Guide for Reading. Vocabulary calorimetry calorimeter enthalpy thermochemical equation heat of reaction heat of combustion.

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1 17.2 Measuring and Expressing Enthalpy Changes 17.2 Connecting to Your World As you know, a burning match gives off heat. When you strike a match, heat is released to the surroundings in all directions. You can classify this reaction as exothermic. In addition to describing the direction of heat flow, you may also want to determine the quantity of heat that is transferred. How much heat does a burning match release to the surroundings? In this section you will learn how you can measure heat flows in chemical and physical processes by applying the concept of specific heat. Calorimetry Heat that is released or absorbed during many chemical reactions can be measured by a technique called calorimetry. Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. In calorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings. The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter. Constant-Pressure Calorimeters Foam cups are excellent heat insulators. Because they do not let much heat in or out, they can be used as simple calorimeters. In fact, the heat flows for many chemical reactions can be measured in a constant-pressure calorimeter similar to the one shown in Figure Because most chemical reactions and physical changes carried out in the laboratory are open to the atmosphere, these changes occur at constant pressure. The heat content of a system at constant pressure is the same as a property called the enthalpy (H ) of the system. The heat released or absorbed by a reaction at constant pressure is the same as the change in enthalpy, symbolized as H. Because the reactions presented in this textbook occur at constant pressure, the terms heat and enthalpy change are used interchangeably. In other words, q H. Figure 17.5 In a simple constant-pressure calorimeter, a thermometer records the temperature change as chemicals react in water. The substances reacting in solution constitute the system. The water constitutes the surroundings. Section Resources Water (where reaction takes place) Print Guided Reading and Study Workbook, Section 17.2 Core Teaching Resources, Section 17.2 Review, Interpreting Graphics Transparencies, T183-T186 Laboratory Manual, Labs 34, 35 Guide for Reading Key Concepts What basic concepts apply to calorimetry? How can you express the enthalpy change for a reaction in a chemical equation? Vocabulary calorimetry calorimeter enthalpy thermochemical equation heat of reaction heat of combustion Reading Strategy Relating Text and Visuals As you read, look at Figure Explain how these diagrams help you understand exothermic and endothermic processes. Thermometer Stirrer Foam lid (loose fitting) Nested foam cups (insulation) Section 17.2 Measuring and Expressing Enthalpy Changes 511 Technology Interactive Textbook with ChemASAP, Problem-Solving 17.13, 17.15, Simulation 22, Assessment 17.2 Go Online, Section FOCUS Objectives Describe how calorimeters are used to measure heat flow Construct thermochemical equations Solve for enthalpy changes in chemical reactions by using heats of reaction. Guide for Reading Build Vocabulary Concept Map Have students make a concept map using the vocabulary terms in this section. Reading Strategy Directed Reading/Thinking Activity Ask students if there is a way to measure how much heat is released from a burning match. (Yes, but only indirectly. If the reaction were confined, then any temperature changes in the surroundings could be attributed to heat transfer from the reaction.) 2 INSTRUCT Tell students that a match won t ignite unless you strike it and add heat to it. Ask, Where does the heat that is added to the match come from? (friction) Is the burning of a match an endothermic reaction? Explain. (No; the reaction releases more energy, in the form of heat and light, than the amount it absorbs to start.) Calorimetry Relate Show students typical utility bills for electricity, gas, or oil. Ask, Why would the size of the bill vary from one household to another? (floor space, number of occupants, thermostat setting, insulation, type of fuel used) Why might the bill vary from month to month? (changes in the gradient between internal and external temperatures, fluctuations in fuel prices) Thermochemistry 511

