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KEY CONCEPT
The masses of reactants
and products are equal.
BEFORE, you learned
NOW, you will learn
• Chemical reactions turn
reactants into products by
rearranging atoms
• Chemical reactions can be
observed and identified
• The rate of chemical reactions
can be changed
• About the law of conservation
of mass
• How a chemical equation
represents a chemical reaction
• How to balance a simple
chemical equation
VOCABULARY
THINK ABOUT
law of conservation
of mass p. 79
coefficient p. 82
What happens to burning matter?
You have probably watched a fire burn in a
fireplace, a campfire, or a candle flame. It looks
as if the wood or candle disappears over time,
leaving a small pile of ashes or wax when the
fire has finished burning. But does matter
really disappear? Combustion is a chemical
reaction, and chemical reactions involve
rearrangements of atoms. The atoms do not
disappear, so where do they go?
Careful observations led to the discovery of
the conservation of mass.
COMBINATION NOTES
Take notes on the
conservation of mass using
combination notes.
The ashes left over from a wood fire contain less mass than the wood.
In many other chemical reactions, mass also appears to decrease.
That is, the mass of the products appears to be less than the mass
of the reactants. In other reactions, the products appear to gain mass.
For example, plants grow through a complex series of reactions, but
where does their extra mass come from? At one time, scientists
thought that chemical reactions could create or destroy matter.
During the 1780s the French chemist Antoine Lavoisier (luh-VWAHzee-ay) showed that matter can never be created or destroyed in a
chemical reaction. Lavoisier emphasized the importance of making very
careful measurements in his experiments. Because of his methods, he
was able to show that reactions that seem to gain mass or lose mass
actually involve reactions with gases in the air. These gases could not
be seen, but their masses could be measured.
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78 Unit: Chemical Interactions
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An example of Lavoisier’s work is his study of the reaction of
the metal mercury when heated in air. In this reaction, the reddishorange product formed has more mass than the original metal.
Lavoisier placed some mercury in a jar, sealed the jar, and recorded
the total mass of the setup. After the mercury had been heated in
the jar, the total mass of the jar and its contents had not changed.
Lavoisier showed that the air left in the jar would no longer
support burning—a candle flame was snuffed out by this air.
He concluded that a gas in the air, which he called oxygen, had
combined with the mercury to form the new product.
Lavoisier conducted many experiments of this type and found in all
cases that the mass of the reactants is equal to the mass of the products.
This conclusion, called the law of conservation of mass, states that in
a chemical reaction atoms are neither created nor destroyed. All atoms
present in the reactants are also present in the products.
Check Your Reading
Lavoisier carefully measured both the reactants
and the products of
chemical reactions.
How did Lavoisier investigate the conservation of mass?
Conservation of Mass
Why is it important to measure the masses of
reactants and products?
SKILL FOCUS
Measuring
PROCEDURE
1
Measure 2 tsp of baking soda. Use a funnel to put the baking
soda in a balloon.
2 Pour 2 tsp of vinegar into the plastic bottle.
3 Secure the balloon over the mouth of the bottle with the balloon
hanging to the side of the bottle. Find and record the mass of the
experimental setup.
4 Lift the balloon so that the baking soda drops into the bottle.
Observe for five minutes, and then find and record the mass
of the setup again.
MATERIALS
•
•
•
•
•
•
•
teaspoon
baking soda
funnel
balloon
vinegar
plastic bottle
balance
TIME
35 minutes
WHAT DO YOU THINK?
• Did the mass of the experimental
setup change?
• How do your observations demonstrate
the conservation of mass?
CHALLENGE What do you think you
would have observed if you had not used
the balloon? Explain.
Chapter 3: Chemical Reactions 79
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Chemical reactions can be described by
chemical equations.
The law of conservation of mass states that in a chemical reaction,
the total mass of reactants is equal to the total mass of products.
For example, the mass of sodium plus the mass of chlorine that reacts
with the sodium equals the mass of the product sodium chloride.
Because atoms are rearranged in a chemical reaction, there must be
the same number of sodium atoms and chlorine atoms in both the
reactants and products.
