Chapter 10 Energy Chapter 10 Table of Contents
Chapter 10 Energy
Chapter 10 Table of Contents 10. 1 10. 2 10. 3 10. 4 10. 5 10. 6 10. 7 10. 8 10. 9 10. 10 The Nature of Energy Temperature and Heat Exothermic and Endothermic Processes Thermodynamics Measuring Energy Changes Thermochemistry (Enthalpy) Hess’s Law Quality Versus Quantity of Energy and Our World Energy as a Driving Force Copyright © Cengage Learning. All rights reserved 2
Section 10. 1 The Nature of Energy • • Ability to do work or produce heat. That which is needed to oppose natural attractions. • Law of conservation of energy – energy can be converted from one form to another but can be neither created nor destroyed. § The total energy content of the universe is constant. Return to TOC Copyright © Cengage Learning. All rights reserved 3
Section 10. 1 The Nature of Energy • • Potential energy – energy due to position or composition. Kinetic energy – energy due to motion of the object and depends on the mass of the object and its velocity. Return to TOC Copyright © Cengage Learning. All rights reserved 4
Section 10. 1 The Nature of Energy Transformations Types of Energy 1. Kinetic energy 2. Potential energy 3. Chemical energy 4. Heat energy 5. Electric energy 6. Radiant energy Return to TOC Copyright © Cengage Learning. All rights reserved 5
Section 10. 1 The Nature of Energy Initial Position • In the initial position, ball A has a higher potential energy than ball B. Return to TOC Copyright © Cengage Learning. All rights reserved 6
Section 10. 1 The Nature of Energy Final Position • After A has rolled down the hill, the potential energy lost by A has been converted to random motions of the components of the hill (frictional heating) and to the increase in the potential energy of B. Return to TOC Copyright © Cengage Learning. All rights reserved 7
Section 10. 1 The Nature of Energy • Heat involves the transfer of energy between two objects due to a temperature difference. • Work – force acting over a distance. • Energy is a state function; work and heat are not: § State Function – property that does not depend in any way on the system’s past or future (only depends on present state). § Changes independently of its pathway Return to TOC Copyright © Cengage Learning. All rights reserved 8
Section 10. 1 The Nature of Energy State Functions? • Distance traveled from one location to another. § Not a state function – depends on route taken • Change in elevation § State function – change in elevation is always the same (doesn’t depend on the route taken) Return to TOC Copyright © Cengage Learning. All rights reserved 9
Section 10. 1 The Nature of Energy Temperature = average kinetic energy. Heat= total energy. Both of the above are at 70 o. F. Which has the most heat? Return to TOC Copyright © Cengage Learning. All rights reserved 10
Section 10. 2 Temperature and Heat Temperature • A measure of the random motions of the components of a substance. Return to TOC Copyright © Cengage Learning. All rights reserved 11
Section 10. 2 Temperature and Heat • A flow of energy between two objects due to a temperature difference between the objects. § Heat is the way in which thermal energy is transferred from a hot object to a colder object. Return to TOC Copyright © Cengage Learning. All rights reserved 12
Section 10. 3 Exothermic and Endothermic Processes • • System – part of the universe on which we wish to focus attention. Surroundings – include everything else in the universe. Return to TOC Copyright © Cengage Learning. All rights reserved 13
Section 10. 3 Exothermic and Endothermic Processes Energy Changes Accompanying the Burning of a Match Return to TOC Copyright © Cengage Learning. All rights reserved 14
Section 10. 3 Exothermic and Endothermic Processes Energy Changes and Chemical Reactions 4 C 3 H 5 N 3 O 9 6 N 2 + 10 H 2 O + 12 CO 2 + 5720 k. J Nitroglycerine Exothermic Return to TOC Copyright © Cengage Learning. All rights reserved 15