2 Section 17.2 (continued) CLASS Activity Calorimetry Measurements Purpose Students will make calorimetry measurements and calculations. Two separate but related oxidationreduction reactions will be studied by different groups of students. The results can then be pooled and compared to provide insight into how calorimetry can be used as a tool to investigate the physical and chemical properties of substances. Materials 1M HCl, foam cup, thermometer, mossy zinc, magnesium turnings Safety Students must wear protective goggles and aprons. Remind them of the safety precautions needed for handling HCl. Use a fume hood if possible. Students should not be allowed to dispense concentrated HCl themselves. Procedure Pour 100 ml of 1M HCl into a foam cup and determine the temperature to the nearest tenth of a degree Celsius. Add 0.5 g of mossy zinc or 0.5 g of magnesium turnings. Expected Outcome For calculations, ignore the mass of the solids added. Assume that HCl is primarily water and has the same specific heat and density of water. H = 100 g 4.18 J/(g C) T H rxn Zn = 150 kj/mol H rxn Mg = 460 kj/mol Discuss sources of error such as heat lost to surroundings. References: 1. Shakhashiri, B.Z. Chemical Demonstrations, Vol. 1, University Press, 1983, Alyea, H.N. J.Chem. Educ. 1970, 47, A-387 For: Links on enthalpy Visit: Web Code: cdn-1172 Figure 17.6 Nutritionists use bomb calorimeters to measure the energy content of the foods you eat. To see some of their data (expressed in Calories per serving), you can look at a nutrition label. Calculating According to the nutrition label below, each serving contains 140 Calories. How many kilojoules is this? 512 Chapter 17 To measure the enthalpy change for a reaction in aqueous solution in a foam cup calorimeter, you dissolve the reacting chemicals (the system) in known volumes of water (the surroundings). Then measure the initial temperature of each solution and mix the solutions in the foam cup. After the reaction is complete, measure the final temperature of the mixed solutions. Because you know the initial and final temperatures and the heat capacity of water, you can calculate the heat absorbed by the surroundings (q surr ) using the formula for specific heat. Electrical leads Oxygen intake valve Firing element Oxygen at high pressure Stirrer q surr m C T In this expression, m is the mass of the water; C is the specific heat of water; and T T f T i. Because the heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system, the enthalpy change for the reaction ( H) can be written as follows. q sys H q surr m C T The sign of H is negative for an exothermic reaction and positive for an endothermic reaction. Constant-Volume Calorimeters Calorimetry experiments can also be performed at constant volume using a device called a bomb calorimeter. In a bomb calorimeter, shown in Figure 17.6, a sample of a compound is burned in a constant-volume chamber in the presence of oxygen at high pressure. The heat that is released warms the water surrounding the chamber. By measuring the temperature increase of the water, it is possible to calculate the quantity of heat released during the combustion reaction. Checkpoint What formula is used to calculate the enthalpy change in a constant-pressure calorimeter? Thermometer Insulated outer container Sample to be burned Steel bomb Water Download a worksheet on Enthalpy for students to complete, and find additional teacher support from NSTA SciLinks. Differentiated Instruction Gifted and Talented L3 A large percentage of the electricity used in the United States is produced by the burning of fossil fuels. Power plants generate electricity by transferring heat to water during the combustion of fossil fuels. The heated water turns to steam, which drives turbines that generate electricity. In order to condense and recycle the steam, a source of coolant water is also needed. Consequently, power plants (including nuclear power plants) are often built near a source of water, such as an ocean, lake, or river. However, when the coolant water is returned to its source, it is usually warmer. This increase in temperature is called thermal pollution. Have students do research to find out if this label is justified. 512 Chapter 17

3 SAMPLE PROBLEM 17.2 Enthalpy Change in a Calorimetry Experiment When 25.0 ml of water containing mol HCl at 25.0 C is added to 25.0 ml of water containing mol NaOH at 25.0 C in a foam cup calorimeter, a reaction occurs. Calculate the enthalpy change in kj) during this reaction if the highest temperature observed is 32.0 C. Assume the densities of the solutions are 1.00 g/ml. Analyze List the knowns and the unknown. Knowns C water 4.18 J/(g C) V final V HCl V NaOH 25.0 ml 25.0 ml 50.0 ml DH m C DT g J/1g C C2 Practice Problems 1463 J J 1.5 kj Practice Problems T i 25.0 C T f 32.0 C Density solution 1.00 g/ml Unknown H? kj Use dimensional analysis to determine the mass of the water. You must also calculate T. Use H q surr m C T to solve for H. Calculate Solve for the unknown. First, calculate the total mass of the water. Now calculate T. m m a 1.00 