Download Describing Chemical Reactions

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Chemical Reactions
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Guided Reading and Study
Describing Chemical Reactions
This section explains how to show chemical reactions with symbols. It also states
the principle of conservation of mass, and identifies three categories of chemical
reactions.
Use Target Reading Skills
After you read the section, reread the paragraphs that contain definitions of Key
Terms. Use all of the information you have learned to write a meaningful sentence
using each Key Term.
a. chemical equation: ______________________________________________________
_________________________________________________________________________
b. reactant: _______________________________________________________________
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c. product: _______________________________________________________________
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d. conservation of mass: ___________________________________________________
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e. open system: ___________________________________________________________
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f. closed system: __________________________________________________________
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g. coefficient: _____________________________________________________________
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h. synthesis: ______________________________________________________________
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i. decomposition: _________________________________________________________
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j. replacement: ___________________________________________________________
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Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Guided Reading and Study
What Are Chemical Equations?
1. What is a chemical equation?
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2. Is the following sentence true or false? Chemical equations use symbols
instead of words to summarize chemical reactions.
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3. If a molecule of carbon dioxide is involved in a chemical reaction, how
is it represented in the chemical equation for the reaction?
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4. The substances you have at the beginning of a chemical reaction are
called the ________________________.
6. What do you read the arrow in a chemical equation as meaning?
________________________
7. Label each formula in the chemical equation below as either a reactant
or a product.
Fe + S → FeS
a. Fe ________________________ b. S ________________________
c. FeS ________________________
8. Circle the letter of each statement that is true about chemical equations.
a. Chemical equations have no real structure.
b. A chemical equation summarizes a reaction.
c. The formulas for the reactants are written on the right.
d. Symbols in the equation show the reactants and the products.
Conservation of Mass
9. Is the following sentence true or false? All the atoms present at the start
of a reaction are present at the end. ________________________
10. At the end of a chemical reaction, what is the total mass of the reactants
compared to the total mass of the products?
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_________________________________________________________________________
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Chemical Reactions
5. The substances you have when a chemical reaction is complete are called
the ________________________.
Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Guided Reading and Study
Describing Chemical Reactions (continued)
11. What is the principle called the conservation of mass?
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12. Describe an open system.
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13. What is an example of a closed system?
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Balancing Chemical Equations
14. When is a chemical equation balanced?
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15. How many atoms of oxygen are there on each side of the following
chemical equation: 2 Mg + O2
2 MgO?
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16. Circle the letter of each chemical equation that is balanced.
a. H2 + O2
H 2O
b. Mg + O2
MgO
c. Na + O2
Na2O
d. 2 H2O2
2 H 2O + O 2
17. A number placed in front of a chemical formula in a chemical equation
is called a(n) ________________________.
18. What does a coefficient tell you?
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Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Guided Reading and Study
19. Tell why this chemical equation is not balanced:
H2 + O 2
H2O.
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20. Write a balanced equation for this reaction: Oxygen reacts with
hydrogen to yield water.
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Classifying Chemical Reactions
21. In what three categories can chemical reactions be classified?
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22. Which category of chemical reactions comes from a term that means “to
put things together”?
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23. Complete the table about the three categories of chemical reactions.
Categories of Chemical Reactions
Category
Description
Example Chemical
Equation
a.
Two or more substances
combine to make a more
complex compound.
2 SO2 + O2 + 2 H2O
H2SO4
Decomposition
b.
2 H 2O 2
c.
One element replaces
another in a compound,
or two elements in
different compounds
trade places.
2 CuO + C
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2 H 2O + O 2
2 Cu + CO2
Chemical Reactions
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Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Guided Reading and Study
Describing Chemical Reactions (continued)
Classify each of the following equations as synthesis, decomposition, or
replacement.
24. a. CaCO3
CaO + CO2________________________
b. 2 Na + Cl2
c. Mg + CuSO4
2 NaCl ________________________
MgSO4 + Cu ________________________
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Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Section Summary
Describing Chemical Reactions
Key Concepts
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What information does a chemical equation contain?
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What does the principle of conservation of mass state?
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What must a balanced chemical equation show?
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What are three categories of chemical reactions?
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Chemical Reactions
A chemical equation is a short, easy way to show a chemical reaction.
