CHAPTER 12 Chemical Reactions and Equations

CHAPTER 12 LESSON 2

Key Concepts

? How can you recognize the type of chemical reaction by the number or type of reactants and products?

? What are the different types of chemical reactions?

Chemical Reactions and Equations

Types of Chemical Reactions

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you've read this lesson, reread the statements to see if you have changed your mind.

Before

Statement

After

3. Reactions always start with two or more substances that react with each other.

4. Water can be broken down into simpler substances.

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Sticky Notes As you read, use sticky notes to mark information that you do not understand. Read the text carefully a second time. If you still need help, write a list of questions to ask your teacher.

Make a four-door book to organize your notes about the different types of chemical reactions.

Patterns in Reactions

If you have ever used hydrogen peroxide, you might have noticed that it comes in a dark bottle. This is because light causes hydrogen peroxide to change into other substances. You might have seen video of an old building or a sports facility being demolished by an explosion. How is the reaction between hydrogen and light similar to a building being demolished? In both, one reactant breaks down into two or more products.

The breakdown of one reactant into two or more products is one of four major types of chemical reaction. Each type of chemical reaction follows a unique pattern in the way atoms in reactants rearrange to form products. In this lesson, you will read how chemical reactions are classified by recognizing patterns in the way the atoms recombine.

Types of Chemical Reactions

Many different types of reactions can occur. It would be impossible to memorize them all. However, most chemical reactions fit into four major categories. Understanding these categories of reactions can help you predict how compounds will react and what products will form.

370 Chemical Reactions and Equations

Types of Chemical Reactions

Synthesis

A synthesis (SIHN thuh sus) is a type of chemical reaction in which two or more substances combine and form one compound. In the synthesis reaction on the left below, magnesium (Mg) reacts with oxygen (O2) in the air and forms magnesium oxide (MgO). You can recognize a synthesis reaction because two or more reactants form only one product.

Visual Check

1. Identify How many

products result from a synthesis reaction?

Synthesis and Decomposition Reactions

Synthesis Reactions

Decomposition Reactions

2Mg + O2 2MgO

2H2O2

magnesium oxygen magnesium oxide hydrogen peroxide

2H2O + O2

water

oxygen

Decomposition

In a decomposition reaction, one compound breaks down and forms two or more substances. You can recognize a decomposition reaction because one reactant forms two or more products. For example, hydrogen peroxide (H2O2), shown on the right above, decomposes and forms water (H2O) and oxygen gas (O2). Notice that decomposition is the reverse of synthesis.

Replacement

In a replacement reaction, an atom or a group of atoms replaces part of a compound. Two types of replacement reactions can occur. The figure below describes both types.

In a single-replacement reaction, one element replaces another element in a compound. In this type of reaction, an element and a compound react and form a different element and a different compound. In a double-replacement reaction, the negative ions in two compounds switch places, forming two new compounds. In this type of reaction, two compounds react and form two new compounds.

Key Concept Check 2. Recognize How can you tell the difference between synthesis and decomposition reactions?

Visual Check 3. Interpret In the singlereplacement equation, which elements switched places?

Replacement Reactions

Single Replacement

Double Replacement

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2AgNO3 + Cu Cu(NO3)2 + 2Ag

silver nitrate copper copper nitrate silver

Types of Chemical Reactions

Pb(NO3)2 + 2KI 2KNO3 + PbI2

lead potassium potassium lead

nitrate iodide

nitrate iodide

Chemical Reactions and Equations 371

Key Concept Check 4. Summarize What are the different types of chemical reactions?

Visual Check 5. Identify Which element is common to all combustion reactions?

Combustion

Combustion is a chemical reaction in which a substance combines with oxygen and releases energy. A combustion reaction usually releases energy as thermal energy and light energy. For example, burning is a common combustion reaction. The burning of fossil fuels produces the energy we use to cook food, power vehicles, and light cities.

The equation in the figure below describes the burning of the fossil fuel propane (C3H8). Combustion reactions always contain oxygen (O2) as a reactant. These reactions often produce carbon dioxide (CO2) and water (H2O).

Combustion Reactions

substance +

O2

C3H8 propane

+

5O2

oxygen

substance(s)

3CO2 + 4H2O carbon water dioxide

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372 Chemical Reactions and Equations

Types of Chemical Reactions

Mini Glossary

combustion: a chemical reaction in which a substance combines with oxygen and releases energy

single replacement: a type of chemical reaction in which one element replaces another element in a compound

decomposition: a type of chemical reaction in which one compound breaks down and forms two or more substances

synthesis (SIHN thuh sus): a type of chemical reaction in which two or more substances combine and form one compound

double replacement: a type of chemical reaction in which the negative ions in two compounds switch places, forming two new compounds

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that identifies the types of energy that combustion usually releases.

2. Identify each type of chemical reaction described in the diagram.

Type: substance + O2

substance(s)

Type:

A + B

AB

Types of Chemical Reactions

Type: AB

A + B

Type:

Replacement

An atom or a group of atoms replaces part of a compound.

Type: An element and a compound form a different element and compound.

What do you think

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

Types of Chemical Reactions

Type:

Two compounds form two new compounds.

Log on to ConnectED.mcgraw- and access your textbook to find this lesson's resources.

END OF LESSON

Chemical Reactions and Equations 373

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Lesson 2 Types of Chemical Reactions

Predict three facts that will be discussed in Lesson 2 after reading the headings. Write your facts in your Science Journal.

Patterns in Reactions

I found this on page .

Types of Chemical Reactions

I found this on page .

Generalize the concept of patterns in chemical reactions.

Number of types:

Major Types of Chemical Reactions

Determined by:

Describe and model synthesis and decomposition reactions. First, describe the reactions. Then draw simple shapes to model how substances behave during these reactions.

Synthesis

Decomposition

Explanation

Diagram

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374 Chemical Reactions and Equations

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