g b 50.0 g ml T T f T i 32.0 C 25.0 C 7.0 C Use the values for m, C water, and T to calculate H. Evaluate Does the result make sense? The sign of H is negative; the reaction releases 1.5 kj of heat to the water. The heat released by the system (the reaction) equals the heat absorbed by the surroundings (the water). About 4 J of heat raises the temperature of 1 g of water 1 C; 50 g of water requires about 200 J. To heat 50 g of water 7 C requires about 1400 J, or 1.4 kj. This estimated answer is very close to the calculated value of H. 12. When 50.0 ml of water containing 0.50 mol HCl at 22.5 C is mixed with 50.0 ml of water containing 0.50 mol NaOH at 22.5 C in a calorimeter, the temperature of the solution increases to 26.0 C. How much heat (in kj) was released by this reaction? 13. A small pebble is heated and placed in a foam cup calorimeter containing 25.0 ml of water at 25.0 C. The water reaches a maximum temperature of 26.4 C. How many joules of heat were released by the pebble? Math For help with dimensional analysis, go to page R66. Problem-Solving Solve Problem 13 with the help of an interactive guided tutorial. withchemasap Sample Problem 17.2 Answers kj J Practice Problems Plus A lead mass is heated and placed in a foam cup calorimeter containing 40.0 ml of water at 17.0 C. The water reaches a temperature of 20.0 C. How many joules of heat were released by the lead? (502 J) Math For a math refresher and practice, direct students to dimensional analysis, page R66. Discuss If possible, show photographs or actual examples of the types of calorimeter(s) used in laboratories today. Discuss the use of calorimetry as an analytical tool; for example, in investigating the caloric content of foods. Make sure students understand all the terms in the equation q sys = H = m C T. Emphasize that the delta notation ( ) represents an algebraic operation involving two separate measurements. When determining the algebraic value of T, the algebraic signs of the final and initial temperatures must be included in the operation. Section 17.2 Measuring and Expressing Enthalpy Changes 513 Answers to... Figure kj Checkpoint q sys = H = q surr = m C T Thermochemistry 513

4 Section 17.2 (continued) Firefighter Firefighters must have a working knowledge of chemistry because they are often called to identify and safely remove hazardous wastes. Knowledge of mechanics, first aid, physics, and record-keeping is also useful. Firefighters may work in industry, nuclear power plants, natural resources, airports, colleges, and in rural or urban firefighting services. Have students research chemistryrelated careers in the library or on the Internet. Students can then construct a table that describes the nature of the work, training requirements, working conditions, and other necessary information. Thermochemical Equations Discuss To illustrate writing a thermochemical equation, refer to two familiar reactions. The combustion of methane in a Bunsen burner adds energy to the surroundings. Thus, the burning of methane has a negative H. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) kj H = 890 kj/mol During the electrolysis of water, electrical energy is used to decompose the water into hydrogen gas and oxygen gas. Energy is added to the system from the surroundings. Thus, the electrolysis of water has a positive H kj + 2H 2 O(l) 2H 2 (g) + O 2 (g) H = kj/mol Remind students that the sign of H indicates whether the reaction is endothermic (+) or exothermic ( ). Although by convention, an exothermic reaction has a negative H, when heat is shown as a product in an equation, it is expressed as an absolute value. Firefighter If you are courageous, teamoriented, and have a strong sense of public service, you may wish to consider a career as a firefighter. Firefighters respond to fires, 514 Chapter 17 Differentiated Instruction Gifted and Talented L3 Iron wool will burn rapidly in high concentrations of oxygen. It will oxidize slowly at the concentrations normally available in air. Rusting of iron is essentially a slow combustion reaction. Have students design an experiment to determine the heat of reaction for medical emergencies, and hazardous chemical spills. In responding to a fire, firefighters must know what types of chemicals can be used on different types of fires. Having knowledge of the requirements of different chemical fires and spills enables firefighters to put out rather than feed fires. Regular reviews of fire science literature keep firefighters up to date on current technology developments and policy changes. Firefighters also engage in physical fitness activities to build stamina and improve agility. Although the job is hazardous and includes unpredictable hours, competition for jobs