nachemte_171.pdf - Simply Chemistry

554A Chapter 17

Thermochemistry Planning GuideThermochePlanning G17

Every chemical process uses or produces energy, often in the form of heat.

Lessons and Objectives Print Resources

For the Student For the Teacher

A-1, A-2, B-3, B-5

17.1 The Flow of Energy p 556–561 17.1.1 Explain the ways in which energy changes

can occur.17.1.2 Explain what happens to the energy of

the universe during a chemical or physical process.

17.1.3 Identify the factors on which the heat capacity of an object depends.

Reading and Study Workbook Lesson 17.1

Lesson Assessment 17.1 p 561

Teaching Resources, Lesson 17.1 Review

Teacher Demo, p 557: An Endothermic Reaction

Class Activity, p 559: Heat Transfer

A-1, A-2, B-3, E-2, F-6, G-1

17.2 Measuring and Expressing Enthalpy Changes p 562–568

17.2.1 Describe how to measure the change in enthalpy of a reaction.

17.2.2 Describe how to express the enthalpy change for a reaction in a chemical equation.




Class Activity, p 563: Calorimetry Measurements

Teacher Demo, p 566: An Exothermic Reaction

A-1, A-2, B-3 17.3 Heat in Changes of State p 569–57517.3.1 Describe the relationship between

molar heat of fusion and molar heat of solidification.

17.3.2 Describe the relationship between molar heat of vaporization and molar heat of condensation.

17.3.3 Describe thermochemical changes that occur when a solution forms.



Quick Lab: Heat of Fusion of Ice, p 571


Class Activity, p 570: Melting and Boiling Points

Teacher Demo, p 572: Exothermic Solidification

Class Activity, p 574: Hot and Cold Packs

A-1, A-2, B-3 17.4 Calculating Heats of Reaction p 578–58217.4.1 Identify two ways that the heat of reaction

can be determined when it cannot be directly measured.



Small-Scale Lab, p 583: Heat of Combustion of a Candle


Essential Questions1. How is energy conserved in a chemical or

physical process?2. How can you determine the amount of energy

absorbed or released in a chemical or physical process?

Study Guide p 584Math Tune-Up p 585STP p 591Reading and Study

Workbook Self-Check and Vocabulary Review Chapter 17

Every chemical process uses or produces energy, often in the form of heat.Every chemical process uses or produces energy

Introducing the BIGIDEA: MATTER AND ENERGY

Essential Questions1. How is energy conserved in a chemical or

Study Guide p 584Math Tune-Up p 58

Assessing the BIGIDEA: MATTER AND ENERGY

NSES

Thermochemistry 554B

For the StudentQuick Lab, p 571

100-mL graduated • cylinderhot water• foam cup• thermometer• ice•

Small-Scale Lab, p 583candle• metric ruler• aluminum foil• balance• safety matches• clock or watch• temperature probe • (optional)

For the TeacherTeacher Demo, p 557

barium hydroxide • octahydrateammonium chloride• 250-mL Erlenmeyer flask • with stopper

Class Activity, p 559sheet metal • (13 cm × 18 cm)foam (13 cm • × 18 cm)wood (40 cm • × 20 cm)glue•

Class Activity, p 5631• M HClfoam cup• thermometer• mossy zinc• magnesium turnings•

Teacher Demo, p 566CaO• metal tray• thin aluminum pan• water• egg (raw)•

Class Activity, p 570beaker• water• boiling chips• hot plate• thermometer• ice cubes• foam cup•

Teacher Demo, p 5723 wide-mouth test tubes• sodium thiosulfate • pentahydratehot water bath• 30• °C water baththermometers• stirring rods •

Class Activity, p 574hot and cold packs•

Online Student Edition Online Teacher’s Edition17.1 Virtual Chem Lab 16: The Specific Heat of a Metal17 Virtual Chem Lab 17: Heat of Fusion of Water 17.2 Virtual Chem Lab 19: Heat of Combustion17.4 Virtual Chem Lab 18: Heats of Reaction

