Unit 8 Heat - Study Guide

Name: _________________________________ Unit 8: Heat - Study Guide Date: ____________ Unit 8: Heat 7.1 Define temperature and heat and describe how heat is transferred. Describe the difference betw een an insulator and a conductor and be able to provide examples of both. 7.2 Identify if energy is absorbed or released during a phase change and state what the temperature of the substance is doing during this time. State what phase change is occurring. 7.3 Define thermal equilibrium and explain in detail what happens when two substances of different temperatures are in contact with one another. Predict and calculate the final temperature when two samples of the same substance are mixed. 7.4 Conceptually explain the relationship between heat, mass, specific heat capacity, and change in temperature. Use the heat equation to solve for unknowns. Use the heat equation to calculate an unknown in a system where two samples of different substances are mixed. Vocabulary Terms From Unit 8 Temperature Thermal Energy Heat specific heat capacity convection conduction radiation Heat equation Mixing Equation(s) Lab Challenges: 1) Fill one flask with 250 mL of HOT water and fill one flask with 250 mL of COLD water. Place the flasks next to each other and place 1 drop of food coloring into each flask. ⮚ Describe what you see and explain this phenomenon using your physics knowledge. You can includ a diagram to assist your explanation. ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ 2) Take beaker of hot water and a beaker of cold water. Record their initial temperatures and plot them on the graph. Every minute, check back and record the temperatures of each. a. Plot these points on a graph. (You may and should do other questions/challenges while you are doing this one) b. Draw a horizontal line that represents a constant room temperature (21 ºC). Describe what you notice in the trends of the temperatures of the two beakers of water. 100 90 80 70 Tem 60 perat 50 ure 40 (ºC) 30 20 10 0 0 1 2 3 4 5 6 7 8 Time (min) 9 10 11 12 13 14 15 c. If we were to continue taking data for another 2 hours, explain what you think would be the trend with the two temperatures and why. __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ Practice Questions: 3) In terms of the molecules or particles of a substance: ● Explain what is happening when the temperature rises. __________________________________________________________________________________________ __________________________________________________________________________________________ ● Explain what is happening when the temperature decreases. __________________________________________________________________________________________ __________________________________________________________________________________________ 4) “Average kinetic energy of the molecules” in a substance is really just referring to the ______________________________ of the substance. 5) If you have two beakers of water with different amounts of water (different masses). After 5 minutes of heating on the same heat source, Beaker A is at a higher temperature than Beaker B. Compare the masses of water in the two beakers and explain your reasoning. 6) Two different substances of the same amount of mass are heated over the same heat source. Substance Q records a change in temperature of +50 ºC and Substance S records a change in temp of +12ºC. Compare and explain what must be true about the two substances. 7) Silver has a specific heat capacity that is almost twice as much as gold’s. Equal masses of silver and gold are placed on the same heat source. Compare the change in temperatures of the wires and explain. 8) On the diagram to the right: ● Which material would have the largest change in temperature if 1000 J of heat was added to it? ● Which material would have the smallest change in temperature of 1000 J of heat was added to it? 9) As a substance is heated, check ☑all of the possible changes that will likely occur (assuming there is no phase change) ● Kinetic energy of the molecules increases ● Temperature of the substance will decrease ● Kinetic energy of the molecules decreases ● Average molecular speed of the substance will increase ● Specific heat of the substance increases ● Specific heat of the substance decreases ● Average molecular speed of the substance will decrease. ● Specific heat of the substance increases ● Mass of the substance will increase ● Temperature of the substance will increase ● Mass of the substance will decrease 10) Four metal spoons (seen below) are at room temperature (~20ºC). They are each placed into separate but equal cups of hot water (85 ºC). 800 g 600 g 400 g 200 g ● What is it called when the spoon and the water reach the same temperature? Define this and explain in detail what this means for heat transfer. ● Which spoon will have the greatest change in temperature once it reaches this point? _______ ● Which spoon will have the least change in temperature once it reaches this point? ________ 11) A cup of hot soup (90 ºC) is place in a large room where the temperature of the air is 20 ºC. After a few hours the soup and the room are in thermal equilibrium. Explain the transfer of thermal energy and the final temperatures of the room and the hot soup. 12) The boiling temperature of steel is 1510 ºC. The specific heat of steel is 0.12 J/gºC. How much energy is required to bring a 1200 g from room temperature (21 ºC) to its boiling point? 