Chapter 6
Section 3 Balancing Chemical
Equations
Objectives
• Demonstrate how to balance chemical equations.
• Interpret chemical equations to determine the
relative number of moles of reactants needed and
moles of products formed.
• Explain how the law of definite proportions allows for
predictions about reaction amounts.
• Identify mole ratios in a balanced chemical equation.
• Calculate the relative masses of reactants and
products from a chemical equation.
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Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions
• One way to record the products and reactants of a reaction is to
write a word equation.
• Example: methane + oxygen → carbon dioxide + water
• A chemical equation is a representation of a chemical reaction
that uses symbols to show the relationship between the reactants
and the products.
• In a chemical equation, such as the one above, the reactants,
which are on the left-hand side of the arrow, form the products,
which are on the right-hand side.
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Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions
• When the number of atoms of reactants matches the
number of atoms of products, then the chemical
equation is said to be balanced.
• Balancing equations follows the law of
conservation of mass.
• You cannot balance chemical equations by changing
chemical formulas themselves, because that would
change the substances involved.
• To balance chemical equations, numbers called
coefficients must be placed in front of the
chemical formulas.
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Chapter 6
Section 3 Balancing Chemical
Equations
Law of Conservation of Mass
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Chapter 6
Section 3 Balancing Chemical
Equations
Describing Reactions, continued
• When the numbers of atoms for each element are the
same on each side, the equation is balanced, as
shown below.
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Chapter 6
Section 3 Balancing Chemical
Equations
Reading a Chemical Equation
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Chapter 6
Section 3 Balancing Chemical
Equations
Chemical Equation
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Chapter 6
Section 3 Balancing Chemical
Equations
Balancing a Chemical Equation by Inspection
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Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills
Balancing Chemical Equations Write the equation
that describes the burning of magnesium in air to
form magnesium oxide.
1. Identify the reactants and products.
Magnesium and oxygen gas are the reactants that
form the product, magnesium oxide.
2. Write a word equation for the reaction.
magnesium + oxygen → magnesium oxide.
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Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
3. Write the equation using formulas for the
elements and compounds in the word equation.
Remember that some gaseous elements, like oxygen, are
molecules, not atoms. Oxygen in air is O2, not O.
Mg + O2 → MgO
4. Balance the equation one element at a time.
The same number of each kind of atom must appear on both
sides. So far, there is one atom of magnesium on each side of
the equation.
But there are two oxygen atoms on the left and only one on
the right.
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Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
4. Balance the equation one element at a time,
continued
To balance the number of oxygen atoms, you need to
double the amount of magnesium oxide:
Mg + O2 → 2MgO
This equation gives you two magnesium atoms on
the right and only one on the left. So you need to
double the amount of magnesium on the left,
as follows.
2Mg + O2 → 2MgO
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Chapter 6
Section 3 Balancing Chemical
Equations
Math Skills, continued
4. Balance the equation one element at a time,
continued
2Mg + O2 → 2MgO
Now the equation is balanced. It has an equal
number of each type of atom on both sides.
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Chapter 6
Section 3 Balancing Chemical
Equations
Determining Mole Ratios
• The law of definite proportions states that a
compound always contains the same elements in the
same proportions, regardless of how the compound
is made or how much of the compound is formed.
• Because the law of definite proportions holds true for
all chemical substances in all reactions, mole ratios
can be derived from balanced equations.
• Mole ratio is the relative number of moles of the
substances required to produce a given amount of
product in a chemical reaction.
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Chapter 6
Section 3 Balancing Chemical
Equations
Law of Definite Proportions
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Chapter 6
Section 3 Balancing Chemical
Equations
Determining Mole Ratios, continued
• The mole ratio for any reaction comes from the balanced
chemical equation.
• Example: The equation for the electrolysis of water
shows that the mole ratio for H2O:H2:O2 is 2:2:1.
• 2H2O → 2H2 + [1]O2
• If you know the mole ratios of the substances in a
reaction, you can find the relative masses of the
substances required to react completely.
• Relative masses can be found by multiplying the
molecular mass of each substance by the mole ratio
from the balanced equation.
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