L A B

VIRTUAL Measuring the Heat of Combustion

Online Student Edition

Additional Digital Resources

Digital Resources

Editable Worksheets PearsonChem.com

Lab 34: The Specific Heat of a Metal

Lab Practical 17-1: Specific Heat

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Calculating the Specific Heat of a Substance

Lab 33: Freezing PointLab 34: The Specific Heat of a

MetalLab 35: Heats of Reaction Lab Practical 17-1: Specific Heat Lab Practical 17-2: Heat of

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Small-Scale Lab Manual Lab 27: Heat of Fusion of Ice

Probeware Lab: Heat of Fusion of Ice

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Exam View Assessment SuiteClassroom Resources Disc

(includes editable worksheets) • Lesson Reviews • Practice Problems • Interpret Graphs • Vocabulary Review • Chapter Quizzes and Tests

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A-1, B-3, B-5, D-1, E-2, F-3, F-4, F-6

National Science Education Standards 554

17Thermochemistry

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This solar furnace in Font Romeu, France converts light from the sun into heat.

554 Chapter 17

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1 CONTENT AND LANGUAGE Write the word thermochemistry on the board Slowly enunciate the word and have students repeat it. Point out the prefix thermo, means “heat.” Have students use this information to predict the meaning of thermochemistry.

BEGINNING

LOW Have students write any unfamiliar words from the lesson titles on cards and practice saying each word.

HIGH Have students use a bilingual dictionary or glossary to find any cognates to any unfamiliar words from the lesson titles.

INTERMEDIATE: LOW/HIGH Have students use an English dictionary or the glossary to define any unfamiliar words from the lesson titles.

ADVANCED: LOW/HIGH Have students use the lesson titles to predict the purpose or main idea of the chapter.

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O E PROBLEM SETS Students can practice key problem-solving skills in an online problems set.

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MATH MATH HELP Identify the students that struggle with math by assigning an online math skills diagnostic test. These students can then improve and practice math skills using the MathXL tutorial system.

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VIRTUAL VIRTUAL LABS Students go on animated

virtual lab tours in which specific heat and heat of combustion are studied in a simulated laboratory environment.

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selected figures from the chapter followed by questions to check for understanding.

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MATTER AND ENERGY

Essential Questions:1. How is energy conserved in a

chemical or physical process?2. How can you determine the amount

of energy absorbed or released in a chemical or physical process?

BIGIDEA

Fighting FrostIt is a cold night in central Florida and weather fore-casters are predicting that temperatures will fall to

6°C. The citrus growers in the area are in a panic. Just an hour or two of temperatures below 0 C could be devastating to the citrus trees and fruit.

Citrus growers can use a number of meth-ods to minimize the damage to their trees and fruit in the event of a frost or freeze. Some growers install heaters to protect their crops. Other farmers use wind machines or helicop-ters to mix the layers of warm and cold air in the atmosphere and raise the temperature at the surface. However, one of the most com-mon methods of protecting citrus trees is to spray water on them. The freezing of the water protects the branches, leaves, and fruit.

Connect to the BIGIDEA As you read about thermochemistry, think about how water freezing can protect citrus trees from frost.

CHEMYSTERY

NATIONAL SCIENCE EDUCATION STANDARDS

A-1, B-3, B-5, D-1, E-2, F-3, F-4, F-6

Thermochemistry 555

Understanding by DesignStudents are building toward describing energy changes during chemical or physical processes by using what they know about matter and energy.

PERFORMANCE GOALS At the end of Chapter 17, students will be able to answer the essential questions by applying their knowledge of thermochemistry. Students will also be able to perform calculations related to heat transfer during a reaction.

ESSENTIAL QUESTIONS Read the essential questions aloud. Ask What is meant by “energy is conserved” in the first question? (Energy cannot be created or destroyed during any chemical or physical process.) Ask How is it possible, then, for energy to be absorbed or released in a reaction? (Energy can change to other forms, or it can move from one substance to another.)

BIGIDEA Use the photo of the solar thermal power plant to help students

connect to the concepts they will learn in this chapter. Activate prior knowledge by reviewing methods of energy transfer. Point out that mirrors direct the Sun’s rays toward the side of the building. The rays then reflect onto collectors in the nearby tower. Ask What type of energy is carried by the Sun’s rays? (radiant energy) Ask What happens to this energy? (It is used as a source of heat.)

