The Flow of Energy - Heat and Work

1. The Flow of Energy - Heat and Work Prentice Hall Chapter 17.1 Dr. Yager

2. Objectives • Explainhow energy, heat and work are related. • Classify processes as either exothermic or endothermic. • Identify the units used to measure heat transfer. • Distinguish between heat capacity and specific heat.

3. 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.

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

5. 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. Go Speed Racer ! Go !

7. When fuel is burned in a car engine, chemical potential energy is released and is used to do work.

8. Go Speed Racer ! Go !

9. Exothermic and Endothermic Processes In anendothermic process the system gains heat as the surroundings cool down. In anexothermic process the system loses heat as the surroundings heat up.

10. A system is defined 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.

11. An endothermic process is one that absorbs heat from the surroundings.

12. An exothermic process is one that releases heat to its surroundings.

15. A container of melted paraffin wax is allowed to stand at room temperature until the wax solidifies. What is the direction of heat flow as the liquid solidifies? Is the process exothermic or endothermic? 1. As the paraffin wax solidifies it must cool or the system loses heat to the surroundings. 2. In an endothermic process the system gains heat as the surroundings cool down. 3. In an exothermic process the system loses heat as the surroundings heat up.

16. A container of melted paraffin wax is allowed to stand at room temperature until the wax solidifies. What is the direction of heat flow as the liquid solidifies? Is the process exothermic or endothermic? 1. As the paraffin wax solidifies it must cool or the system loses heat to the surroundings. 2. In an endothermic process the system gains heat as the surroundings cool down. 3. In an exothermic process the system loses heat as the surroundings heat up. Exothermic

17. 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.

18. Heat capacity is the amount of heat needed to increase the temperature of an object 1°C . The heat capacity of an object depends on both its mass and its chemical composition.

19. The specific heat capacity, or simply the specific heat “C”, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C.

20. Water releases a lot of heat as it cools. During freezing weather, farmers protect citrus crops by spraying them with water.

21. Water is considered the standard by definition

22. Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.

25. How much heat “q” is required to raise the temperature of 250.0g of mercury 52oC?

26. Heat Capacity of Water • Why is the heat capacity of liquid water so high? How does water store thermal energy? • Thermal energy is stored as kinetic energy, and also as potential energy in the modes of vibration of molecules and solids. • Potential energy is energy stored in a system due to the position or configuration of different parts of that system. It has the potential to be converted into a different form of energy, such as kinetic energy. • Vibration modes represent degrees of freedom of movement for atoms. There are six total degrees of freedom, three potential and three kinetic. • Quantum Theory can be used to quantitatively predict specific heat capacities in simple systems. • It is the heat capacity per-mole-of-atoms, not per-mole-of-molecules, which comes closest to being a constant for all substances at high temperatures. • The fact that specific heat capacities are approximately equal when corrected by the weight of the atoms of solids supports the atomic theory of matter. • Hydrogen bonds are another place to store thermal energy as potential energy of vibration. Water’s hydrogen bonds allow it to approach the theoretical limit of 3 R per mole of atoms of energy storage (R = gas constant)

27. 1. The energy released when a piece of wood is burned has been stored in the wood as • sunlight. • heat. • calories. • chemical potential energy.

28. 1. The energy released when a piece of wood is burned has been stored in the wood as • sunlight. • heat. • calories. • chemical potential energy.

29. 2. Which of the following statements about heat is false? • Heat is the same as temperature. • Heat always flows from warmer objects to cooler objects. • Adding heat can cause an increase in the temperature of an object. • Heat cannot be specifically detected by senses or instruments.

30. 2. Which of the following statements about heat is false? • Heat is the same as temperature. • Heat always flows from warmer objects to cooler objects. • Adding heat can cause an increase in the temperature of an object. • Heat cannot be specifically detected by senses or instruments.

31. Choose the correct words for the spaces: In an endothermic process, the system ________ heat when heat is ________ its surroundings, so the surroundings _________. • gains, absorbed from, cool down. • loses, released to, heat up. • gains, absorbed from, heat up. • loses, released to, cool down.

32. Choose the correct words for the spaces: In an endothermic process, the system ________ heat when heat is ________ its surroundings, so the surroundings _________. • gains, absorbed from, cool down. • loses, released to, heat up. • gains, absorbed from, heat up. • loses, released to, cool down.

33. 4. Which of the relationships listed below can be used to convert between the two units used to measure heat transfer? • 1 g = 1ºC • 1 J = 0.2390 cal • 1ºC = 1 cal • 1 g = 4.184 J

34. 4. Which of the relationships listed below can be used to convert between the two units used to measure heat transfer? • 1 g = 1ºC • 1 J = 0.2390 cal • 1ºC = 1 cal • 1 g = 4.184 J

35. 5. Assuming that two samples of different materials have equal mass, the one that becomes hotter from a given amount of heat is the one that • has the higher specific heat capacity. • has the higher molecular mass. • has the lower specific heat capacity. • has the higher density.

36. 5. Assuming that two samples of different materials have equal mass, the one that becomes hotter from a given amount of heat is the one that • has the higher specific heat capacity. • has the higher molecular mass. • has the lower specific heat capacity. • has the higher density.