and silicon dioxide. At the momentof a crash, an electronic sensor in the cardetects the sudden change in speed. Thesensor sends a small electric current to thegas generator. This provides the activationenergy for the chemicals in the gas generator.
The rate at which the reaction happensis very fast. In �
215� of a second, the gas
formed in the reaction inflates the bag. Theair bag fills upward and outward. By fillingthe space between a person and the car’sdashboard, the air bag protects him orher from getting hurt.
1 Which of these happens first? A The sensor sends an electric current to
the gas generator.B The air bag inflates.C The air bag fills the space between the
person and the dashboard.D The sensor detects a change in speed.
2 The author probably wrote this passage toF convince people to wear a seat belt.G describe the series of events that
inflate an air bag.H explain why air bags are an important
safety feature in cars.J show how chemical reactions protect
pedestrians.
Read the passage. Then read each question that follows the passage. Decide which is the best answer to each question.
with a fire retardant. This extra time allows theground team to create a fire line that will finallystop the fire.
Neon Isn’t NecessaryOnce a fire is put out, the slimy, red streaks lefton the blackened ground can be an eyesore. Tosolve the problem, scientists have made specialdyes for the retardant. These dyes make thegoop neon colors when it is first dropped. Aftera few days in the sun, though, the goop turns anatural brown shade.
What Do They Study?� Do some research to learn about a fire-fighter’s training. What classes and tests mustfirefighters pass? How do they keeptheir certifications once they becomefirefighters? Why do you think it isimportant for firefighters to under-stand the chemistry of fire?
133
Slime That Fire!Once a fire starts in the hard-to-reach mountains of the western United States, it isdifficult to stop. Trees, grasses, and brush canprovide a lot of fuel. In order to stop a fire,firefighters make a fire line. A fire line is anarea where all the burnable materials areremoved from the ground. How would you slowdown a fire to give a ground team more timeto build a fire line? How about dropping waterfrom a plane? That is not a bad idea. Whatwould happen if you had something even better than water—like some slimy, red goop?
Red Goop Goes the DistanceThe slimy, red goop is in fact a powerful fireretardant. A retardant is something that slowsthe progress of a chemical reaction. The goop is a mixture of a powder and water. It can beloaded directly onto a plane. Carrying between4,500 and 11,000 L of the slime, the planedrops it all in front of the raging flames whenthe pilot presses the button.
By changing the thickness of the red goop,the firefighters can control how it falls. Theamount of water added to the powder depends
on where the fire is taking place. If a fire is burning over bushes and grasses, more
water is needed. In this form, the goop actually rains down to the ground through
the tops of trees. But if a fire is burning intall trees, less water is used. This way theslime will glob onto the branches and ooze down very slowly.
Failed FlamesThe burning of trees, grass, and brush is anexothermic reaction. A fire retardant slows orstops this self-feeding reaction. A fire retardantincreases the activation energy for the materials
to which it sticks. Firefighters on the groundcan gain valuable time when a fire is slowed
� This plane is dropping fire retardant on a forest fire.