CHEMYSTERY Have students read over the CHEMystery. Connect the

CHEMystery to the Big Idea of Matter and Energy by explaining that freezing occurs because of the transfer of heat energy. Have students use this fact to predict why the growers might protect the citrus trees by spraying them with water. As a hint, remind students that a change in the state of matter involves a gain or loss of heat.

Introduce the ChapterIDENTIFYING MISCONCEPTIONS Since students may know that some reactions reactions release heat, they may assume that other reactions release cold, rather than absorbing heat. Use the activity to identify students who may have this misconception.

Activity You’ll need two or more chemical heat packs or hand warmers, and two or more chemical cold packs. Ask How does a chemical reaction affect the total energy of the reactants? (Answers will vary. Students may say that the reactants lose energy.) Follow the directions for activating the heat packs, and pass them around the room. Ask Why does the heat pack feel warm? (Sample answer: heat is flowing from the pack to my hand.) Follow the directions for activating the cold packs, and pass them around the room. Ask Why does the cold pack feel cold? (Sample answer: cold is flowing from the pack to my hand.) Explain that cold is not flowing from the pack to the students’ hands, but that heat is flowing from their hands to the pack. Sensors in their skin detect the change in temperature, and their brains interpret this as a feeling of cold. Tell students to keep this activity in mind as they read the chapter.

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National Science Education Standards

CHEMISTRY YOU YYY&

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The Flow of Energy17.1

Key Questions What are the ways in which

energy changes can occur?

What happens to the energy of the universe during a chemical or physical process?

On what factors does the heat capacity of an object depend ?

Vocabulary

Q: Why does lava cool faster in water than in air? Lava flowing out of an erupting volcano is very hot. Its temperature ranges from 550°C to 1400°C. As lava flows down the side of a volcano, it loses heat and begins to cool slowly. In some instances, the lava may flow into the ocean, where it cools more rapidly. In this lesson, you will learn about heat flow.

Energy Transformations What are the ways in which energy changes can occur?

Energy is the capacity for doing work or supplying heat. Unlike mat-ter, energy has neither mass nor volume. Energy is detected only because of its effects. For example, a car moves because of the energy supplied by the fuel. Thermochemistry is the study of energy changes that occur dur-ing chemical reactions and changes in state. Every substance has a certain amount of energy stored inside it. The energy stored in the chemical bonds of a substance is called chemical potential energy. The kinds of atoms and the arrangement of the atoms in a substance determine the amount of energy stored in the substance.

During a chemical reaction, a substance is transformed into another sub-stance with a different amount of chemical potential energy. When you buy gasoline, shown in Figure 17.1, you are actually buying the stored potential energy it contains. The controlled explosions of the gasoline in a car’s engine transform the potential energy into useful work, which can be used to pro-pel the car. At the same time, however, heat is also produced, making the car’s engine extremely hot. Energy changes occur as either heat transfer or work, or a combination of both.

Heat, represented by q, is energy that transfers from one object to another because of a temperature differ-ence between the objects. One of the effects of adding heat to an object is an increase in its temperature. Heat flows spontaneously from a warmer object to a cooler object. If two objects remain in contact, heat will flow from the warmer object to the cooler object until the temperature of both objects is the same.

Figure 17.1 Chemical Potential EnergyChemical potential energy is stored within the bonds of the molecules in gasoline.

556 Chapter 17 • Lesson 1

Focus on ELL

1 CONTENT AND LANGUAGE Divide the class into five groups and assign each group two key vocabulary words from the lesson. Have each group pronounce the words, define the words, give a visual example of the words, and create a non-linguistic representation of the words. Have each group teach their words to the class.

2 FRONTLOAD THE LESSON Write the word calorie on the board. Discuss as a class the context in which students are most familiar with this word. Ask students to share three things that they have heard about calories. Explain how food calories are related to the unit of measure, calorie. Clarify any misconceptions between these two concepts.

