1. 17.2 Measuring and Expressing Enthalpy Changes 1 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 17 Thermochemistry 17.1 The Flow of Energy 17.2 Measuring and Expressing Enthalpy Changes 17.3 Heat in Changes of State 17.4 Calculating Heats of Reaction

2. 17.2 Measuring and Expressing Enthalpy Changes 2 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. How can you measure the amount of heat released when a match burns? CHEMISTRY & YOU Remember: The concept of specific heat allows you to measure heat flow in chemical and physical processes.

3. 17.2 Measuring and Expressing Enthalpy Changes 3 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry Calorimetry How can you measure the change in enthalpy of a reaction?

4. 17.2 Measuring and Expressing Enthalpy Changes 4 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes.

5. 17.2 Measuring and Expressing Enthalpy Changes 5 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. In a calorimetry experiment involving an endothermic process, the heat absorbed by the system is equal to the heat released by its surroundings. In an exothermic process, the heat released by the system is equal to the heat absorbed by its surroundings. Calorimetry

6. 17.2 Measuring and Expressing Enthalpy Changes 6 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter. Calorimetry

7. 17.2 Measuring and Expressing Enthalpy Changes 7 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters Foam cups can be used as simple calorimeters because they do not let much heat in or out. Most chemical reactions and physical changes carried out in the laboratory are open to the atmosphere and thus occur at constant pressure. Calorimetry

8. 17.2 Measuring and Expressing Enthalpy Changes 8 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The enthalpy (H) of a system accounts for the heat flow of the system at constant pressure. The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy, symbolized as ΔH. Calorimetry

9. 17.2 Measuring and Expressing Enthalpy Changes 9 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at constant pressure. In this textbook, the terms heat and enthalpy change are used interchangeably. In other words, q = ΔH. Calorimetry

10. 17.2 Measuring and Expressing Enthalpy Changes 10 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters To measure the enthalpy change for a reaction in aqueous solution in a foam cup calorimeter, dissolve the reacting chemicals (the system) in known volumes of water (the surroundings). Calorimetry

11. 17.2 Measuring and Expressing Enthalpy Changes 11 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters 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. Calorimetry

12. 17.2 Measuring and Expressing Enthalpy Changes 12 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters You can calculate the heat absorbed or released by the surroundings (q surr ) using the formula for the specific heat, the initial and final temperatures, and the heat capacity of water. q surr = m  C  ΔT Calorimetry

13. 17.2 Measuring and Expressing Enthalpy Changes 13 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters q surr = m  C  ΔT m is the mass of the water. C is the specific heat of water. ΔT = T f – T i Calorimetry

14. 17.2 Measuring and Expressing Enthalpy Changes 14 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system. q surr = –q sys Calorimetry

15. 17.2 Measuring and Expressing Enthalpy Changes 15 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The enthalpy change for the reaction (ΔH) can be written as follows: q sys = ΔH = –q surr = – m  C  ΔT The sign of ΔH is positive for an endothermic reaction and negative for an exothermic reaction. Calorimetry

16. 17.2 Measuring and Expressing Enthalpy Changes 16 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Volume Calorimeters Calorimetry experiments can also be performed at a constant volume using a device called a bomb calorimeter. Calorimetry

17. 17.2 Measuring and Expressing Enthalpy Changes 17 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Volume Calorimeters In a bomb calorimeter, a sample of a compound is burned in a constant-volume chamber in the presence of oxygen at high pressure. Calorimetry

18. 17.2 Measuring and Expressing Enthalpy Changes 18 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry Constant-Volume Calorimeters 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.

19. 17.2 Measuring and Expressing Enthalpy Changes 19 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What type of calorimeter would you use to measure the heat released when a match burns? Describe the experiment and how you would calculate the heat released. CHEMISTRY & YOU

20. 17.2 Measuring and Expressing Enthalpy Changes 20 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What type of calorimeter would you use to measure the heat released when a match burns? Describe the experiment and how you would calculate the heat released. CHEMISTRY & YOU A constant-volume, or bomb, calorimeter would be used to measure the heat released when a match burns. The match would be ignited in the chamber. By measuring the temperature increase in the water and using the equation q = –m  C  ΔT, the heat released, q, can be calculated.

21. 17.2 Measuring and Expressing Enthalpy Changes 21 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. When 25.0 mL of water containing 0.025 mol HCl at 25.0°C is added to 25.0 mL of water containing 0.025 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 that the densities of the solutions are 1.00 g/mL and the volume of the final solution is equal to the sum of the volumes of the reacting solutions. Sample Problem 17.3 Enthalpy Change in a Calorimetry Experiment

22. 17.2 Measuring and Expressing Enthalpy Changes 22 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 Use dimensional analysis to determine the mass of the water. You must also calculate ΔT. Use ΔH = –q surr = –m  C  ΔT to solve for ΔH. Sample Problem 17.3

23. 17.2 Measuring and Expressing Enthalpy Changes 23 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 KNOWNSUNKNOWN C water = 4.18 J/(g·°C) V final = V HCl + V NaOH = 25.0 mL + 25.0 mL = 50.0 mL T i = 25.0°C T f = 32.0°C density solution = 1.00 g/mL ΔH = ? kJ Sample Problem 17.3

