CK-12 Physics Concepts - Intermediate Answer Key Chapter 10: Thermal Energy 10.1 Heat, Temperature, and Thermal Energy Transfer Practice Questions 1. Which material was a better conductor of heat? 2. Explain why metals feel cold even when they are at room temperature? Answers 1. The aluminum melted the ice cube faster because it was able to conduct the heat better. 2. The heat from your finger is able to be dissipated throughout the metal so more heat leaves your body. When you touch styrofoam, the heat remains on the surface and it feels warmer. Review Questions 1. 2. 3. 4. Convert 4.22 K to °C. Convert 37°C to K. If you had beeswax attached to one end of a metal skewer and you placed the other end of the skewer in a flame, what would happen after a few minutes? Which contains more heat, a coffee cup of boiling water or a bathtub of room temperature water? Answers 1. 273.15 - 4.22 = 268.93 °C 2. 37 + 273.15 = 310.15 K 3. After a few minutes, the beeswax would begin to melt because of the heat transfer along the metal skewer. 4. The bathtub has more heat, but a lower temperature. 10.2 Specific Heat Practice 1 Added an interactive link Review Questions 1. How much heat is absorbed by 60.0 g of copper when it is heated from 20.0°C to 80.0°C? 2. A 40.0 kg block of lead is heated from -25°C to 200.°C. How much heat is absorbed by the lead block? 3. The cooling system of an automobile motor contains 20.0 kg of water. What is the of the water if the engine operates until 836,000 J of heat have been added to the water? Answers 𝐽 1. Using 𝑄 = 𝑚𝑐∆𝑡 ∶ (0.06𝑘𝑔) (385 𝑘𝑔 𝐾) (60°) = 1,386 J 𝐽 2. Using 𝑄 = 𝑚𝑐∆𝑡 ∶ (40𝑘𝑔) (130 𝑘𝑔 𝐾) (225°) = 1,170,000 J 𝑄 3. Using ∆𝑡 = 𝑚𝑐 ∶ 836,000𝐽 𝐽 𝐾) 𝑘𝑔 (20.0𝑘)(4180 ∶ 10.3 Calorimetry Practice Questions 1. What is the number 4.18 J/gºC in the video? 2. In the equation, q = mc∆t, what does c represent? 3. What does it mean if the temperature in the calorimeter goes down? Answers 1. The number is water's specific heat capacity. 2. C represents the material's specific heat capacity. 3. Heat was absorbed and the reaction is endothermic. Review Questions 1. A 300.0 g sample of water at 80.0ºC is mixed with 300.0 g of water at 10.0ºC. Assuming no heat loss to the surroundings, what is the final temperature of the mixture? 2 2. 3. A 400.0 g sample of methanol at 16.0ºC is mixed with 400.0 g of water at 85.0ºC. Assuming no heat loss to the surroundings, what is the final temperature of the mixture? The specific heat of methanol is 2450 J/kg•ºC. A 100.0 g brass block at 100.0ºC is placed in 200.0 g of water at 20.0ºC. The specific heat of brass is 376 J/kg•ºC. Assuming no heat loss to the surroundings, what is the final temperature of the mixture? Answers 1. 2. 3. The mixture contains equal volumes of the same material (water), so the temperature will be the arithmetic mean of the two: 45 C. Using 𝑚𝑚 𝑐𝑚 (𝑡2 − 𝑡1 )𝑚 = 𝑚𝑤 𝑐𝑤 (𝑡2 − 𝑡1 )𝑤 ∶ 𝐽 𝐽 . 4𝑘𝑔 (2450 𝑘𝑔 𝐶) (16 − 𝑥) = .4𝑘𝑔 (4180 𝑘𝑔 𝐶) (𝑥 − 85) = 59.5 C. Using 𝑚𝑏 𝑐𝑏 (𝑡2 − 𝑡1 )𝑏 = 𝑚𝑤 𝑐𝑤 (𝑡2 − 𝑡1 )𝑤 ∶ 𝐽 𝐽 . 1𝑘𝑔 (376 𝑘𝑔 𝐶) (100 − 𝑥) = .2𝑘𝑔 (4180 𝑘𝑔 𝐶) (𝑥 − 20) = 23.4 C. 10.4 Change of State Practice Questions 1. For water, which takes more energy, melting or evaporating? 2. When are there two phases present at the same time in the pot? Answers 1. Evaporation has a higher energy required. 2. At 0ºC and 100ºC, the water is in transition and two phases are present. Review Questions 1. A 200. g sample of water at 60.0°C is heated to water vapor at 140.0°C. How much heat was absorbed? 2. A 175 g lump of molten lead at its melting point (327°C) is placed into 55.0 g of water at 20.0°C. The specific heat of lead is 130 J/kg °C and the Hf of lead is 20,400 J/kg. a. When the lead has become solid but is still at the melting point, what is the temperature of the water? b. When the lead and the water have reached equilibrium, what is the temperature of the mixture? 3 Answers 𝐽 𝐽 1. Using (. 2𝑘𝑔 (4187 𝑘𝑔 𝐶) (100 − 60)) + (. 2𝑘𝑔 (2260000 𝑘𝑔)) + 𝐽 (. 2𝑘𝑔 (1996 𝑘𝑔 𝐶) (140 − 100)) = 5.01 x 105 J 2. 𝑄 a. Using 𝑚𝑐 = ∆𝑡 ∶ 0.175𝑘𝑔∗20,400 𝐽 𝑘𝑔 𝐽 𝐶) 𝑘𝑔 .055𝑘𝑔(4187 = ∆𝑡 = 15.5 ∶ 20.0 + 15.5 = The water is 35.5 °C. b. Using 𝑚𝑙 𝑐𝑙 (𝑡2 − 𝑡1 )𝑙 = 𝑚𝑤 𝑐𝑤 (𝑡2 − 𝑡1 )𝑤 ∶ 𝐽 𝐽 .175𝑘𝑔 (130 𝑘𝑔 𝐶) (327 − 𝑥) = .055𝑘𝑔 (4180 𝑘𝑔 𝐶) (𝑥 − 35.5) At equilibrium, the temperature is 61.8 °C. 4