3 COMPREHENSIBLE INPUT When discussing direction of heat flow with respect to the system, sketch the images from the various photos in the lesson on the board. Label the system and surroundings in each and draw arrows to indicate the direction of heat flow with respect to the system. Have students classify each process as endothermic or exothermic.

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Key Objectives17.1.1 EXPLAIN the ways in which energy

changes can occur.

17.1.2 EXPLAIN how the energy of the universe before a chemical or physical process is related to the energy of the universe after a chemical or physical process.

17.1.3 IDENTIFY two factors on which the heat capacity of an object depends.

Additional ResourcesReading and Study Workbook, Lesson 17.1

Available Online or on Digital Media:

• Teaching Resources, Lesson 17.1 Review• Laboratory Manual, Lab 34• Virtual Chemistry Labs, Lab 16

EngageCHEMISTRY YOU YOYY U&& Ask What happens to

the heat in lava once it flows out of a volcano? (The heat is released.) Ask Why does lava cool more quickly in water than on land? (Sample answer: Water has a greater capacity to absorb heat than does air; the temperature difference between the lava and water may be greater than between the lava and the air.)

Activate Prior KnowledgeTo assess whether students know the difference between temperature and energy, Ask What does a thermometer measure? (average kinetic energy) Ask Does a thermometer measure heat? (no)

Heat

SurroundingsSurroundings

SystemSystemHeat

a b

Thermochemistry 557

Figure 17.2 Heat FlowHeat flow is defined from the point of view of the system. a. In an endothermic process, heat flows into the system from the surroundings. b. In an exothermic process, heat flows from the system to the surroundings. In both cases, energy is conserved.Apply Concepts In which process does q have a negative value?

Endothermic and Exothermic Processes What happens to the energy of the universe during a chemical or

physical process?Chemical reactions and changes in physical state generally involve either the absorption or the release of heat. In studying energy changes, you can define a system as the part of the universe on which you focus your atten-tion. Everything else in the universe makes up the surroundings. In thermo-chemical experiments, you can consider the region in the immediate vicinity of the system as the surroundings. Together, the system and its surroundings make up the universe.

A major goal of thermochemistry is to examine the flow of heat between the system and its surroundings. The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. During any chemical or physical process, the energy of the universe remains unchanged. If the energy of the system increases dur-ing that process, the energy of the surroundings must decrease by the same amount. Likewise, if the energy of the system decreases during that process, the energy of the surroundings must increase by the same amount.

Direction of Heat Flow In thermochemical calculations, the direction of heat flow is given from the point of view of the system. Heat is absorbed from the surroundings in an endothermic process. In an endothermic pro-cess, the system gains heat as the surroundings lose heat. In Figure 17.2a, the system (the body) gains heat from its surroundings (the fire). Heat flowing into a system from its surroundings is defined as positive; q has a positive value. An exothermic process is one that releases heat to its surroundings. In an exothermic process, the system loses heat as the surroundings gain heat. In Figure 17.2b, the system (the body) loses heat to the surroundings (the perspiration on the skin, and the air). Heat flowing out of a system into its surroundings is defined as negative; q has a negative value.

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Explain

Energy TransformationsUSE VISUALS Direct students’ attention to Figure 17.1. Have students consider the use of fuel in automobiles. Ask What work is done as the chemical potential energy of the fuel is released? (The automobile moves along the road.) Ask In addition to work, what other type of energy change occurs as the fuel is burned? (heat is released)

Exothermic and Endothermic ProcessesMAKE A CONNECTION Discuss with students the distinction between kinetic and potential energy. Kinetic energy is the energy associated with an object because of its motion. Potential energy is the energy associated with an object because of its position in a field of force or due to its particular chemical composition. Explain that the potential energy of a reactant or a product in a chemical reaction is determined by the strengths of the attractive and repulsive forces between atoms. In a chemical reaction, atoms are rearranged into new groupings that have different relative potential energies. The change in potential energy is either the result of absorption of energy from the surroundings (endothermic reaction) or the release of energy to the surroundings (exothermic reaction).

Explore

Teacher DemoPURPOSE Students will observe an endothermic reaction between two solids.

