BD 78 Unit: Chemical Interactions

VOCABULARY

law of conservation of mass p. 79

coefficient p. 82

BEFORE, you learned

• Chemical reactions turn reactants into products by rearranging atoms

• Chemical reactions can beobserved and identified

• The rate of chemical reactionscan be changed

NOW, you will learn

• About the law of conservationof mass

• How a chemical equation represents a chemical reaction

• How to balance a simple chemical equation

KEY CONCEPT

The masses of reactantsand products are equal.

THINK ABOUT

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 ofthe conservation of mass.

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-VWAH-

zee-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.

COMBINATION NOTES

Take notes on the conservation of mass usingcombination notes.

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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 reddish-

orange 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 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 How did Lavoisier investigate the conservation of mass?

law of conservation of mass,

Conservation of MassConservation of Mass

Lavoisier carefully meas-ured both the reactantsand the products ofchemical reactions.

Why is it important to measure the masses ofreactants and products?

PROCEDURE

Measure 2 tsp of baking soda. Use a funnel to put the baking

soda in a balloon.

Pour 2 tsp of vinegar into the plastic bottle.

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.

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.

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.

4

3

2

1

Chapter 3: Chemical Reactions 79

SKILL FOCUSMeasuring

MATERIALS• teaspoon• baking soda• funnel• balloon• vinegar• plastic bottle• balance

TIME35 minutes

BD

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BD

Chemical reactions can be described bychemical 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 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.

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.

reactants direction of reaction product

C + O2 CO2

Carbon dioxide is a gasthat animals exhale.

80 Unit: Chemical Interactions

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BDChapter 3: Chemical Reactions 81

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.

Unbalanced Equation

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.

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 O2molecule on the left.

Balanced Equation

CH4 O2 CO2 H2O+ +C C HH

H

H

H

O

O O

O H

O

reminder

Oxygen is always a reactantin a combustion reaction.

CH4 O2 CO2 H2O H2OO2+ + + +C C HH

H

H

H

H

O

O

O

O

O O H

H

O

O

reading tip

As you read how to balancethe equation, look at theillustrations and count theatoms. The number of eachtype of atom is shownbelow the formula.

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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 equa-

tion 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 coeffi-

cients. 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.

Balanced Equation with Coefficients

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 can-

not 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 repre-

sents 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 Why are coefficients usedto balance equations?

Coefficients

CH4 CO2 2H2O2O2+ +

coefficient subscript

reminder

A subscript shows the number of atoms in a molecule. If a subscript ischanged, the molecule represented by the formulais changed.

The combustion ofmethane (CH4) is usedto melt glass.

82 Unit: Chemical InteractionsBD

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Balancing Equations with Coefficients

Chapter 3: Chemical Reactions 83

1

2

3

Count the atoms. Neither Nnor H is balanced. The reactants contain two atomseach of N and H, but theproduct contains one Natom and three H atoms.

Use a coefficient to addatoms to one side of theequation. A coefficient of 2on NH3 balances the numberof N atoms.

Add a coefficient toanother reactant or product. Adding a coefficient of 3 to H2on the left side of theequation balances thenumber of H atoms onboth sides. Now theequation is balanced.

Tip: Listing the number ofatoms of each elementmakes it easy to see whichelements must be balanced.

Tip: When adding coeffi-cients, start with thereactant or product thatcontains the greatest num-ber of different elements.

Tip: Make sure that thecoefficients in your bal-anced equation are thesmallest whole numberspossible—that is, theyhave no common factorother than 1.

The steps below show how to balance the equation for thesynthesis reaction between nitrogen (N2) and hydrogen (H2),which produces ammonia (NH3).

BD

APPLYBalance the following equations.

1. Hg + O2 HgO2. Zn + HCl ZnCl2 + H2

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BD 84 Unit: Chemical Interactions

KEY CONCEPTS

1. State 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 C + O2

CRITICAL THINKING

4. Communicate Describe

Lavoisier’s experiment with mer-

cury. How does this experiment

show the law of conservation

of mass?

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?

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 conserva-

tion 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 Why must chemical equations be balanced?

The decomposition ofsodium azide is used to inflate air bags in automobiles.

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