Chemical Reactions Back

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Chemical
Reactions
Forming New
Substances . . . . . . . . . 112
QuickLab
. . . . . 114
Internet Connect . . . . 114
Chemical Formulas
and Equations . . . . . . 115
MathBreak . . . . . 116, 119
Self-Check
. . . . 118
QuickLab
. . . . . 120
Internet Connect . . . . 120
Energy and Rates of
Chemical Reactions . . 121
QuickLab
. . 121, 122
Apply . . . . . . . . . . . . . 123
QuickLab
. . 124, 125
Internet Connect . . . . 125
Chapter Lab
. . . . . . . 126
Chapter Review
. . . . . 129
TEKS/TAKS
Practice Tests . . . . . . 131, 132
Feature Article . . . . . . . . . . 133
LabBook
. . . 656, 657, 658
Pre-Reading
Questions
1. What clues can show that
a chemical reaction is
taking place?
2. Why are chemical formulas
important?
3. How can you change the
rate of a chemical reaction?
110
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A MODEL FORMULA
Reaction to the Rescue
A car slams into a wall at 97 km/h (60 mph) during a crash
test. Although both dummies are wearing seat belts, one
suffers a crushing blow to the head as it strikes the dashboard. The other suffers only minor bruises thanks to an air
bag. Air bags can inflate rapidly due to a chemical reaction
that produces gas at a very fast rate. In this chapter you
will learn how to identify and describe a chemical reaction.
You will also learn about factors that affect the rate of a
reaction.
Chemicals react in very precise ways.
In this activity, you will model a
chemical reaction and predict how
chemicals react.
Procedure
1. You will receive several
marshmallow models.
The models are marshmallows
stuck together with toothpicks.
Each of these models
is a model A.
2. Your teacher will show you model
B and model C. Take apart one or
more model A’s to make copies of
model B and model C.
3. Do you have any pieces left over?
If so, use them to make more
model B’s and/or model C’s. Do
you need more parts to complete
model B or model C? If so, take
apart another model A.
Reaction to the Rescue
A car slams into a wall at 97 km/h (60 mi/h)
during a crash test. Although both dummies are
wearing seat belts, one suffers a crushing blow to
the 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 chemical
reaction that produces gas at a very fast rate. In
this chapter, you will learn how to identify and
describe a chemical reaction. You will also learn
about factors that affect the rate of a reaction.
4. If needed, repeat step 3 until you
do not have any parts left over.
Analysis
5. How many model A’s did you use
to make copies of model B and
model C?
6. How many model B’s did you
make? How many model C’s did
you make?
7. Suppose you needed to make six
model B’s. How many model A’s
would you need? How many
model C’s could you make with
the leftover parts?
Chemical Reactions
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111
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Forming New Substances
Each fall, a beautiful change takes place. Leaves change
color. You see bright reds, oranges, and yellows that had
been hidden by green all year. What causes this change?
Terms to Learn
chemical reaction
What You’ll Do
●
Name the clues that show
that a chemical reaction
might be taking place.
●
Demonstrate that substances
may react chemically to form
new substances.
To answer this question, you need to know what causes leaves
to be green in the first place. Leaves are green because they
contain a green substance called chlorophyll (KLAWR uh FIL).
During the spring and summer, the leaves contain
a lot of chlorophyll. But in the fall, when
temperatures drop and there are fewer hours
of sunlight, the chlorophyll goes through a
chemical change. The chlorophyll changes
into new substances that have no color. When
this happens, the red, orange, and yellow
colors that were always present in the leaves
can now be seen.
Chemical Reactions
The chemical change that happens as chlorophyll breaks down
into 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 physical
properties from the original substances. Many of the changes
you see every day are chemical reactions. For example, striking
a match, taking a photograph, and using a battery all involve
chemical reactions. Check out Figure 1 to see two more examples of chemical reactions.
Figure 1
Examples of Chemical Reactions
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 the
tasty treat its spongelike texture.
112
A reaction between gasoline and oxygen
forms carbon dioxide, water, and other
substances. The formation of the new
substances causes the
pistons to move,
which makes
the car run.
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Clues to Chemical Reactions
How can you tell when a chemical reaction is taking place?
There are several clues to look for. Look at Figure 2. A change
in color can be one clue. The formation of gas bubbles or a
solid also may show that a reaction is happening. A solid formed
in a solution as a result of a chemical reaction is called a precipitate (pree SIP uh TAYT). Also, during many chemical reactions, energy is taken in or given off as light, thermal, or
electrical energy. The more of these clues you see, the more
likely it is that a chemical reaction is taking place.
