Reminder!

1. Reminder! • Labs due today (place in red box) • Section 7.1 Quiz tomorrow! • Write any questions you have on scrap paper at the front Brainteaser How many words in the English language end in “-gry”?

2. 7.2 Changes of State December 7, 2010

3. What do you think? • Is the transfer of thermal energy constant? • When does a change in state occur? • What is latent heat? • What happens to the temperature during a phase change?

4. Heating and Cooling Curves • There are two methods of heating and cooling: • Constant Power • This method increases (or decreases) the temperature of an object at a constant rate

5. Heating and Cooling Curves • Fixed Temperature • This method raises (or lowers) the temperature of an object quickly at first, then slows down until it reaches the temperature of the surrounding area

6. Energy and States of Matter • Physical states, or phases, are determined by the organization of particles in matter. • Solids • Liquids • Gasses • During a phase change, particles get enough energy, they can break the bonds between nearby particles • When this happens, the potential energy increases, but average kinetic energy remains constant

7. http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/lat_heat3.htmlhttp://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/lat_heat3.html

8. Heating (and Cooling) at a Constant Rate • Heating a solid at a constant rate results in the following: http://resources.yesican-science.ca/lpdd/g07/lp/nelson/nel10.html

9. How Much Energy Does it Need? • Latent Heat: The amount of thermal energy needed for a phase change • Specific Latent Heat of Fusion: The amount of thermal energy needed to melt 1 kg of a substance at its melting point • Specific Latent Heat of Vaporization: The amount of thermal energy needed to vaporize 1kg of a substance at its boiling point

10. New Equations! • Specific Latent Heat of Fusion Where: LF = Specific Latent Heat of Fusion (kJ/kg) QF = Thermal Energy transferred (kJ) m = mass of substance (kg) • Specific Latent Heat of Vaporization Where: LV = Specific Latent Heat of Vaporization (kJ/kg) QV = Thermal Energy transferred (kJ) m = mass of substance (kg)

11. Let’s Do an Example!Specific Latent Heat of Fusion Practice Problems #2. pg. 224 The specific latent heat of fusion of silver is 88.0 kJ/kg. How much energy is released when 6.20 μg of silver solidify? (1 μg = 1 x 10-6g) Given: LF = 88.0 kJ/kg m = 6.20 x 10-6 g = 6.20 x 10-9 kg R.T.F.: QF = ? Solution: LF = QF m • QF = LFm • = (88.0 kJ/kg)(6.20 x 10-9kg) • = 5.46 x 10-7 kJ • = 5.46 x 10-4 J

12. Let’s Do Another Example!Specific Latent Heat of Vaporization Practice Problems #2. pg. 225 The boiling point of Freon-12 is -29.8°C. How much thermal energy is absorbed by 455 g of Freon-12 going from the liquid phase at -29.8°C to the gaseous phase at 20.02°C? The specific latent heat of vaporization of Freon-12 is 143 kJ/kg and its specific heat capacity is 950 J/(kg•K). Given: m = 455 g = 0.455 kg Ti = -29.8°C Tf = 20.02°C ΔT = 20.02°C – (-29.8°C) = 49.82°C or 49.82 K LV = 143 kJ/kg c = 950 J/(kg•K) • R.T.F.: • QV = ? • Q = ? • Solution: • QV = Lvm • = (143 kJ/kg)(0.455 kg) • = 65.1 kJ Q = mc ΔT = (0.455 kg)(950 J/kg•K)(49.82 K) = 21534.7 J = 21.5 kJ QTOT = QV + Q = 65.1 kJ + 21.5 kJ = 86.6 kJ