Chapter 5 Thermochemistry Chapter 6 Table of Contents Copyright Cengage Learning. All rights reserved The Nature of Energy 6.2 Enthalpy and Calorimetry 6.3 Hesss Law 6.4 Standard Enthalpies of Formation 6.5 Present Sources of Energy 6.6New Energy Sources Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 3 Capacity to do work or to 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 6.1 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 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 5 In the initial position, ball A has a higher potential energy than ball B. Initial Position Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 6 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 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 7 Heat involves the transfer of energy between two objects due to a temperature difference. Work force acting over a distance. Energy Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 8 System part of the universe on which we wish to focus attention. Surroundings include everything else in the universe. Chemical Energy Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 9 Endothermic Reaction: Heat flow is into a system. Absorb energy from the surroundings. Exothermic Reaction: Energy flows out of the system. Energy gained by the surroundings must be equal to the energy lost by the system. Chemical Energy Section 6.1 The Nature of Energy Return to TOC Diagram of Exothermic and Endothermic Process Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 11 Concept Check Is the freezing of water an endothermic or exothermic process? Explain. Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 12 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 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 13 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 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 14 Concept Check Hydrogen gas and oxygen gas react violently to form water. Explain. Which is lower in energy: a mixture of hydrogen and oxygen gases, or water? Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 15 Law of conservation of energy is often called the first law of thermodynamics. 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: E = q + w q represents heat w represents work Internal Energy Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 16 Work vs. Energy Flow Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 17 Sign reflects the systems point of view. Endothermic Process: q is positive Exothermic Process: q is negative Internal Energy Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 18 Sign reflects the systems point of view. System does work on surroundings: w is negative Surroundings do work on the system: w is positive Internal Energy Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 19 Work = P A h = PV P is pressure. A is area. h is the piston moving a distance. V is the change in volume. Work Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 20 For an expanding gas, V is a positive quantity because the volume is increasing. Thus V and w must have opposite signs: w = PV To convert between Latm and Joules, use 1 Latm = J. Work Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 21 Exercise Which of the following performs more work? a)A gas expanding against a pressure of 2 atm from 1.0 L to 4.0 L. b) A gas expanding against a pressure of 3 atm from 1.0 L to 3.0 L. They perform the same amount of work. Section 6.1 The Nature of Energy Return to TOC Copyright Cengage Learning. All rights reserved 22 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| E = negative E = positive E = negative Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 23 State function H = q at constant pressure H = H products H reactants Change in Enthalpy Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 24 Exercise Consider the combustion of propane: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) H = 2221 kJ Assume that all of the heat comes from the combustion of propane. Calculate H in which 5.00 g of propane is burned in excess oxygen at constant pressure. 252 kJ Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 25 Science of measuring heat Specific heat capacity: The energy required to raise the temperature of one gram of a substance by one degree Celsius. Molar heat capacity: The energy required to raise the temperature of one mole of substance by one degree Celsius. Calorimetry Section 6.2 Enthalpy and Calorimetry Return to TOC Specific Heat Capacities of Common Substances Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 27 If two reactants at the same temperature are mixed and the resulting solution gets warmer, this means the reaction taking place is exothermic. An endothermic reaction cools the solution. Calorimetry Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 28 A CoffeeCup Calorimeter Made of Two Styrofoam Cups Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 29 Energy released (heat) = s m T s = specific heat capacity (J/Cg) m = mass (g) T = change in temperature (C) Calorimetry Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 30 Concept Check A g sample of water at 90C is added to a g sample of water at 10C. The final temperature of the water is: a) Between 50C and 90C b) 50C c) Between 10C and 50C Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 31 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 50C and 90C b) 50C c) Between 10C and 50C Calculate the final temperature of the water. 