Figure 2
Some Clues to Chemical Reactions
Gas Formation
Bubbles of carbon dioxide
form when hydrochloric
acid is placed
on a piece of
limestone.
Solid Formation
Here you see potassium chromate
solution being added to a silver
nitrate solution. The dark red solid
is a precipitate of silver chromate.
Energy Change
The chemical reaction that takes
place in a fire gives off light and
thermal energy. During some other
reactions, energy is taken in.
Color Change
Don’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.
Chemical Reactions
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113
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Figure 3
Reaction of Hydrogen and Chlorine
Breaking bonds
Making bonds
Hydrogen
Hydrogen chloride
Chlorine
The elements hydrogen and chlorine are diatomic.
Diatomic molecules are made of two atoms bonded
together. For these molecules to react, the bonds
joining the atoms must break.
Molecules of the new substance, hydrogen chloride,
are formed as new bonds are made between
hydrogen atoms and chlorine atoms.
Breaking and Making Bonds
Reaction Ready
1. Place a piece of
chalk in a plastic
cup.
2. Add 1 tsp of vinegar to the cup.
Record your observations in your
ScienceLog.
3. What evidence do you see
that a chemical reaction is
taking place?
4. What type of new substance was formed?
In a chemical reaction, bonds are broken and new bonds are
formed. When chemical bonds in the starting substances are
broken, new bonds can form between different atoms to make
the new substances. Look at the model in Figure 3 to understand how this process happens.
New Substances Form in a Chemical Reaction
You’ve read that a color change is one clue to a chemical
reaction. A color change may indicate that a new substance
has formed. For example, a color change takes place when
chlorine gas reacts with hydrogen gas. Chlorine gas is greenish yellow in color. Hydrogen is a flammable, colorless gas.
The two substances combine to form hydrogen chloride, a
nonflammable, colorless gas. Hydrogen chloride has different
chemical and physical properties than the substances that
combined to form it. So, hydrogen chloride is a different substance than hydrogen and chlorine.
What is a chemical reaction?
List four clues that can help you demonstrate that a
chemical reaction is taking place.
TOPIC: Chemical Reactions
sciLINKS NUMBER: HSTP330
In a chemical reaction, what must happen to the
chemical bonds of the reacting substances in order to
make a new substance?
Applying Concepts Is a chemical reaction taking place
when the metal of a car body rusts? Explain your answer.
114
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Chemical Formulas
and Equations
Think about how many words you can create using the
26 letters of the alphabet. Hundreds of thousands of
words are possible!
Terms to Learn
chemical formula
chemical equation
reactant
product
law of conservation of mass
What You’ll Do
●
Interpret and write simple
chemical formulas and
equations that express
what happens in a
chemical reaction.
●
Explain how a balanced
equation shows the law
of conservation of mass.
Figure 4
In the same way that letters are used to form words, chemical
symbols are used to represent substances. You can think of
chemical formulas as “words” made from chemical symbols.
Chemical Formulas
All substances are formed from about 100 elements. Each element has its own chemical symbol. A chemical formula is a
shorthand notation that uses chemical symbols and numbers
to represent a substance. A chemical formula shows how many
of each kind of atom are present in a molecule.
Look at Figure 4. The chemical formula for water is H2O. This
formula tells you that one water molecule is made of two
atoms of hydrogen and one atom of oxygen. The small 2 in
the formula is a subscript. A subscript is a number written below
and to the right of a chemical symbol in a formula. When a
symbol, such as the O for oxygen in water’s formula, has no
subscript, only one atom of that element is present. Figure 4
also explains two more chemical formulas.
Chemical Formulas of Different Substances
Water
Oxygen
Glucose
H2O
O2
C6H12O6
Water molecules are made up of
3 atoms—2 atoms of hydrogen
bonded to 1 atom of oxygen.
Oxygen is a diatomic element.
Each molecule of oxygen gas
has 2 atoms of oxygen bonded
together.
Glucose molecules have 6 atoms
of carbon, 12 atoms of hydrogen,
and 6 atoms of oxygen. (Glucose
is the sugar made by plants
during photosynthesis.)
Chemical Reactions
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115
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Writing Formulas for Covalent Compounds
Prefixes Used in
Chemical Names
mono-
1
hexa-
6
di-
2
hepta-
7
tri-
3
octa-
8
tetra-
4
nona-
9
penta-
5
deca-
10
You can often write a chemical formula if you know the name
of the substance. Covalent compounds are usually made of
two nonmetals. The names of covalent compounds use prefixes to tell you how many atoms of each element are in the
formula. Each prefix represents a number. The prefixes are
shown in the table at left. Figure 5 shows two examples of
how to write a chemical formula of a covalent compound.
