Changes of State and thermodynamics.pptx

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1. Changes of State and Thermodynamics Lesson 2

2. Focus Question Why doesn't a bowl of hot soup spontaneously freeze?

3. New Vocabulary heat of fusion heat of vaporization first law of thermodynamics heat engine second law of thermodynamics entropy

4. Review Vocabulary joule (J): the SI unit of work and energy; 1 J of work is done when a force of 1 N acts on an object over displacement of 1 m.

5. Change of State The three most common states of matter are solids, liquids, and gases. Suppose you heat solid water (ice) and make a graph of the temperature versus the amount of energy you add (next slide). As you heat the ice, the temperature rises. The slope of the line between A and B is the specific heat of ice.

6. Change of State

7. Changes of State At some point, the added thermal energy causes the particles of a solid to move rapidly enough that their motion overcomes the forces holding them together in a fixed location. The particles are still touching each other, but they have more freedom of movement. Eventually, the particles become free enough to slide past each other. That is, the solid melts. The temperature at which this change occurs is the substance’s melting point.

8. Changes of State When a substance is melting, all of the added thermal energy goes to overcome the forces holding the particles together in the solid state. None of the added thermal energy increases the kinetic energy of the particles, as indicated by the horizontal line between B and C. The energy difference between C and B is the heat of fusion. The heat of fusion ( H f ) of a substance is the amount of energy needed to melt 1 kg of the substance. Heat Required to Melt a Solid Q = mH f

9. Changes of State If you continue heating the water, its temperature will rise. The slope of the line between C and D is the specific heat of water. As the temperature increases further, some particles in the liquid acquire enough energy to break free from the other particles. At a specific temperature, known as the boiling point, further addition of energy causes the substance to undergo another change of state.

10. Changes of State All the added thermal energy converts the substance from the liquid state to the gaseous state. As in melting, the temperature does not rise while a liquid boils. Energy difference between D and E is the heat of vaporization. The heat of vaporization ( H v ) of a substance is the thermal energy needed to vaporize 1 kg of the substance. Heat Required to Vaporize a Liquid Q = mH v

11. Changes of State As you continue to add energy, the temperature of the water vapor increases. The slope of the line after E is the specific heat of water vapor.

12. Changes of State EVALUATE THE ANSWER Energy is measured in joules, so the units are correct. Use with Example Problem 3. Problem 1.0 kg of water is warmed from room temperature (25.0°C) to boiling, then half of its initial volume is vaporized. How many joules of thermal energy must the stove provide to do this? Response SKETCH AND ANALYZE THE PROBLEM Sketch the situation. List the knowns and unknowns. KNOWN UNKNOWN m = 1.0 kg m vapor = ½ m Q = ? T i = 25.0°C T f = 100.0°C C water = 2.26×10 6 J/(kg · °C) H v, water = 2.26×10 6 J/kg SOLVE FOR THE UNKNOWN The total energy transfer is the sum of the energy needed to heat the water and the energy needed to boil half the water. Initial 1.0 kg water at 25.0 ° C Final 0.5 kg water at 100.0 ° C 0.5 kg vapor at 100.0 ° C

13. The First Law of Thermodynamics The first law of thermodynamics states that the change in thermal energy (Δ U ) of an object is equal to the heat ( Q ) that is added to the object minus the work ( W ) done by the object. First Law of Thermodynamics Δ U = Q − W Note that Δ U , Q , and W are all measured in joules, the unit of energy. The first law of thermodynamics is merely a restatement of the law of conservation of energy, which states that energy is neither created nor destroyed but can be changed into other forms.

14. The First Law of Thermodynamics A device that is able to continuously convert thermal energy to mechanical energy is called a heat engine.

15. The First Law of Thermodynamics Internal combustion engines are one type of heat engine.

16. The First Law of Thermodynamics When the engine is working continuously, the internal energy of the engine does not change, or Δ U = 0 = Q – W . The work done by the engine is W = Q H – Q L . The energy ( Q L ) transferred out of the engine is called waste heat, which is heat that has not been converted into work. Because there is always waste heat, even the most efficient engines fall short of 100 percent efficiency.

17. The First Law of Thermodynamics It is possible to remove thermal energy from a colder object and add it to a warmer object if work is done. A refrigerator is a common example of a device that accomplishes this transfer.

18. The First Law of Thermodynamics Electric energy runs a motor that does work on a gas and compresses it. The gas draws thermal energy from the interior of the refrigerator, passes through the condenser coils on the outside of the refrigerator, and cools into a liquid. Thermal energy is transferred into the air in the room. The liquid is vaporized in the expansion valve and the process repeats.

19. The First Law of Thermodynamics The overall change in the thermal energy of the gas is zero. Thus, according to the first law of thermo- dynamics, the sum of the heat removed from the refrigerator’s contents and the work done by the motor is equal to the heat expelled, as shown in the figure.

20. The First Law of Thermodynamics A heat pump is a refrigerator that can be run in two directions. In the summer, the pump removes heat from a house and thus cools the house. In the winter, heat is removed from the cold outside air and transferred into the warmer house. In both cases, mechanical energy is required to transfer heat from a cold object to a warmer one.

21. Second Law of Thermodynamics We take for granted that many daily events occur spontaneously only in one direction. For example, you are not surprised when a metal spoon, heated at one end, soon becomes uniformly hot. Consider your reaction, however, if a spoon lying on a table suddenly, on its own, became red hot at one end and icy cold at the other. This reverse process would not violate the first law of thermodynamics—the thermal energy of the spoon would remain the same.

22. Second Law of Thermodynamics Many processes that are consistent with the first law of thermodynamics have never been observed to occur spontaneously, because of the second law of thermodynamics. The second law of thermodynamics states that whenever there is an opportunity for energy dispersal, the energy always spreads out.

23. Second Law of Thermodynamics In other words, when a hot object is placed in contact with cooler surroundings, the thermal energy in the hot object has the opportunity to disperse, or spread out more. Some of the thermal energy moves into the cold object, warming it and therefore cooling the originally hotter object.

24. Second Law of Thermodynamics The measure of this dispersal of energy is known as entropy (S). Entropy is a property of a system. Another way of stating the second law of thermodynamics is that natural processes go in a direction that maintains or increases the total entropy of the universe.

25. Second Law of Thermodynamics If thermal energy is added to a system, the entropy increases. If thermal energy is removed from a system, its entropy decreases. If a system does work on its surroundings without any transfer of thermal energy, the entropy does not change.

26. Second Law of Thermodynamics For a reversible process at a constant temperature, the change in entropy (ΔS) is expressed by the following equation, in which entropy has units of J/K and the temperature is measured in kelvins. Change in Entropy Entropy is often used as a measure of the unavailability of useful energy.

27. Quiz specific heat of water D heat of fusion C melting point B specific heat of ice A What is the slope of the line between A and B? 1. CORRECT

28. Quiz What is the amount of thermal energy needed to melt 1 kg of a substance called? 2. heat of freezing D heat of fusion C heat of melting B heat of vaporization A CORRECT

29. Quiz reservoir heat D waste heat C efficient heat B thermal heat A What is the energy from an automobile engine that is released into the environment called? 3. CORRECT

30. Quiz stable D unstable C unusable B useable A The second law of thermodynamics states that in an uncontrolled system, whenever there is an opportunity for energy dispersal, the energy always spreads out toward a more ________ state. 4. CORRECT

31. B A Quiz 1/ J  K D J/K J  K K/J What are the units for entropy? 5. CORRECT C