MATERIALS barium hydroxide octahydrate, ammonium chloride, 250-mL Erlenmeyer flask with stopper

SAFETY Barium salts are poisonous if ingested. Inhalation of concentrated ammonia vapors can be dangerous. Perform this demo in a fume hood or a well-ventilated room. Disposal: Flush the solution down the drain with excess water.

PROCEDURE Mix 32 g of barium hydroxide octahydrate (Ba(OH)2 · 8H2O) with 11 g of ammonium chloride (NH4Cl) in a 250-mL Erlenmeyer flask fitted with a stopper. Swirl to mix.

EXPECTED OUTCOME The flask becomes extremely cold. Place the flask on a wet piece of wood and the flask will freeze to the wood.

AnswersFIGURE 17.2b In Figure 17.2b, The system loses heat

to its surroundings, so q is negative.

Differentiated InstructionLPR LESS PROFICIENT READERS Have students write definitions for each of the key terms in this chapter. Special attention should be given to those terms that are defined by mathematical relationships. Example calculations showing how to apply the formulas should be included in their notes.

L1 SPECIAL NEEDS STUDENTS Provide tactile examples of endothermic and exothermic reactions tailored to the main senses that the student relies on for learning.

ELL ENGLISH LANGUAGE LEARNERS Have students compare and contrast the prefixes endo- and exo-. Guide students to create symbols they can use to mark a reaction as heat absorbing or heat releasing.

Sample Problem 17.1

First, identify the system and

surroundings in each situation. Then,

determine the direction of heat flow.

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Units for Measuring Heat Flow Describing the amount of heat flow requires units different than those used to describe temperature. Heat flow is measured in two common units, the calorie and the joule. You have probably heard of someone exercising to “burn calories.” During exercise, your body breaks down sugars and fats in a process that releases heat. Although there is not an actual fire burning the sugars and fats within your body, chemical reactions accomplish the same result. For example, in breaking down 10 g of sugar, your body releases the same amount of heat that would be released if 10 g of sugar were completely burned in a fire.

A calorie (cal) is defined as the quantity of heat needed to raise the tem-perature of 1 g of pure water 1°C. The word calorie is written with a small c except when referring to the energy contained in food. The dietary Calorie, written with a capital C, always refers to the energy in food. One dietary Calorie is equal to one kilocalorie, or 1000 calories.

1 Calorie 1 kilocalorie 1000 calories

Recognizing Endothermic and Exothermic ProcessesOn a sunny winter day, the snow on a rooftop begins to melt. As the melted water drips from the roof, it refreezes into icicles. Describe the direction of heat flow as the water freezes. Is this process endothermic or exothermic?

Analyze Identify the relevant concepts. Heat flows from a warmer object to a cooler object. An endothermic process absorbs heat from the surroundings. An exothermic pro-cess releases heat to the surroundings.

Solve Apply the concepts to this problem.

A container of melted wax stands at room 1. temperature. What is the direction of heat flow as the liquid wax solidifies? Is the process endothermic or exothermic?

When barium hydroxide octahy-2. drate, Ba(OH)2 8H2O is mixed in a beaker with ammonium thiocyanate, NH4SCN, a reaction occurs. The bea-ker becomes very cold. Is the reaction endothermic or exothermic?

System: water Surroundings: air

In order for water to freeze, its temperature must decrease. Heat flows out of the water and into the air.

Heat is released from the system to the surroundings. The process is exothermic.

Determine the direction of heat flow.

First, identify the system and the surroundings.

Determine if the process is endothermic or exothermic.


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Explain

Endothermic and Exothermic ProcessesMAKING CONNECTIONS As a class, brainstorm everyday processes students experience that absorb or release heat. Create a list on the board and randomly select students to write –heat or +heatnext to each process to indicate the direction of heat flow.

Sample Practice ProblemDuring photosynthesis, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. Is photosynthesis an endothermic or exothermic process? (endothermic)

Misconception AlertMake sure students understand the terms introduced in this section and how to convert from one to another. Students may have difficulty with the distinction between calorie and Calorie because everyday usage of the terms is often not accurate. The distinction between heat capacity and specificheat can be hard to remember because the terms are so closely related and sound so similar. Tell students to remember that specific heats of different materials can be compared because the quantity of matter involved (1 g) is specified.

