Chemical Reactions - Denton ISD

Back

Chemical Reactions

Forming New

Substances . . . . . . . . . 112

QuickLab

. . . . . 114

Internet Connect . . . . 114

Chemical Formulas

and Equations . . . . . . 115

MathBreak . . . . . 116, 119

Self-Check

. . . . 118

QuickLab

. . . . . 120

Internet Connect . . . . 120

Energy and Rates of

Chemical Reactions . . 121

QuickLab

. . 121, 122

Apply . . . . . . . . . . . . . 123

QuickLab

. . 124, 125

Internet Connect . . . . 125

Chapter Lab

. . . . . . . 126

Chapter Review

. . . . . 129

TEKS/TAKS Practice Tests . . . . . . 131, 132

Feature Article . . . . . . . . . . 133

LabBook

. . . 656, 657, 658

Pre-Reading Questions

1. What clues can show that a chemical reaction is taking place?

2. Why are chemical formulas important?

3. How can you change the rate of a chemical reaction?

110 Chapter 5

Copyright ? by Holt, Rinehart and Winston. All rights reserved.

Back

Reaction to the Rescue

A car slams into a wall at 97 km/h (60 mph) during a crash test. Although both dummies are wearing seat belts, one suffers a crushing blow to the head as it strikes the dashboard. The other suffers only minor bruises thanks to an air bag. Air bags can inflate rapidly due to a chemical reaction that produces gas at a very fast rate. In this chapter you will learn how to identify and describe a chemical reaction. You will also learn about factors that affect the rate of a reaction.

Reaction to the Rescue

A car slams into a wall at 97 km/h (60 mi/h) during a crash test. Although both dummies are wearing seat belts, one suffers a crushing blow to the head as it strikes the dashboard. The other suffers only minor bruises thanks to an air bag. Air bags can inflate rapidly because of a chemical reaction that produces gas at a very fast rate. In this chapter, you will learn how to identify and describe a chemical reaction. You will also learn about factors that affect the rate of a reaction.

Copyright ? by Holt, Rinehart and Winston. All rights reserved.

A MODEL FORMULA

Chemicals react in very precise ways. In this activity, you will model a chemical reaction and predict how chemicals react.

Procedure

1. You will receive several marshmallow models. The models are marshmallows stuck together with toothpicks. Each of these models is a model A.

2. Your teacher will show you model B and model C. Take apart one or more model A's to make copies of model B and model C.

3. Do you have any pieces left over? If so, use them to make more model B's and/or model C's. Do you need more parts to complete model B or model C? If so, take apart another model A.

4. If needed, repeat step 3 until you do not have any parts left over.

Analysis

5. How many model A's did you use to make copies of model B and model C?

6. How many model B's did you make? How many model C's did you make?

7. Suppose you needed to make six model B's. How many model A's would you need? How many model C's could you make with the leftover parts?

Chemical Reactions 111

Back

Terms to Learn

chemical reaction

What You'll Do

q Name the clues that show that a chemical reaction might be taking place.

q Demonstrate that substances may react chemically to form new substances.

Forming New Substances

Each fall, a beautiful change takes place. Leaves change color. You see bright reds, oranges, and yellows that had been hidden by green all year. What causes this change?

To answer this question, you need to know what causes leaves to be green in the first place. Leaves are green because they contain a green substance called chlorophyll (KLAWR uh FIL). During the spring and summer, the leaves contain a lot of chlorophyll. But in the fall, when temperatures drop and there are fewer hours of sunlight, the chlorophyll goes through a chemical change. The chlorophyll changes into new substances that have no color. When this happens, the red, orange, and yellow colors that were always present in the leaves can now be seen.

Chemical Reactions

The chemical change that happens as chlorophyll breaks down into new substances is one example of a chemical reaction. A chemical reaction is the process by which one or more substances change to produce one or more different substances. These new substances have different chemical and physical properties from the original substances. Many of the changes you see every day are chemical reactions. For example, striking a match, taking a photograph, and using a battery all involve chemical reactions. Check out Figure 1 to see two more examples of chemical reactions.

Figure 1 Examples of Chemical Reactions

When you mix water with baking powder, substances in baking powder react to form bubbles of carbon dioxide gas. When you bake a muffin, the bubbles give the tasty treat its spongelike texture.

A reaction between gasoline and oxygen forms carbon dioxide, water, and other substances. The formation of the new substances causes the pistons to move, which makes the car run.

112 Chapter 5

Copyright ? by Holt, Rinehart and Winston. All rights reserved.

Back

Clues to Chemical Reactions How can you tell when a chemical reaction is taking place? There are several clues to look for. Look at Figure 2. A change in color can be one clue. The formation of gas bubbles or a solid also may show that a reaction is happening. A solid formed in a solution as a result of a chemical reaction is called a precipitate (pree SIP uh TAYT). Also, during many chemical reactions, energy is taken in or given off as light, thermal, or electrical energy. The more of these clues you see, the more likely it is that a chemical reaction is taking place.

Figure 2 Some Clues to Chemical Reactions

Gas Formation Bubbles of carbon dioxide form when hydrochloric acid is placed on a piece of limestone.

Solid Formation Here you see potassium chromate solution being added to a silver nitrate solution. The dark red solid is a precipitate of silver chromate.

Energy Change The chemical reaction that takes place in a fire gives off light and thermal energy. During some other reactions, energy is taken in.

Copyright ? by Holt, Rinehart and Winston. All rights reserved.

Color Change Don't accidentally spill chlorine bleach on your jeans! The bleach reacts with the blue dye on the cloth, which causes the cloth's color to change.

Chemical Reactions 113

Back

Figure 3 Reaction of Hydrogen and Chlorine Breaking bonds

Making bonds

Hydrogen

Chlorine

The elements hydrogen and chlorine are diatomic. Diatomic molecules are made of two atoms bonded together. For these molecules to react, the bonds joining the atoms must break.

Hydrogen chloride

Molecules of the new substance, hydrogen chloride, are formed as new bonds are made between hydrogen atoms and chlorine atoms.

Reaction Ready

1. Place a piece of chalk in a plastic cup.

2. Add 1 tsp of vinegar to the cup. Record your observations in your ScienceLog.

3. What evidence do you see that a chemical reaction is taking place?

4. What type of new substance was formed?

Breaking and Making Bonds In a chemical reaction, bonds are broken and new bonds are formed. When chemical bonds in the starting substances are broken, new bonds can form between different atoms to make the new substances. Look at the model in Figure 3 to understand how this process happens.

New Substances Form in a Chemical Reaction You've read that a color change is one clue to a chemical reaction. A color change may indicate that a new substance has formed. For example, a color change takes place when chlorine gas reacts with hydrogen gas. Chlorine gas is greenish yellow in color. Hydrogen is a flammable, colorless gas. The two substances combine to form hydrogen chloride, a nonflammable, colorless gas. Hydrogen chloride has different chemical and physical properties than the substances that combined to form it. So, hydrogen chloride is a different substance than hydrogen and chlorine.

TOPIC: Chemical Reactions sciLINKS NUMBER: HSTP330

114 Chapter 5

What is a chemical reaction? List four clues that can help you demonstrate that a chemical reaction is taking place. In a chemical reaction, what must happen to the chemical bonds of the reacting substances in order to make a new substance? Applying Concepts Is a chemical reaction taking place when the metal of a car body rusts? Explain your answer.

Copyright ? by Holt, Rinehart and Winston. All rights reserved.

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download