Carbon dioxide
Dinitrogen monoxide
CO2
N2O
Having no prefix indicates
one carbon atom.
The prefix di- indicates two
nitrogen atoms.
The prefix di- indicates two
oxygen atoms.
The prefix mono- indicates
one oxygen atom.
Figure 5 The formulas of these covalent compounds
can be written using the prefixes in their names.
Writing Formulas for Ionic Compounds
Counting Atoms
Some formulas have parentheses.
When counting atoms, multiply
everything inside the parentheses
by the subscript. For example,
Ca(NO3)2 has
1 calcium atom
2 nitrogen atoms (2 1)
6 oxygen atoms (2 3)
Now It’s Your Turn
Find the number of atoms of each
element in the formulas Mg(OH)2
and Al2(SO4)3.
If the name of a compound has the name of a metal and a
nonmetal, the compound is probably ionic. To write the formula for an ionic compound, you must make sure the compound’s overall charge is zero. In other words, the formula must
have subscripts that cause the charges of the ions to cancel
out. Figure 6 shows two examples of how to write a chemical
formula from the name of an ionic compound.
Sodium chloride
Magnesium chloride
NaCl
MgCl2
A sodium ion has a
1 charge.
A magnesium ion has a
2 charge.
A chloride ion has a
1 charge.
A chloride ion has a
1 charge.
One sodium ion and one
chloride ion have an overall
charge of (1) (1) 0.
One magnesium ion and two
chloride ions have an overall
charge of (2) 2(1) 0.
Figure 6 The formula of an ionic compound is written by using
enough of each ion so that the overall charge is zero.
116
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Chemical Equations
Think about a piece of music, such as the one in Figure 7.
Someone writing music must communicate to the musician
what notes to play, how long to play each note, and how each
one should be played. He or she does not use words to describe
the piece. Instead, musical symbols are used to communicate
in a way that can be understood by anyone who can read music.
Equations Describe Reactions
In the same way, chemists around the world must communicate about reactions clearly. Descriptions using sentences would
need translations into other languages. Instead of using long
sentences, chemists can use chemical equations to describe
reactions. A chemical equation uses chemical formulas as a shorthand description of a chemical reaction. A chemical equation
is short and understood by anyone who understands chemical
formulas. Each element’s chemical symbol is understood around
the world. So, a chemical equation does not need translation.
Figure 7 The symbols on this
music are understood around the
world—just like chemical symbols.
Reactants Yield Products
When carbon burns, it reacts with oxygen to form carbon
dioxide. How would a chemist describe this reaction? Look at
Figure 8. The starting materials in a chemical reaction are
reactants (ree AK tuhnts). The substances formed from a reaction are products. In this example, carbon and oxygen are
reactants. Carbon dioxide is the product.
Figure 8
The Parts of a Chemical Equation
Charcoal is used to cook food on a barbecue.
When carbon in charcoal reacts with oxygen in
the air, the primary product is carbon dioxide.
The formulas of the reactants
are written before the arrow.
CO2
A plus sign separates the formulas of two or more reactants
or products from one another.
The formulas of the products
are written after the arrow.
CO2
The arrow is called the yields
sign. It separates the formulas
of the reactants from the formulas of the products.
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117
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Figure 9
Examples of Similar Symbols and Formulas
CO2
The chemical formula for the
compound carbon dioxide is
CO2. Carbon dioxide is a
colorless, odorless gas.
CO
The chemical formula for the
compound carbon monoxide is
CO. Carbon monoxide is a
colorless, odorless, poisonous gas.
Co
The chemical symbol for the
element cobalt is Co. Cobalt
is a hard, bluish gray metal.
Accuracy Is Important
Hydrogen gas, H2, is an important fuel that may help reduce
air pollution. Water is the only
product formed as hydrogen
burns. That’s why there is little
air pollution from a car that
uses hydrogen as fuel.
The symbol or formula for each substance in the equation
must be written correctly. For a compound, decide if it is covalent or ionic. Then write the correct formula. For an element,
use the proper chemical symbol. Be sure to use a subscript of
2 for the diatomic elements. (The seven diatomic elements are
hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and
iodine.) An equation with the wrong chemical symbol or
formula will not correctly describe the reaction. In fact, even
a simple mistake can make a huge difference, as in Figure 9.
An Equation Must Be Balanced
In a chemical reaction, every atom in the reactants becomes
part of the products. Atoms are never lost or gained in a chemical 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 those
elements in the products.
Self-Check
Why is it important to use correct chemical formulas in chemical
equations? (See page 698 to check your answer.)