13) It takes 12,040 J to heat a 1,300 g stack of newspapers from 30 ºC to 35 ºC. What is the specific heat of the newspapers? 14) A 400 g block gold (c = 0.129 J/gºC) is at 15 ºC. If 5,490 J of thermal energy is added to the block of gold, what is its new temperature. 15) Explain the 3 different forms of heat transfer and provide 3 examples of each (total of 9 examples). Heat Transfer type Definition 3 Examples 16) During an investigation, a student used two insulated containers, container 1 and container 2. The student added equal amounts of water to the containers: 95ºC water to container 1 and 5ºC water to container 2. The student closed the containers and placed a thermometer in each. The student then placed one end of a U-shaped piece of metal into the water in container 1 and the other end into the water in container 2, as shown (the metal allows heat to transfer between the two containers). a) The average molecular kinetic energy of the substance in Container 1 is _______________(higher, lower, the same as) than the average molecular kinetic energy of the substance in Container 2. b) Heat will flow from Container ________ to Container ________. c) Calculate the temperature of the water in the containers when they reach equilibrium. 17) Equal masses of Substance X and Substance Y are mixed together in an insulator cup. a. The amount of heat lost by Substance X is ____________ (larger than, smaller than, or equal to) the amount of heat gained by Substance Y. b. Which substance has a greater specific heat capacity? ________________ Explain your reasoning. Substance X Substance Y c. What are two possible things that you could change about the lab set-up to result in a different equilibrium temperature? Change 1: Change 2: 18) A Temperature vs Time graph is shown for an unknown substance that has heat being added at a constant rate. Temperature vs Time for an Unknown Substance 70 60 Temperature 50 40 30 20 10 0 Time Range 0 2 4 6 8 1 Time 1 Phase(s) Present During the Time Period circle your choice(s) 0 - 2 min solid liquid gas 2 - 6 min solid liquid gas 6 - 10 min solid liquid gas 10 - 20 min solid liquid gas 20 - 24 min solid liquid gas Melting: The substance is melting from _____ min to _____ min. 1 1 1 2 2 2 Heat is being… (check your choice) added removed neither The melting point is ________°C. Boiling: The substance is boiling from _____ min to _____ min. The boiling point is ________°C. Extra Pracitce 19) A student heats a 300 g sample of aluminum from 20°C to 220°C. The specific heat of aluminum is 0.9 J/g°C. Calculate the thermal energy absorbed by the aluminum. 20) 43,200 J of energy are added to 600 g of oil. The specific heat of oil is 1.9 J/g°C. a. How much does the oil’s temperature change? b. If the oil started at 5°C, what is its final temperature? 21) ● ● ● ● Which of the following substances has the lowest average molecular kinetic energy? a. nitrogen at 75°C b. quartz at 25°C c. steel at 50°C d. water at 10°C 22) A friend says that a cup of water and a large lake have the same amount of thermal energy. You say that they both have the same temperature. Who is correct? Could you both be correct? Define “thermal energy” and “temperature” in your answer. Explain your answer. 23) A student touches a metal sink with her finger and notices that her finger feels cold. a. Which object was warmer to start: her finger or the sink? Explain your answer. b. A student then touches a wooden spoon near the sink. The spoon and sink have been in the same room all day long and the temperature of each is shown to be exactly the same. Explain why the student feels the metal sink as “colder” than the wooden spoon. 24) A student is trying to determine the specific heat of a metal sample. The student heats the sample and then places it in an insulated container with 300 g of 20°C water. The student knows that the specific heat of water is 4.19 J/g°C. a. Select three additional measurements the student must make to determine the specific heat of the metal sample. ● ● ● ● the mass of the sample the volume of the water the volume of the sample the initial temperature of the sample before it is placed in the insulated container ● the amount of time it takes the sample and the water to reach thermal equilibrium ● the final temperature of the water after the sample and the water reach thermal equilibrium b. Given the following information, calculate specific heat capacity of the mystery metal. Measured quantities: Space for calculations: mmetal = 100 g Vwater = 300 mL Vmetal = 37 cm3 Ti, metal = 100 ºC time until thermal equilibrium = 3.8 minutes Tfinal = 25.34 ºC 25) You have 1 kg of concrete (cconcrete = 880 J/kg°C) and 1 kg of silver (csilver = 235 J/kg°C) in front of you. You place each on a burner. Which one heats up more quickly? Explain your answer in terms of each material’s specific heat. 26) A student heats a 200 g sample of water from 20°C to 80°C. The specific heat of water is 4.18 J/g • °C. i. Calculate the thermal energy absorbed by the water. Show your calculations and include units in your answer. The student then boils the water. ii. Describe what happens to the temperature of the water as it boils. Explain your answer. Answer Key Heat equation Q = mc∆T Mixing Equation(s) m1c1∆T1 = Q = – m2c2∆T2 (m1c1+m2c2)•Tf = m1c1T1i + m2c2T2i 1) The hot water mixes the dye around more quickly. The molecules in the hot water have a higher average kinetic energy (they are vibrating/moving around more), so they are able to mix the dye around faster. 