Heat and the First Law of Thermodynamics PDF

Heat and the first law of thermodynamics A thunderstorm is driven by the violent collision of warm air with cooler air. 1  Outline...

Heat and the first law of thermodynamics A thunderstorm is driven by the violent collision of warm air with cooler air. 1  Outline Chapter SPCIFIC DEFINITION 18.1 18.6 (18) HEAT OF OF HEAT SOLIDS AND FLUIDS 18.7 LATENT HEAT AND PHASE TRANSITIONS 2  First Law of Thermodynamics 1 This chapter examines the nature of heat. 2 Heat is a form of energy that is transferred into or out of a system. 3 Heat is governed by a more general form of the law of conservation of energy, known as the First Law of Thermodynamics. 4 Heat is essential to life processes; no life could exist on earth without heat from the sun or from the earth’s interior. 3  Definition of Heat What is heat ?  Heat is one of the most common forms of energy in the universe, and we all experience it every day.  Heat is energy in transfer: to or from an object.  It’s symbol is Q and it is measured by Joules = J.  Heat is the reason substances change from: Solids liquids gases  Heat always flows from high temperatures to low temperatures.  Definition of Heat A cup of water at 20 °C (temperature of sample Ts) is placed in a room on a hot day 30°C (temperature of environment Te ). The cup gains heat energy (Q) and heats up. Its temperature increases until it reaches the temperature of the air in the room. If thermal energy is transferred into a system, then Q > 0. A cup of water at 20 °C (temperature of sample Ts) is placed in a room on a cold day 10°C (temperature of environment Te ). The cup loses heat energy (Q) and cools down. Its temperature decrease until it reaches the temperature of the air in the room. If thermal energy is transferred from a system, then Q < 0  Definition of Heat A cup of water at 20 °C (temperature of sample Ts) is placed in a room at 20°C (temperature of environment Te ). The cup neither gains not loses heat energy (Q). Its temperature remains 20 °C. If the system and its environment have the same temperature, then Q = 0. If Ts ≠ Te , then the temperature of the system changes until it is equal to the temperature of the environment (thermal equilibrium). At thermal equilibrium, the water, the cup, and the air in the room are all at the same temperature.  Definition of Heat 1 What does a positive value for Q mean? 2 When does heat transfer between a system and its environment? 3 What is the meaning of thermal equilibrium?  Specific Heat of Solids and Fluids Heat capacity (C):- Is the amount of heat energy required to increase the temperature of a substance 1°C. Heat (J) Heat capacity change in temperature (K) Or (°C) The SI unit of heat capacity is: (J/K) or (J/ °C ) Notice: the abbreviation of heat capacity is capital letter C Specific Heat of Solids and Fluids  Specific Heat of Solids and Fluids The specific heats of various materials are given in the table.  Extra Exercise Q1: You have 2.00 kg of water at a temperature of 20.0 °C. How much energy is required to raise the temperature of that water to 95.0 °C? 11  Extra Exercise Q1: You have 2.00 kg of water at a temperature of 20.0 °C. How much energy is required to raise the temperature of that water to 95.0 °C? Ans: The energy required to warm 2.00 kg of water from 20.0 °C to 95.0 °C is: There is no need to convert °C to K cwater From table because the difference in temperature of both scales are the same. 𝛥𝛥TC = 𝛥𝛥TK 12  Extra Exercise Q2: A certain amount of heat will warm 1gm of material A by 3 ˚C and 1gm of material B by 4˚C. Find the ratio between their specific heats? 3CA=4CB CA/CB=4/3 13  Extra Exercise Q2: A certain amount of heat will warm 1gm of material A by 3 ˚C and 1gm of material B by 4˚C. Find the ratio between their specific heats? 