Chapter 10 Energy

Chapter 10 Table of Contents Copyright © Cengage Learning. All rights reserved The Nature of Energy 10.2 Temperature and Heat 10.3 Exothermic and Endothermic Processes 10.4 Thermodynamics 10.5 Measuring Energy Changes 10.6Thermochemistry (Enthalpy) 10.7 Hess’s Law 10.8 Quality Versus Quantity of Energy 10.9 Energy and Our World 10.10Energy as a Driving Force

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 3 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. Energy

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 4 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. Energy

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 5 Types of Energy 1. Kinetic energy 2. Potential energy 3. Chemical energy 4. Heat energy 5. Electric energy 6. Radiant energy Energy Transformations

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 6 In the initial position, ball A has a higher potential energy than ball B. Initial Position

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 7 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. Final Position

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 8 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 Energy

Section 10.1 The Nature of Energy Return to TOC Copyright © Cengage Learning. All rights reserved 9 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) State Functions?

Section 10.2 Temperature and Heat Return to TOC Copyright © Cengage Learning. All rights reserved 10 A measure of the random motions of the components of a substance. Temperature

Section 10.2 Temperature and Heat Return to TOC Copyright © Cengage Learning. All rights reserved 11 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. Heat

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 12 System – part of the universe on which we wish to focus attention. Surroundings – include everything else in the universe.

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 13 Energy Changes Accompanying the Burning of a Match

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved Energy Changes and Chemical Reactions 4 C 3 H 5 N 3 O 9  6 N H 2 O + 12 CO 2 + O kJ Nitroglycerine Exothermic

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 15 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.

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 16 Concept Check Is the freezing of water an endothermic or exothermic process? Explain.

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 17 Concept Check Classify each process as exothermic or endothermic. Explain. The system is underlined in each example. a)Your hand gets cold when you touch ice. b)The ice gets warmer when you touch it. c)Water boils in a kettle being heated on a stove. d)Water vapor condenses on a cold pipe. e)Ice cream melts. Exo Endo Exo Endo

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 18 Concept Check For each of the following, define a system and its surroundings and give the direction of energy transfer. a)Methane is burning in a Bunsen burner in a laboratory. b)Water drops, sitting on your skin after swimming, evaporate.

Section 10.3 Exothermic and Endothermic Processes Return to TOC Copyright © Cengage Learning. All rights reserved 19 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.

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 20 Study of energy Law of conservation of energy is often called the first law of thermodynamics.  The energy of the universe is constant.

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 21 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 Internal Energy

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 22 Sign reflects the system’s point of view. Endothermic Process:  q is positive Exothermic Process:  q is negative Internal Energy

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 23 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 Internal Energy

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 24

Section 10.4 Thermodynamics Return to TOC Copyright © Cengage Learning. All rights reserved 25 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|  |work| < |heat| b)Work is done on a gas and the process is exothermic.  |work| > |heat|  |work| < |heat| Δ E = negative Δ E = positive Δ E = negative

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 26 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 = joules

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 27 Convert 60.1 cal to joules. Example

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 28 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. Energy (Heat) Required to Change the Temperature of a Substance Depends On:

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 29 Specific Heat Capacities of Some Common Substances

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 30 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) To Calculate the Energy Required for a Reaction:

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 31 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 311,000 cal = cal/g o C x g x ? o C 1 gallon of water weighs g 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) Heat = SH x m x ΔT

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 32 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. Example

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 33 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.  J/g°C How do we get there? Example

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 34 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)0.052 g b)4.6 g c)19 g d)590 g

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 35 Concept Check A g sample of water at 90°C is added to a 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 (100g)+ 10 o (100g))/200g T = 50 o

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 36 Concept Check A g sample of water at 90.°C is added to a 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 (100g)+ 10 o (500g))/600g T = 23 o