Chemical equations use chemical formulas and other symbols instead of
words to summarize a reaction. All chemical equations have a common
structure. A chemical equation tells you the substances you start with in a
reaction and the substances you get at the end. The substances you have at
the beginning are called the reactants. When the reaction is complete, you
have new substances called the products. The formulas for the reactants are
written on the left side of the equation, followed by an arrow ( ). You read
the arrow as “yields.” The formulas for the products are written on the right
side of the equation. When there are two or more reactants or products, they
are separated by plus signs.
The principle called conservation of mass was first demonstrated in the
late 1700s. The principle of conservation of mass states that in a chemical
reaction, the total mass of the reactants must equal the total mass of the
products. In an open system, matter can enter from or escape to the
surroundings. A match burning in the air is an example of an open system.
You cannot measure the mass of all the reactants and products in an open
system. A closed system is a system in which matter cannot enter from or
escape to the surroundings. A sealed plastic bag is an example of a closed
system. A closed system allows you to measure the mass of all reactants and
products in a reaction.
To describe a reaction accurately, a chemical equation must show the
same number of each type of atom on both sides of the equation. An
equation is balanced when it accurately represents conservation of mass. To
balance a chemical equation, you may have to use coefficients. A coefficient
is a number placed in front of a chemical formula in an equation. It tells you
how many atoms or molecules of a reactant or a product take part in the
reaction.
Many chemical reactions can be classified in one of three categories:
synthesis, decomposition, or replacement. When two or more elements or
compounds combine to make a more complex substance, the reaction is
called a synthesis reaction. The reaction of hydrogen and oxygen to make
water is a synthesis reaction. A reaction called a decomposition reaction
breaks down compounds into simpler products. For example, hydrogen
peroxide decomposes into water and oxygen gas. When one element
replaces another in a compound, or when two elements in different
compounds trade places, the reaction is called a replacement reaction.
Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Review and Reinforce
Describing Chemical Reactions
Understanding Main Ideas
Balance the equations on the lines below. State whether the reaction is a synthesis,
decomposition, or replacement reaction.
Given Equation
Balanced Equation
Type of Reaction
1. FeS + HCl
a.
b.
a.
b.
a.
b.
2. Na + F2
3. HgO
FeCl2 + H2S
NaF
Hg + O2
4. Describe in words the chemical composition of the molecules involved
and the reaction represented by the equation: 2H2 + O2
2 H 2O
5. Use the principle of conservation of mass to explain why the equation in
question 4 is balanced.
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition in
the right column on the line beside the term in the left column.
6. chemical equation
a. substance present after a reaction
____
7. decomposition reaction
b. reaction in which substances combine to
form a more complex compound
____
8. coefficient
c. uses symbols and formulas to show
chemical reactions
9. product
d. reaction in which one element replaces
another in a compound
____
____
e. substance present before a reaction
____ 10. reactant
____ 11. conservation of mass
f. number telling how many molecules
of a substance are involved in a chemical
reaction
____ 12. synthesis reaction
g. reaction in which compounds are broken
down into simpler products
____ 13. replacement reaction
h. principle that states that matter is not
created or destroyed during a chemical
reaction
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Chemical Reactions
Answer questions 4 and 5 on a separate sheet of paper.
Name ____________________________ Date ____________________ Class ____________
Chemical Reactions
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Enrich
The Decomposition of Water
You learned in Section 2 that hydrogen gas and oxygen gas can react to
produce water. The reverse of this reaction can also occur. In other words,
water can be broken down to make hydrogen gas and oxygen gas. The
breakdown of water is a decomposition reaction. The unbalanced equation
for this reaction is shown below.
H 2O
H2
+
O2
Water
Hydrogen gas
Oxygen gas
For this reaction to occur, there must be an electric current through the
water as shown in the figure below. Two wires are connected to a battery, and
the free ends of the wires are put into a beaker of water that contains a small
amount of sulfuric acid. The sulfuric acid helps to increase the flow of
current through the water.
Wires
Beaker
Battery
Hydrogen
gas
Oxygen
gas
Water with a small
amount of sulfuric acid
Answer the following questions on a separate sheet of paper.
1. Write a balanced equation for the decomposition of water.
2. How many atoms of hydrogen are on the left side of the balanced
equation? How many oxygen atoms? How many hydrogen atoms are on
the right side of the balanced equation? How many oxygen atoms?
3. The water in a beaker has a mass of 18 g. An electric current is turned on
in the water for two hours. Afterward the water has a mass of only 16 g.
What happened to the missing mass?
4. Water decomposes to make 4 g of hydrogen gas and 32 g of oxygen gas.
What mass of water decomposed? How do you know?
5. Look at the figure above. How can you tell that a reaction is occurring?
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