is relatively high. To become a firefighter, you must be at least 18 years of age with a high school education or equivalent. Selection for positions in fire departments often depends on passing a written test as well as a medical examination. Once hired, training consists of instruction in firefighting and rescue techniques, emergency medical procedures, basic chemistry of fires and firefighting, and instruction in local building codes and fire prevention methods. For: Careers in Chemistry Visit: PHSchool.com Web Code: cdb-1172 Thermochemical Equations If you mix calcium oxide with water, the water in the mixture becomes warm. This exothermic reaction occurs when cement, which contains calcium oxide, is mixed with water to make concrete. When 1 mol of calcium oxide reacts with 1 mol of water, 1 mol of calcium hydroxide forms and 65.2 kj of heat is released. In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product. In the equation describing the exothermic reaction of calcium oxide and water, the enthalpy change can be considered a product. CaO(s) H 2 O(l) Ca(OH) 2 (s) 65.2 kj This equation is presented visually in Figure 17.7a. A chemical equation that includes the enthalpy change is called a thermochemical equation. The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. You will usually see heats of reaction reported as H, which is equal to the heat flow at constant pressure. The physical state of the reactants and products must also be given. The standard conditions are that the reaction is carried out at kpa (1 atmosphere) and that the reactants and products are in their usual physical states at 25 C. The heat of reaction, or H, in the above example is 65.2 kj. Each mole of calcium oxide and water that react to form calcium hydroxide produces 65.2 kj of heat. CaO(s) H 2 O(l) Ca(OH) 2 (s) H 65.2 kj the rusting of iron wool. (Wrapping moist iron wool around the bulb of a thermometer demonstrates the heat released by this reaction. A foam cup calorimeter can be used to quantify the thermodynamic parameters of the reaction.) 514 Chapter 17

5 a CaO(s) + H 2 O(l) H = 65.2 kj Other reactions absorb heat from the surroundings. For example, baking soda (sodium bicarbonate) decomposes when it is heated. The carbon dioxide released in the reaction causes a cake to rise while baking. This process is endothermic. 2NaHCO 3 (s) 129 kj Na 2 CO 3 (s) H 2 O(g) CO 2 (g) Remember that H is positive for endothermic reactions. Therefore, you can write the reaction as follows. 2NaHCO 3 (s) Na 2 CO 3 (s) H 2 O(g) CO 2 (g) H 129 kj Figure 17.7b shows the enthalpy diagram for this reaction. Chemistry problems involving enthalpy changes are similar to stoichiometry problems. The amount of heat released or absorbed during a reaction depends on the number of moles of the reactants involved. The decomposition of 2 mol of sodium bicarbonate, for example, requires 129 kj of heat. Therefore, the decomposition of 4 mol of the same substance would require twice as much heat, or 258 kj. In this and other endothermic processes, the potential energy of the product(s) is higher than the potential energy of the reactant(s). The physical state of the reactants and products in a thermochemical reaction must also be stated. To see why, compare the following two equations for the decomposition of 1 mol H 2 O. Although the two equations are very similar, the different physical states of H 2 O result in different H values. In one case, the reactant is a liquid; in the other case, the reactant is a gas. The vaporization of 1 mole of liquid water to water vapor at 25 C requires an extra 44.0 kj of heat. H 2 O(l) H 2 O(g) H 44.0 kj Checkpoint Ca(OH) 2 (s) Exothermic Reaction H 2 O1l 2 H 2 1g2 1 2 O 2 1g2 H 2 O1g 2 H 2 1g2 1 2 O 2 1g2 DH kj DH kj difference 44.0 kj Why must the physical states of the reactants and products be stated in a thermochemical equation? Differentiated Instruction Gifted and Talented L3 Many metals react with oxygen to produce metal oxides. Have students research the combustion reactions of a series of selected metals (e.g., Mg, Ca, Fe). Students should write thermochemical equations for each reaction and compare the relative amounts of heat released in each case. Ask students to describe any trend(s) they observe. b Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) 2NaHCO 3 (s) H = 129 kj Endothermic Reaction Figure 17.7 These enthalpy diagrams show exothermic and endothermic processes: a the reaction of calcium oxide and water and b the decomposition of sodium bicarbonate. Identifying In which case is the enthalpy of the reactant(s) higher than that of the product(s)? Simulation 22 Simulate a combustion reaction and compare the H results for several compounds. withchemasap