14 Concept Check 18.1 Suppose you raise the temperature of: copper block 1 from –10 °C to +10 °C, copper block 2 from +20 °C to +40 °C, and copper block 3 from +90 °C to +110 °C. Assuming that the blocks have the same mass, to which one did you add the most heat? A. block 1 B. block 2 C. block 3 D. All of the blocks received the same amount of heat. Chapter 1 15 Concept Check 18.1 Suppose you raise the temperature of: copper block 1 from –10 °C to +10 °C, copper block 2 from +20 °C to +40 °C, and copper block 3 from +90 °C to +110 °C. Assuming that the blocks have the same mass, to which one did you add the most heat? A. block 1 B. block 2 C. block 3 D. All of the blocks received the same amount of heat. Ans: D (as the blocks are all the same material with the same rise in temperature). Chapter 1 16  Extra Exercise Q3: 5.00 kg of some liquid at 10.0°C is mixed with 1.00 kg of the same liquid at 40.0°C. What is the final equilibrium temperature? Ignore any heat flow between the containers and/or surroundings. A. 12.0°C B. 15.0°C C. 18.0°C D. 25.0°C 17 Extra Exercise 18 Sample Problem 18.1 Water and Lead p 242 PROBLEM: A metalsmith pours 3.00 kg of lead shot at a temperature of 94.7 °C into 1.00 kg of water at 27.5 °C in an insulated container. What is the final temperature of the mixture? 19  Sample Problem 18.1 Water and Lead p 242 SOLUTION: The sum of the heat lost by the lead shot and the heat gained by the water is zero, because the process took place in an insulated container and because the total energy is conserved. 20  Latent Heat and Phase Transition The three common states of matter are solid, liquid, and gas. If enough heat is added to a solid, it melts into a liquid. If enough heat is added to a liquid, it vaporizes into a gas. These are phase changes, or phase transitions.  0°C -1°C -3°C After some time After some time 0°C Why is the temperature is not changing while we continue giving heat? 0°C Where did the heat that was given to the ice go? Where is the heat getting hidden? What will happen if we continue heating? After some time After some time 22  100°C 60°C 22°C After some time After some time 100°C 100°C Why is the temperature not changing while we continue giving heat?? Where does the heat that was given to the water go? Where is the heat getting hidden? What will happen if we continue heating? After some time After some time 23  Latent Heat and Phase Transition 100 °C Latent heat (L):- It is the process through which a substance melts or evaporates. During this process some heat is 100 °C released or absorbed in the form 0 °C of thermal energy while the temperature remains constant. 0 °C  Latent Heat and Phase Transition Latent Heat Latent heat of fusion, Lfusion Latent heat of vaporization, Lvaporization A change of phase of a A change of phase of a substance substance from solid to liquid from liquid to gas at same at the same temperature. temperature. The SI unit of latent heat is (J/Kg). The latent heat of fusion for a given substance is different from the latent heat of vaporization for that substance. Sample Problem 18.5 Ice to Water and Water to Steam p 245 How much heat, Q, is required to convert 0.500 kg of ice at a temperature of –30 °C to steam at 140 °C? Q1 Q2 Q3 Q4 Q5 Sp. H Lat. H Sp. H Lat. H Sp. H Ice Ice water water steam steam -30 °C 0°C 0°C 100°C 100°C 140°C 26 Sample Problem 18.5 Ice to Water and Water to Steam p 245 How much heat, Q, is required to convert 0.500 kg of ice at a temperature of –30 °C to steam at 140 °C? SOLUTION: Q1 Q2 Q3 Q4 Q5 Sp. H Lat. H Sp. H Lat. H Sp. H Ice Ice water water steam steam -30 °C 0°C 0°C 100°C 100°C 140°C We solve this problem in steps, with each step corresponding to either a rise in temperature or a phase change. 1. We first calculate how much heat is required to raise the temperature of the ice from –30 °C to 0 °C. 2. The specific heat of ice is 2.06 kJ/(kg K), so the heat required is: 27 Sample Problem 18.5 Ice to Water and Water to Steam p 245 3. We continue to add heat to the ice until it melts. 4. The temperature remains at 0 °C until all the ice is melted. 5. The latent heat of fusion for ice is 334 kJ/kg, so the heat required to melt all of the ice is: 6. Once all the ice has melted to water, we continue to add heat until the water reaches the boiling point, at 100 °C. 7. The required heat for this step is: 8. We continue to add heat to the water until it vaporizes. The heat required for vaporization at 100 °C is: 28 Sample Problem 18.5 Ice to Water and Water to Steam p 245 9. We now warm the steam and raise its temperature from 100 °C to 140 °C. 10. The heat necessary for this step is: 11. The total heat required is: 12. Note that almost three quarters of the total heat in this entire process of warming the sample is spent converting the liquid water to steam in the process of vaporization. We can look at a plot of the warming process. 29 Sample Problem 18.5 Ice to Water and Water to Steam p 245 30  Equation summary (1) Heat capacity (2) Specific heat Q=CΔT Q=mcΔT (3) Latent heat Q=mL 31 The END OF CHAPTER (18) 32