2) c. Eventually both cups of water would reach 21ºC (room temp) and then remain at that temperature. 3) a. average KE increases, molecules move faster on average. b. average KE decreases, molecules move slower on average. 4) Temperature 5) Beaker A has less mass because it had a higher change in temperature ∆T ~ 1/m 6) Substance Q has a LOWER specific heat capacity than Substance R because given the same amoung of heat (and same mass), Q had a larger change in temperature. ∆T ~ 1/c 7) The change in temperature of gold should be about twice as much as the ∆T for silver. ∆T ~ 1/c 8) a. largest ∆T would happen to the substance with the lowest specific heat capacity (Diamond) b. smallest ∆T would happen to the substance witht the largest specific heat capacity (Helium) 9) Avg KE of moleucles increases (on average), Temp increases, Avg molecular speed increases 10) Thermal Equilibrium, when two objects in thermal contact reach the same temperature. At this point, no heat is transferred between objects (heat flows from the hot to cold, and since neither is at a higher temperature, heat doesn’t flow) a. greatest ∆T is smallest spoon b. least ∆T is largest spoon. 11) Thermal Energy from the soup is released into the room. The soup will decrease in temperature until it reaches the temperature of the room (The air’s temp may increase slightly, but since there is LOTS more air probably, its temperature increase may be negligible). You can expect both to be at approximately room temperature after a long while (maybe a hair above if its totally contained environment). 12) 214,416 J of energy 13) 1.852 J/gºC 14) 121.4 ºC 15) Conduction - Heat transfer via physical contact (or through a material) Convection - heart transfer via circulating substance within a liquid or gas due to hot fluid rising due to its lower density, and colder fluid falling due to its higher density. This mixing causes heat the be transferred. Radiation - heat transfer via electromagnetic radiation/waves (EM waves) 16) a. Higher b. 1 to 2 c. Since they have same c, and same m, Tfinal will just be average of temperatures (50ºC) 17) a. equal (conservation of energy) b. Substance X (it’s changing temperature more slowly given the same amount of heat being transferred) c. Change the amount of one of the substances (ie, more Subtance X would mean a higher final temperature when they reach Thermal Equilibrium). Change the initial temperature of one of the substances (ie, start Substance X at a lower temp and then the final temp would be lower.) 18) Heat is being… Phase(s) Present During the Time Time Range (check your choice) Period circle your choice(s) added removed neither 0 - 2 min solid liquid gas x 2 - 6 min solid liquid gas x 6 - 10 min solid liquid gas x 10 - 20 min solid liquid gas x 20 - 24 min solid liquid gas x Melting: The substance is melting from 2-6 min. The melting point is 100 °C. Boiling: The substance is boiling from 10-20 min The boiling point is 550ºC. 19) 54,000 J 20) a. +37.9 ºC b. 42.9ºC 21) lowest temp = water 22) The lake probably has more thermal energy (the amount of molecules factors into the total amount of thermal energy, lake has WAY more molecules). If the temperatures are the same, then the two would have the same AVERAGE kinetic energy, but the lake would have way more total Thermal Energy. If the cup of water was hotter (at a higher temp), it would have an higher AVERAGE kinetic energy of the molecules but the lake will likely still have more thermal energy because of the big difference in the amount of molecules in the lake. Thermal energy (total amount of kinetic energy of all moleucels added together) Temperature (related to AVERAGE kinetic energy of the molecules) 23) a. “feeling cold” means your fingers are losing thermal energy, so your fingers must bet a higher temperature because heat flows from hotter objects to colder ones. b. The wooden spook transfers thermal energy more slowly (its an thermal insulator), so you don’t lose energy as quickly so it doesn’t feel as cold. The sink takes your energy must faster (its a thermal conductor), and thus you loose heat faster and it feels colder. 24) mass of sample, initial temp of sample, final temp of water Q gained by water = 6712.38 J = Q lost by metal 0.899 J/gºC (which is close to specific heat capacity for aluminum at 0.902 J/gºC) 25) Silver will heat up more quickly because it has a lower specific heat capacity. 26) i. 50,160 J ii. As water boils (going through a phase change) its temperature will be constant until it all completely changes into gas.