23C Section 6.2 Enthalpy and Calorimetry Return to TOC Copyright Cengage Learning. All rights reserved 32 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/Cg and s Fe = 0.45 J/Cg) The final temperature of the water is: a) Between 50C and 90C b) 50C c) Between 10C and 50C Calculate the final temperature of the water. 18C Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 33 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 6.3 Hesss Law Return to TOC Standard Enthalpies of Formation for Compounds at 25 degrees C Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 35 This reaction also can be carried out in two distinct steps, with enthalpy changes designated by H 2 and H 3. N 2 (g) + O 2 (g) 2NO(g) H 2 = 180 kJ 2NO(g) + O 2 (g) 2NO 2 (g) H 3 = 112 kJ N 2 (g) + 2O 2 (g) 2NO 2 (g) H 2 + H 3 = 68 kJ H 1 = H 2 + H 3 = 68 kJ N 2 (g) + 2O 2 (g) 2NO 2 (g) H 1 = 68 kJ Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 36 The Principle of Hesss Law Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 37 Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 38 If a reaction is reversed, the sign of H is also reversed. The magnitude of H 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 H is multiplied by the same integer. Characteristics of Enthalpy Changes Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 39 Consider the following data: Calculate H for the reaction Example Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 40 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 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 41 Reverse the two reactions: Desired reaction: Example Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 42 Multiply reactions to give the correct numbers of reactants and products: 4( ) 4( ) 3( ) 3( ) Desired reaction: Example Section 6.3 Hesss Law Return to TOC Copyright Cengage Learning. All rights reserved 43 Final reactions: Desired reaction: H = kJ Example Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 44 Change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states. Standard Enthalpy of Formation (H f ) Section 6.4 Standard Enthalpies of Formation Return to TOC Standard Enthalpies of Formation for Compounds at 25 degrees C Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 46 For a Compound For a gas, pressure is exactly 1 atm. For a solution, concentration is exactly 1 M. Pure substance (liquid or solid) For an Element The form [N 2 (g), K(s)] in which it exists at 1 atm and 25C. Heat of formation is zero. Conventional Definitions of Standard States Section 6.4 Standard Enthalpies of Formation Return to TOC Figure 6.2: Combusion of Methane Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 48 A Schematic Diagram of the Energy Changes for the Reaction CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) H reaction = (75 kJ) (394 kJ) + (572 kJ) = 891 kJ Section 6.4 Standard Enthalpies of Formation Return to TOC Figure 6.8: Pathway for Combustion of Methane Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 50 1.When a reaction is reversed, the magnitude of H remains the same, but its sign changes. 2.When the balanced equation for a reaction is multiplied by an integer, the value of H for that reaction must be multiplied by the same integer. Problem-Solving Strategy: Enthalpy Calculations Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 51 3.The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products: H rxn = n p H f (products) - n r H f (reactants) 4.Elements in their standard states are not included in the H reaction calculations because H f for an element in its standard state is zero. Problem-Solving Strategy: Enthalpy Calculations Section 6.4 Standard Enthalpies of Formation Return to TOC Copyright Cengage Learning. All rights reserved 52 Exercise Calculate H for the following reaction: 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Given the following information: H f (kJ/mol) Na(s)0 H 2 O(l) 286 NaOH(aq) 470 H 2 (g)0 H = 368 kJ Section 6.5 Present Sources of Energy Return to TOC Copyright Cengage Learning. All rights reserved 53 Fossil Fuels Petroleum, Natural Gas, and Coal Wood Hydro Nuclear Section 6.5 Present Sources of Energy Return to TOC Copyright Cengage Learning. All rights reserved 54 Energy Sources Used in the United States Section 6.5 Present Sources of Energy Return to TOC Copyright Cengage Learning. All rights reserved 55 Transparent to visible light from the sun. Visible light strikes the Earth, and part of it is changed to infrared radiation. Infrared radiation from Earths surface is strongly absorbed by CO 2, H 2 O, and other molecules present in smaller amounts in atmosphere. Atmosphere traps some of the energy and keeps the Earth warmer than it would otherwise be. The Earths Atmosphere Section 6.5 Present Sources of Energy Return to TOC Copyright Cengage Learning. All rights reserved 56 The Earths Atmosphere Section 6.6 New Energy Sources Return to TOC Copyright Cengage Learning. All rights reserved 57 Coal Conversion Hydrogen as a Fuel Other Energy Alternatives Oil shale Ethanol Methanol Seed oil