Thermochemistry

1. Thermochemistry Chapter 17

2. 17.1 The Flow of Energy—Heat and Work • The temperature of lava from a volcano ranges from 550°C to 1400°C. As lava flows, it loses heat and begins to cool. You will learn about heat flow and why some substances cool down or heat up more quickly than others.

3. 17.1 Energy Transformations • Energy Transformations • In what direction does heat flow?

4. 17.1 Energy Transformations • Heat, represented by q, is energy that transfers from one object to another because of a temperature difference between them. • Heat always flows from a warmer object to a cooler object.

5. 17.1 Energy Transformations • Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state. • The energy stored in the chemical bonds of a substance is called chemical potential energy.

6. 17.1 Energy Transformations • When fuel is burned in a car engine, chemical potential energy is released and is used to do work.

7. 17.1 Exothermic and Endothermic Processes • Exothermic and Endothermic Processes • What happens in endothermic and exothermic processes?

8. 17.1 Exothermic and Endothermic Processes • In an endothermic process, the system gains heat as the surroundings cool down. • In an exothermic process, the system loses heat as the surroundings heat up.

9. 17.1 Exothermic and Endothermic Processes • In studying energy changes, you can define a system as the part of the universe on which you focus your attention. The surroundings include everything else in the universe. • The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed.

10. 17.1 Exothermic and Endothermic Processes • An endothermic process is one that absorbs heat from the surroundings.

11. 17.1 Exothermic and Endothermic Processes • An exothermic process is one that releases heat to its surroundings.

12. Conceptual Problem 17.1

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14. 17.1 Units for Measuring Heat Flow • Units for Measuring Heat Flow • In what units is heat flow measured?

15. 17.1 Units for Measuring Heat Flow • Heat flow is measured in two common units, the calorie and the joule. • The energy in food is usually expressed in Calories.

16. 17.1 Heat Capacity and Specific Heat • Heat Capacity and Specific Heat • On what two factors does the heat capacity of an object depend?

17. 17.1 Heat Capacity and Specific Heat • The heat capacity of an object depends on both its mass and its chemical composition. • The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object.

18. 17.1 Heat Capacity and Specific Heat • 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. • Specific heat of water = 4.18 J/g-°C • ref. table B

19. 17.1 Heat Capacity and Specific Heat • Water releases a lot of heat as it cools. During freezing weather, farmers protect citrus crops by spraying them with water.

20. 17.1 Heat Capacity and Specific Heat

21. 17.1 Heat Capacity and Specific Heat • Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.

22. 17.1

23. for Sample Problem 17.1

24. 17.2 Measuring and Expressing Enthalpy Changes • A burning match releases heat to its surroundings in all directions. How much heat does this exothermic reaction release? You will learn to measure heat flow in chemical and physical processes by applying the concept of specific heat.

25. 17.2 Calorimetry • Calorimetry • What basic concepts apply to calorimetry?

26. 17.2 Calorimetry • Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes.

27. 17.2 Calorimetry • 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. • Ref. table T – heat transfer equation: • q = heat absorbed or released • M = mass of water • C= specific heat • ΔT = temperature change q = m x C x ΔT

28. 17.2 Calorimetry • The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.

29. 17.2 Calorimetry • Constant-Pressure Calorimeters • The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system.

30. 17.2 Calorimetry • Constant-Volume Calorimeters • Calorimetry experiments can be performed at a constant volume using a bomb calorimeter.

31. 17.2

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33. 17.2 Thermochemical Equations • Thermochemical Equations • How can you express the enthalpy (heat) change for a reaction in a chemical equation?

34. 17.2 Thermochemical Equations • In a chemical equation, the enthalpy (heat) change for the reaction can be written as either a reactant or a product. • In exothermic reactions, heat is given off to the surroundings, and shown as a product. • In endothermic reactions, heat is taken in from the surroundings, and is shown as a reactant.

35. 17.2 Thermochemical Equations • A chemical equation that includes the enthalpy change is called a thermochemical equation.

36. 17.2 Thermochemical Equations • The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. • Exothermic reactions have a negative heat of reaction, and endothermic reactions have a positive heat of reaction. • Reference table I

37. 17.2 Thermochemical Equations Exothermic Reaction

38. 17.2 Thermochemical Equations Endothermic Reaction

39. 17.3

40. for Sample Problem 17.3

41. 17.2 Thermochemical Equations

42. 17.2 Thermochemical Equations • The heat of combustion is the heat of reaction for the complete burning of one mole of a substance. Reference table I

43. 17.3 Heat in Changes of State • During a race, an athlete can burn a lot of calories that either do work or are released as heat. This section will help you to understand how the evaporation of sweat from your skin helps to rid your body of excess heat.

44. 17.3 Heats of Fusion and Solidification • Heats of Fusion and Solidification • How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies?

45. Heats of Fusion and Solidification • The heat of fusion(Hf) is the heat absorbed by one gram of a solid substance as it melts to a liquid at a constant temperature. • The heat of solidification(Hs) is the heat lost when one mole of a liquid solidifies at a constant temperature. • For water: Hf = 334 J/g Reference table B

46. 17.3 Heats of Fusion and Solidification • The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies; that is, Hf = -Hs • To calculate heat gained during melting: • q = mHf • Reference table T

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49. 17.3 Heats of Vaporization and Condensation • Heats of Vaporization and Condensation • How does the quantity of heat absorbed by a vaporizing liquid compare to the quantity of heat released when the vapor condenses?

50. 17.3 Heats of Vaporization and Condensation • The amount of heat necessary to vaporize one mole of a given liquid is called its heat of vaporization(Hv). • The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its heat of condensation(Hc). • For water: Hv= 2260 J/g