Chapter 5: Thermochemistry - Chemistry Lecture PDF

Chemistry: The Central Science Fifteenth Edition Chapter 5 Thermochemistry Copyright © 2023 Pearson Education, Inc. All Rights Reserved Energy From Chapter 1, energy is the ability to do...

Chemistry: The Central Science Fifteenth Edition Chapter 5 Thermochemistry Copyright © 2023 Pearson Education, Inc. All Rights Reserved Energy From Chapter 1, energy is the ability to do work or transfer heat. This chapter is about thermodynamics, which is the study of energy and its transformations. Specifically, thermochemistry is the study of chemical reactions and the energy changes that involve heat. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.1 The Nature of Chemical Energy The most important form of potential energy in charged particles is electrostatic potential energy Eel : kQ1Q2 Eel  d Reminder: The unit of energy commonly used is the Joule: kg m2 1 J 1 2 s Copyright © 2023 Pearson Education, Inc. All Rights Reserved Attraction Between Ions Electrostatic attraction is seen between oppositely charged ions. Energy is released when chemical bonds are formed Eel  0 ; energy is consumed E el  0 when chemical bonds are broken. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.2 First Law of Thermodynamics Energy can be converted from one form to another, but it is neither created nor destroyed. To heat your home, chemical energy needs to be converted to heat. Sunlight is converted to chemical energy in green plants. There are many more examples of conversion of energy. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Definitions: System and Surroundings The portion of the universe that we single out to study is called the system (here, the hydrogen and oxygen molecules). – A chemical reaction represents a system. The surroundings are everything else (here, the cylinder, piston, and everything beyond). Copyright © 2023 Pearson Education, Inc. All Rights Reserved Types of Systems 1) Open System: A region of the universe being studied that can exchange heat and mass with its surroundings. 2) Closed System: A region of the universe being studied that can only exchange heat with its surroundings (not mass). 3) Isolated System: A region of the universe that cannot exchange heat or mass with its surroundings. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Internal Energy By definition, the change in internal energy, E, is the final energy of the system minus the initial energy of the system: E Efinal  Einitial Delta E is a state function which depends only on the initial and final states. It does not depend on path. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Thermodynamic Quantities Have Three Parts 1) A number 2) A unit 3) A sign Note about the sign: – A positive E results when the system gains energy from the surroundings. – A negative E results when the system loses energy to the surroundings. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Internal Energy (1 of 3) The internal energy of a system is the sum of all kinetic and potential energies of all components of the system. Represented by the symbol E. We generally don’t know E, only how it changes ( E ). Copyright © 2023 Pearson Education, Inc. All Rights Reserved Internal Energy (2 of 3) IF : E  0, Efinal  Einitial (the red arrow), the system released energy to the surroundings. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Internal Energy (3 of 3) IF : E  0, Efinal  Einitial (the blue arrow), the system absorbed energy from the surroundings. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Relating E , to Heat and Work delta E, When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w). That is, E q  w. Copyright © 2023 Pearson Education, Inc. All Rights Reserved  EE,, Q, W, and Their Signs delta Table 5.1 Sign Conventions for q, w, and E For q + means system gains heat  means system loses heat For w + means work done on system  means work done by system + means net gain of energy by  means net loss of energy by For E system system Copyright © 2023 Pearson Education, Inc. All Rights Reserved Exchange of Heat Between System and Surroundings (1 of 2) When heat is absorbed by the system from the surroundings, the process is endothermic. Note the temperature drop. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Exchange of Heat Between System and Surroundings (2 of 2) When heat is released by the system into the surroundings, the process is exothermic. Copyright © 2023 Pearson Education, Inc. All Rights Reserved State Functions (1 of 3) Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem. However, we do know that the internal energy of a system is independent of the path by which the system achieved that state. – In the system below, the water could have reached room temperature from either direction. Copyright © 2023 Pearson Education, Inc. All Rights Reserved State Functions (2 of 3) Therefore, internal energy is a state function. It depends only on the present state of the system, not on the path by which the system arrived at that state. And so, E depends only on Einitial and Efinal. Copyright © 2023 Pearson Education, Inc. All Rights Reserved State Functions (3 of 3) Note q and w are not state functions. They are path dependent. Whether the battery is shorted out or is discharged by by running the fan, its E is the same, but q and w are different in the two cases. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.3 Enthalpy (1 of 3) If a process takes place at constant pressure (and we usually work at atmospheric pressure) and the only work done is this pressure–volume work, we can account for heat flow during the process by measuring the enthalpy (H) of the system. Enthalpy is defined as the internal energy plus the product of pressure and volume: H E  PV Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.3 Enthalpy (2 of 3) When the system changes at constant pressure, the change in enthalpy, H, is H (E  PV ) This can also be written H E  P V Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.3 Enthalpy (3 of 3) Since E q  w and w  P V , we can substitute these into the enthalpy expression: H E  P V H (q  w )  w H q So, at constant pressure, the change in enthalpy is the heat gained or lost. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Enthalpy Change and Heat A process is endothermic when H is positive. A process is exothermic when H is negative. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Pressure–Volume Work (1 of 2) Usually the only work done by chemical or physical change is the mechanical work associated with a change in volume of gas. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Pressure–Volume Work (2 of 2) We can measure the work done by the gas in a reaction done in a vessel that has been fitted with a piston: w  P V The work is negative because it is work done by the system. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.4 Enthalpies of Reaction The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants: Hrxn Hproducts  Hreactants Copyright © 2023 Pearson Education, Inc. All Rights Reserved Heat of Reaction This quantity, Hrxn , is called the enthalpy of reaction, or the heat of reaction. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Enthalpy Guidelines 1. Enthalpy is an extensive property. Depends upon amount, i.e., moles of reactant. 2. The enthalpy change for a reaction is equal in magnitude, but opposite in sign, to H for the reverse reaction. If the forward reaction enthalpy change is  890 kJ, the reverse reaction change is +890 kJ. 3. The enthalpy change for a reaction depends on the states (phases) of the reactants and the products. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.5 Calorimetry Since we cannot know the exact enthalpy of the reactants and products, we measure H using calorimetry, the measurement of heat flow. The instrument used to measure heat flow is called a calorimeter. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Heat Capacity and Specific Heat The amount of energy required to raise the temperature of a substance by 1K (1°C) is its heat capacity. If the amount of the substance heated is one gram, it is the specific heat. If the amount is one mole, it is the molar heat capacity. Table 5.2 Specific Heats of Some Substances at 298 K Elements Elements Compounds Compounds Substance Specific Heat Substance Specific Heat J g - K  J g - K  left parenthesis joule per gram kelvin right parenthesis left parenthesis joule per gram kelvin right parenthesis N2 g  N 2, gaseousN 2, gas 1.04 H2 O l  H 2 O, liquid 4.18 Al(g) 0.90 CH4 g  C H 4, gas 2.20 Fe(s) 0.45 CO2 g  C O 2, gas 0.84 Hg(l). 0.14 CaCO3 s  C a C O 3, solid 0.82 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Constant-Pressure Calorimetry By carrying out a reaction in aqueous solution in a simple calorimeter, the heat change for the system can be found by measuring the heat change for the water in the calorimeter. The specific heat for water is 4.184 J/g K. We use this value for dilute solutions. We can calculate H for the reaction with this equation: qsoln Cs msoln T  qrxn Copyright © 2023 Pearson Education, Inc. All Rights Reserved Bomb Calorimetry (Constant Volume) (1 of 2) Reactions can be carried out in a sealed “bomb” such as this one. The heat absorbed (or released) by the water is a very good approximation of the enthalpy change for the reaction. qrxn  Ccal T Copyright © 2023 Pearson Education, Inc. All Rights Reserved Bomb Calorimetry (Constant Volume) (2 of 2) Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H. For most reactions, the difference is very small and we can equate the two. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.6 Hess’s Law (1 of 2) H is well known for many reactions. It is often inconvenient to measure​ H for every reaction in which we are interested.​ However, we can calculate H using published H standard values and the properties of enthalpy.​ Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.6 Hess’s Law (2 of 2) Hess’s law: If a reaction is carried out in a series of steps, H for the overall reaction equals the sum of the enthalpy changes for the individual steps. Because H is a state function, for a particular set of reactants and products, H is the same whether the reaction takes place in one step or in a series of steps. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.7 Enthalpies of Formation An enthalpy of formation,​ Hf , is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms, i.e., for ammonia, it is nitrogen(g) and hydrogen(g).​ – The coefficient of the compound must be one. 1 2 N2 ( g)  3/ 2 H2 ( g) NH3 ( g) when necessary, use fractional coefficients Copyright © 2023 Pearson Education, Inc. All Rights Reserved Standard Enthalpies of Formation (1 of 4) Standard enthalpies of formation, H f , are measured under standard conditions (25  C and 1.00 atm pressure). Copyright © 2023 Pearson Education, Inc. All Rights Reserved Standard Enthalpies of Formation (2 of 4) Table 5.3 Standard Enthalpies of Formation, Hf , at 298 K Substance Formula ΔH°f kJ mol  delta H degree sub f, kilo joule per mole Acetylene C2H2 (g ) C 2 H 2, gas 226.7 Ammonia NH3 (g ) N H 3, gas  46.19 negative 46.19 Benzene C6H6 (l ) C 6 H 6, liquid 49.0 Calcium carbonate CaCO3 (s ) C a C O 3, solid  1207.1 negative 1207.1 Calcium oxide CaO (s ) C a O, solid  635.5 negative 635.5 Carbon dioxide CO2 (g ) C O 2, gas  393.5 negative 393.5 Carbon monoxide CO (g ) C O, gas  110.5 negative 110.5 Diamond C (s ) C, solid 1.88 Ethane C2H6 (g ) C 2 H 6, gas  84.68 negative 84.68 Ethanol C2H5OH(l ) C 2 H 5 O H, liquid  277.7 negative 277.7 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Standard Enthalpies of Formation (3 of 4) Table 5.3 [continued] Substance Formula ΔH°f kJ mol  delta H degree sub f, kilo joule per mole Ethylene C2H4 (g ) C 2 H 4, gas 52.30 Glucose C6H12O6 (s ) C 6 H 12 O 6, solid  1273 negative 1273 Hydrogen bromide HBr (g ) H B r, gas  36.23 negative 36.23 Hydrogen chloride HCl(g ) H C l, gas  92.30 negative 92.30 Hydrogen fluoride HF (g ) H F, gas  268.60 negative 268.60 Hydrogen iodide HI(g ) H l, gas 25.9 Methane CH4 (g ) C H 4, gas  74.80 negative 74.80 Methanol CH3OH(l ) C H 3 O H, liquid  238.6 negative 238.6 Propane C3H8 (g ) C 3 H 8, gas  103.85 negative 103.85 Silver chloride AgCl (s ) A g C l, solid  127.0 negative 127.0 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Standard Enthalpies of Formation (4 of 4) Table 5.3 [continued] Substance Formula ΔH°f kJ mol  delta H degree sub f, kilo joule per mole Sodium bicarbonate NaHCO3 (s ) N a H C O 3, solid  947.7 negative 947.7 Sodium carbonate Na2CO3 (s ) N a 2 C O 3, solid  1130.9 negative 1130.9 Sodium chloride NaCl(s ) N a C l, solid  410.9 negative 410.9 Sucrose C12H22O11 (s ) C 12 H 22 O 11, solid  2221 negative 2221 Water H2O (l ) H 2 O, liquid  285.8 negative 285.8 Water vapor H2O (g ) H 2 O, gas  241.8 negative 241.8 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Breaking Down H delta H (1 of 5) C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) Imagine this reaction as occurring in three steps: 1) Decomposition of the reactant propane to the elements: C3H8 (g ) 3 C(graphite)  4 H2 (g ) Copyright © 2023 Pearson Education, Inc. All Rights Reserved Breaking Down H delta H (2 of 5) C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) 2) Formation of the product CO2 : 3 C(graphite)  3 O2 (g ) 3 CO2 (g ) Copyright © 2023 Pearson Education, Inc. All Rights Reserved Breaking Down H delta H (3 of 5) C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) 3) Formation of the product H2O : 4 H2 (g )  2 O2 (g ) 4 H2O(l ) Copyright © 2023 Pearson Education, Inc. All Rights Reserved Breaking Down H delta H (4 of 5) C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) Putting the steps together: C3H8 (g ) 3 C(graphite)  4 H2 (g ) 3 C(graphite)  3 O2 (g ) 3 CO2 (g ) 4 H2 (g )  2 O2 (g ) 4 H2O(l ) Copyright © 2023 Pearson Education, Inc. All Rights Reserved Breaking Down H delta H (5 of 5) C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) The sum of these equations is the overall equation. C3H8 (g ) 3 C(graphite)  4 H2 (g ) 3 C(graphite)  3 O2 (g ) 3 CO2 (g ) 4 H2 (g )  2 O2 (g ) 4 H2O(l ) C3H8 (g )  5 O2 (g ) 3 CO 2 (g )  4 H2O(l ) Copyright © 2023 Pearson Education, Inc. All Rights Reserved How to Calculate H delta H We can apply Hess’s law using tabulated values to obtain a numerical value for the heat of reaction: H   nHff, products   mH , reactants where n and m are the stoichiometric coefficients and heats of formation are taken from published values. The Hf 0 for an element in its standard state is zero by definition, i.e., H0f of O2 ( g) 0 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Use Values from the Standard Enthalpy Table C3H8 (g )  5 O2 (g ) 3 CO2 (g )  4 H2O(l ) H [3(  393.5 kJ)  4(  285.8 kJ)]  [1(  103.85 kJ)  5(0 kJ)] [(  1180.5 kJ)  (  1143.2 kJ)]  [(  103.85 kJ)  (0 kJ)] (  2323.7 kJ)  (  103.85 kJ)  2219.9 kJ Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.8 Bond Enthalpies The enthalpy associated with breaking one mole of a particular bond in a gaseous substance. The bond enthalpy is always positive because energy is required to break chemical bonds. The greater the bond enthalpy, the stronger the bond. Energy is always released when a bond forms between gaseous fragment. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Bond Enthalpies Table 5.4 Average Bond Enthalpies (kJ / mol) Bond Enthalpies left parenthesis kilo Joule per mole right parenthesis Bond Enthalpies left parenthesis kilo Joule per mole right parenthesis Bond Enthalpies left parenthesis kilo Joule per mole right parenthesis Bond Enthalpies left parenthesis kilo Joule per mole right parenthesis (kJ / mol) (kJ / mol) (kJ / mol) (kJ / mol) C H C single bond H 413 N H N single bond H 391 O H O single bond H 463 F F F single bond F 155 C C C single bond C 348 N N N single bond N 163 O OO single bond O 146 Blank Blank C  C C double bond C 614 N O N single bond O 201 O  O O double bond O 495 CI   F C l single bond F 253 C N C single bond N 293 N F N single bond F 272 O  F O double bond F 190 CI   CI C l single bond C l 242 C O C single bond O 358 N   CI N single bond C l 200 O   CI O single bond C l 203 Blank Blank C  O C double bond O 799 N   Br N single bond B r 243 O IO