Section 10.5 Measuring Energy Changes Return to TOC Copyright © Cengage Learning. All rights reserved 37 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 H2O = 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

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 38 State function ΔH = q at constant pressure (ΔH p = heat) Change in Enthalpy

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 39 Consider the reaction: S(s) + O 2 (g) → SO 2 (g) ΔH = –296 kJ 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 kJ of energy is released. We have 2.10 g S. Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 40 Consider the reaction: S(s) + O 2 (g) → SO 2 (g) ΔH = –296 kJ 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? Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 41 Exercise Consider the combustion of propane: C 3 H 8 (g) + 5O 2 (g) → 3CO 2 (g) + 4H 2 O(l) ΔH = –2221 kJ 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 kJ

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 42 Calorimetry Enthalpy, H is measured using a calorimeter.

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 43 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.

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 44 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) → 2NO(g) ΔH 2 = 180 kJ 2NO(g) + O 2 (g) → 2NO 2 (g) ΔH 3 = – 112 kJ N 2 (g) + 2O 2 (g) → 2NO 2 (g) ΔH 2 + ΔH 3 = 68 kJ ΔH 1 = ΔH 2 + ΔH 3 = 68 kJ N 2 (g) + 2O 2 (g) → 2NO 2 (g) ΔH 1 = 68 kJ

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 45 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. Characteristics of Enthalpy Changes

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 46 Consider the following data: Calculate ΔH for the reaction Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 47 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. Problem-Solving Strategy

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 48 Reverse the two reactions: Desired reaction: Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 49 Multiply reactions to give the correct numbers of reactants and products: 4( ) 4( ) 3( ) 3( ) Desired reaction: Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 50 Final reactions: Desired reaction: ΔH = kJ Example

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 51 Concept Check Calculate ΔH for the reaction: SO 2 + ½O 2 → SO 3 ΔH = ? Given:(1) S + O 2 → SO 2 ΔH = –297 kJ (2) 2S + 3O 2 → 2SO 3 ΔH = –792 kJ a)–693 kJ b)101 kJ c)693 kJ d)–99 kJ Turn the first reaction around and divide the second reaction by 2. Do the same with the ΔH’s. -792/ = -99 kJ

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 52 When we use energy to do work we degrade its usefulness. While the total amount or quantity of energy in the universe is constant (1st Law) the quality of energy is degraded as it is used. Burning of petroleum:

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 53 Carbon based molecules from decomposing plants and animals  Energy source for United States Fossil Fuels

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 54 Thick liquids composed of mainly hydrocarbons.  Hydrocarbon – compound composed of C and H. Petroleum

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 55 Gas composed of hydrocarbons. Natural Gas

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 56 Formed from the remains of plants under high pressure and heat over time. Coal

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 57 Greenhouse Effect Effects of Carbon Dioxide on Climate

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 58 Atmospheric CO 2  Controlled by water cycle  Could increase temperature by 10 o C Effects of Carbon Dioxide on Climate

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 59 Solar Nuclear Biomass Wind Synthetic fuels New Energy Sources

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 60 Natural processes occur in the direction that leads to an increase in the disorder of the universe. Example  Consider a gas trapped as shown

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 61 What happens when the valve is opened?

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 62 Energy spread Matter spread Two Driving Forces Gives off energy when solid forms. Spreads out when dissolves.

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 63 In a given process concentrated energy is dispersed widely. This happens in every exothermic process. Energy Spread

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 64 Molecules of a substance spread out to occupy a larger volume. Processes are favored if they involve energy and matter spread. Matter Spread

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 65 Function which keeps track of the tendency for the components of the universe to become disordered. Measure of disorder or randomness Entropy, S

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 66 What happens to the disorder in the universe as energy and matter spread? Entropy, S

Section 10.6 Thermochemistry (Enthalpy) Return to TOC Copyright © Cengage Learning. All rights reserved 67 The entropy of the universe is always increasing. Second Law of Thermodynamics 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.