Section 17.2 Measuring and Expressing Enthalpy Changes 515 Less Proficient Readers L1 Special attention should be given to vocabulary terms in the chapter that are defined by mathematical relationships. Have students write definitions of these terms. Sample calculations showing how to apply the formulas should be included in students' notes. Use Visuals L1 Figure 17.7 Ask students to draw similar diagrams for the burning of CH 4 and the electrolysis of H 2 O. For each reaction, ask, Is the potential energy of the reactants higher or lower than the products? (higher for CH 4, lower for H 2 O) In what direction does heat move in diagram 17.7a? (into the surroundings) In what direction does heat move in diagram 17.7b? (absorbed from the surroundings by the reaction) TEACHER Demo An Exothermic Reaction Purpose Students will observe an exothermic reaction and write a thermochemical equation. Materials CaO, metal tray, thin aluminum pan, water, egg (raw) Safety CaO is corrosive. Procedure Place 100 g of CaO in a metal tray. Place a thin aluminum pan or pie tin in the tray so that its bottom is completely in contact with the calcium oxide. Pour just enough water into the tray to cover the calcium oxide. Then break a fresh egg into the aluminum pan. Ask, Is this an exothermic or endothermic reaction? (Exothermic; 65.2 kj of heat is released per mole of CaO.) Have students write a thermochemical equation to describe the reaction. Check to see that students express the enthalpy of reaction as a negative value. Have students use the thermochemical equation to answer stoichiometric questions such as: How much heat is released when g of calcium oxide reacts with excess water? (116 kj) Expected Outcome Students should observe that enough heat is liberated by the reaction to cook the egg. Answers to... Figure 17.7 for the exothermic reaction: CaO(s) + H 2 O(l) Ca(OH) 2 (s) Checkpoint Different physical states of the same substance have different H values. Thermochemistry 515

6 Section 17.2 (continued) Sample Problem 17.3 Answers kj kj Practice Problems Plus The burning of magnesium is a highly exothermic reaction. 2Mg(s) + O 2 (g) 2MgO(s) kj How many kilojoules of heat are released when 0.75 mol of Mg burn in an excess of O 2? (450 kj) Conversion Problems A conversion factor relating kilojoules of heat and moles of sodium bicarbonate is needed to solve the Sample Problem. It is important to label the units of the conversion factor so that the answer contains the correct units. Math For a math refresher and practice, direct students to conversion problems, page R66. Relate The decomposition of sodium bicarbonate to form carbon dioxide gas is essential for successful baking. Sodium bicarbonate, also called baking soda, is the source of the carbon dioxide that causes cakes, muffins, and quick breads to rise in the oven. Without this substance, these foods would be flat and heavy. Conversion Problems A conversion factor is a ratio of two quantities that are equal to one another. When doing conversions, write the conversion factors so that the unit of a given measurement cancels, leaving the correct unit for your answer. Note that the equalities needed to write a particular conversion factor may be given in the problem. In other cases, you will need to know or look up the necessary equalities. Math 516 Chapter 17 For help with conversion problems, go to page R66. Problem-Solving Solve Problem 15 with the help of an interactive guided tutorial. withchemasap SAMPLE PROBLEM 17.3 Using the Heat of Reaction to Calculate Enthalpy Change Using the thermochemical equation in Figure 17.7b on page 515, calculate the amount of heat (in kj) required to decompose 2.24 mol NaHCO 3 (s). Analyze List the knowns and the unknown. Knowns 2.24 mol NaHCO 3 (s) decomposes H 129 kj (for 2 mol NaHCO 3 ) Use the thermochemical equation, Practice Problems Unknown H? kj 2NaHCO 3 (s) 129 kj Na 2 CO 3 (s) H 2 O(g) CO 2 (g), to write a conversion factor relating kilojoules of heat and moles of NaHCO 3. Then use the conversion factor to determine H for 2.24 mol NaHCO 3. Calculate Solve for the unknown. The thermochemical equation indicates that 129 kj are needed to decompose 2 mol NaHCO 3 (s). Use this relationship to write the following conversion factor. 129 kj 2 mol NaHCO 3 1s2 Using dimensional analysis, solve for H. 129 kj DH 2.24 mol NaHCO 3 1s2 2 mol NaHCO 3 1s2 144 kj Evaluate Does the result make sense? Because the H 129 kj refers to the decomposition of 2 mol NaHCO 3 (s), the decomposition of 2.24 mol should absorb about 10% more heat than 129 kj, or slightly more than 142 kj. The answer of 144 kj is consistent with this estimate. 