118
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How to Balance an Equation
Writing a balanced equation calls for the use of coefficients
(KOH uh FISH uhnts). A coefficient is a number placed in front
of a chemical symbol or formula. When counting atoms, you
multiply a coefficient by the subscript of each of the elements
that follows the coefficient in the formula. This means that
2CO2 stands for two carbon dioxide molecules. Together the
two carbon dioxide molecules have two carbon atoms and four
oxygen atoms.
Coefficients are used to balance equations. The reason is
that the subscripts in chemical formulas cannot be changed.
Changing a subscript changes a formula so that it no longer
stands for the correct substance. Study Figure 10 to see how to
use coefficients to balance a simple equation—the formation
of water. Then you can practice balancing equations by doing
the MathBreak at right.
Balancing Act
When balancing an equation,
place coefficients in front of an
entire chemical formula. Never
place them in the middle of a
formula. For example:
F2 + 2KCl
2KF + Cl2
Now It’s Your Turn
Balance the following equations:
HCl + Na2S
H2S + NaCl
Al + Cl2
AlCl3
Figure 10 Balancing a Chemical Equation
Follow these steps to write a balanced
equation for H2 O2
H2O.
1
Reactants
Products
H2 O2
H2O
Count the atoms of each element in the
reactants and in the products. You can see
that there are fewer oxygen atoms in the
products than in the reactants.
H2
2
To balance the oxygen atoms, place the
coefficient 2 in front of the formula for
water. This gives you two oxygen atoms in
both the reactants and the products. But
now there are too few hydrogen
atoms in the reactants.
To balance the hydrogen atoms, place the
coefficient 2 in front of hydrogen’s formula.
To be sure your answer is correct, always
double-check your work!
H 2 O 1
Reactants
Products
H2 O2
2H2O
H2
3
O2
O2
H 4 O 2
Reactants
Products
2H2 O2
2H2O
H4
O2
H 4 O 2
Chemical Reactions
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Mass Is Conserved—It’s a Law!
Figure 11 Magnesium in the
flashbulb of a camera reacts with
oxygen. The mass is the same
before and after the reaction.
The practice of balancing equations comes from the work of
a French chemist, Antoine Lavoisier (lah vwah ZYAY). In the
1700s, Lavoisier carefully measured and compared the masses
of the substances in chemical reactions. He found that the
total mass of the reactants was the
same as the total mass of the
products. Lavoisier’s work led to
the law of conservation of mass.
This law states that mass is
neither created nor destroyed in
ordinary chemical and physical
changes. This law means that a
chemical equation must show the
same number and kind of atom
on both sides of the arrow. You
can see how this law works in
Figure 11 and in the QuickLab
below.
Mass Conservation
1. Place 1 tsp of baking soda into a sealable
plastic bag.
2. Place 1 tsp of vinegar into a plastic film
canister. Close the lid.
3. Place the canister into the bag. Squeeze the
air out of the bag. Seal it tightly.
4. Use a balance to find the mass of the bag
and its contents.
5. Keeping the bag closed, open the canister in
the 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 before
and after the reaction took place.
Explain why chemical formulas and chemical equations
are important.
TOPIC: Chemical Formulas
sciLINKS NUMBER: HSTP335
TOPIC: Chemical Equations
sciLINKS NUMBER: HSTP340
Write the chemical formulas for carbon tetrachloride
and calcium bromide.
Explain how a balanced chemical equation illustrates
that mass is never lost or gained in a chemical reaction.
Applying Concepts Write a balanced chemical equation
to express what happens when methane, CH4, and oxygen gas react to form water and carbon dioxide.
120
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Energy and Rates of
Chemical Reactions
What’s the difference between eating a meal and running
a mile? You could say that one gives you energy, while
the other uses up energy.
Terms to Learn
exothermic
endothermic
law of conservation
of energy
activation energy
What You’ll Do
●
Identify and demonstrate
that loss of energy happens
during exothermic
reactions.
●
Identify and demonstrate
that gain of energy happens
during endothermic
reactions.
●
Explain activation energy
and energy diagrams.
●
Describe the factors that
affect the rate of reaction.
Chemical reactions can be described in the same way. Some
reactions give off energy. Other reactions absorb energy. In this
section, you will learn how to classify a chemical reaction in
terms of the reaction’s energy. You will also learn how to change
the rate at which a chemical reaction happens.
Every Reaction Involves Energy
Chemical energy is part of all chemical reactions. During a
reaction, as chemical bonds in the reactants break, energy is
absorbed. As new bonds form in the products, energy is given
off. You can compare the chemical energy of the reactants
with the chemical energy of the products. This is how you
determine whether energy is given off or absorbed in the
overall reaction.