Section 10. 3 Exothermic and Endothermic Processes • Endothermic Process: § Heat flow is into a system. § Absorb energy from the surroundings. • Exothermic Process: § Energy flows out of the system. • Energy gained by the surroundings must be equal to the energy lost by the system. Return to TOC Copyright © Cengage Learning. All rights reserved 16
Section 10. 3 Exothermic and Endothermic Processes Concept Check Is the freezing of water an endothermic or exothermic process? Explain. Return to TOC Copyright © Cengage Learning. All rights reserved 17
Section 10. 3 Exothermic and Endothermic Processes Concept Check Classify each process as exothermic or endothermic. Explain. The system is underlined in each example. Exo Endo a) b) c) d) e) Your hand gets cold when you touch ice. The ice gets warmer when you touch it. Water boils in a kettle being heated on a stove. Water vapor condenses on a cold pipe. Ice cream melts. Copyright © Cengage Learning. All rights reserved Return to TOC 18
Section 10. 3 Exothermic and Endothermic Processes Concept Check For each of the following, define a system and its surroundings and give the direction of energy transfer. a) b) Methane is burning in a Bunsen burner in a laboratory. System=burner, surroundings= lab, energy to system Water drops, sitting on your skin after swimming, evaporate. System=drops, surroundings= air, energy to surroundings Return to TOC Copyright © Cengage Learning. All rights reserved 19
Section 10. 3 Exothermic and Endothermic Processes Concept Check Hydrogen gas and oxygen gas react violently to form water. § Which is lower in energy: a mixture of hydrogen and oxygen gases, or water? Explain. Return to TOC Copyright © Cengage Learning. All rights reserved 20
Section 10. 4 Thermodynamics • • Study of energy Law of conservation of energy is often called the first law of thermodynamics. § The energy of the universe is constant. Return to TOC Copyright © Cengage Learning. All rights reserved 21
Section 10. 4 Thermodynamics Internal Energy • • Internal energy E of a system is the sum of the kinetic and potential energies of all the “particles” in the system. To change the internal energy of a system: ΔE = q + w q represents heat w represents work Return to TOC Copyright © Cengage Learning. All rights reserved 22
Section 10. 4 Thermodynamics Internal Energy • • Sign reflects the system’s point of view. Endothermic Process: § q is positive • Exothermic Process: § q is negative Return to TOC Copyright © Cengage Learning. All rights reserved 23
Section 10. 4 Thermodynamics Internal Energy • • Sign reflects the system’s point of view. System does work on surroundings: § w is negative • Surroundings do work on the system: § w is positive Return to TOC Copyright © Cengage Learning. All rights reserved 24
Section 10. 4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 25
Section 10. 4 Thermodynamics Concept Check Determine the sign of E for each of the following with the listed conditions: a) An endothermic process that performs work. § |work| > |heat| Δ E = negative § |work| < |heat| Δ E = positive b) Work is done on a gas and the process is exothermic. § |work| > |heat| Δ E = positive § |work| < |heat| Δ E = negative Return to TOC Copyright © Cengage Learning. All rights reserved 26
Section 10. 5 Measuring Energy Changes • The common energy units for heat are the calorie and the joule. § calorie – the amount of energy (heat) required to raise the temperature of one gram of water 1 o. C. § Joule – 1 calorie = 4. 184 joules Return to TOC Copyright © Cengage Learning. All rights reserved 27
Section 10. 5 Measuring Energy Changes Example Convert 60. 1 cal to joules. Return to TOC Copyright © Cengage Learning. All rights reserved 28