Chemical equations represent how atoms are rearranged in a
chemical reaction. The atoms in the reactants are shown on the left
side of the equation. The atoms in the products are shown on the right
side of the equation. Because atoms are rearranged and not created or
destroyed, the number of atoms of each different element must be the
same on each side of the equation.
Check Your Reading
Carbon dioxide is a gas
that animals exhale.
How does a chemical equation show the conservation of mass?
In order to write a chemical equation, the information that you
need to know is
• the reactants and products in the reaction
• the atomic symbols and chemical formulas of the reactants and
products in the reaction
• the direction of the reaction
The following equation describes the formation of carbon
dioxide from carbon and oxygen. In words, this equation says
“Carbon reacts with oxygen to yield carbon dioxide.” Notice
that instead of an equal sign, an arrow appears between the
reactants and the products. The arrow shows which way the
reaction proceeds—from reactants on the left to the product
or the products on the right.
reactants
C + O2
direction of reaction
product
CO2
Remember, the numbers below the chemical formulas for
oxygen and carbon dioxide are called subscripts. A subscript
indicates the number of atoms of an element in a molecule.
You can see in the equation above that the oxygen molecule
has two oxygen atoms, and the carbon dioxide molecule also
has two oxygen atoms. If the chemical formula of a reactant
or product does not have a subscript, it means that only one
atom of each element is present in the molecule.
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80 Unit: Chemical Interactions
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Chemical equations must be balanced.
Remember, chemical reactions follow the law of conservation of mass.
Chemical equations show this conservation, or equality, in terms of
atoms. The same number of atoms of each element must appear on
both sides of a chemical equation. However, simply writing down the
chemical formulas of reactants and products does not always result in
equal numbers of atoms. You have to balance the equation to make
the number of atoms equal on each side of an equation.
Balancing Chemical Equations
To learn how to balance an equation, look at the example of the
combustion of natural gas, which is mostly methane (CH4). The
reactants are methane and oxygen. The products are carbon dioxide
and water. You can write this reaction as the following equation.
reminder
Oxygen is always a reactant
in a combustion reaction.
Unbalanced Equation
CH 4
C
+
O2
H
O
H
O
+
CO 2
C
H2 O
O
H
O
H
O
H
H
This equation is not balanced. There is one C on each side of the
equation, so C is balanced. However, on the left side, H has a subscript
of 4, which means there are four hydrogen atoms. On the right side, H
has a subscript of 2, which means there are two hydrogen atoms. Also,
there are two oxygen atoms on the left and three oxygen atoms on the
right. Because of the conservation of mass, you know that hydrogen
atoms do not disappear and oxygen atoms do not suddenly appear.
reading tip
As you read how to balance
the equation, look at the
illustrations and count the
atoms. The number of each
type of atom is shown
below the formula.
You can balance a chemical equation by changing the amounts
of reactants or products represented.
•
•
To balance H first, add another H2O molecule on the right. Now,
both C and H are balanced.
There are now two oxygen atoms on the left side and four
oxygen atoms on the right side. To balance O, add another O2
molecule on the left.
Balanced Equation
CH 4 + O 2 + O 2
C
H
O
O
H
O
O
CO 2 + H 2 O + H 2 O
C
O
H
O
H
O
H
O
H
H
H
Chapter 3: Chemical Reactions 81
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Using Coefficients to Balance Equations
The balanced equation for the combustion of methane shows that one
molecule of methane reacts with two molecules of oxygen to produce
one molecule of carbon dioxide and two molecules of water. The equation can be simplified by writing 2O2 instead of O2 + O2, and 2H2O
instead of H2O + H2O.
The numbers in front of the chemical formulas are called coefficients. Coefficients indicate how many molecules take part in the
reaction. If there is no coefficient, then only one molecule of that type
takes part in the reaction. The balanced equation, with coefficients, for
the combustion of methane is shown below.
reminder
A subscript shows the
number of atoms in a
molecule. If a subscript is
changed, the molecule
represented by the formula
is changed.
Balanced Equation with Coefficients
CH 4
+
2O 2
coefficient
CO 2
+
2H 2 O
subscript
Chemical formulas can have both coefficients and subscripts.
In these cases, multiply the two numbers together to find the number
of atoms involved in the reaction. For example, two water molecules
(2H2O) contain 2 • 2 = 4 hydrogen atoms and 2 • 1 = 2 oxygen atoms.