Dietary Calories

Your proper caloric intake depends on your level of physical activity. In an eight-hour day at a desk, you burn about 800 Calories. This is about the number of Calories in two helpings of spaghetti. When exercising, however, you become a relative biochemical blast furnace. In vigorous activities such as running and jumping, you expend 7–10 Calories per minute, or 420–600 Calories per hour. At these rates, a runner who covers a 26-mile marathon course in 3 hours might expend 1800 Calories, or the equivalent of 4.5 helpings of spaghetti.

Interpret DataSpecific Heats of Some Common Substances

SubstanceSpecific heat

J/(g °C) cal/(g °C)

Liquid water 4.18 1.00

Ethanol 2.4 0.58

Ice 2.1 0.50

Steam 1.9 0.45

Chloroform 0.96 0.23

Aluminum 0.90 0.21

Iron 0.46 0.11

Silver 0.24 0.057

Mercury 0.14 0.033

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Figure 17.3 Heat CapacityA massive steel cable has a higher heat capacity than a steel nail.Compare Which has a greater heat capacity: a cup of water or a drop of water?

The joule (J) is the SI unit of energy. One joule of heat raises the tem-perature of 1 g of pure water 0.2390°C. You can convert between calories and joules using the following relationships:

1 J 0.2390 cal 4.184 J 1 cal

Heat Capacity and Specific Heat On what factors does the heat capacity of an object depend ?

The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object. The heat capacity of an object depends on both its mass and its chemical composition. The greater the mass of the object, the greater its heat capacity. One of the massive steel cables on the bridge in Figure 17.3, for example, requires much more heat to raise its temperature 1°C than a small steel nail does.

Different substances with the same mass may have different heat capac-ities. On a sunny day, a 20-kg puddle of water may be cool, while a nearby 20-kg iron sewer cover may be too hot to touch. This situation illustrates how different heat capacities affect the temperature of objects. Assuming that both the water and the iron absorb the same amount of radiant energy from the sun, the temperature of the water changes less than the tempera-ture of the iron in the same amount of time because the specific heat capac-ity of water is larger than the specific heat capacity of iron.

The specific heat capacity, or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C. Table 17.1 gives specific heats for some common substances. Water has a very high specific heat compared with the other substances in the table. Metals generally have low specific heats. The same amount of heat affects the temperature of objects of the same mass with a high specific heat much less than the temperature of those with a low specific heat.

Hint: For part c, use the relationship 1 J0.2390 cal to write the appropriate conversion factor.

Table 17.1 The specific heat of a substance can be expressed in J/(g °C) or cal/(g °C).

Read Tables a. What is the specific heat of chloroform in cal/(g °C)?

Compare b. Which metal in the table has the highest specific heat?

Calculate c. Show how to convert the specific heat of liquid water from J/(g °C) to cal/(g °C).

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Answers1. Heat flows from the system (wax) to the

surroundings (air). The process is exothermic.2. Since the beaker becomes cold, heat is

absorbed by the system (chemicals within the beaker) from the surroundings (beaker and surrounding air). The process is endothermic.

FIGURE 17.3 a cup of water

INTERPRET DATAa. 0.23 cal/(g∙8C)b. aluminum

c. 4.18 J/ 1g # °C 2 30.2390 cal

1 J5 1.00 cal/ 1g # °C 2

Explain

Heat Capacity and Specific HeatUSE VISUALS Have students study Table 17.1. Ask Which has a higher specific heat capacity—water or steam? (water) Ask What factors do you think affect the specific heat of each substance? (Specific heat is affected by the amount of heat and the change in temperature.)

Explore

Class ActivityPURPOSE Students will compare heat transfer of different materials.

MATERIALS sheet metal (13 cm × 18 cm), foam (13 cm × 18 cm), wood (40 cm × 20 cm), glue

PROCEDURE Glue a 13 cm × 18 cm piece of sheet metal and a 13 cm × 18 cm piece of foam to a piece of wood that is about 40 cm × 20 cm. Pass the wood around the classroom, and ask students to put their hands on each of the three surfaces and describe the temperature of each.