Section 10. 5 Measuring Energy Changes Energy (Heat) Required to Change the Temperature of a Substance Depends On: 1. The amount of substance being heated (number of grams). 2. The temperature change (number of degrees). 3. The identity of the substance. § Specific heat capacity is the energy required to change the temperature of a mass of one gram of a substance by one Celsius degree. Return to TOC Copyright © Cengage Learning. All rights reserved 29
Section 10. 5 Measuring Energy Changes Specific Heat Capacities of Some Common Substances Return to TOC Copyright © Cengage Learning. All rights reserved 30
Section 10. 5 Measuring Energy Changes To Calculate the Energy Required for a Reaction: • Energy (heat) required, Q = s × m × ΔT Q = energy (heat) required (J) s = specific heat capacity (J/°C·g) m = mass (g) ΔT = change in temperature (°C) Return to TOC Copyright © Cengage Learning. All rights reserved 31
Section 10. 5 Measuring Energy Changes Energy of a Snickers Bar There are 311 Cal in a Snickers Bar. If you added this much heat to a gallon of water at 25 o. C what would be the final temperature? 311 Cal = 311, 000 cal 1 gallon of water weighs 3960. 4 g Heat = SH x m x ΔT 311, 000 cal = 1. 000 cal/go. C x 3960. 4 g x ? o. C Solving for the change of temperature gives us 78. 5 o. C Add this to the 25 o. C to get 104 o. C which is above the boiling point of water (100 o. C) Return to TOC Copyright © Cengage Learning. All rights reserved 32
Section 10. 5 Measuring Energy Changes Example Calculate the amount of heat energy (in joules) needed to raise the temperature of 6. 25 g of water from 21. 0°C to 39. 0°C. Where are we going? • We want to determine the amount of energy needed to increase the temperature of 6. 25 g of water from 21. 0°C to 39. 0°C. What do we know? • The mass of water and the temperature increase. Return to TOC Copyright © Cengage Learning. All rights reserved 33
Section 10. 5 Measuring Energy Changes Example Calculate the amount of heat energy (in joules) needed to raise the temperature of 6. 25 g of water from 21. 0°C to 39. 0°C. What information do we need? • We need the specific heat capacity of water. § 4. 184 J/g°C How do we get there? Return to TOC Copyright © Cengage Learning. All rights reserved 34
Section 10. 5 Measuring Energy Changes Exercise A sample of pure iron requires 142 cal of energy to raise its temperature from 23ºC to 92ºC. What is the mass of the sample? (The specific heat capacity of iron is 0. 45 J/gºC. ) a) b) c) d) 0. 052 g 4. 6 g 19 g 590 g Return to TOC Copyright © Cengage Learning. All rights reserved 35
Section 10. 5 Measuring Energy Changes Concept Check A 100. 0 g sample of water at 90°C is added to a 100. 0 g sample of water at 10°C. The final temperature of the water is: a) Between 50°C and 90°C b) 50°C c) Between 10°C and 50°C (90 o(100 g)+ 10 o(100 g))/200 g. T = 50 o Return to TOC Copyright © Cengage Learning. All rights reserved 36
Section 10. 5 Measuring Energy Changes Concept Check A 100. 0 g sample of water at 90. °C is added to a 500. 0 g sample of water at 10. °C. The final temperature of the water is: a) Between 50°C and 90°C b) 50°C c) Between 10°C and 50°C Calculate the final temperature of the water. (90 o(100 g)+ 10 o(500 g))/600 g. T = 23 o Return to TOC Copyright © Cengage Learning. All rights reserved 37
Section 10. 5 Measuring Energy Changes Concept Check You have a Styrofoam cup with 50. 0 g of water at 10. C. You add a 50. 0 g iron ball at 90. C to the water. (s. H 2 O = 4. 18 J/°C·g and s. Fe = 0. 45 J/°C·g) The final temperature of the water is: a) Between 50°C and 90°C b) 50°C c) Between 10°C and 50°C Calculate the final temperature of the water. 18°C 50. 0 g. Fe(0. 45 SHFe)(90 -Xo. C)=50. 0 gw(4. 18 SHw)(X-10 o. C) Return to TOC Copyright © Cengage Learning. All rights reserved 38