single bond l 234 Br   F B r single bond F 237 C  F C single bond F 485 Blank blank Blank Blank Br   CI B r single bond C l 218 C   CI C single bond C l 328 H H H single bond H 436 Blank Blank Br   Br B r single bond B r 193 C   Br C single bond B r 276 H F H single bond F 567 Blank Blank Blank Blank C I C single bond l 240 H   CI H single bond C l 431 Blank Blank I   CI I single bond C l 208 Blank Blank H   Br H single bond B r 366 Blank Blank I   Br I single bond B r 175 Blank Blank H I H single bond l 299 Blank Blank I I I single bond I 151 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Bond Enthalpies and Enthalpies of Reaction (1 of 2) We can predict whether a chemical reaction will be endothermic or exothermic using bond energies. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Bond Enthalpies and Enthalpies of Reaction (2 of 2) Add bond energy values for all bonds made (+) Subtract bond energy values for all bonds broken ( ) The result is an estimate of H.  bond enthalpies   bond enthalpies  Hrxn        of bonds broken   of bonds fromed  CH4  Cl2 HCl  CH3Cl 4C  H Cl  Cl H  Cl C  Cl 3C  H Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.9 Energy in Foods (1 of 3) The energy released when one gram of food is combusted is its fuel value. Table 5.5 Compositions and Fuel Values of Some Common Foods Approximate Approximate Approximate Blank Fuel Value Fuel Value Composition Composition Composition kilo Joule per gram kJ g Kilo calorie per gram left parenthesis Calorie per gram right parenthesis (% by Mass) (% by Mass) (% by Mass) kcal g (Cal g ) Carbohydrate Fat Protein Carbohydrate 100 Blank Blank 17 4 Fat Blank 100 Blank 38 9 Protein Blank Blank 100 17 4 Apples 13 0.5 0.4 2.5 0.59 Beer a 1.2 Blank 0.3 1.8 0.42 Bread 52 3 9 12 2.8 Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.9 Energy in Foods (2 of 3) Table 5.5 [continued] Approximate Approximate Approximate Blank Fuel Value Fuel Value Composition Composition Composition kilo Joule per gram Kilo calorie per gram left parenthesis Calorie per gram right parenthesis (% by Mass) (% by Mass) (% by Mass) kJ g kcal g (Cal g ) Carbohydrate Fat Protein Cheese 4 37 28 20 4.7 Eggs 0.7 10 13 6.0 1.4 Fudge 81 11 2 18 4.4 Green beans 7.0 Blank 1.9 1.5 0.38 Hamburger Blank 30 22 15 3.6 Milk (whole) 5.0 4.0 3.3 3.0 0.74 Peanuts 22 39 26 23 5.5 a Beer typically contains 3.5% ethanol, which has fuel value. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5.9 Energy in Foods (3 of 3) Most of the energy in foods comes from carbohydrates, fats, and proteins. Carbohydrates (17 kJ / g) : C6H12O6 (s ) + 6 O2 (g ) 6 CO2 (g ) + 6 H2O(l ) H ° =  2803 kJ Fats (38 kJ / g) : 2 C57H110O6 (s ) + 163 O 2 (g ) 114 CO 2 (g ) + 110 H2O(l ) H ° = 275,520 kJ Proteins produce 17 kJ / g (same as carbohydrates): Their chemical reaction in the body is not the same as in a calorimeter. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Energy in Fuels (1 of 2) The vast majority of the energy consumed in this country comes from fossil fuels. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Energy in Fuels (2 of 2) Table 5.6 Fuel Values and Compositions of Some Common Fuels Approximate Approximate Approximate Approximate Blank Elemental Elemental Elemental Elemental Composition Composition Composition Composition (Mass %) (Mass %) (Mass %) (Mass %) C H O Fuel Value left parenthesis Kilo Joule per gram right parenthesis Wood (pine) 50 6 44 18 (kJ g ) Anthracite coal 82 1 2 31 (Pennsylvania) Bituminous coal 77 5 7 32 (Pennsylvania) Charcoal 100 0 0 34 Crude oil (Texas) 85 12 0 45 Gasoline 85 15 0 48 Natural gas 70 23 0 49 Hydrogen 0 100 0 142 Copyright © 2023 Pearson Education, Inc. All Rights Reserved Other Energy Sources Nuclear fission produces 8.6% of the U.S. energy needs. Renewable energy sources, like solar, wind, geothermal, hydroelectric, and biomass sources produce 9.9% of the U.S. energy needs. Copyright © 2023 Pearson Education, Inc. All Rights Reserved Copyright This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Copyright © 2023 Pearson Education, Inc. All Rights Reserved