EXPECTED OUTCOME Usually students indicate that the metal feels the coldest and the foam feels the same temperature as their hands. Explain that all of the surfaces are colder than their hand, and that heat is being transferred from their hand to those surfaces. Convey that metals conduct heat away from the hand more efficiently than foam, so they feel colder than foam.

Check for UnderstandingWhat happens to the energy of the universe during a chemical

or physical process?

Access students understanding of energy flow by using the following activity. Have each student take an ice cube out of a cooler. Ask them to identify the surroundings, if the ice cube is the system. Then, have them explain the direction of energy flow that occurs as it relates to the ice cube. (The hand is the surroundings. Energy flows from the skin to the ice cube, giving a sense that it is cold.) Ask Is the process endothermic or exothermic?

ADJUST INSTRUCTION If students are having difficulty identifying the surroundings, have students reread the first paragraph of the Endothermic and Exothermic Processes section. Then, ask students where the heat transfer occurs between the skin and the ice cube. (The transfer is occurring between the surface molecules of the skin and the surface molecules of the ice.) Explain that the process is endothermic because the ice gains heat from the hand.

Eureka65/53

Redding109/70

Tahoe Valley88/48

Sacramento105/62

Yosemite98/65

Fresno109/70 Bakersfield

109/74

Santa Barbara80/62

Death Valley125/85

Barstow115/75

Los Angeles92/68

Palm Springs119/83

San Diego74/66

Blythe118/83

San Francisco78/56

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Specific Heat of Water Just as it takes a lot of heat to raise the tem-perature of water, water also releases a lot of heat as it cools. Water in lakes and oceans absorbs heat from the air on hot days and releases it back into the air on cool days. As illustrated in Figure 17.4, this prop-erty of water is responsible for moderate climates in coastal areas. Citrus farmers often spray their trees with water to protect the fruit from frost damage during icy weather. As the water freezes, it releases heat, which helps prevent the fruit from freezing. When a freshly baked apple pie, such as the one shown in Figure 17.5, comes out of the oven, both the filling and crust are at the same temperature. However, the filling, which is mostly water, has a higher specific heat than the crust. In order to cool down, the filling must give off a lot of heat. This release of heat is why you have to be careful not to burn your tongue when eating hot apple pie.

Calculating Specific Heat To calculate the specific heat (C) of a sub-stance, you divide the heat input by the mass of the substance times the temperature change.

In the equation above, q is heat and m is mass. The symbol T (read “delta T”) represents the change in temperature. T is calculated from the equation T Tf Ti, where Tf is the final temperature and Ti is the initial temperature. As you can see from the equation and in Table 17.1 on the previous page, heat may be expressed in terms of joules or calories. Therefore, the units of specific heat are either J/(g °C) or cal/(g °C).

qm T

qm TTT

C heat (J or cal)

mass (g) change in temperature (°C)(( )) hhh ii t t (°(°C)C)

Figure 17.4 Temperature ModerationSan Francisco is located on the Pacific coast. The high specific heat of the water in the ocean helps keep the temperature in San Francisco much more moderate than that of the towns and cities farther inland.Compare Describe how the ocean affects the temperature of coastal areas in the summer and in the winter.

Figure 17.5 Cooling of WaterThe filling of a hot apple pie is mostly water, so it is much more likely to burn your tongue than the crust.

Q: Heat will flow from the lava to the surroundings until the lava and surroundings are at the same tem-perature. Air has a smaller specific heat than water. Why would lava then cool more quickly in water than in air?

CHEMISTRY YOU YYYYY&


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ExplainCRITICAL THINKING Direct students to Figure 17.4 and discuss the difference in temperature between a coastal city such as San Francisco and an inland city such as Fresno. State the following a remark once made by Mark Twain: “The coldest winter I ever spent was the summer I spent in San Francisco.” Ask How does this quote relate to the temperature difference between a coastal city and an inland city? (The relatively high heat capacity of the large bodies of water near San Francisco help to moderate local temperatures. During summer, the ocean water warms slowly as it absorbs large amounts of heat from warm air masses. Onshore breezes off the cool water keep coastal land temperatures from rising to match those of inland areas. Condensation of water vapor in the air produces a cool fog, which probably enhanced Mark Twain’s impression of the cool of San Francisco summers.)