Section 10. 6 Thermochemistry (Enthalpy) Change in Enthalpy • • State function ΔH = q at constant pressure (ΔHp = heat) Return to TOC Copyright © Cengage Learning. All rights reserved 39
Section 10. 6 Thermochemistry (Enthalpy) Example Consider the reaction: S(s) + O 2(g) → SO 2(g) ΔH = – 296 k. J per mole of SO 2 formed. Calculate the quantity of heat released when 2. 10 g of sulfur is burned in oxygen at constant pressure. Where are we going? • We want to determine ΔH for the reaction of 2. 10 g of S with oxygen at constant pressure. What do we know? • • When 1 mol SO 2 is formed, – 296 k. J of energy is released. We have 2. 10 g S. Copyright © Cengage Learning. All rights reserved Return to TOC 40
Section 10. 6 Thermochemistry (Enthalpy) Example Consider the reaction: S(s) + O 2(g) → SO 2(g) ΔH = – 296 k. J per mole of SO 2 formed. Calculate the quantity of heat released when 2. 10 g of sulfur is burned in oxygen at constant pressure. How do we get there? Return to TOC Copyright © Cengage Learning. All rights reserved 41
Section 10. 6 Thermochemistry (Enthalpy) Exercise Consider the combustion of propane: C 3 H 8(g) + 5 O 2(g) → 3 CO 2(g) + 4 H 2 O(l) ΔH = – 2221 k. J Assume that all of the heat comes from the combustion of propane. Calculate ΔH in which 5. 00 g of propane is burned in excess oxygen at constant pressure. – 252 k. J Return to TOC Copyright © Cengage Learning. All rights reserved 42
Section 10. 6 Thermochemistry (Enthalpy) Calorimetry • Enthalpy, H is measured using a calorimeter. Return to TOC Copyright © Cengage Learning. All rights reserved 43
Section 10. 6 Thermochemistry (Enthalpy) • In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. Return to TOC Copyright © Cengage Learning. All rights reserved 44
Section 10. 6 Thermochemistry (Enthalpy) N 2(g) + 2 O 2(g) → 2 NO 2(g) • ΔH 1 = 68 k. J This reaction also can be carried out in two distinct steps, with enthalpy changes designated by ΔH 2 and ΔH 3. N 2(g) + O 2(g) → 2 NO(g) + O 2(g) → 2 NO 2(g) N 2(g) + 2 O 2(g) → 2 NO 2(g) ΔH 2 = 180 k. J ΔH 3 = – 112 k. J ΔH 2 + ΔH 3 = 68 k. J ΔH 1 = ΔH 2 + ΔH 3 = 68 k. J Return to TOC Copyright © Cengage Learning. All rights reserved 45
Section 10. 6 Thermochemistry (Enthalpy) Characteristics of Enthalpy Changes • • If a reaction is reversed, the sign of ΔH is also reversed. The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of ΔH is multiplied by the same integer. Return to TOC Copyright © Cengage Learning. All rights reserved 46
Section 10. 6 Thermochemistry (Enthalpy) Example • Consider the following data: • Calculate ΔH for the reaction Return to TOC Copyright © Cengage Learning. All rights reserved 47
Section 10. 6 Thermochemistry (Enthalpy) Problem-Solving Strategy • • • Work backward from the required reaction, using the reactants and products to decide how to manipulate the other given reactions at your disposal. Reverse any reactions as needed to give the required reactants and products. Multiply reactions to give the correct numbers of reactants and products. Return to TOC Copyright © Cengage Learning. All rights reserved 48
Section 10. 6 Thermochemistry (Enthalpy) Example • Reverse the two reactions: • Desired reaction: Return to TOC Copyright © Cengage Learning. All rights reserved 49
Section 10. 6 Thermochemistry (Enthalpy) Example • Multiply reactions to give the correct numbers of reactants and products: 4( 3( • ) 4( ) 3( ) ) Desired reaction: Return to TOC Copyright © Cengage Learning. All rights reserved 50
Section 10. 6 Thermochemistry (Enthalpy) Example • Final reactions: • Desired reaction: ΔH = +1268 k. J Return to TOC Copyright © Cengage Learning. All rights reserved 51