Energy Is Released in Exothermic Reactions
Sometimes, the chemical energy of the reactants is greater than
the chemical energy of the products. This kind of reaction gives
off energy. A chemical reaction in which energy is given off is
called exothermic. Exo- means “go out” or “exit,” and -thermic
means “heat” or “energy.” A chemical reaction can give off
energy in several different forms, as shown in Figure 12 on the
next page. The energy given off in an exothermic reaction is
often written as a product, as in this equation:
2Na + Cl2
The Temperature Tells
1. Fill a plastic cup half full
with hydrogen peroxide
solution.
2. Measure the temperature
of the solution with a
thermometer.
3. Carefully add 20 cubes
of raw potato to the cup.
4. Record your observations in
your ScienceLog.
5. When the reaction has
stopped, record the temperature of the solution.
2NaCl + energy
6. What evidence did you
see that an exothermic
reaction took place?
(Hint: Was energy lost or
gained?)
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Figure 12 Types of Energy Released in Reactions
Light energy is given off in the
reaction taking place in these
light sticks.
Electrical energy is given off in
the reaction taking place in the
dry cells in this flashlight.
Light and thermal energy are
given off in the reaction taking
place in this campfire.
Energy Is Absorbed in Endothermic Reactions
Sometimes, the chemical energy of the reactants is less than
the chemical energy of the products. In this kind of reaction,
energy is absorbed. A chemical reaction in which energy is
absorbed is called endothermic. Endo- means “go in.” The energy
absorbed in an endothermic reaction is often written as a
reactant, as in this equation:
Endo Alert
1. Fill a plastic cup
half full with calcium
chloride solution.
2. Measure the temperature of the
solution with a
thermometer.
3. Carefully add 1 tsp
of baking soda.
4. Record your observations
in your ScienceLog.
5. When the reaction has
stopped, record the temperature of the solution.
6. What evidence did you
see that an endothermic
reaction took place?
(Hint: Was energy lost
or gained?)
122
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 neither
created nor destroyed. The energy given off in exothermic
reactions was originally stored in the reactants. And the energy
absorbed in endothermic reactions is stored in the products
that form. Imagine that you could measure all the energy in
a 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 Happen
A match can be used to light a campfire—but only if the match
is lit! A strike-anywhere match has all the reactants it needs
to be able to burn, but the reactants will not ignite by themselves. Energy is needed for the reaction to occur. Rubbing the
tip of a match on a rough surface provides the energy needed
for the chemicals to react. The smallest amount of energy
needed for substances to react is called activation energy.
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Where Does Activation Energy Come From?
The friction of striking a match heats the substances on the
match, breaking bonds in the reactants. This allows the new
bonds in the products to form. Chemical reactions need some
energy to occur. An electric spark in a car’s engine provides
activation energy for the burning of gasoline. Light can also
provide the activation energy for a reaction. You can better
understand activation energy by studying the diagrams in
Figure 13. You can also use the diagrams to compare exothermic reactions and endothermic reactions.
Fresh Hydrogen Peroxide
Hydrogen peroxide solution is used as a disinfectant for
small scrapes and cuts. This is because it decomposes
into oxygen gas and water. These products help clean
the wound. Explain why hydrogen peroxide must be
stored in a dark bottle to keep it fresh. (Hint: What type
of energy would be blocked by this type of bottle?)
Figure 13 Energy Diagrams
Endothermic Reaction
Exothermic Reaction
Activation
energy
Energy
Energy
Products
Activation
energy
Reactants
Energy
given off
Energy
absorbed
Reactants
Products
Once begun, an exothermic reaction can continue
to react. The energy given off as the product forms
continues to supply the activation energy needed
for the substances to react.
Reaction progress
An endothermic reaction continues to absorb energy.
Energy must be absorbed to provide the activation
energy needed for the substances to react.
Chemical Reactions
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Reaction progress
123
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Factors Affecting Rates of Reactions
Which Is Quicker?
1. Fill a plastic cup
half full with warm
water. Fill a second
plastic cup half full
with cold water.
1 effervescent
2. Place 4
tablet in each of the two
cups of water at the same
time.
3. Record your observations
in your ScienceLog.
4. In which cup did the reaction happen at a faster
rate? Explain.
You can think of a reaction as happening only if the particles
of reactants collide with enough energy to break the right
bonds. So, how much energy the particles have and how often
they collide affect the rate of reaction. The rate of a reaction
is 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 presence
of a catalyst or inhibitor.
Temperature
A higher temperature means a faster rate of reaction. At higher
temperatures, particles of reactants move faster. This means
they collide more often and with more energy. More particles
then 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.