24. 17.2 Measuring and Expressing Enthalpy Changes 24 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. First calculate the total mass of the water. Calculate Solve for the unknown. 2 m water = 50.0 mL  = 50.0 g 1 mL 1.00 g Assume that the densities of the solutions are 1.00 g/mL to find the total mass of the water. Sample Problem 17.3

25. 17.2 Measuring and Expressing Enthalpy Changes 25 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Now calculate ΔT. Calculate Solve for the unknown. 2 ΔT = T f – T i = 32.0°C – 25.0°C = 7.0°C Sample Problem 17.3

26. 17.2 Measuring and Expressing Enthalpy Changes 26 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use the values for m water, C water, and ΔT to calculate ΔH. Calculate Solve for the unknown. 2 Use the relationship 1 kJ = 1000 J to convert your answer from J to kJ. Sample Problem 17.3 ΔH = –q surr = –m water  C water  ΔT = –(50.0 g)(4.18 J/(g· o C))(7.0°C) = –1500 J = –1.5 kJ

27. 17.2 Measuring and Expressing Enthalpy Changes 27 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The temperature of the solution increases, which means that the reaction is exothermic, and thus the sign of ΔH should be negative. About 4 J of heat raises the temperature of 1 g of water 1°C, so 200 J of heat is required to raise 50 g of water 1°C. Raising the temperature of 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 ΔH. Evaluate Does the result make sense? 3 Sample Problem 17.3

28. 17.2 Measuring and Expressing Enthalpy Changes 28 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The initial temperature of the water in a constant-pressure calorimeter is 24°C. A reaction takes place in the calorimeter, and the temperature rises to 87°C. The calorimeter contains 367 g of water, which has a specific heat of 4.18 J/(g·°C). Calculate the enthalpy change during this reaction.

29. 17.2 Measuring and Expressing Enthalpy Changes 29 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. ΔH = –m  C  ΔT = –367 g  4.18 J/(g·°C)  (87°C – 24°C) = –97000 J = –97 kJ The initial temperature of the water in a constant-pressure calorimeter is 24°C. A reaction takes place in the calorimeter, and the temperature rises to 87°C. The calorimeter contains 367 g of water, which has a specific heat of 4.18 J/(g·°C). Calculate the enthalpy change during this reaction.

30. 17.2 Measuring and Expressing Enthalpy Changes 30 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations How can you express the enthalpy change for a reaction in a chemical equation?

31. 17.2 Measuring and Expressing Enthalpy Changes 31 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations In a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product.

32. 17.2 Measuring and Expressing Enthalpy Changes 32 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations 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 Calcium oxide is one of the components of cement.

33. 17.2 Measuring and Expressing Enthalpy Changes 33 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations CaO(s) + H 2 O(l) → Ca(OH) 2 (s) + 65.2 kJ A chemical equation that includes the enthalpy change is called a thermochemical equation.

34. 17.2 Measuring and Expressing Enthalpy Changes 34 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. Heats of reaction are reported as ΔH. The physical state of the reactants and products must also be given. The standard conditions are that the reaction is carried out at 101.3 kPa (1 atm) and 25°C. Heats of Reaction

35. 17.2 Measuring and Expressing Enthalpy Changes 35 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations CaO(s) + H 2 O(l) → Ca(OH) 2 (s) Each mole of calcium oxide and water that reacts to form calcium hydroxide produces 65.2 kJ of heat. ΔH = –65.2 kJ In exothermic processes, the chemical potential energy of the reactants is higher than the chemical potential energy of the products. Heats of Reaction

36. 17.2 Measuring and Expressing Enthalpy Changes 36 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g) Baking soda (sodium bicarbonate) decomposes when it is heated. This process is endothermic. Heats of Reaction The carbon dioxide released in the reaction causes muffins to rise while baking.

37. 17.2 Measuring and Expressing Enthalpy Changes 37 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g) Heats of Reaction Remember that ΔH is positive for endothermic reactions. Therefore, you can write the reaction as follows: 2NaHCO 3 (s) → Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g)ΔH = 85 kJ

38. 17.2 Measuring and Expressing Enthalpy Changes 38 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations The amount of heat released or absorbed during a reaction depends on the number of moles of the reactant involved. The decomposition of 2 mol of sodium bicarbonate requires 85 kJ of heat. Therefore, the decomposition of 4 mol of the same substance would require twice as much heat, or 170 kJ. Heats of Reaction

39. 17.2 Measuring and Expressing Enthalpy Changes 39 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Reaction To see why the physical state of the reactants and products must be stated, compare the following two equations. difference = 44.0 kJ H 2 O(l) → H 2 (g) + O 2 (g)ΔH = 285.8 kJ 1 2 H 2 O(g) → H 2 (g) + O 2 (g)ΔH = 241.8 kJ 1 2