Section 10. 6 Thermochemistry (Enthalpy) Concept Check Calculate ΔH for the reaction: SO 2 + ½O 2 → SO 3 ΔH = ? Given: (1) S + O 2 → SO 2 (2) 2 S + 3 O 2 → 2 SO 3 a) b) c) d) – 693 k. J 101 k. J 693 k. J – 99 k. J ΔH = – 297 k. J ΔH = – 792 k. J Turn the first reaction around and divide the second reaction by 2. Do the same with the ΔH’s. -792/2 + 297 = -99 k. J Return to TOC Copyright © Cengage Learning. All rights reserved 52
Section 10. 6 Thermochemistry (Enthalpy) • • • When we use energy to do work we degrade its usefulness. While the total amount or quantity of energy in the universe is constant (1 st Law) the quality of energy is degraded as it is used. Burning of petroleum: Return to TOC Copyright © Cengage Learning. All rights reserved 53
Section 10. 6 Thermochemistry (Enthalpy) Fossil Fuels • Carbon based molecules from decomposing plants and animals § Energy source for United States Return to TOC Copyright © Cengage Learning. All rights reserved 54
Section 10. 6 Thermochemistry (Enthalpy) Petroleum • Thick liquids composed of mainly hydrocarbons. § Hydrocarbon – compound composed of C and H. Return to TOC Copyright © Cengage Learning. All rights reserved 55
Section 10. 6 Thermochemistry (Enthalpy) Natural Gas • Gas composed of hydrocarbons. Return to TOC Copyright © Cengage Learning. All rights reserved 56
Section 10. 6 Thermochemistry (Enthalpy) Coal • Formed from the remains of plants under high pressure and heat over time. Return to TOC Copyright © Cengage Learning. All rights reserved 57
Section 10. 6 Thermochemistry (Enthalpy) Effects of Carbon Dioxide on Climate • Greenhouse Effect Return to TOC Copyright © Cengage Learning. All rights reserved 58
Section 10. 6 Thermochemistry (Enthalpy) Effects of Carbon Dioxide on Climate • Atmospheric CO 2 § Controlled by water cycle § Could increase temperature by 10 o. C Return to TOC Copyright © Cengage Learning. All rights reserved 59
Section 10. 6 Thermochemistry (Enthalpy) New Energy Sources • • • Solar Nuclear Biomass Wind Synthetic fuels Return to TOC Copyright © Cengage Learning. All rights reserved 60
Section 10. 6 Thermochemistry (Enthalpy) • • Natural processes occur in the direction that leads to an increase in the disorder of the universe. Example § Consider a gas trapped as shown Return to TOC Copyright © Cengage Learning. All rights reserved 61
Section 10. 6 Thermochemistry (Enthalpy) • What happens when the valve is opened? Return to TOC Copyright © Cengage Learning. All rights reserved 62
Section 10. 6 Thermochemistry (Enthalpy) Two Driving Forces • • Energy spread Matter spread Gives off energy when solid forms. Spreads out when dissolves. Return to TOC Copyright © Cengage Learning. All rights reserved 63
Section 10. 6 Thermochemistry (Enthalpy) Energy Spread • In a given process concentrated energy is dispersed widely. • This happens in every exothermic process. Return to TOC Copyright © Cengage Learning. All rights reserved 64
Section 10. 6 Thermochemistry (Enthalpy) Matter Spread • Molecules of a substance spread out to occupy a larger volume. • Processes are favored if they involve energy and matter spread. Return to TOC Copyright © Cengage Learning. All rights reserved 65
Section 10. 6 Thermochemistry (Enthalpy) Entropy, S • • Function which keeps track of the tendency for the components of the universe to become disordered. Measure of disorder or randomness Return to TOC Copyright © Cengage Learning. All rights reserved 66
Section 10. 6 Thermochemistry (Enthalpy) Entropy, S • What happens to the disorder in the universe as energy and matter spread? Return to TOC Copyright © Cengage Learning. All rights reserved 67
Section 10. 6 Thermochemistry (Enthalpy) Second Law of Thermodynamics • The entropy of the universe is always increasing. The heat death of the universe will occur when all particles of matter ultimately have the same average kinetic energy and exist in a state of maximum disorder. Return to TOC Copyright © Cengage Learning. All rights reserved 68
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