14. When carbon disulfide is formed from its elements, heat is absorbed. Calculate the amount of heat (in kj) absorbed when 5.66 g of carbon disulfide is formed. C(s) 2S(s) CS 2 (l) H 89.3 kj 15. The production of iron and carbon dioxide from iron(iii) oxide and carbon monoxide is an exothermic reaction. How many kilojoules of heat are produced when 3.40 mol Fe 2 O 3 reacts with an excess of CO? Fe 2 O 3 (s) 3CO(g) 2Fe(s) 3CO 2 (g) 26.3 kj Facts and Figures Keeping Warm How do animals such as polar bears and seals survive the cold land and water temperatures where they live? A good coat of fur helps, but it is not enough to keep them warm. These animals also have unique fat cells that help generate heat. These fat cells are in tissue called brown fat. The cells of brown fat are unlike other fat cells in these animals bodies. Most other cells store chemical energy released during the breakdown of carbohydrates and fatty acids in the bonds of adenosine triphosphate (ATP). ATP acts as the central source of energy and growth of all organisms. Heat is mostly a waste product in ATP-producing cells. The heat generated by the brown fat tissue, however, helps the animal keep relatively comfortable even at subzero temperatures. 516 Chapter 17

7 Table 17.2 Heats of Combustion at 25 C Substance Formula H (kj/mol) Hydrogen H 2 (g) 286 Carbon C(s), graphite 394 Methane CH 4 (g) 890 Acetylene C 2 H 2 (g) 1300 Ethanol C 2 H 5 OH(l) 1368 Propane C 3 H 8 (g) 2220 Glucose C 6 H 12 O 6 (s) 2808 Octane C 8 H 18 (l) 5471 Sucrose C 12 H 22 O 11 (s) 5645 Table 17.2 lists heats of combustion for some common substances. The heat of combustion is the heat of reaction for the complete burning of one mole of a substance. The combustion of natural gas, which is mostly methane, is an exothermic reaction used to heat many homes around the country. CH 4 (g) 2O 2 (g) CO 2 (g) 2H 2 O(l) 890 kj You can also write this equation as follows. CH 4 (g) 2O 2 (g) CO 2 (g) 2H 2 O(l) H 890 kj Burning 1 mol of methane releases 890 kj of heat. The heat of combustion ( H) for this reaction is 890 kj per mole of carbon burned. Like other heats of reaction, heats of combustion are reported as the enthalpy changes when the reactions are carried out at kpa of pressure and the reactants and products are in their physical states at 25 C Section Assessment 16. Key Concept Calorimetry is based on what basic concepts? 17. Key Concept How are enthalpy changes treated in chemical equations? 18. When 2 mol of solid magnesium (Mg) combines with 1 mole of oxygen gas (O 2 ), 2 mol of solid magnesium oxide (MgO) is formed and 1204 kj of heat is released. Write the thermochemical equation for this combustion reaction. 19. Gasohol contains ethanol (C 2 H 5 OH)(l), which when burned reacts with oxygen to produce CO 2 (g) and H 2 O(g). How much heat is released when 12.5 g of ethanol burns? C 2 H 5 OH(l) 3O 2 (g) 2CO 2 (g) 3H 2 O(l) H 1368 kj Assessment 17.2 Test yourself on the concepts in Section withchemasap Figure 17.8 The combustion of natural gas is an exothermic reaction. As bonds in methane (the main component of natural gas) and oxygen are broken and bonds in carbon dioxide and water are formed, large amounts of energy are released. 20. Explain the term heat of combustion. Group 6A Look up the important chemical reactions involving Group 6A elements. Write two examples of thermochemical equations one describing an exothermic reaction and another describing an endothermic reaction. 3 ASSESS Evaluate Understanding To determine students knowledge about measuring and expressing heat changes, ask, Explain the difference between H and H. (H is enthalpy or heat content; H represents a change in heat content.) Use the data in Table 17.2 to write the thermochemical equation for the combustion of propane. (C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l); H = 2220 kj) Draw an enthalpy diagram for this reaction like those in Figure Calculate the amount of heat produced when 64.0 grams of propane burns completely. ( kj) Reteach L1 Select a substance from Table 17.2, and write the thermochemical equation on the chalkboard. Discuss the H for the reaction, why it is negative or positive, and draw the enthalpy diagram for the reaction. Remind students that heat is the energy that flows into or out of a thermodynamic system because of differences in temperature between the system and its surroundings. Remind students that the total energy (potential and kinetic) of the system and the surroundings must remain the same in any chemical or physical process. Elements Example exothermic reactions: 4K(s) + O 2 (g) 2K 2 O(s) kj/mol C 6 H 12 O 6 (s) + 6O 2 (g) 6CO 2 (g) + 6H 2 O(g) kj/mol Example endothermic reaction: 3O 2 (g) kj/mol 2O 3 (g) Section 17.2 Measuring and Expressing Enthalpy Changes The heat released by the system is equal to the heat absorbed by the surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings. 17. The enthalpy change in a chemical reaction can be written as either a reactant or a product. Section 17.2 Assessment 18. 2Mg(s) + O 2 (g) 2MgO(s) kj, or 2Mg(s) + O 2 (g) 2MgO(s) H = 1204 kj kj 20. Heat of combustion is the heat of reaction for the complete burning of one mole of a substance. If your class subscribes to the Interactive Textbook, use it to review key concepts in Section with ChemASAP Thermochemistry 517