Concentration
Generally, a higher concentration of reactants means a faster
rate of reaction, as shown in Figure 14. Concentration is a
measure 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 smaller
distance between particles. The particles run into each other
more often. So, more particles react each second.
Figure 14 Two Factors That Affect Rates of Reactions
Temperature
The light stick on the right glows brighter than the
one on the left. The higher temperature causes a
faster rate of reaction.
124
Concentration
The reaction on the right produces bubbles of
hydrogen gas at a faster rate, because the
concentration of hydrochloric acid used is higher.
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Surface Area
When the surface area, or the amount of exposed surface, of
solid reactants is greater, the rate of a reaction is faster. Grinding
a solid into a powder exposes more particles of the reactant
to other reactant particles. The particles collide more often.
So, the rate of the reaction is faster. You can see this effect in
the QuickLab at right.
Catalysts and Inhibitors
Some chemical reactions would be too slow to be useful without a catalyst (KAT uh LIST). A catalyst is a substance that
speeds up a reaction without being permanently changed. A
catalyst lowers the activation energy of a reaction. This allows
the reaction to happen more rapidly. Most reactions in your
body are sped up using catalysts called enzymes. Catalysts are
even found in car parts, such as the one shown in Figure 15.
An inhibitor (in HIB it uhr) is a substance that slows down
or stops a chemical reaction. Preservatives added to foods are
inhibitors. They slow down reactions in the bacteria or fungi
that can spoil food. Many poisons are also inhibitors.
4. Record your observations
in your ScienceLog.
Energy
TOPIC: Exothermic and Endothermic
Reactions
sciLINKS NUMBER: HSTP345
Reaction progress
Interpreting Graphics How
can you tell that this energy
diagram shows an endothermic reaction?
2. Fold a sheet of
1
paper around 4
effervescent tablet.
Carefully crush the tablet.
1 efferves3. Get a second 4
cent tablet. Carefully pour
the crushed tablet into one
cup. At the same time,
place the uncrushed tablet
in the second cup.
List four ways to make the
rate of a reaction faster.
Comparing Concepts Compare
exothermic and endothermic
reactions.
1. Fill two plastic cups
half full with roomtemperature water.
5. In which cup did the reaction happen at a faster
rate? Explain.
Figure 15 This catalytic converter contains platinum
and palladium. These two catalysts speed up
the rate of reactions that make a car’s
exhaust less polluting.
What is activation energy?
I’m Crushed!
Chemical Reactions
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125
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Putting Elements Together
You have learned that the products
of a chemical reaction have different
chemical and physical properties than
the reactants. In this activity, you will
observe the reaction between copper
and oxygen to form copper(II) oxide.
• metric balance
• evaporating dish
• weighing paper
• copper powder
• ring stand and ring
• wire gauze
• Bunsen burner or
•
•
•
126
portable burner
spark igniter
tongs
computer (optional)
Procedure
1
Copy the table shown on the next page into your
ScienceLog, 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 evaporating
dish. 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 protective
gloves 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 wire
gauze on top of the ring. Carefully place the
evaporating dish on the wire gauze.
6
Place the Bunsen burner under the ring and wire
gauze. Use the spark igniter to light the Bunsen
burner. Heat the evaporating dish for 10 minutes.
Caution: Use extreme care when working near an
open flame.
7
Turn off the burner, and allow the evaporating
dish to cool for 10 minutes. Use tongs to remove
the evaporating dish, and place it on the balance
to find the mass. Record the mass in the table.
Chapter 5
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Data Collection Table
Object
Mass (g)
Evaporating dish
Copper powder
Copper + evaporating dish after heating
Copper(II) oxide
8
Find the mass of the product of the
reaction—copper(II) oxide—by subtracting
the mass of the evaporating dish from
the combined mass of the evaporating
dish and copper powder after heating.
Record this mass in the table.
12 Why was the copper heated?
(Hint: Look in your book for the
discussion of activation energy.)
13 The copper bottoms of cooking pots can
turn black when used. How is that change
similar to the results you got in this lab?
Analysis
9
What evidence of a chemical reaction did
you 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?)
Chemical Reactions
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127
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Section 1
Vocabulary
chemical reaction (p. 112)
Section Notes
• Chemical reactions form new
substances that have different properties than the
starting substances.
• Clues that a chemical reac-
tion is taking place include
formation of a gas or solid,
a color change, and an
energy change.
Section 2
Vocabulary
chemical 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 uses
chemical symbols and subscripts to describe the composition of a compound.
Subscripts are small numbers written below and to
the right of a symbol in a
chemical formula.