CHEMISTRY YOU YOYY U&& A given mass of water can absorb more heat from the lava than an equal mass of air.

Foundations for MathSYMBOLS Students often struggle with calculations involving the delta (Δ) symbol, as some calculations require the subtraction of the smallest value from the largest, while in others the initial value is subtracted from the final value. Explain that in specific heat calculations, the presence of Δ in the denominator of the equation indicates that the initial temperature should be subtracted from the final temperature.

In Sample Problem 17.2, the initial temperature is 25oC and the final temperature is 48.0oC. Explain to students that they can verify that their order is correct by looking for words that indicate the direction of change, such as increase and decrease. In this problem, the phrase increases from 25.08C provides additional evidence that the lower value is is the initial value.

Sample Problem 17.2T U T O R

CHEM

T

C M

KNOWNS

mCu 95.4 g

T 48.0 C 25.0 C 23.0 C

q 849 J

UNKNOWN

CCu J/(g C)

CCu

CCu

qmCu T

849 J

95.4 g 23.0 C 0.387 J/(g C)

Start with the equation for specific heat.

Substitute the known quantities into the equation to calculate the unknown value CCu.

Thermochemistry 561

LessonCheckPR O B L E M

S

ONLINE

RO B L E MS

M

O E 17.15. Review What are the ways that energy

conversion can occur?

6. Describe What happens to the energy of the universe during a physical or chemical process?

7. List On what two factors does the heat capacity of an object depend?

Classify 8. On a cold night you use an electric blanket to warm your body. Describe the direc-tion of heat flow. Is this process endothermic or exothermic?

Calculate 9. A chunk of silver has a heat capacity of 42.8 J/°C and a mass of 181 g. Calculate the specific heat of silver.

Calculate 10. Using calories, calculate how much heat 32.0 g of water absorbs when it is heated from 25.0°C to 80.0°C. How many joules is this?

BIGIDEA MATTER AND ENERGY

How is the energy of the universe conserved dur-11. ing the combustion of gasoline in a car engine?

Calculating the Specific Heat of a SubstanceThe temperature of a 95.4-g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper?

Analyze List the knowns and the unknown. Use the known values and the definition of specific heat.

Calculate Solve for the unknown.

Evaluate Does the result make sense? Remember that liquid water has a specific heat of 4.18 J/(g °C). Metals have specific heats lower than water. Thus the calculated value of 0.387 J/(g °C) seems reasonable.

When 435 J of heat is added to 3. 3.4 g of olive oil at 21°C, the tem-perature increases to 85°C. What is the specific heat of the olive oil?

How much heat is required to raise 4. the temperature of 250.0 g of mercury 52°C?

You can find the specific heat

of mercury on Table 17.1.

Thermochemistry 561

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Answers3. 2.0 J/(g ∙ °C)4. 1.8 kJ

Sample Practice ProblemHow much heat is required to raise the temperature of 3.5 g of grain alcohol 15ºC? (1.26 × 102 J)

Evaluate

Informal AssessmentPlace an ice cube in a beaker of cool water. Have students discuss the flow of heat when the ice and water are the system and surroundings, respectively, and when the ice/water mixture and a 37°C room are the system and surroundings, respectively. Then have students complete the 17.1 Lesson Check.

Reteach Emphasize that substances vary in their response to an input of heat; a given amount of heat raises the temperature of some substances (such as metals) far more than others (such as nonmetals). Point out that specific heat is a property of a substance; it is a measure of the ability of a substance to store heat.

Lesson Check Answers 5. Energy changes occur as either heat

transfer or work, or a combination of both.

6. The energy of the universe remains unchanged.

7. mass and chemical composition8. Heat flows from the blanket to your

body. If you body is the system, the process is endothermic.

9. 2.36 × 10−1 J/(g ∙ oC) 10. 1.76 × 103 cal (1.76 kcal);

7.36 × 103 J (7.36 kJ)11. Chemical energy in the gasoline is

transformed into energy to move the car. None of the energy is lost in the process.

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