8 Section 17.2 (continued) Solar Power Plants Background Solar One, an early demonstration solar energy plant in the Mojave Desert in California, used water as the fluid in the receiver. The receiver in the tower generated steam to drive a turbine to generate electricity. The plant was later converted to Solar Two, which used molten salt as the fluid. The hot salt can be stored, then used when needed to boil water into steam that drives the turbine. CSP is the least expensive solar electricity technology for large-scale power generation and has the potential to make solar power available at a very competitive rate. As a result, government, industry, and utilities are forming partnerships with the goal of reducing the manufacturing cost of CSP technologies. Solar Power Plants When you consider energy sources other than coal, oil, or natural gas, you may think of the sun. Solar energy, or energy from the sun, is free, clean, plentiful, and readily available. The daily amount of solar energy received by Earth is about 200,000 times the total world electricalgenerating capacity. However, the process of collecting solar energy and using it to generate electricity can be expensive, complex, and space-consuming. Large-scale solar power plants use concentrating solar power (CSP) technologies, which convert solar energy into heat and then electrical energy. Interpreting Diagrams In which part of a power tower is solar energy transformed into heat? Solar power by the tower A power tower is a CSP technology that uses a vast field of sun-tracking mirrors, called heliostats, to concentrate sunlight on a receiver that sits on top of a tower. Before the solar energy can be used to do work, it must first be transformed into heat by means of a fluid such as steam or a molten salt. From sun to socket Because a heliostat field collects solar energy faster than the system can use it, some of the energy is stored (as a molten salt) for later use. Energy storage enables a power tower to supply electricity during cloudy weather or even at night. CLASS Activity Energy Conversions Have students construct a concept map or flow chart depicting the conversion of solar energy into other forms of energy. For example, solar energy can be used to heat water, which can be used to generate electricity, which in turn can be converted into light (e.g., a reading lamp) or chemical energy (e.g., recharging a battery). 518 Chapter Chapter 17

9 CLASS Activity Sun Heliostats In a molten-salt power tower, a liquid mixture of 60% NaNO 3 and 40% KNO 3 is used as the heat-transfer fluid. This mixture melts at 221 C. In the receiver, the collected solar energy is transformed into heat as the molten salt reaches a temperature of 565 C. Receiver Tower Hot molten salt (565 C) The heated salt is stored in an insulated storage tank, then pumped to a steam generator, where heat is transferred from the salt to liquid water. The water vaporizes, forming high-pressure steam that drives a turbine. An electric generator transforms the mechanical energy of the turbine into electrical energy. Hot storage tank Steam Power line tower Turbine and electric generator Developments in Solar Power Have students research and write a report on a topic related to solar power. Possible topics include: hybrid solar power plants that combine solar energy collection with fossil fuel combustion mirror technologies used in solar power plants heat-transfer fluids used in CSP technologies (e.g., air, steam, molten salt, oil) houses and other buildings that have solar-powered heating and hot-water systems photovoltaic technologies used in solar power plants, solar cars, and solar houses Cooled molten salt (290 C) Liquid water Cold storage tank The cooled molten salt, now at a temperature of about 290 C, is diverted to another storage tank until it is pumped back to the receiver and reheated. Field of mirrors Surrounding the tower is a circular field of hundreds or thousands of heliostats that collect solar energy. Each heliostat rotates on an axis as it follows the sun s movement during the day. Technology and Society 519 Answers to... Interpreting Diagrams The receiver on the power tower converts solar energy to heat. Thermochemistry 519