• Chemical formulas can often
be written from the names
of covalent compounds and
ionic compounds.
128
• A chemical equation uses
chemical formulas, chemical
symbols, and coefficients to
describe a reaction.
• Balancing an equation
requires that the numbers
of atoms of each element
in the reactants equal the
number of atoms of those
elements in the products.
• A balanced equation uses
coefficients to illustrate the
law of conservation of mass—
mass is neither created nor
destroyed during ordinary
physical and chemical
changes.
LabBook
• The law of conservation
of energy states that energy
is neither created nor
destroyed.
• Activation energy is the
energy needed for a
reaction to occur.
• Energy diagrams indicate
whether a reaction is exothermic or endothermic by
showing whether energy is
given off or absorbed during
the reaction.
• The rate of a chemical reac-
Finding a Balance (p. 656)
Section 3
Vocabulary
exothermic (p. 121)
endothermic (p. 122)
law of conservation of energy
(p. 122)
activation energy (p. 122)
Section Notes
• Energy is given off in exo-
tion is affected by temperature, concentration, surface
area, and the presence of a
catalyst or inhibitor.
• Higher temperatures, higher
concentrations, greater surface areas, and the addition
of a catalyst can make the
rate of a reaction faster.
LabBook
Cata-what? Catalyst! (p. 657)
Speed Control (p. 658)
thermic reactions. The energy
given off can be written as
a product in a chemical
equation.
• Energy is absorbed in endo-
thermic reactions. The energy
absorbed can be written as
a reactant in a chemical
equation.
Chapter 5
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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8. Which is an example of the use of
activation energy?
a. plugging in an iron
b. playing basketball
c. holding a lit match to paper
d. eating
USING VOCABULARY
Complete the following sentences by choosing
the 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 or
endothermic)
3. A shorthand notation that uses chemical
symbols and numbers to represent a substance is a ? . (chemical formula or
chemical equation)
4. The 2 in the formula Ag2S is a
(subscript or coefficient)
?
.
9. Enzymes in your body act as catalysts.
Thus, the role of enzymes is to
a. 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 that
you could speed up
the rate of a chemical
reaction. Name two
ways that you could
slow down the rate of
a chemical reaction.
5. The starting materials in a chemical
reaction are ? . (reactants or products)
UNDERSTANDING CONCEPTS
Multiple Choice
6. Balancing a chemical equation so that the
same number of atoms of each element
is found in both the reactants and the
products is an illustration of
a. activation energy.
b. the law of conservation of energy.
c. the law of conservation of mass.
d. an exothermic reaction.
11. After carrying out a chemical reaction in
lab, you notice that condensation on the
outside of the beaker formed ice crystals.
Is the chemical reaction exothermic or
endothermic? Explain.
12. Acetic acid, a compound found in vinegar,
reacts with baking soda to form carbon
dioxide, water, and sodium acetate. Without
writing an equation, name the reactants
and the products of this reaction.
7. What is the correct chemical formula for
calcium chloride?
a. CaCl
b. CaCl2
c. Ca2Cl
d. Ca2Cl2
Chemical Reactions
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129
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Write one or two sentences to answer the
following questions:
14. Your friend is very worried by rumors he has
heard about a substance called dihydrogen
monoxide, which is found in the city’s
tap water. What could you say to your
friend to calm his fears? (Hint: Write the
formula of the substance.)
20. Use the energy diagram below to answer
the questions that follow.
CRITICAL THINKING AND
PROBLEM SOLVING
19. During an experiment,
a scientist observes the
event shown in the
photo at right. She
hypothesizes that a
chemical reaction is
taking place. What
evidence supports her
hypothesis?
B
A
Energy
13. Use the following terms to
create a concept map:
chemical reaction,
chemical equation,
chemical formulas,
reactants, products,
coefficients, and
subscripts.
INTERPRETING GRAPHICS
D
C
15. As long as proper safety precautions are
taken, why can explosives be transported
long distances without exploding?
MATH IN SCIENCE
16. Calculate the number of atoms of each
element shown in each of the following:
c. Fe(NO3)2
a. CaSO4
b. 4NaOCl
d. 2Al2(CO3)3
17. Write balanced equations for the following:
Fe2O3
a. Fe O2
b. Al CuSO4
Al2(SO4)3 Cu
BaSO4 HCN
c. Ba(CN)2 H2SO4
18. Write balanced chemical equations from
each 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.
130
Reaction progress
CONCEPT MAPPING
a. Which letter represents the energy of
the products?
b. Which letter represents the activation
energy of the reaction?
c. Is energy given off or absorbed by this
reaction?
d. Is this reaction an exothermic or
endothermic reaction? Explain.