40. 17.2 Measuring and Expressing Enthalpy Changes 40 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Reaction To see why the physical state of the reactants and products must be stated, compare the following two equations. The vaporization of 1 mol of liquid water to water vapor at 25°C requires 44.0 kJ of heat. H 2 O(l) → H 2 O(g) ΔH = 44.0 kJ difference = 44.0 kJ H 2 O(l) → H 2 (g) + O 2 (g)ΔH = 285.8 kJ 1 2 H 2 O(g) → H 2 (g) + O 2 (g)ΔH = 241.8 kJ 1 2

41. 17.2 Measuring and Expressing Enthalpy Changes 41 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate the amount of heat (in kJ) required to decompose 2.24 mol NaHCO 3 (s). Using the Heat of Reaction to Calculate Enthalpy Change 2NaHCO 3 (s) + 85 kJ → Na 2 CO 3 (s) + H 2 O(l) CO 2 (g) Sample Problem 17.4

42. 17.2 Measuring and Expressing Enthalpy Changes 42 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 KNOWNS UNKNOWN amount of NaHCO 3 (s) that decomposes = 2.24 mol ΔH = 85 kJ for 2 mol NaHCO 3 ΔH = ? kJ for 2.24 mol NaHCO 3 Use the thermochemical equation to write a conversion factor relating kJ of heat and moles of NaHCO 3. Then use the conversion factor to determine ΔH for 2.24 mol NaHCO 3. Sample Problem 17.4

43. 17.2 Measuring and Expressing Enthalpy Changes 43 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Write the conversion factor relating kJ of heat and moles of NaHCO 3. Calculate Solve for the unknown. 2 The thermochemical equation indicates that 85 kJ are needed to decompose 2 mol NaHCO 3 (s). 85 kJ 2 mol NaHCO 3 (s) Sample Problem 17.4

44. 17.2 Measuring and Expressing Enthalpy Changes 44 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Using dimensional analysis, solve for ΔH. Calculate Solve for the unknown. 2 Sample Problem 17.4 85 kJ 2 mol NaHCO 3 (s) ΔH = 2.24 mol NaHCO 3 (s)  = 95 kJ

45. 17.2 Measuring and Expressing Enthalpy Changes 45 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The 85 kJ in the thermochemical equation refers to the decomposition of 2 mol NaHCO 3 (s). Therefore, the decomposition of 2.24 mol should absorb more heat than 85 kJ. The answer of 95 kJ is consistent with this estimate. Evaluate Does the result make sense? 3 Sample Problem 17.4

46. 17.2 Measuring and Expressing Enthalpy Changes 46 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Combustion The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.

47. 17.2 Measuring and Expressing Enthalpy Changes 47 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Thermochemical Equations Heats of Combustion Small amounts of natural gas within crude oil are burned off at oil refineries. This is an exothermic reaction. Burning 1 mol of methane releases 890 kJ of heat. The heat of combustion (ΔH) for this reaction is –890 kJ per mole of methane burned. CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(l) + 890 kJ

48. 17.2 Measuring and Expressing Enthalpy Changes 48 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Like other heats of reaction, heats of combustion are reported as the enthalpy changes when the reactions are carried out at 101.3 kPa and 25°C. Interpret Data Heats of Combustion at 25°C SubstanceFormulaΔH (kJ/mol) HydrogenH2(g)H2(g) –286 Carbon C(s, graphite) –394 MethaneCH 4 (g) –890 AcetyleneC2H2(g)C2H2(g)–1300 EthanolC 2 H 6 O(l)–1368 PropaneC3H8(g)C3H8(g)–2220 GlucoseC 6 H 12 O 6 (s)–2808 OctaneC 8 H 18 (l)–5471 SucroseC 12 H 22 O 11 (s)–5645

49. 17.2 Measuring and Expressing Enthalpy Changes 49 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following thermochemical equations represents an endothermic reaction? A.C graphite (s) + 2 kJ C diamond (s) B.2H 2 (g) + O 2 (g) 2H 2 O + 483.6 kJ

50. 17.2 Measuring and Expressing Enthalpy Changes 50 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Which of the following thermochemical equations represents an endothermic reaction? A.C graphite (s) + 2 kJ C diamond (s) B.2H 2 (g) + O 2 (g) 2H 2 O + 483.6 kJ

51. 17.2 Measuring and Expressing Enthalpy Changes 51 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts & Key Equation The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at a constant pressure. In a chemical equation, the enthalpy change for the reaction can be written as either a reaction or a product. q sys = ΔH = –q surr = –m  C  ΔT

52. 17.2 Measuring and Expressing Enthalpy Changes 52 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms calorimetry: the precise measurement of heat flow out of a system for chemical and physical processes calorimeter: an insulated device used to measure the absorption or release of heat in chemical or physical processes enthalpy (H): the heat content of a system at constant pressure

53. 17.2 Measuring and Expressing Enthalpy Changes 53 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms thermochemical equation: a chemical equation that includes the enthalpy change heat of reaction: the enthalpy change for a chemical equation exactly as it is written heat of combustion: the heat of reaction for the complete burning of one mole of a substance

54. 17.2 Measuring and Expressing Enthalpy Changes 54 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The heat of reaction or process can be determined experimentally through calorimetry. BIG IDEA Matter and Energy

55. 17.2 Measuring and Expressing Enthalpy Changes 55 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF 17.2