Remember, coefficients in a chemical equation indicate how many
molecules of each type take part in the reaction.
Only coefficients can be changed in order to balance a chemical
equation. Subscripts are part of the
chemical formula for reactants or
products and cannot be changed
to balance an equation. Changing
a subscript changes the substance
represented by the formula.
For example, the equation for
the combustion of methane cannot be balanced by changing the
formula CO2 to CO. The formula
CO2 represents carbon dioxide gas,
which animals exhale when they
breathe. The formula CO represents carbon monoxide gas, which
is a very different compound
from CO2. Carbon monoxide gas
is poisonous, and breathing too
much of it can be fatal.
Check Your Reading
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82 Unit: Chemical Interactions
Why are coefficients used
to balance equations?
The combustion of
methane (CH4) is used
to melt glass.
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Balancing Equations with Coefficients
The steps below show how to balance the equation for the
synthesis reaction between nitrogen (N2) and hydrogen (H2),
which produces ammonia (NH3).
1
Count the atoms. Neither N
nor H is balanced. The
reactants contain two atoms
each of N and H, but the
product contains one N
atom and three H atoms.
2
Use a coefficient to add
atoms to one side of the
equation. A coefficient of 2
on NH3 balances the number
of N atoms.
3
Add a coefficient to
another reactant or
product. Adding a
coefficient of 3 to H2
on the left side of the
equation balances the
number of H atoms on
both sides. Now the
equation is balanced.
Tip: Listing the number of
atoms of each element
makes it easy to see which
elements must be
balanced.
Tip: When adding coefficients, start with the
reactant or product that
contains the greatest number of different elements.
Tip: Make sure that the
coefficients in your balanced equation are the
smallest whole numbers
possible—that is, they
have no common factor
other than 1.
APPLY
Balance the following equations.
1. Hg + O2
2. Zn + HCl
HgO
ZnCl2 + H2
Chapter 3: Chemical Reactions 83
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The decomposition of
sodium azide is used
to inflate air bags in
automobiles.
Using the Conservation of Mass
A balanced chemical equation shows that no matter how atoms are
rearranged during a chemical reaction, the same number of atoms
must be present before and after the reaction. The following example
demonstrates the usefulness of chemical equations and the conservation of mass.
The decomposition of sodium azide (NaN3) is used to inflate
automobile air bags. Sodium azide is a solid, and the amount of
sodium azide needed in an air bag fills only a small amount of space.
In fact, the amount of sodium azide used in air bags is only about
130 grams—an amount that would fit in a large spoon. An inflated
air bag, though, takes up much more space even though it contains
the same number of atoms that entered the reaction. The reason is
illustrated by the chemical equation for this reaction.
Balanced Equation
2NaN3
2Na + 3N2
According to the balanced equation shown above, three molecules
of nitrogen gas are formed for every two molecules of sodium azide
that decompose. Because the nitrogen is a gas, it fills a much greater
volume than the original sodium azide. In fact, 67 liters of nitrogen
gas are produced by the 130 grams of sodium azide in the reaction.
This amount of nitrogen is enough to quickly inflate the air bag
during a collision—the decomposition of sodium azide to sodium
and nitrogen takes 0.03 seconds.
Check Your Reading
KEY CONCEPTS
CRITICAL THINKING
1. State the law of conservation
of mass.
4. Communicate Describe
Lavoisier’s experiment with mercury. How does this experiment
show the law of conservation
of mass?
2. Write the chemical equation
that shows sodium (Na) and
chlorine (Cl2) combining to
form table salt (NaCl).
3. Is the following equation
balanced? Why or why not?
CO
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Why must chemical equations be balanced?
C + O2
84 Unit: Chemical Interactions
5. Synthesize Suppose a log’s
mass is 5 kg. After burning, the
mass of the ash is 1 kg. Explain
what may have happened to
the other 4 kg of mass.
CHALLENGE
6. Synthesize Suppose a
container holds 1000 hydrogen
molecules (H2) and 1000
oxygen molecules (O2) that
react to form water. How many
water molecules will be in the
container? Will anything else
be in the container?
If so, what?