Reading
Check-up
Take a minute to review
your answers to the
Pre-Reading Questions
found at the bottom
of page 110. Have your answers changed? If
necessary, revise your answers based on what
you have learned since you began this chapter.
Chapter 5
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Chapter 5
1
C6H12O6
Glucose molecule
According to this chemical formula, all
of the following elements are found in
a glucose molecule EXCEPT
A carbon.
B nitrogen.
C hydrogen.
D oxygen.
2
Fe2O3
Ferric oxide molecule
According to this chemical formula,
how many atoms of oxygen are in a
ferric oxide molecule?
F 1
G 2
H3
J 4
3 A student collecting data in a laboratory
investigation measured liquid using a
graduated cylinder. What is the volume
of water in this graduated cylinder?
A 24.3 mL
B 24.4 mL
24.5
C 24.5 mL
D 24.8 mL
24.0
4 A scientist carries out a reaction in a
test tube. After the bubbling stops, she
notices that the test tube is very warm.
What might she conclude about the
reaction?
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 chemical
equation, how many reactants are
needed to form water and carbon
dioxide?
H2CO3
H2O CO2
A
B
C
D
1
2
3
4
6 Look at the equations below. Which of
the following statements is true?
1
CH4 2O2
energy
CO2 2H2O 2
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 exothermic reactions.
J It is not possible to determine the
type of reaction.
mL
Chemical Reactions
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131
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Chapter 5
Math
1 Nina has 15 pens in her backpack. There
are 3 red pens, 10 black pens, and 2 blue
pens. If Ben selects a pen to borrow at
random, what is the probability that
the pen selected is red?
A 2
15
B 1
5
C 1
3
D 2
3
2 Which letter best represents the number
on the number line?
23
5
P
0
F
G
H
J
Q
1
R S
2
3
P
Q
R
S
Reading
Read the passage. Then read each question that follows the passage.
Decide which is the best answer to each question.
The key to an air bag’s success during a
crash is the speed at which it inflates. Inside
the bag is a gas generator that contains
the compounds sodium azide, potassium
nitrate, and silicon dioxide. At the moment
of a crash, an electronic sensor in the car
detects the sudden change in speed. The
sensor sends a small electric current to the
gas generator. This provides the activation
energy for the chemicals in the gas generator.
The rate at which the reaction happens
of a second, the gas
is very fast. In 21
5
formed in the reaction inflates the bag. The
air bag fills upward and outward. By filling
the space between a person and the car’s
dashboard, the air bag protects him or
her from getting hurt.
132
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 to
F 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.
Chapter 5
Copyright © by Holt, Rinehart and Winston. All rights reserved.
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Slime That Fire!
Once a fire starts in the hard-to-reach
mountains of the western United States, it is
difficult to stop. Trees, grasses, and brush can
provide a lot of fuel. In order to stop a fire,
firefighters make a fire line. A fire line is an
area where all the burnable materials are
removed from the ground. How would you slow
down a fire to give a ground team more time
to build a fire line? How about dropping water
from a plane? That is not a bad idea. What
would happen if you had something even
better than water—like some slimy, red goop?
Red Goop Goes the Distance
The slimy, red goop is in fact a powerful fire
retardant. A retardant is something that slows
the progress of a chemical reaction. The goop
is a mixture of a powder and water. It can be
loaded directly onto a plane. Carrying between
4,500 and 11,000 L of the slime, the plane
drops it all in front of the raging flames when
the pilot presses the button.
By changing the thickness of the red goop,
the firefighters can control how it falls. The
amount 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 in
tall trees, less water is used. This way the
slime will glob onto the branches and
ooze down very slowly.
Failed Flames
The burning of trees, grass, and brush is an
exothermic reaction. A fire retardant slows or
stops this self-feeding reaction. A fire retardant
increases the activation energy for the materials
to which it sticks. Firefighters on the ground
can gain valuable time when a fire is slowed
This plane is dropping fire retardant on a
forest fire.
with a fire retardant. This extra time allows the
ground team to create a fire line that will finally
stop the fire.
Neon Isn’t Necessary
Once a fire is put out, the slimy, red streaks left
on the blackened ground can be an eyesore. To
solve the problem, scientists have made special
dyes for the retardant. These dyes make the
goop neon colors when it is first dropped. After
a few days in the sun, though, the goop turns a
natural brown shade.
What Do They Study?
Do some research to learn about a firefighter’s training. What classes and tests must
firefighters pass? How do they keep
their certifications once they become
firefighters? Why do you think it is
important for firefighters to understand the chemistry of fire?
133
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