Lesson 12.2
Name
Date
Period __________
Chapter 12: Lesson 2- Describing Chemical Reactions
What Information Does a Chemical Equation Contain?
1. EXPLAIN: What do the formulas, arrows, and plus signs tell you in a chemical equation?
2. INTERPRET DATA: Write the chemical equation for the following reaction: The elements sodium and
chlorine combine to yield the compound sodium chloride.
How Is Mass Conserved During a Chemical Reaction?
3. INFER: If the total mass of the products in a reaction is 90 grams, what was the total mass of the reactants?
4. DEMONSTRATE: Explore the law of conservation of mass by balancing the equations.
Al + CuO → Al2O3 + Cu
Fe2O3 + C → Fe + CO2
SO2 + O2 → SO3
5. ANSWER: How is matter conserved in a chemical reaction?
Building Vocabulary
Write a definition for each of these terms on the lines below.
6. chemical equation: ___________________________________________________________________
__________________________________________________________________________________________
7. coefficient: ___________________________________________________________________
__________________________________________________________________________________________
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Lesson 12.2
8. Identify the basic structure of a chemical equation and explain why an equation must be balanced.
Understanding Main Ideas
Balance each equation.
9. FeS + HCl → FeCl2 + H2S
10. Na + F2 → NaF
11. HgO → Hg + O2
12. What are the two reactants in the equation below? What is the product?
2H2 + O2 → 2H2O
13. Use the law of conservation of mass to explain why the equation in question 12 is balanced.
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Lesson 12.2
Read the passage and study the figure. Then answer the questions that follow.
The Decomposition of Water
You have learned 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 an example of a decomposition reaction. This is a chemical reaction in
which compounds break down into simpler products.
H2O
Water
→
H2
Hydrogen gas
+
O2
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.
14. Write a balanced equation for the decomposition of water.
15. 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?
16. The water in a beaker has a mass of 200 g. An electric current is turned on in the water for two hours.
Afterward the water has a mass of only 176 g. What happened to the missing mass?
17. Looking at the figure above, how can you tell that a reaction is occurring?
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Lesson 12.2
Write the letter of the correct answer on the line at the left.
18. ____ In a balanced chemical equation,
A atoms are conserved
B coefficients are equal
C molecules are equal
D energy is not conserved
19. _____ When the equation Al + Br 2 → AlBr3
is balanced, the coefficient for Al is
A 1
B 2
C 3
D 4
20. _____ In the chemical equation
Na + Cl2 → NaCl, Na is considered
A an atom
B a reactant
C a product
D a coefficient
21. _____ All of the following statements about
coefficients are true EXCEPT
A They tell you the amount of a reactant or
a product that takes part in a reaction
B They apply to every atom of the formula
they appear in front of
C They need to appear even when there is
1 atom
D in the chemical expression SO2, there
are two coefficients
Fill in the blank to complete each statement.
22. A number written in front of a chemical formula is a(n) ________________.
23. According to the law of ______________________ _____ ___________, the total number of atoms of
each element in the reactants must equal the total number of atoms of each element in the products.
24. All of the _______________ present before a physical or chemical change are present after the change.
25. In a chemical equation, the arrow is read as ________________.
26. In the balanced chemical equation for the formation of ammonia (NH 3) from nitrogen (N2) and hydrogen
(H2), the sum of the coefficients is _________________.
27. A(n) _________________ sign is used to join reactants in a chemical equation.
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